United States Department of Agriculture

Forest 

Service

Pacific Southwest 

Research Station

General Technical Report

PSW-GTR-254 August 

2017

cover

The California Spotted Owl: Current 

State of Knowledge 
In accordance with Federal civil rights law and U.S. Department of Agriculture 

(USDA) civil rights regulations and policies, the USDA, its Agencies, offices, and 

employees, and institutions participating in or administering USDA programs are 

prohibited from discriminating based on race, color, national origin, religion, sex, 

gender identity (including gender expression), sexual orientation, disability, age, 

marital status, family/parental status, income derived from a public assistance program, political beliefs, or reprisal or retaliation for prior civil rights activity, in any program or activity conducted or funded by USDA (not all bases apply to all programs). 

Remedies and complaint filing deadlines vary by program or incident. 

Persons with disabilities who require alternative means of communication for 

program information (e.g., Braille, large print, audiotape, American Sign Language, 

etc.) should contact the responsible Agency or USDA’s TARGET Center at (202) 

720-2600 (voice and TTY) or contact USDA through the Federal Relay Service at 

(800) 877-8339. Additionally, program information may be made available in languages other than English. 

To file a program discrimination complaint, complete the USDA Program Discrimination Complaint Form, AD-3027, found online at http://www.ascr.usda.gov/complaint_

filing_cust.html and at any USDA office or write a letter addressed to USDA and 

provide in the letter all of the information requested in the form. To request a copy 

of the complaint form, call (866) 632-9992. Submit your completed form or letter to 

USDA by: (1) mail: U.S. Department of Agriculture, Office of the Assistant Secretary 

for Civil Rights, 1400 Independence Avenue, SW, Washington, D.C. 20250-9410; 

(2) fax: (202) 690-7442; or (3) email: program.intake@usda.gov. 

USDA is an equal opportunity provider, employer, and lender.

The use of trade or firm names in this publication is for reader information and 

does not imply endorsement by the U.S. Department of Agriculture of any product 

or service. 

Technical Editors 

R.J. Gutiérrez is a professor and Gordon Gullion Endowed Chair Emeritus, University of Minnesota, 2003 Upper Buford Cir., St. Paul, MN 55108; Patricia N. Manley 

is a research program manager, Conservation of Biodiversity Program, U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station, 2480 Carson 

Rd., Placerville, CA 95667; and Peter A. Stine is a biogeographer and retired director 

of Partnerships and Collaboration, U.S. Department of Agriculture, Forest Service, 

Pacific Southwest Research Station, John Muir Institute for the Environment, University of California–Davis, 1 Shields Ave., Davis, CA 95616. 
Authors 

Brandon M. Collins is a research fire scientist, U.S. Department of Agriculture, 

Forest Service, Pacific Southwest Research Station and the University of 

California–Berkeley, 140 Mulford Hall, Berkeley, CA 94720. 

R.J. Gutiérrez is a professor and Gordon Gullion Endowed Chair Emeritus, 

University of Minnesota, 2003 Upper Buford Cir., St. Paul, MN 55108. 

John J. Keane is a research wildlife ecologist, U.S. Department of Agriculture, 

Forest Service, Pacific Southwest Research Station, 1731 Research Park Dr., 

Davis, CA 95618. 

Maggi Kelly is a geographer, Cooperative Extension specialist and professor, 

Department of Environmental Science, Policy, and Management, University of 

California–Berkeley, 130 Mulford Hall, Berkeley, CA 94720. 

Patricia N. Manley is a research program manager, Conservation of Biodiversity 

Program, U.S. Department of Agriculture, Forest Service, Pacific Southwest 

Research Station, 2480 Carson Rd., Placerville, CA 95667. 

Malcolm P. North is a research forest ecologist, U.S. Department of Agriculture, 

Forest Service, Pacific Southwest Research Station, 1731 Research Park Dr., Davis, 

CA 95618. 

M. Zachariah Peery is an associate professor, Department of Forest and Wildlife 

Ecology, University of Wisconsin, 1630 Linden Dr., Madison, WI 53706. 

Susan L. Roberts is a wildlife ecologist and private consultant, P.O. Box 2163, 

Wawona, CA 95389. 

Mark W. Schwartz is a professor and director of the John Muir Institute of the 

Environment, University of California–Davis, 1 Shields Ave., Davis, CA 95616. 

Peter A. Stine is a biogeographer and retired director of Partnerships and Collaboration, U.S. Department of Agriculture, Forest Service, Pacific Southwest Research 

Station, John Muir Institute for the Environment, 1 Shields Ave., University of 

California–Davis, CA 95616. 

Douglas J. Tempel is a postdoctoral research associate, Department of Forest and 

Wildlife Ecology, University of Wisconsin, 1630 Linden Dr., Madison, WI 53706. 

i 
The California Spotted Owl: Current State 

of Knowledge 

R.J. Gutiérrez, Patricia N. Manley, and Peter A. Stine, 

Technical Editors 

U.S. Department of Agriculture, Forest Service 

Pacific Southwest Research Station 

Albany, California 

General Technical Report PSW-GTR-254 

August 2017 

ii 
Abstract 

Gutiérrez, R.J.; Manley, Patricia N.; Stine, Peter A., tech. eds. 2017. The 

California spotted owl: current state of knowledge. Gen. Tech. Rep. PSWGTR-254. Albany, CA: U.S. Department of Agriculture, Forest Service, 

Pacific Southwest Research Station. 294 p.

This conservation assessment represents a comprehensive review by scientists of 

the current scientific knowledge about the ecology, habitat use, population dynamics, and current threats to the viability of the California spotted owl (Strix occidentalis occidentalis). It is based primarily on peer-reviewed published information 

 with an emphasis on new scientific information that has emerged since the first 

technical assessment for the California spotted owl (CASPO) was conducted in 

1992. Substantial new information and insights exist for owls inhabiting the Sierra 

Nevada, but much less exists for populations inhabiting the central and southern 

California parts of its range. Spotted owls are habitat specialists that are strongly 

associated with mature forests that are multistoried or complex in structure, and 

have high canopy cover, and an abundance of large trees and large coarse woody 

debris. Most California spotted owl habitat is concentrated in mid-elevation forests 

of the Sierra Nevada, which consist primarily of ponderosa pine (Pinus ponderosa 

Lawson and C. Lawson), mixed-conifer, white fir (Abies concolor (Gord. & Glend.) 

Lindl. ex Hildebr.), and mixed-evergreen forest types. Currently, there are about 

~2 million ha (~5 million ac) of suitable habitat in the Sierra Nevada, with 75 percent occurring on national forests. These habitat conditions have been demonstrated 

to have a strong positive association with key vital rates (e.g., occupancy, adult 

survival, reproductive success), which drive population persistence. 

All studies published since CASPO have demonstrated that owl populations 

on national forests in the Sierra Nevada have declined over the past 20 years. A 

preponderance of evidence suggests that the past century’s combination of timber 

harvest and fire suppression has resulted in forests that have a considerably higher 

density of trees but a reduced density of large-diameter trees and logs, a greater 

density of shade-tolerant fire-sensitive tree species, and an increase in forest fuels. 

These conditions have resulted in reduced habitat quality, increased habitat fragmentation, and increased risk of high-severity fire in the Sierra Nevada. Climate 

change is projected to have significant effects on Sierra Nevada forests, including 

exacerbating the risk and impacts from high-severity fires, which in turn is likely to 

affect spotted owl habitat and populations. The specter of additional threats in the 

iii 
form of competition from the newly invading barred owl (Strix varia) and environmental contaminants, as well as the continuing dearth of information on central and 

southern California populations, further raises concerns about the fate of California 

spotted owl populations. Maintenance of a viable population of spotted owls in 

the Sierra Nevada and throughout its range will depend on effective, long-term 

owl conservation practices embedded in an overall management strategy aimed at 

restoring resilient forest structure, composition, and function, including reducing 

the risk of large-scale high-severity fires while reducing the risk of habitat loss to 

the owls. 

Keywords: California spotted owl, Strix occidentalis occidentalis, conservation 

assessment, national forest, Sierra Nevada, forest resilience, USDA Forest Service, 

viability, wildfire. 

iv 
 

Contents 

1 Chapter 1: Introduction 

Peter A. Stine and Patricia N. Manley 

1 Regulatory Context 

2 1992 California Spotted Owl Technical Assessment 

3 Lessons Learned From the Northern Spotted Owl 

4 Expanding Challenges in Spotted Owl Conservation 

6 This Conservation Assessment 

8 Information Sources Consulted 

8 The Process and Product 

9 Literature Cited 

11 Chapter 2: The Biology of the California Spotted Owl 

R.J. Gutiérrez, Douglas J. Tempel, and M. Zachariah Peery 

11 Introduction 

12 Taxonomy 

14 Ecology 

15 Behavior 

19 Reproductive Ecology 

23 Habitat Associations 

28 Diet 

30 Cause-Specific Mortality

31 Chapter Summary 

34 Literature Cited 

47 Acknowledgments 

49 Chapter 3: California Spotted Owl Habitat 

Characteristics and Use 

Susan L. Roberts 

49 Introduction 

49 Habitat Characteristics 

49 Nest and Nest Tree Characteristics 

50 Nest Stand Characteristics 

52 Core Area Habitat Characteristics 

54 Foraging Habitat Characteristics 

v 
56 Prey Habitat Characteristics 

58 Home Range Characteristics 

60 Effects of Fire on Spotted Owl Habitat 

62 New Findings Relative to Management Guidelines 

63 Chapter Summary 

65 Literature Cited 

75 Chapter 4: Population Distribution and Trends of California 

Spotted Owls 

Douglas J. Tempel, R.J. Gutiérrez, and M. Zachariah Peery 

75 Distribution 

75 Geographic Range 

78 Distribution of Owls and Gaps in Distribution 

79 Demographic Rates 

79 History of Demographic Research in the Sierra Nevada 

81 Reproduction 

84 Survival 

85 Population Size and Trends 

85 Population Size 

85 Population Trends 

90 Site Occupancy 

92 Population and Conservation Genetics of California Spotted Owls 

92 Conservation Units 

94 Connectivity in Fragmented Populations 

95 Genetic Variation Within Populations 

97 Characterizing Demographic History 

98 Chapter Summary 

101 Literature Cited 

109 Chapter 5: Current and Projected Condition of Mid-Elevation 

Sierra Nevada Forests 

Malcolm P. North, Mark W. Schwartz, Brandon M. Collins, 

and John J. Keane 

109 Introduction 

109 Forest Management 

vi 
109 Ownerships 

110 Historical Management Practices 

111 Forest Management Since 1990 

123 Current Status of Forests With Potential California Spotted Owl Habitat 

127 Historical Fire Effects on Mid-Elevation Forests 

129 Drivers of Forest Change 

129 Current and Projected Fire Effects 

130 Postfire Forest Management

131 Climate Change 

134 Future Management of Mid-Elevation Forests 

135 Creating Forest Heterogeneity 

137 Chapter Summary 

140 Literature Cited 

155 Appendix 5-1: Information on Source and Data Quality Issues for Timber 

Harvest Volume, Silvicultural Prescriptions, and Habitat Data 

159 Chapter 6: Mapping Forest Conditions: Past, Present, 

and Future 

Maggi Kelly 

159 Introduction 

159 Owl Habitat Mapping Methods, Strengths, and Weaknesses 

159 Historical Mapping Technology 

160 Aerial Photography 

161 Landsat 

163 High Spatial Resolution Imagery 

164 Current and Emerging Technology 

164 LiDAR 

164 LiDAR Metrics 

165 Characterizing Habitat Across Scales 

167 Mapping Nests and Nest Trees 

167 Mapping Nest Stand Characteristics 

169 Mapping Core Use Area Characteristics 

171 Mapping Characteristics of Foraging Habitat 

173 Mapping Home Ranges 

vii 
 

174 Accuracy Assessment 

176 Chapter Summary 

177 Literature Cited 

185 Chapter 7: Threats to the Viability of California Spotted Owls 

John J. Keane 

185 Introduction 

185 Evaluation of Threats Identified in CASPO

185 Forest Management 

196 Wildfire

204 Integration of Forest Management and Wildfire

204 Areas of Concern: Gaps in the Distribution of California Spotted Owls in 

the Sierra Nevada 

207 Human Development 

207 Evaluation of Emerging Threats 

207 Barred Owls 

211 Climate Change 

216 Disease, Parasites, and Contaminants 

217 Human Recreation and Disturbance 

218 Genetics 

219 Chapter Summary 

221 Literature Cited 

233 Appendix 7-1: Distribution of Forest Management Treatments and 

Wildfire During 1990–2014 Within the Areas of Concern 

239 Chapter 8: The Spotted Owl in Southern and Central 

Coastal California 

R.J. Gutiérrez, Douglas J. Tempel, and M. Zachariah Peery 

239 Introduction 

240 CASPO Assessment of Areas of Concern 

240 Distribution and Metapopulation 

244 General Ecology 

245 Habitat 

246 Population Dynamics 

247 Density 

viii 
 

247 Reproduction 

249 Survival 

250 Dispersal 

251 Occupancy 

252 Population Trends 

253 Threats 

253 Natural Connectivity Among Populations 

254 Integrity of Habitat Supporting Each Population 

254 Water Diversion and Stream Channelization 

254 Wildfire

255 Human Recreation 

255 Drought 

255 Air Pollution 

255 Mining 

255 Marijuana Cultivation 

256 Cumulative Effect of Small-Scale Management Actions 

256 Invasive Species and Disease 

256 Climate Change 

257 Chapter Summary 

258 Literature Cited 

263 Chapter 9: Synthesis and Interpretation of California 

Spotted Owl Research Within the Context of Public 

Forest Management 

M. Zachariah Peery, R.J. Gutiérrez, Patricia N. Manley, 

Peter A. Stine, and Malcolm P. North 

263 Introduction 

263 Implications of Recent Research for Spotted Owl Conservation 

264 Conservation of Spotted Owls in the Context of Ecosystem Restoration 

268 Desired Conditions for Areas of Ecological Importance 

270 The Science Behind Scale-Specific Desired Conditions: Implications for

Forest Management 

276 Postfire Management

277 Barred Owl Range Expansion, Monitoring, and Control 

ix 
278 Establishing Benchmarks for Conservation Success 

279 Progress on CASPO Uncertainties and Remaining Knowledge Gaps 

279 Inventory 

280 Monitoring 

281 Research 

281 Priority Research Needs 

281 Identifying Environmental Causes of Population Declines 

284 Effects of Fuel Treatments and Wildfire on Population Viability

286 Historical (Pre-Euro-American) Abundance of California Spotted Owls 

287 Enhancing Foraging and Prey Habitat 

288 Impacts of Climate Change to Spotted Owls and Their Habitat 

288 Habitat Evaluation Tools 

289 Literature Cited 

292 Glossary 

x 
Preface 

The chapters that comprise this compendium provide an assessment of recent 

scientific knowledge of the California spotted owl—the information that has been 

published since “The California Spotted Owl: A Technical Assessment of Its Current Status” in 1992. In addition, we included two chapters that are relevant to spotted owl conservation (fire, forests, and climate change in chapter 5; and threats to 

the owl in chapter 7). As noted in chapter 1, the assessment team limited the scope 

of the summary of recent California spotted owl research, conducted no original 

analysis (other than to attempt a synthesis, chapter 9), and limited the number of 

topics. 

The process followed by the assessment team (consisting of the authors of the 

chapters) began with a meeting in April 2014 in Davis, California, to outline the 

scope and goals of the assessment, assign individual chapters, and discuss issues 

related to the owl and its conservation. Individual authors then worked alone or in 

collaboration with coauthors to complete the chapters which they were assigned. 

I reviewed and edited draft chapters, which were then sent to all remaining team 

members for internal team review. Second revisions were returned to me, and, if 

necessary, I provided additional editing to prepare the papers for peer reviews. 

Chapter 9 was a synthesis and context of chapter information, written by Dr. 

Zachariah Peery. A first draft was reviewed and discussed by coauthors and team 

members over several months. To facilitate completion of chapter 9 and the final 

details of other chapters, the team convened for a second time in December 2014 

in Davis, California. Following this meeting, team members revised their chapters 

again, and I reviewed and compiled them into a single file. This file was then sent 

by Dr. Peter Stine, team leader, to the California Regional Office of the U.S. Forest 

Service in June 2015. 

Prior to completion of the chapters, the Pacific Southwest Region (Region 5) 

selected anonymous reviewers and handled the review process. The team received 

the anonymous peer review comments in early August 2015 and the Region 5 staff 

review comments in mid-August. Team members then created individual response 

documents to these reviews. I edited and compiled the individual responses into a 

composite response document using a consistent style, but did not alter the content 

of responses. This response document was then returned to Region 5 peer-review 

coordinator. 

xi 
The team did not create a detailed response document to specific comments 

made by the regional staff because many of the comments overlapped those of the 

peer reviewers, and others were not relevant given the goals and constraints of the 

assessment. Regardless, the chapter authors made a good-faith effort to incorporate 

the suggestions made by regional staff so that they could be accommodated without 

compromising the scientific and professional integrity of the document. 

The team then revised their chapters based on the anonymous peer reviews 

and the Region 5 staff reviews and in accordance with their commitments made in 

their responses to reviewers. I read these revised chapters as they were completed, 

and they were then sent individually to PSW for a policy review between midOctober and early November 2015. Chapter 9 was completed last owing to multiple 

discussions among the coauthors to settle on language that was acceptable to all 

coauthors. The authors of chapter 9 acted in good faith to achieve language that 

they felt was scientifically defensible yet reflected the existence of differences of 

interpretation of information or the implications of the published information. After 

several policy reviews were conducted by PSW from October 15, 2015, to April 

2016, I made specific recommendations on the policy comments to each chapter 

author. The authors edited their chapters once again and returned them to PSW 

in January 2016. A final courtesy review of the final draft chapter was sent to the 

Region, which resulted in additional requests for revision. 

The team held a final conference call in late February to discuss the issues 

related to the demarcation between what might be considered implied direction 

to managers by scientists and what constituted a direct, logical interpretation of 

scientific information. Authors agreed to continue with the review process, but with 

heightened attention to the issues of scientific independence and integrity of the 

document. The team received a final station’s policy review of the January version of chapter 9 on March 10. The authors addressed the review comments to the 

extent they felt was appropriate but resisted any changes that would have altered the 

context of the original text. At the beginning of May, after 11 months of reviews, 

the full manuscript was accepted by PSW. 

In summary, the roles of the technical editors were to ensure consistency 

of writing style among chapters, to facilitate responses by authors to reviewer 

comments at several stages of review, and to recommend changes or additions to 

authors about content and prose of chapters. However, the editors had no ultimate 

xii 
authority or discretion to reject specific chapters or to force authors to comply with 

editorial and content recommendations of any type. Moreover, the review process 

was completely outside the jurisdiction of the editors (as it should have been, given 

that editors were also authors of chapters). 

Although the release of the GTR was delayed because of disagreements 

between the authors and managers about what constituted management recommendations vs. implications of research, we feel the chapters provided an accurate 

summary and synthesis of recent California spotted owl research. The knowledge 

gained about California spotted owls since the publication of the 1992 technical 

assessment led by Dr. Jared Verner will provide a substantial basis for constructing 

a scientifically defensible conservation strategy for the California spotted owl. 

R.J. Gutiérrez 

xiii 
Acknowledgments 

The editors thank the authors for their outstanding commitment to a very long 

and arduous process. We thank U.S. Forest Service Pacific Southwest Region 

(Region 5), Pacific Southwest Research Station, the University of Wisconsin, and 

Minnesota Agricultural Experiment Station for providing support for the authors 

and the publication process. Especially, Rachel White gave initial advice in planning the structure of the document, Cindy Minor and Keith Routman advised us on 

preparing and publishing the document, and Carolyn Wilson provided the overall 

editing and shepherding of the document through to publication. Valerie Gallup and 

Lorna Wren assisted in formatting the final document. We especially thank Sara 

Sawyer for organizing and implementing the blind peer-review of the draft assessment. Diana Craig, Deb Whitman, Don Yasuda, and the staff of Region 5 provided 

various forms of assistance during the process. Kevin Roberts, Ed Murphy, and 

Tom Engstrom shared information with the assessment team about their spotted owl 

work on behalf of Sierra Pacific Industries. Finally, the anonymous peer reviewers 

and Region 5 reviewers contributed to an improved assessment. 

The chapter authors’ specific acknowledgments are as follows: R. J. Gutiérrez 

(chapters 2 and 8) thanks Kaye Westcott-Gutiérrez for help in proofreading chapter 

2, chapter 8, and other sections of the proof; Gavin Jones provided many useful 

comments on chapter 2; and William Berigan created figures 2-1 and 8-1. Susan 

Roberts (chapter 3) thanks M. Meyer for his insightful review and J. Kane for creating figures 3-1 and 3-2. Douglas Tempel (chapter 4) thanks Sheila Whitmore for 

organizing the database of historic owl locations and creating figures 4-1 and 4-2. 

Malcolm North (chapter 5) thanks Marc Meyer, USFS Southern Sierra Province 

Ecologist, for discussing and reviewing chapter 5. Maggie Kelly (chapter 6) thanks 

Marek Jakubowski, Stefania DiTommaso, and Qinghua Guo for their work on Lidar 

and vegetation mapping. John Keane (chapter 7) thanks Ross Gerrard and Claire 

Gallagher for generating and formatting numerous tables and figures. And Zach 

Peery (chapter 9) thanks the four anonymous peer reviewers for stimulating many 

discussions among the chapter’s authors. 

xiv 
Executive Summary 

Introduction 

The California spotted owl (Strix occidentalis occidentalis) occurs in the Sierra 

Nevada, the mountains of central coastal California, and the peninsula and 

Transverse Ranges of southern California. It is a species of conservation concern 

because of the potential impacts of forest management and high-severity fire on its 

habitat—primarily closed-canopy forest. The first California spotted owl technical 

assessment “The California Spotted Owl: A technical Assessment of it’s current 

status” (CASPO) was published in 1992. It was developed to help guide forest management in the Sierra Nevada and southern California mountains. Since CASPO, 

much has been learned about the ecology of California spotted owls, but the complexity of managing owl habitat, forests, and wildlife also has increased because of 

declining forest health, climate change, diseases, and invasive species. Moreover, 

the population status of owls in the Sierra Nevada is no longer uncertain—populations are declining on national forests. 

This document represents a comprehensive overview of the current 

knowledge about the ecology, habitat use, and population dynamics of the 

California spotted owl as well as existing and potential threats to its viability. 

For this assessment (as in CASPO), we divided the range of the California spotted 

owl into two major physiographic provinces: the Sierra Nevada and the mountains 

of southern California (including the Transverse Ranges of southern California 

and portions of the Coast Range of central California). Tehachapi Pass was used 

as the demarcation between the regions. The majority of new information pertains 

to the Sierra Nevada population, so all but chapter 8 primarily address the Sierra 

Nevada population. The science team that produced the assessment was assembled 

to provide expertise in owl biology and other relevant disciplines (experts in climate 

change, fire and fuels management, forest ecology, remote sensing, and vegetation 

ecology). Ideally, this assessment will help inform future options for management 

and activities ranging in scales from site-specific projects to large landscapes to the 

entire range of the owl. 

Biology and Ecology 

The three subspecies of the spotted owl are recognized by the American Ornithologists’ Union: northern (S. o. caurina), California (S. o. occidentalis), and 

Mexican (S. o. lucida). The ranges of the northern and California spotted owls are 

parapatric (ranges immediately adjacent). For purposes of owl management and 

conservation, Pit River has been recommended as the management dividing line 

xv 
between these two subspecies although there is evidence that both subspecies occur 

on either side of the river. 

Spotted owls have a monogamous mating system, with territorial pairs 

forming relatively long-term pair bonds and occupying large home range 

areas. Spotted owls sometimes break pair bonds (i.e., “divorce”); birds that break 

pair bonds or whose mate has died form new pair bonds with other birds, often in 

different territories. Spotted owls are territorial (i.e., exclude other pairs or individuals from the core of their home range) and exhibit strong fidelity to their territory. 

The territory is typically smaller than a home range. Although the sizes of territories have not been estimated, home ranges are relatively large (about 400 to 1200 ha 

[1,000 to 3,000 ac]), and home ranges of adjacent owls often overlap. 

Spotted owls are primarily active at night when they hunt, defend, socialize, and conduct exploratory movements. They sleep, conduct self-maintenance, 

and guard young during the day while roosting in complex-structured forests. 

These forests provide thermal and protective cover, and the same roost sites are 

often used consistently over many years. The areas around nest and roost sites serve 

as the center of activity for spotted owls. An owl can forage anywhere within its 

home range. 

Owls have evolved long lifespans and low reproductive rates as mechanisms 

to mitigate the negative effects of short-term, unpredictable environmental 

conditions (such as weather variability and disturbance frequency). Annual 

reproduction by California spotted owls is extremely variable, ranging from no 

young produced within an area to nearly all birds producing young. These biological features have led some scientists to suggest that the owl exhibits a “bet hedging” 

life history strategy, meaning that the lack of reproduction at a site for one or more 

years does not necessarily reflect low site quality, but rather it could reflect temporarily poor environmental conditions that cause owls to postpone reproduction until 

conditions improve. 

Spotted owls prey primarily on medium-sized small mammals, particularly dusky-footed and big-eared woodrats (Neotoma spp.) at lower elevations 

and flying squirrels (Glaucomys sabrinus) at higher elevations. However, they 

prey on many other species, such as mice, pocket gophers, voles, birds, lizards, and 

insects. 

Predators of spotted owls include the great horned owl (Bubo virginianus), 

northern goshawk (Accipiter gentilis), and red-tailed hawk (Buteo jamaicensis). The invasion of the barred owl (Strix varia) in western North America has 

been of substantial concern for spotted owl conservation because it is a dominant 

competitor. 

xvi 
Habitat Characteristics and Use 

Spotted owls are habitat specialists that are strongly associated with mature 

forests that are multistoried or complex in structure and have larger trees, 

higher canopy cover, and more coarse woody debris than does the general 

landscape. Several hypotheses have been generated to explain why owls select old/ 

mature forests (such as nest site requirements, ambient temperature moderation, or 

prey availability). They use large, old trees and snags as structures for nests. Here 

they nest in cavities, broken tree tops, and occasionally on debris platforms such 

as nests of other species or mistletoe brooms. In mixed-conifer forests, the average 

nest tree is 124 cm (49 in) in diameter at breast height (d.b.h.) and 31 m (103 ft) tall 

with an average nest height of 23 m (74 ft). Nests trees in hardwood forests have an 

average diameter of 76 cm (30 in) and an average nest height of 12 m (38 ft). Owl 

site occupancy and adult survivorship increase when there is a greater proportion 

of area of the nest stand containing high canopy cover and high basal area in an owl 

territory. 

Spotted owls are central-place foragers so they concentrate their activities 

in a “core area” around nests and roosts, with foraging activity decreasing as 

distance increases from nests or roosts. The “core area” refers to the area that 

contains the nesting, roosting, and foraging habitat that is essential to each pair’s 

survival and reproductive success. It is commonly considered to be consistent 

with the territory and is often portrayed in analyses as a circle with a radius that is 

half the average distance between adjacent nests (i.e., nearest neighbor distance). 

Occupancy, site colonization, adult survival, and reproductive success are positively 

associated with the proportion of the core area containing structurally complex 

conifer forest with large trees and high canopy cover. Concomitantly, reproductive 

success is negatively correlated with the proportion of nonforested areas and forest 

types that are not used by owls for nesting or foraging. 

Current management on the National Forest System (NFS) centers on 

protection of 121 ha (300 ac) of high-quality habitat (protected activity center 

[PAC]) around the nest site as a means of maintaining average habitat conditions within the core use area. One study showed that the current size specified 

for spotted owl PACs may be adequate to maintain occupancy of territories. 

Another study showed that mechanical tree removal on ≥20 ha (49 ac) of a PAC was 

negatively correlated with site colonization and occupancy. Because of the limited 

number of studies, the contribution of PACs to owl conservation still needs study. 

xvii 
Spotted owl foraging habitat is characterized by a mosaic of vegetation 

types and seral stages including but not limited to mature forest. Spotted owls 

often forage in areas having high-contrast edges as well as in interior forest patches 

(i.e., with few edges). The juxtaposition of mature closed-canopy forest with other 

cover types is correlated with higher reproductive output and intermediate survival 

rates in northern spotted owls, which in turn may reflect higher prey diversity and 

abundance where there is a mosaic of cover types available to owls. 

Habitat characteristics of most spotted owl prey have remained largely 

unstudied in the Sierra Nevada. In general, the dominance of flying squirrels in 

the diet increases as elevation increases—the reverse is true for woodrats. In the 

Sierra Nevada, northern flying squirrels are associated with mature forest stands 

with patches of moderate-to-high canopy closure (>70 percent), large live or dead 

trees (>75 cm d.b.h. [>30 in]), thick litter layers (≥2.5 cm [≥0.1 in]), and sparsely 

distributed coarse woody debris or understory cover. In lower elevation forests, 

woodlands, and shrublands of the west-side Sierra Nevada, woodrats are positively 

associated with oak cover or large oak density (>32 cm [>13 in] d.b.h.). 

A home range is the area used by an individual to meet its requirements for 

survival and reproduction—consistently, owl home ranges contain a greater 

abundance of large trees and greater proportion of mature forest than is available on the landscape. Generally, California spotted owl home ranges are larger 

in the northern Sierra Nevada ([>1000 ha [>2,500 ac]) and smaller in the southern 

Sierra Nevada (<1000 ha). Owl home ranges contain a mosaic of cover types; 

however, home range size increases as heterogeneity increases, suggesting that 

high cover type heterogeneity can negatively affect habitat quality. Data sources to 

describe habitat characteristics across large geographic areas such as home ranges 

have proven to be inaccurate and inconsistent among studies, making it difficult to 

derive specific and reliable home range characteristics to inform management. 

Population Distribution and Trends 

As reported in CASPO, there appear to be no significant gaps in distribution 

of owls in the Sierra Nevada. The majority of owls occur within the mid-elevation, 

mixed-conifer forests on the west slope of the Sierra Nevada. We could not determine the relative population density or size between public and private land in the 

Sierra Nevada because there has been no published estimate of the number of owls 

occupying private land in the Sierra Nevada. 

xviii 
Since CASPO, data collected on five long-term California spotted owl 

study areas have provided substantial empirical data on demographic rates 

and population trends. Of these five study areas, four were in the Sierra Nevada— 

three on national forests (Lassen, Eldorado, and Sierra) and one within Sequoia and 

Kings Canyon National Parks. Meta-analyses of these data have shown substantial 

variation in reproductive rates (number of young fledged per territorial female for 

which reproduction was assessed) among the four areas. Reproduction has declined 

over time on the Eldorado but has been relatively constant on the other study areas. 

Reproductive rates have been correlated with both climatic conditions and 

habitat characteristics. Reproductive rates were negatively correlated with higher 

precipitation and colder temperatures during the previous winter or early nesting 

season and positively correlated with the presence of closed-canopied forest, respectively. Key vital rates—reproductive rates at both the nest and territory scales, as 

well as the survival of adults—appear to be closely tied to habitat characteristics, 

namely a positive association with the amount of forest with dense canopy and 

larger trees. 

All studies published since CASPO have demonstrated that owl populations on national forests in the Sierra Nevada have declined over the past 20 

years. Both the finite rate of a population increase (λ) and the realized rate of 

change (Δ ) have shown negative trends over the past 20 years. The greatest popula- t 

tion declines have occurred on the Lassen and Eldorado National Study Areas. 

However, one study in two national parks, Sequoia and Kings Canyon, showed a 

stable population. These findings removed the uncertainty expressed in CASPO 

about the status of population trends in the California spotted owl. 

Sierra Nevada Forest Conditions 

Most of the California spotted owl’s habitat is concentrated in mid-elevation 

forests of the Sierra Nevada, which are made up primarily of ponderosa pine, 

mixed-conifer, white fir, and mixed-evergreen forest types. The majority of the 

range of California spotted owl is occupied by NFS lands, with private lands and 

national parks making up almost all the rest. These forests have changed substantially since the arrival of Europeans. Management practices (fire suppression, logging, and grazing) on national forests and private lands have largely shaped current 

forest conditions in the Sierra Nevada. There are five national parks in the Sierra 

Nevada and southern Cascades covering 1.74 million ac that contain substantial 

suitable spotted owl habitat. Management of these parks is directed toward building 

ecosystem resilience to cope with changing climates, primarily using prescribed 

fire and managed wildfire to accomplish conservation objectives. 

xix 
Until 1990, similar management objectives and silvicultural prescriptions 

were used on both NFS and private lands. Prior to 1900, logging occurred mainly 

near mining operations and communities, with most logging occurring on private 

lands. Timber harvest in the Sierra Nevada peaked in the post-World War II years 

and then stabilized starting in the 1960s. The CASPO report noted four key changes 

in forest conditions that occurred from 1850 to 1992: (1) the loss of old, largediameter trees and associated large downed logs; (2) a shift in species composition 

toward shade-tolerant, fire-sensitive tree species (i.e., from pines to fir and cedar); 

(3) increases in fuel loads associated with the mortality of small-diameter trees; and 

(4) the presence of fuel ladders (ground to canopy) that facilitate crown fire. We 

found no new information that was contrary to this historical view (but see climate 

change effects below). 

With the adoption of the California spotted owl guidelines following 

CASPO, management of national forest and private forests diverged significantly in the mid-1990s. Timber harvest dramatically decreased on NFS lands 

such that private lands produced more than 80 percent of the timber volume from 

1990 to 2013. Nearly 400 000 ha [1 million ac] were logged on private land between 

1990 and 2013 in the Sierra Nevada, with most logging occurring in the southern 

Cascades and northern Sierra Nevada. In contrast, about (265 000 ha [665,000 ac]) 

were logged during about the same time on national forests, with most logging 

occurring in the northern Sierra Nevada. 

Current estimates are (about 4.9 million ac [2 million ha] ) of suitable habitat, with about 75 percent, 7 percent, and 18 percent occurring on NFS, national 

parks, and either private or other government lands, respectively. About half of all 

suitable habitat is classified as the Sierra Nevada mixed-conifer vegetation type, 

and this type is mostly ( about 75 to 80 percent) on NFS lands, which demonstrates 

the critical role of NFS lands for owl conservation in the Sierra Nevada. 

The preponderance of evidence suggests forests have a considerably higher 

density of trees than forests of presettlement times because of fire exclusion 

and logging that allowed the regrowth of dense tree stands. Moreover average 

canopy cover of presettlement forests has been estimated to be as low as 22 percent 

but ranging from 8 to 37 percent. However, a less accepted estimate of presettlement forests based on Forest Inventory and Analysis data and on historical tree data 

suggests that presettlement forests had greater density than suggested by others. 

Despite these differences, most studies suggest presettlement forests were spatially 

complex across landscapes, including the presence of early seral vegetation (e.g., 

dense conifer regeneration, and shrubs) and denser mature forest stands within a 

xx 
matrix of generally low-density stands. Numerous studies have demonstrated that 

high frequency (5- to 15-year return intervals) of low-severity fire maintained lowdensity stands across much of the landscape, resulting in the dominance of large, 

fire-resistant trees. This is particularly the case for yellow pine (Pinus ponderosa 

and P. jeffreyi) and mixed-conifer forest types within the Sierra Nevada. 

Fire is a critical ecosystem process throughout the Sierra Nevada, but 

that process is changing because of fire exclusion and climate change. Current 

trajectories of fire size and impact, along with a predicted doubling of the likelihood of future fires, suggest a future in which the frequency and proportion of 

stand-replacing fires in the Sierra Nevada will exceed both current and past levels. 

Such changes have the potential to reduce forest regeneration. 

Estimates of the effects of climate change predict an upward elevation shift 

of plant species and communities, an expansion of grassland, savannah and 

shrub-dominated ecosystems, and a general reorganization of forested ecosystems. Increases in tree stress and large-tree mortality are expected among these 

vegetation changes. Climate projections also suggest the potential for conversions 

of all vegetation type with increasing warmer and drier future climate scenarios, 

including the forests upon which California spotted owls currently depend. 

Forest heterogeneity in the Sierra Nevada is strongly influenced by water 

availability and fire. Mesic and riparian sites are dominated by the greatest 

densities of large overstory trees, have high basal area and canopy cover, and have 

an abundance of large snags and logs. Slope steepness and slope position (e.g., 

ridgetop, midslope, valley bottom) influence forest heterogeneity because they 

affect the reception and retention of water. While overstory forest patterns are 

closely associated with climatic water deficit, understory conditions are strongly 

shaped by fire. 

Heterogeneity within forest types that are fire adapted (historically 

affected by frequent fires) can be characterized by the interspersion of individual trees, clumps of trees, and openings or gaps (i.e., ICO structure). The 

small-scale heterogeneity characteristic of historical forest conditions are hypothesized to confer multiple desirable functions: openings may inhibit crown fire 

spread under most weather conditions and may be as effective as fuel breaks with 

regularly spaced trees with wide crown separations; and the variable microclimate 

and vegetation conditions between the three conditions may enhance forest drought 

resilience and provide greater habitat diversity for both plants and animals. Intentionally creating these conditions by either mechanical treatments or prescribed fire 

is likely to be challenging. 

xxi 
Mapping Forest Conditions Past, Present, and Future 

Mapped data are essential to public land managers and researchers to identify 

and characterize wildlife habitat across scales, to monitor species and habitat 

change, and to predict and plan for future scenarios. National forest managers in 

the Sierra Nevada use maps to aid conservation planning for sensitive species. They 

require the ability to estimate important habitat metrics accurately across spatial 

scales to account for variation in a species’ needs. However, creating accurate maps 

can be challenging because landscapes exhibit great variability in composition, 

cover, and topography, and reflect a complex legacy of fire and logging effects. 

Aerial photographs provide spatially detailed records and remain a valuable data source for habitat despite the increase in the number and types of 

digital sensors available to managers and scientists. Aerial photographs predate 

satellite imagery; in California, imagery archives include images from the 1930s 

onward. Further, the spatial detail provided by aerial photography is high, even 

when analog photographs are digitized. Finally, when digitized, aerial photographs 

(e.g., digital orthophoto quadrangles) can be analyzed with powerful image analysis 

techniques. 

Approaches to mapping wildlife habitat have been varied. Data used to create maps describing owl habitat have been gathered from field surveys; black and 

white or color air photos; or digital aerial imagery, and other maps such as timber 

survey; Landsat-derived vegetation; and fire-severity maps. Remotely sensed 

imagery at both fine spatial resolution (e.g., 1 m [3 ft]) and moderate resolution 

(e.g., 30 m [98 ft]) has also been used to create maps. 

Vegetation maps derived from Landsat data have been used widely to study 

California spotted owl habitat. The broad spatial coverage of Landsat and the 

spectral detail of its sensors have been found useful to map species groups and canopy cover but cannot detect the residual tree component of forests in forests dominated by the medium-sized trees necessary for owls to use these forests. Additional 

sources of vegetation data useful to create maps of owl habitat are color infrared 

aerial photographs, National Agricultural Imagery Program (NAIP) imagery, and 

Google Earth, but have yielded varying levels of success. Some studies have shown 

that some Landsat maps have error rates that can lead to erroneous conclusions 

about changes in habitat conditions. The NAIP imagery has shown greater promise 

for accurately mapping tree size, canopy cover, and vegetation type. 

LiDAR (Light Detection and Ranging) is a laser-based technology that 

provides detailed, extensive, and accurate vegetation structure data, which are 

key elements of species’ habitats. LiDAR provides data suitable to estimate many 

vegetation characteristics typically associated with California spotted owl habitat. 

xxii 
In particular, the ability to map individual trees, tree sizes, and canopy cover are 

enhanced by analysis of LiDAR data. 

An essential step in map development is assessing the map’s accuracy. 

The best method for doing this is to compare classified map values against fieldverified values (creating an error matrix). Using field validation methods, spotted 

owl researchers generally have created maps with greater than 80 percent accuracy 

using aerial photography and 76 percent accuracy using Landsat. But all such largescale maps of owl habitat thus far created have not contained the “residual tree” 

component that appears critical for owls. Mapping technology has been and will 

continue to be critical to understanding owl habitat relationships and inform their 

conservation. 

Population and Habitat Threats 

The CASPO provided four factors as either threats or potential threats to the 

viability of California spotted owl populations: (1) timber harvest and forest 

management, (2) wildfire, (3) development of gaps in owl distribution across 

the Sierra Nevada, and (4) human population growth and development. Since 

then, most of these conditions persist or have worsened, while additional factors 

have emerged as threats to California spotted owl population viability: invasion of 

the barred owl (Strix varia) into the Sierra Nevada; climate change that could affect 

owls and their habitat; and disease, parasites, and contaminants (namely rodenticides) that could lead to owl mortality. 

Three studies have explicitly addressed the threat of timber harvest and 

wildfire at territory and landscape spatial scales with mixed results. Whereas 

multiple studies have shown the importance of critical amounts of mature conifer 

forest for owls, they have been mixed in their ability to elucidate their cause-andeffect relationships. For example, one study showed that loss of >20 ha (50 ac) of 

mature forest within a territory resulted in a decline in the probability of territory 

occupancy. Further, territories with greater amounts of mature conifer forest had 

higher probabilities of being colonized and lower probability of being unoccupied 

relative to territories with lower amounts of mature conifer forest. Another study 

was unable to relate habitat change due to fire or logging directly to owl vital 

rates, but the amount of mature, high canopy cover forest was positively related to 

owl survival, reproduction, population growth rate, and occupancy. There is only 

a single published study on the effects of logging on the owl, which showed a 43 

percent reduction in occupied owl sites. Although causative linkages have not been 

established, these high rates of decline are coincident with the greater amount and 

extent of logging on public and private lands. 

xxiii 
Studies relating owl demographic parameters to habitat patterns indicate 

the importance of territory-scale habitat conditions such as the amount of 

complex-structured mature forest present and an intermediate amount of 

habitat edge between forest and other vegetation types. This pattern has also 

been reported for owls whose territories have been affected by mixed-severity 

fires, including low amounts of stand-replacing fires. However, there is significant 

uncertainty about the amounts of edge and fine-scale heterogeneity that might be 

beneficial to owls or how best to achieve this heterogeneity. 

Recent research indicates that California spotted owls can persist on territories burned by low-moderate severity and mixed-severity (i.e., low-moderate 

fires with inclusions of high severity) wildfire. The amount of high-severity fire 

that owls can tolerate within their territory is unknown. Occupancy of sites by owls 

after fire appears to be a function of the amount of suitable habitat remaining after 

fire, the amount of suitable habitat burned at high severity, and whether postfire salvage logging was conducted. Postfire salvage logging may negatively affect postfire 

habitat suitability and confounds our understanding of owl response to fire. 

Development of gaps in owl distribution in the Sierra Nevada could have 

negative demographic effects because dispersal among geographic areas likely 

would be reduced. Spotted owls in the Sierra Nevada have low genetic diversity so 

fragmentation and isolation of owl populations in the future could lead to increased 

risk to long-term viability. The CASPO had a list of eight land areas of concern 

(AOCs) within the Sierra Nevada where potential gaps in the distribution could 

develop because (1) naturally fragmented distribution of habitat and owls occurs, (2) 

populations become isolated, (3) habitat becomes highly fragmented, and (4) areas 

occur where crude density of owls becomes low. Evidence indicates that the threat 

of gaps in distribution has likely increased since CASPO as a function of habitat 

loss and fragmentation. Documented owl population declines in Lassen and Eldorado National Forests (AOCs 2 and 4, respectively), along with uncertainty about 

the status of owls in the northern Lassen, Tahoe, Stanislaus, and Sequoia National 

Forests (AOCs 1, 3, 5, and 8, respectively) where extensive forest management 

treatments have occurred contribute to the increased threat of gaps developing in 

the distribution of owls. 

Human population continues to grow in the main area of owl distribution 

on the west slope of the Sierra Nevada, which has raised the risk to owl habitat. 

Wildland-urban interface (WUI) zones are typically heavily managed to reduce 

fuels and the risk of fire to protect communities. About 50 percent of known owl 

sites occur within areas designated as WUIs. Disturbance resulting from human 

recreation and management activities also can potential affect California spotted 

xxiv 
owls. Impacts from recreation can range from the presence of hikers near owl nests 

and roosts to loud noises made by motorized vehicles. Research studies have varied 

in their findings about the effects of disturbances on owls. 

Barred owls have invaded the range of both northern and California spotted owls. Because barred owls are having a major negative impact on northern 

spotted owls, it is predicted they will have a similar impact on spotted owls in 

California. Competition between barred and spotted owls occurs because of broad 

overlap in habitat use, similar diets, and choice of nests. Barred owls are behaviorally dominant. Through 2013, 51 barred and 27 “sparred” (hybrids between the two 

species) owls, and 1 unknown have been detected in the Sierra Nevada. No barred 

owls have been reliably documented in either southern or central coastal California. 

Experiments are occurring to test the effects of barred owl removal on northern 

spotted owls and to assess whether removal is a feasible management strategy 

to reduce competition with spotted owls. If left unchecked, barred owls have the 

potential to extirpate spotted owls from the Sierra Nevada. 

Climate change is projected to have significant effects on Sierra Nevada 

forests, which in turn would affect spotted owls. Increases in temperature and 

changes in precipitation patterns may have direct effects on spotted owl physiology, 

survival, reproduction, recruitment, and population growth. Climate change may 

also precipitate indirect effects through mechanisms such as (1) changes in habitat 

distribution, abundance, and quality; (2) increasing high-severity wildfire; (3) 

increasing mature/large-tree mortality caused by drought, insects, and disease; (4) 

changes in prey distribution, abundance, and population dynamics; (5) changes in 

interspecific interactions with competitors and predators; and (6) changes in disease 

dynamics associated with changing temperature and precipitation patterns. 

Although little information exists on the threat of disease, parasites, and 

contaminants on spotted owl populations, the potential for impacts from these 

elements is concerning. The primary threats are West Nile Virus, ectoparasites, 

and endoparasites. West Nile virus is primarily a mosquito-borne flavivirus that 

has recently invaded North America and is highly lethal to owls. Several species of 

ectoparasites and endoparasites have been identified in spotted owls. Diseases and 

parasites can interact with other stressors to affect either the condition or survival 

of individuals. Environmental contaminants have not been identified as current 

ecological stressors on California spotted owls; however, recent reports of high 

exposure rates of fisher (Pekania pennanti) to rodenticides across the southern 

Sierra Nevada are likely to have implications for spotted owls because they feed on 

rodents. For example, 62 percent (44 of 71 owls) of barred owls tested positive for 

rodenticides on the Hoopa Reservation in northern California. 

xxv 
The Spotted Owl in Southern and Central Coastal California 

Spotted owls in southern and central coastal California have received much 

less attention than those inhabiting the Sierra Nevada because of economic 

(effect of habitat conservation measures on timber harvest) and social issues 

(community desire for naturally functioning ecosystems). Yet there has been 

continued concern over the status of owl populations in this region since CASPO. 

The owl in this region is distributed from Monterey County and Tehachapi Pass 

south through the coastal, Peninsular, and Transverse Ranges to Mount Palomar 

near the Mexican border. The presumption is that owls in the Sierra San Pedro 

Martir in Baja California Norte are California spotted owls as well. 

There are four major cover types used by spotted owls in southern California: riparian/hardwood forests and woodlands, live oak (Quercus chrysolepis 

Liebm.)/big cone-fir (Pseudotsuga macrocarpa (Vasey) Mayr) forests, mixedconifer forests, and redwood (Sequoia sempervirens (Lamb. ex D. Don) Endl.)/ 

California laurel (Umbellularia californica (Hook. & Arn.) Nutt.) forests. 

Unlike in the Sierra Nevada, most owls occur in cover types other than mixedconifer forest because mixed-conifer forest is only found at the highest elevations 

in most of these isolated mountain ranges. Thus, they are found over gradients of 

habitat within these mountain ranges. Yet, site-specific characteristics of territories 

and nest sites follow patterns seen in the Sierra Nevada owl habitat selection. 

The spotted owl in southern California is unique among west coast spotted 

owl populations because it occurs as a presumed metapopulation—distinct 

populations that function independently, yet their dynamics are interrelated 

because of dispersal among populations. Metapopulation structure is presumed, 

but there is a lack of documented movement among populations to confirm this 

presumption. One analysis in CASPO revealed key properties of this theoretical 

metapopulation. One property was that the San Bernardino population was critical 

to the persistence of the entire metapopulation because the many small populations 

in the region would benefit from having this large population be a source of immigrants. A later simulation study suggested that the metapopulation would likely 

either go extinct within the next 30 to 40 years or would undergo a substantial 

decline but not go extinct. If there is little or no dispersal among populations, as 

current studies indicate, the risk of local population extinctions increases. 

Crude densities (density across the landscape) of owls in southern California are lower than densities in other areas of California, which suggests that 

there is higher spatial fragmentation of suitable habitat within populations 

in southern California; however, ecological density (density within suitable 

xxvi 
cover types across the landscape) is comparable to at least one population of 

northern spotted owls prior to its recent decline. This suggests that the habitat in 

southern California has a similar capacity for supporting spotted owls as the more 

mesic forests in northwestern California. Like populations in the Sierra Nevada, 

fecundity of owls is variable among years and influenced by the age of owls 

(subadults have lower fecundity than adults) and weather. In southern California, 

survival was related to age (survival higher in older age classes) and precipitation in 

the preceding winter. 

The most complete data on territory occupancy and population trend 

in southern California exists for the San Bernardino Mountains within the 

San Bernardino National Forest, and suggests that this key population has 

declined. This study occurred from 1987 through 1998, with additional monitoring 

of known owl territories from 2003 through 2011. The San Jacinto population was 

studied less intensively and sporadically from 1988 through 2011. Both populations 

have shown significant declines (about 50 percent) in territory occupancy and for 

the San Bernardino a significant decline based on estimates of vital rates. 

Connectivity among populations is critical to the persistence of the spotted owl in this region, and it is a function of barriers and dispersal habitat. 

In CASPO, urban and suburban development and the loss of riparian areas were 

reported to be threats to the metapopulation because they were barriers to dispersal 

among populations. The current situation is worse than at the time of CASPO 

because development continues unabated within both the Los Angeles Basin and 

the surrounding deserts. Further, two new types of barriers pose potential threats to 

dispersal: wind farms and large reservoirs. Many wind turbines have been erected 

in several areas that could serve as potential dispersal corridors between mountain 

ranges and between the southern California region and the Sierra Nevada. At the 

time of CASPO, reservoirs were not specifically considered a barrier to dispersal, 

but at least one owl drowned in its apparent attempt to cross one in the area between 

the San Bernardino and San Gabriel Mountains. 

Habitat loss could result from fires and salvage logging, as well as habitat 

loss and disturbance from urban development and recreation. There are as yet 

no restrictions on logging of trees on private land within the range of the owl other 

than those imposed by the California Forest Practices Act. Habitat is also being lost 

or fragmented as a result of primary and secondary (i.e., vacation) home building. 

However, there is no longer any commercial timber harvest on national forests 

within the owl’s range in southern California. Post-CASPO assessments of riparian 

xxvii 
habitat found no evidence for loss of riparian habitat owing to the water diversion threat that was listed as a potential threat in CASPO. Yet such loss remains a 

potential threat as does the threat of channelization to control waterflow (i.e., flood 

protection). Wildfire has long been a concern because of its potential impact on 

owls and their habitat, but its overall effect on owl populations is not clear. Given 

the loss of habitat owing to other factors (e.g., urbanization and drought), fires are 

likely a contributing factor to owl declines. A myriad of additional threats to habitat 

and owls exist in the southern California and coastal populations, including disturbance from human recreation, drought, air pollution, mining, marijuana cultivation, 

invasive species, disease, cumulative effects of small-scale management actions, 

and climate change. 

Synthesis and Interpretation Within the Context of Public Forest 

Management 

In this final chapter, we identify and discuss key scientific findings that have 

emerged since the CASPO report in 1992. We also discuss priorities for future 

research that could enhance the successful conservation of California spotted 

owls and their habitat, and we acknowledge when uncertainty limits well-founded 

conclusions and articulate potential differences in interpretation of the scientific 

literature where such differences exist. 

Conservation of California spotted owls in the Sierra Nevada will require 

maintaining a well-distributed population of owls of sufficient abundance 

that the population will be resilient to the effects of climate change and other 

environmental stressors. Establishing a set of biologically based conservation 

benchmarks would be valuable to indicate the status of spotted owl populations and 

to prompt additional or alternate conservation measures. 

Maintaining a viable population of spotted owls on public lands in the 

Sierra Nevada will be an outcome of effective, long-term owl conservation 

practices embedded in an overall management strategy aimed at restoring 

resilient forest structure, composition, and function. Conserving spotted owl 

populations and restoring ecosystem resilience are complementary objectives when 

management activities reduce the loss of old forest and owl habitat to drought and 

large high-severity fires. A reasonable guiding philosophy is to manage Sierra 

Nevada forests in ways that combine the objectives of spotted owl conservation, 

fuels management, and drought resilience, while also recognizing that forests are 

dynamic ecosystems that will support a range of vegetation types and structures 

that vary over space and time. In practice, however, implementing effective firemanagement and ecosystem restoration programs that do not also pose risks to 

spotted owls will be challenging. 

xxviii 
Two paradigms emerged as part of this assessment regarding tradeoffs 

between the potential short-term negative impacts and possible long-term 

benefits of fuel and restoration treatments on spotted owls. 

• One paradigm holds that forest management treatments within spotted owl 

habitat pose risks to spotted owls because their populations have declined 

and restoration treatments commonly entail the reduction of canopy cover 

and canopy complexity, and even the removal of some large trees. Thus, a 

strategy focused on conserving and enhancing existing owl habitat would 

be the most effective approach to conservation. 

• The alternative paradigm holds that increases in the spatial extent of highseverity fire and other disturbances to forests (e.g., prolonged drought, 

insects, and disease) pose the primary proximate threat to spotted owl population persistence, owl habitat, and forest ecosystems in the Sierra Nevada. 

Thus, a strategy that reduces the risk of large, high-severity fires would be 

the most effective approach to conservation. 

The following key findings and points of consensus regarding new scientific information are relevant to both owl conservation and forest restoration in 

the Sierra Nevada: 

• Spotted owls have declined in abundance on some national forest lands in 

the Sierra Nevada over the past two decades. 

• The density of large and defect trees has declined in Sierra Nevada forest 

as a result of historical (pre-CASPO) timber harvesting; these habitat elements may well be contributing to recent spotted owl population declines, 

and restoring large trees is expected to benefit both spotted owls and forest 

resilience. 

• A century of fire exclusion has led to an increase in the size of high-severity fires owing to the accumulation of surface and ladder fuels, and a concomitant high risk of habitat loss resulting from large high-severity fires. 

• Restoring low- to moderate-severity fire regimes to the mixed-conifer zone 

could help achieve both spotted owl conservation and forest restoration 

goals. 

• Habitat conditions in owl territories that are located in areas with high 

burn probabilities or low drought tolerance may not be viable in the long 

term—conservation and restoration focused in areas that can sustain suitable habitat conditions may align the distribution of owl habitat with forest 

restoration goals. 

xxix 
Different habitat features are important to spotted owls at each of several 

spatial scales, and considering these scale-specific requirements will facilitate the 

development of forest conditions that minimize risk to owls and promote resilient 

forest ecosystems. The scales of greatest importance are the owl’s activity center, 

territory, and home range, embedded within the broader forested landscape. Desired 

conditions for each scale of ecological importance, as well as the implications of 

recent research for achieving these conditions via forest management are as follows: 

• Activity center scale: Maintaining high-quality nesting and roosting habitat at known spotted owl activity centers will likely enhance occupancy 

and demographic performance. Forest structural characteristics known 

to be important at this scale are likely to be maintained or even enhanced 

through low-intensity vegetation treatments intended to reduce the risk of 

high-severity fire and drought-induced large-tree mortality. 

• Territory scale (outside of activity centers): Spotted owl occupancy and 

fitness within territories appear to be positively related to the acreage of 

high-quality habitat. Given climate change predictions and the likely implications for fire and tree mortality, reducing these risks to forests within territories is likely to benefit spotted owl populations. 

• Home range scale (outside of territories): Spotted owl home ranges are 

characterized by heterogeneous forests containing a mosaic of vegetation 

conditions. At this scale, greater emphasis can be placed on fuels management and forest restoration, particularly approaches that enhance forest 

resilience, landscape heterogeneity, and spotted owl foraging habitat. 

• Landscape scale (matrix between home ranges): A landscape of heterogeneous forests containing a mosaic of vegetation conditions including 

patches of old forest is likely to promote the recruitment of new spotted 

owl territories. Fuels and restoration treatments (including prescribed and 

managed fire) that promote landscape heterogeneity in forest conditions and 

reduce risks for high-severity fire are likely to be beneficial to spotted owl 

conservation in the longer term. 

Recent research indicates that California spotted owls persist in territories 

that experience low-moderate severity and mixed-severity wildfire and that 

small patches of high-severity fire may enhance foraging conditions for spotted owls. However, high-severity fire can also have a negative effects on spotted 

owls. Salvage harvesting within such landscapes, particularly high-intensity salvage 

(removal of most snags), could invoke or exacerbate negative impacts on spotted 

owl habitat via the removal of snags and ultimately the reduction of coarse woody 

debris on the forest floor. 

xxx 
Barred owl range expansion into the northern Sierra Nevada, particularly 

given the profound impacts they have had on northern spotted owls, could 

warrant control measures. Control measures would be most effective while barred 

owls still occur at low densities in the Sierra Nevada. The momentum of range 

expansion and abundance is expected to increase exponentially once barred owls 

have reached a critical, as yet unknown, density. 

A set of “conservation benchmarks” would be valuable to indicate the 

status of California spotted owl populations. Such benchmarks could be used 

to evaluate monitoring results and gauge whether management activities are 

effectively accomplishing their intended objective of conserving spotted owls, 

or whether additional conservation measures need to be implemented, within an 

adaptive management framework. For example, potential demographic metrics of 

spotted owl population status upon which conservation benchmarks could be based 

include abundance, population trends, and geographic distribution. 

Despite considerable resources devoted to improving our understanding of 

the ecology and status of the California spotted owl, important uncertainties 

and knowledge gaps remain that could be addressed through future monitoring or research investments. 

• It remains unclear what environmental or anthropogenic factors are responsible for observed population declines of the California spotted owl. 

• A greater understanding about the effects of fuel and restoration treatments 

and wildfire on California spotted owls is needed to inform forest management that is intended to recover owl populations and restore ecosystem 

resilience in Sierra Nevada forests. 

• Considerable uncertainty remains about the owl’s distribution and winter 

ranges in the mountain ranges of southern California, the foothills of the 

western Sierra Nevada, and the Coast Ranges, and the significance of these 

local and regional owl populations to the species’ rangewide persistence. 

• To achieve a regional-scale inference based on sampling of owls, a regionalscale, occupancy-based monitoring program would be highly complementary to the information provided by the demographic monitoring and would 

facilitate the assessment of barred owl impacts. 

• Nonforested vegetation (e.g., montane chaparral) distributed within a 

mosaic of forest types may constitute important foraging habitat, particularly when juxtaposed with closed-canopy forests and may confer fitness 

(survival and reproduction); however, such linkages have not yet been demonstrated conclusively for California spotted owls. 

xxxi 
• A greater understanding of the vegetation conditions that shape the abundance and distribution of important prey species in the Sierra Nevada 

would inform the development of effective stand- and landscape-scale forest management strategies to enhance spotted owl foraging habitat. 

• Future studies could further our understanding of potential climate change 

effects by linking expected changes in owl distribution to shifts in vegetation communities and change in fire dynamics—an effort that would benefit from integrative efforts involving wildlife, forest, and fire ecologists. 

• There are many outstanding needs for mapping of wildlife habitat, including the mapping of snags, large trees, and large broken-top trees; the development of improved metrics to quantify vertical canopy structure; and the 

development of tree species distributions in mixed-conifer forests. 

xxxii 
The California Spotted Owl: Current State of Knowledge

Chapter 1: Introduction 

Peter A. Stine and Patricia N. Manley1 

The California spotted owl (Strix occidentalis occidentalis) occurs across a large 

portion of California, including the portion of the southern Cascade Range south 

of the Pit River that abuts the Sierra Nevada and throughout the Sierra Nevada, the 

mountains of central coastal California, and the Peninsular and Transverse Ranges 

of southern California. The future of the California spotted owl is of concern 

because of population trends over the past few decades, the potential impacts of 

forest management, and the threat of high-severity fire on its primary habitat of 

closed-canopy forest. Data from demographic studies conducted in three locations in the Sierra Nevada show that populations have been declining over the past 

20-plus years (e.g., Conner et al. 2013, Tempel et al. 2014). The majority of the 

current range of the owl occurs on public lands, primarily national forests. 

Regulatory Context 

The U.S. Forest Service (USFS) is required, under the new 2012 National Forest 

Management Act (NFMA 2012) Planning Rule (36 CFR 219; Federal Register 

2012), to identify potential species of conservation concern and provide an assessment of existing information for those species. A species of conservation concern is 

defined as: 

...a species, other than federally recognized threatened, endangered, 

proposed, or candidate species, that is known to occur in the plan area 

and for which the regional forester has determined that the best available 

scientific information indicates substantial concern about the species’ ֻ

capability to persist over the long-term in the plan area. 

The California spotted owl is considered a species of conservation concern (FS 

Handbook 1909.12 § 12.52c-d), thus the USFS is directed to identify and assess 

information relevant to this species. A technical assessment of its status and threats 

is a valuable, if not essential, step in informing effective conservation measures and 

strategies. Among other applications, the information presented in this assessment 

will inform revisions of the USFS Land and Resource Management Plans for the 15 

national forests within its range. 

1 Peter A. Stine is a biogeographer and retired Director of Partnerships and Collaboration, 

U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station, John 

Muir Institute for the Environment, 1 Shields Ave., University of California–Davis, CA 

95616; Patricia N. Manley is a research program manager, U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station, Conservation of Biodiversity 

Program, 2480 Carson Rd., Placerville, CA 95667. 

1 
2 

GENERAL TECHNICAL REPORT PSW-GTR-254

The first technical assessment for the California spotted owl “The California 

Spotted Owl: a Technical Assessment of Its Current Status” or (CASPO) was 

initiated in 1991 (Verner et al. 1992). It was developed to help guide management 

direction for national forest project planning in the Sierra Nevada and southern 

California mountains (Verner et al. 1992; see below for more details). The interim 

management guidelines recommended in CASPO were followed by the national forests in the Sierra Nevada until the Sierra Nevada Forest Plan Amendment record of 

decision (ROD) was released in 2001 and then amended in 2004. The ROD (USDA 

FS 2004) provided some specific standards and guidelines for California spotted 

owl habitat based on a science synthesis created for the forest plan amendment 

process. Additionally, the 2004 ROD focused on fuels treatments because loss of 

owls and their habitat as a result of catastrophic wildfires was considered a significant threat to the species. Much has been learned about the ecology of California 

spotted owls since CASPO, but there has been no comprehensive assessment of this 

new information. New information in the published literature needs to be assessed 

to improve our understanding of its significance and relevance to management and 

future research. Periodic syntheses and assessments also help bring cohesion to 

the interpretation of new information, thereby guiding its application to management. Over the 25-year period since CASPO, many site-specific research projects, 

long-term demographic monitoring, basic ecological research, and specific project 

monitoring activities have occurred throughout the range of the California spotted 

owl. Dozens of peer reviewed publications have resulted from these activities (e.g., 

see chapters 2, 3, 4, 7, and 8), and given forest plan revision activities and growing 

concerns for the status of the spotted owl, it was timely to generate an updated 

assessment to support and inform conservation and management efforts. 

1992 California Spotted Owl Technical Assessment 

The information benchmark for our assessment was CASPO (Verner et al. 1992). 

The CASPO represents the last effort to comprehensively summarize what is 

known about this subspecies throughout its range. It was developed over a 1-year 

period by a technical team dedicated solely to this task. The CASPO was conducted 

in response to the designation of the northern spotted owl (S. o. caurina) in 1990 as 

a federally listed threatened species, the recognition that forest managers in California would benefit from an assessment of the current status of the California subspecies, and the need to develop a scientifically defensible plan for the conservation 

of the California spotted owl. The CASPO received direction from the California 

Spotted Owl Assessment Team Steering Committee, whose members represented 
3 

The California Spotted Owl: Current State of Knowledge

several state of California (Resources Agency, Board of Forestry, Department of 

Fish and Game, and Department of Forestry and Fire Protection) and federal entities (U.S. Department of Agriculture Forest Service; and U.S. Department of the 

Interior Bureau of Land Management, Fish and Wildlife Service, and National Park 

Service). 

The charter for CASPO specified submission of a report to the steering committee on the current status of the California spotted owl following “accepted 

scientific standards and practices.” The 285-page CASPO report was intended to 

provide guidance for managing owl habitat as forest plans were revised and more 

information about the owl was learned to justify either deviation from the strategy 

or to support a long-term conservation strategy based on fulfilling critical information needs of the owl. The technical assessment had the following objectives: 

1. Present, analyze, and interpret relevant information currently available on 

the biology of the owl—its distribution, abundance, density, movements, 

breeding biology, diet, demography, habitat associations, etc. 

2. To the extent possible, characterize the attributes of various habitats used 

for foraging, roosting, and nesting by the owl throughout its range in 

California. 

3. Evaluate current land management practices throughout the range of the 

owl, recognizing that more detailed information may be available for some 

land ownerships than for others. 

4. Evaluate a range of options to achieve an amount and configuration of suitable habitat to provide for the long-term maintenance of the owl throughout 

its range. 

5. Identify research, monitoring, and inventory programs needed to answer 

existing critical questions and to provide for adaptive management of the 

owl in the future. 

Lessons Learned From the Northern Spotted Owl 

Although the northern spotted owl (Strix occidentalis caurina) is a different subspecies within a different ecoregion, many of the challenges and successes in conservation efforts associated with this federally threatened species over the past 24 years 

are applicable to the California spotted owl (USFWS 2011). Our assessment makes 

no attempt to incorporate the large body of work published on the northern spotted 

owl, but the five primary topic areas of the 2011 Recovery Plan clearly reflect the 
4 

GENERAL TECHNICAL REPORT PSW-GTR-254

critical information needed to address the conservation and management of spotted 

owl populations and habitat: 

1. Conservation of existing spotted owl sites and high-value spotted owl 

habitat. 

2. Ecological forestry and active forest restoration approaches to meet the 

challenges of climate change and altered ecological processes. 

3. The threat posed by barred owls (Strix varia) and management options to 

address those threats. 

4. The potential need for state and private lands to contribute to spotted owl 

recovery in areas with substantial mixed ownership. 

5. Development of a population and habitat modeling framework as a 

decision-support tool to better inform future land management decisions. 

Expanding Challenges in Spotted Owl Conservation 

The complexities of managing habitat that supports viable populations of species 

associated with mature forests have continued to grow in recent decades. At the 

time of CASPO, concern about fire centered on sufficient fire suppression measures, climate change was not a primary focus for Forest Service scientists and 

managers, and as the barred owl had just recently invaded the range of the northern 

spotted owl, there was uncertainty about its impact on the spotted owl (Verner et al. 

1992). Since CASPO, we have observed significant declines in spotted owl populations across its range (Conner et al. 2013, LaHaye and Gutiérrez 2005, Tempel et al. 

2014; chapters 4 and 8); an increase in the size and severity of wildfires (chapter 5); 

a growing recognition of the essential role of active fire in forest restoration, and 

the dual role that fire can play as a destructive and constructive process (chapters 

5 and 9); the challenge of balancing forest restoration using fire and increasingly 

strict air quality objectives and constraints (Quinn-Davidson and Varner 2012); a 

clearer understanding of the impact of high-severity burned forests in sustaining 

owls in a dynamic landscape where fire is likely to become more prevalent (Bond 

et al. 2009; chapter 7); the emergence of diseases, such as West Nile virus (Ishak 

et al. 2008; chapter 7); and the significant threat that the invasion of the barred owl 

has on spotted owl population persistence (Gutiérrez et al. 2007; chapter 7). Against 

this background, uncertainty posed by climate change in California, most notably 

in the form of extended droughts, is predicted to exacerbate many of these observed 

challenges in addition to unforeseen effects on owl populations and their prey (Millar and Stephenson 2015). These emerging issues have joined, not replaced, those 
5 

The California Spotted Owl: Current State of Knowledge

recognized at the time of CASPO, namely the impact of habitat loss and fragmentation through logging and urbanization, the uncertain impact of water diversions to 

serve increasing demands with (now) decreasing supplies, and the uncertainty of 

imperfect information about the ecology, vulnerabilities, and primary drivers of 

population trends. Perhaps the most troubling of all is the overwhelming evidence 

that uncertainties about how to conserve the California spotted owl are now confounded by the uncertainty about how to conserve forest ecosystems in light of the 

increasing threat of high-severity fires and climate change. This excerpt from the 

northern spotted owl recovery plan pertaining to habitat conservation and management in dry forest ecosystems illustrates the conservation conundrum we face 

(USFWS 2011: III-20): 

Changing climate conditions, dynamic ecological processes, and a variety 

of past and current management practices render broad management 

generalizations impractical. Recommendations for spotted owl recovery 

in this area also need to be considered alongside other land management 

goals—sometimes competing, sometimes complimentary—such as 

fuels management and invasive species control. In some cases, failure 

to intervene or restore forest conditions may lead to dense stands heavy 

with fuels and in danger of stand-replacing fires and insect and disease 

outbreaks. In general, we recommend that dynamic, disturbance-prone 

forests ...should be actively managed in a way that reconciles the 

overlapping goals of spotted owl conservation, responding to climate 

change and restoring dry forest ecological structure, composition and 

processes, including wildfire and other disturbances. 

The management of forested landscapes entails many considerations. In addition to the concerns raised above, there are other species of conservation concern 

(including candidate species for listing under the Endangered Species Act of 1973 

[ESA 1973]) that also occupy the mid-elevation multilayered, mature forests that 

California spotted owls use. In particular, the pacific fisher (Pekania pennanti), 

another old-forest-associated species of concern, is found in the southern Sierra 

Nevada where its range almost entirely overlaps that of the California spotted owl. 

Therefore, management of California spotted owl habitat may have complementary 

or competing objectives with other old-forest-associated species. This could be 

especially important when considering the cumulative effects of multiple conservation strategies on meeting ecosystem management and ecological restoration 

objectives. 
6 

GENERAL TECHNICAL REPORT PSW-GTR-254

This Conservation Assessment 

This conservation assessment for the California spotted owl was initiated in 

response to a request by the Pacific Southwest Region of the USFS to provide 

a scientific foundation for a comprehensive conservation strategy for National 

Forest System lands within the owl’s range. The intention of this assessment was 

to provide a comprehensive overview of the best available scientific information 

about the ecology, habitat use, population dynamics, and existing and potential 

threats throughout the geographic range of the California spotted owl, as well as its 

implications for land management within the context of the broader landscape. It 

was also intended to specify and clarify to the degree possible the complex interactions of owl populations, forests, and landscape dynamics, and address these topics 

from the perspective of different areas of scientific expertise in order to provide a 

more comprehensive perspective on management challenges and opportunities. In 

a few cases, different authors reached different conclusions about particular topics; 

when this occurred, authors worked together to provide additional clarification on 

the range of perspectives and their respective foundations. Ideally, this assessment 

will help inform future options for management ranging in scale from site-specific 

projects to large landscapes to the entire range of the owl. Land managers must reconcile many objectives and demands, including conserving the full suite of native 

species associated with forest ecosystems within their jurisdiction. We intended this 

assessment to inform that reconciliation. 

The geographic coverage of this assessment includes the entire range of the 

subspecies—the Sierra Nevada, the Transverse Ranges of southern California, and 

portions of central coastal California (Gutiérrez et al. 1995). However, the majority 

of the owl population occurs in the Sierra Nevada, and this is also where most of the 

new information has been generated over the past 20 years. Thus, this assessment 

is focused primarily on the Sierra Nevada, with a largely independent update of the 

southern and coastal California populations addressed only in chapter 8. 

The Owl Assessment Team members, the authors of this assessment, were 

assembled to represent expertise relevant to the conservation and management of the 

California spotted owl and its habitat (see appendix). This team provided expertise not 

only in owl biology but also in several related disciplines, including climate change, 

fire and fuels management, forest ecology, remote sensing, and vegetation ecology. 

The forests in which the owl lives have changed significantly in composition and 

structure over at least the past 100 years as a result of human activities and will be 

subject to additional human influences and other stressors in the coming decades. 

This assessment provides a comprehensive summary of the state of knowledge in 
7 

The California Spotted Owl: Current State of Knowledge

relevant topic areas, many of which were also addressed in CASPO. The latest 

understanding of the biology of the spotted owl is provided in chapter 2. Chapter 3 

provides a thorough reporting of the current knowledge of habitat associations and 

use. Population distribution and trends based on demographic monitoring and other 

research results are summarized in chapter 4. The assessment then transitions to the 

environmental context for owl populations and habitat, starting with an overview of 

current and projected future forest conditions in mid-elevation conifer forests of California in chapter 5, followed by habitat mapping and analysis technology in chapter 

6. Chapter 7 summarizes the primary threats that owl populations and habitat face 

currently and into the foreseeable future. Given the relative paucity of information 

on spotted owl population status and trends in southern California, an overview of 

what we do know is presented entirely in chapter 8. The assessment concludes with a 

synthesis and interpretation of California spotted owl research within the context of 

broader land management challenges presented in chapter 9. 

This assessment does not evaluate explicit habitat management options for 

the long-term maintenance of the owl throughout its range, but rather it attempts 

to identify key elements that appear to be critical to the success of any conservation effort, and it explores implications for future research and management. This 

assessment does not provide an evaluation of monitoring and inventory program 

needs necessary to provide for adaptive management of the owl, but it does identify 

future research investments that could reduce key uncertainties as management 

proceeds. 

Although this assessment addresses only one species, it reflects the situation 

facing old-forest ecosystems and associated species in dry forest ecosystems of the 

Sierra Nevada and southern California. The essential role of fire in restoring the 

resilience of forests in the range of the California spotted owl, juxtaposed with the 

threat that high-intensity wildlife poses to suitable spotted owl habitat given current forest conditions, and the uncertainty of impacts to owls of forest treatments 

to reduce the risk of high-intensity fire creates challenges for both scientists and 

managers. The role of forest management and the use of managed wildfire and prescribed fire emerge as dominant themes across the chapters of this assessment, and 

the final chapter provides an evaluation of common ground between owl conservation approaches and management to restore forest resilience. 

Information Sources Consulted 

Substantial monitoring of owl populations, field research, and analysis of data have 

been completed since CASPO. Five geographically distinct demographic study 

areas were established either before or approximately coincident with CASPO to 
8 

GENERAL TECHNICAL REPORT PSW-GTR-254

examine apparent survival probability, reproductive output, and population trends. 

The five areas were on the Lassen National Forest, Eldorado National Forest, Sierra 

National Forest, the adjoining Sequoia and Kings Canyon National Parks, and the 

San Bernardino Mountains. These study areas represented a broad spectrum of 

habitat and management conditions in the Sierra Nevada and the largest population 

in southern California (Franklin et al. 2004). Although the San Bernardino Mountains demography study ended in 1998, it has been the source of the majority of 

information from southern California. A summary of the latest scientific information on the southern California owl populations was developed in 2011 by the San 

Bernardino National Forest2 but not published. Many other research projects were 

conducted throughout the range of this subspecies within this time period, contributing significantly to the current body of knowledge about the owl. 

The information used in this assessment is based almost exclusively on peerreviewed, published literature. The team was not solely dedicated to this task but, as 

it was a minor allocation of their professional responsibilities, we limited the review 

to (1) published information), (2) only a limited scope of northern spotted owl work, 

and (3) no data compilation or analysis of raw data. As such, it was a much more 

limited and constrained (by time and money) effort relative to CASPO. It was not 

possible within the constraints of this assessment to access unpublished reports 

and archived data that may exist and that could be relevant to this assessment. 

Resources used to inform this assessment are individually referenced and cited 

in each chapter. 

The Process and Product 

This assessment is published as a Pacific Southwest Research Station (PSW) 

general technical report, as was the previous CASPO report. General technical 

reports provide the opportunity for a detailed reporting of information, a rigorous 

peer review process, and an easily accessible outlet. The technical peer review was 

conducted by an anonymous, independent group of four scientists who represented 

the same scientific disciplines covered by the content of the report. The comments 

of the reviewers were individually addressed through a scrupulous revision process. 

The document also was subject to management review by Region 5 staff, and to a 

policy review by PSW. 

2 Eliason, E.; Loe S. 2011. Unpublished report. On file with: USDA Forest Service, San 

Bernadino National Forest, 602 S Tippecanoe Ave., San Bernardino, CA 92408. 
9 

The California Spotted Owl: Current State of Knowledge

Literature Cited 

Bond, M.L.; Lee, D.E.; Siegel, R.B.; Ward, J.P. 2009. Habitat use and selection 

by California spotted owls in a postfire landscape. Journal of Wildlife 

Management. 73: 1116–1124. 

Conner, M.M.; Keane, J.J.; Gallagher, C.V.; Jehle, G.J.; Munton, T.E.; 

Shaklee, P.A.; Gerrard, R.A. 2013. Realized population change for long-term 

monitoring: California spotted owl case study. Journal of Wildlife Management. 

77: 1449–1458. 

Endangered Species Act of 1973 [ESA]; 16 U.S.C. 1531–1543, 1538-1540.

Federal Register. 2012. National Forest System Land Management Planning. 

77(68): 21162–21276. 

Franklin, A.B.; Gutiérrez, R.J.; Nichols, J.D.; Seamans, M.E.; White, G.C.; 

Zimmerman, G.S.; Hines, J.E.; Munton, T.E.; LaHaye, W.S.; Blakesley, 

J.A.; Steger, G.N.; Noon, B.R.; Shaw, D.W.H.; Keane, J.J.; McDonald, T.R.; 

Britting, S. 2004. Population dynamics of the California spotted owl (Strix 

occidentalis occidentalis): a meta-analysis. Ornithological Monographs. 54. 

Gutiérrez, R.J.; Cody, M.; Courtney, S.; Franklin, A.B. 2007. The invasion 

of barred owls and its potential effect on the spotted owl: a conservation 

conundrum. Biological Invasions. 9: 181–196. 

Gutiérrez, R.J.; Franklin, A.B.; LaHaye, W.S. 1995. Spotted owl (Strix 

occidentalis). In: Poole, A.; Gill, F., eds. The birds of North America No. 179: 

life histories for the twenty-first century. Washington, DC: The Philadelphia 

Academy of Sciences and the American Ornithologists’ Union. 

Ishak, H.D.; Dumbacher, J.P.; Anderson, N.L.; Keane, J.J.; Valkiūnas, G.; 

Haig, S.M.; Tell, L.A.; Sehgal, R.N.M. 2008. Blood parasites in owls with 

conservation implications for the spotted owl (Strix occidentalis). PLoS ONE. 

3(5): e2304. 

LaHaye, W.S.; Gutiérrez, R.J. 2005. The spotted owl in southern California: 

ecology and special concerns for maintaining a forest-dwelling species in a 

human-dominated desert landscape. Gen. Tech. Rep. PSW-GTR-195. Albany, 

CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research 

Station. 11 p. 

Millar, C.I.; Stephenson, N.L. 2015. Temperate forest health in an era of emerging 

megadisturbance. Science. 349(6250): 823–826. 
10 

GENERAL TECHNICAL REPORT PSW-GTR-254

National Forest Management Act of 1976 [NFMA]; Act of October 22, 1976; 

16 U.S.C. 1600. 

Quinn-Davidson, L.; Varner, J.M. 2012. Impediments to prescribed fire 

across agency, landscape and manager: an example from northern California. 

International Journal of Wildland Fire. 21(3): 210–218. 

Spies, T.A.; Hemstrom, M.A.; Youngblood, A.; Hummel, S. 2006. Conserving 

old-growth forest diversity in disturbance-prone landscapes. Conservation 

Biology. 20: 351–362. 

Tempel, D.J.; Gutiérrez, R.J.; Whitmore, S.A.; Reetz, M.J.; Stoelting, 

R.E.; Berigan, W.J.; Seamans, M.E.; Peery, M.Z. 2014. Effects of forest 

management on California spotted owls: implications for reducing wildfire risk 

in fire-prone forests. Ecological Applications. 24: 2089–2106.

U.S. Department of Agriculture, Forest Service [USDA FS]. 2002. Sensitive 

species—key policies and requirements. 8 p. http://www.fs.fed.us/r6/sfpnw/ 

issssp/documents/ag-policy/20021200-fs-sensitive-species-key-policies.pdf. 

(16 September 2016). 

U.S. Department of Agriculture, Forest Service [USDA FS]. 2004. Sierra 

Nevada Forest Plan Amendment Final Environmental Impact Statement, 

Record of Decision. 72 p. http://www.fs.usda.gov/detail/r5/landmanagement/ 

planning/?cid=stelprdb5349922. (16 September 2016). 

U.S. Department of Agriculture, Forest Service [USDA FS]. 2013. Region 5 

regional forester’s 2013 sensitive animal species list. http://www.fs.usda.gov/ 

main/r5/plants-animals. (16 September 2016). 

U.S. Department of the Interior, Fish and Wildlife Service [USDI FWS]. 2011. 

Revised recovery plan for the northern spotted owl (Strix occidentalis caurina). 

Portland, OR. 258 p. 

Verner, J.; McKelvey, K.S.; Noon, B.R.; Gutiérrez, R.J.; Gould, G.I., Jr.; Beck, 

T.W., tech. coords. 1992. The California spotted owl: a technical assessment of 

its current status. Gen. Tech. Rep. PSW-GTR-133. Albany, CA: U.S. Department 

of Agriculture, Forest Service, Pacific Southwest Research Station. 285 p.
11 

The California Spotted Owl: Current State of Knowledge

Chapter 2: The Biology of the California Spotted Owl 

R.J. Gutiérrez, Douglas J. Tempel, and M. Zachariah Peery1 

Introduction 

The spotted owl (Strix occidentalis) is one of the most studied raptors in the world 

(Lõmus 2004) because forest management throughout its range has the potential 

to negatively affect owl populations. Information on the California spotted owl (S. 

o. occidentalis) has been summarized in several literature reviews (e.g., Gutiérrez 

1996; Gutiérrez and Carey 1985; Gutiérrez et al. 1995; Keane 2014; Roberts and 

North 2012; USFWS 1990, 1993, 2003, 2006; Verner et al. 1992a). However, the 

first comprehensive review of the biology of the California spotted owl was conducted by Verner et al. (1992a). Verner et al. (1992a) also served as a foundational 

chapter for the California spotted owl technical assessment “The California Spotted 

Owl: A technical Assessment of it’s current status” (CASPO) and its recommended 

owl management strategy (Verner et al. 1992b). Much has been learned about the 

biology of the California spotted owl since CASPO including new methods of 

data analysis to provide scientifically defensible results (Gutiérrez 2004, 2008). In 

this chapter, we summarize new information on the natural and life history of the 

California spotted owl that has been gathered primarily since CASPO (Verner et al. 

1992b), but we also include new research about other owl subspecies (northern and 

Mexican spotted owls, S. o. caurina and S. o. lucida, respectively) when it is applicable to the California spotted owl. We cite the Birds of North America spotted owl 

account (Gutiérrez et al. 1995) for most information about northern and Mexican 

spotted owls published before 1995 rather than citing the original sources. Relatively more ecological knowledge is available for the northern spotted owl owing 

to its longer history of conservation concern and its commingling with old-growth 

forest protection issues (Gutiérrez et al. 1995, 2015; Redpath et al. 2013). Although 

this chapter is about the general biology of California spotted owls, most of the 

salient ecological information on habitat use and population dynamics is treated 

separately in chapters 3 and 4, respectively, of this assessment because those topics 

are particularly critical for understanding current population trends and developing 

1 R.J. Gutiérrez is a professor and Gordon Gullion Endowed Chair Emeritus, University 

of Minnesota, 2003 Upper Buford Circle, St. Paul, MN 55108; Douglas J. Tempel is a 

postdoctoral research associate, Department of Forest and Wildlife Ecology, University 

of Wisconsin, 1630 Linden Dr., Madison, WI 53706; M. Zachariah Peery is an associate professor, Department of Forest and Wildlife Ecology, University of Wisconsin, 500 

Lincoln Dr., Madison, WI 53706. 
12 

GENERAL TECHNICAL REPORT PSW-GTR-254

future forest management plans. Indeed, we know more about spotted owl habitat 

and population dynamics than of most other species of conservation concern 

(Gutiérrez 2008, Lõhmus 2004). Finally, we occasionally include the theoretical 

underpinnings to support inferences we make about some new research findings. 

Taxonomy 

The three owl subspecies named above (northern, California, and Mexican) are the 

only subspecies recognized by the American Ornithologists’ Union (AOU 1957). 

Whether the subspecies is a useful or valid taxonomic delineation is a much debated 

topic among ornithologists (e.g., Barrowclough 1982, Mayr 1982). This seemingly 

esoteric subject has been elevated as a topic of significance for the conservation of 

spotted owls because “subspecies” is a category recognized under the Endangered 

Species Act as a biological unit that can be considered for listing. This general 

subspecies controversy has led some to question the validity of some listing decisions using subspecies as a conservation unit because it has not always been clear 

that designated subspecies were phylogenetically distinct from other populations of 

a species (e.g., Zink 2004). In the case of the spotted owl, the subspecies boundaries are well defined so the subspecies as currently recognized are valid taxa (Zink 

2004). 

Typically, subspecies in birds have been recognized on the basis of plumage 

variation. For the spotted owl, the northern subspecies has the darkest brown 

plumage with the smallest white spots, and the Mexican subspecies has the lightest plumage with the largest white spots. California spotted owls are thought to 

be intermediate between them. However, these plumage characteristics exhibit 

clinal variation so they have not been useful for identification of subspecies in 

the field (Barrowclough 1990). Recent research using DNA analysis now shows 

a clear genetic differentiation among the subspecies (Barrowclough et al. 1999, 

2005: chapter 4; Haig et al. 2004). Interestingly, California spotted owls are more 

closely related to Mexican than to northern spotted owls (Barrowclough et al. 1999, 

Haig et al. 2004). VanGelder (2003) also showed that vocal structures of the three 

subspecies supported the subspecies relationships that were defined by the mtDNA 

analysis of Barrowclough et al. (1999). 

While the distributions of the two west coast subspecies and the Mexican 

subspecies are allopatric (separated in space), the distributions of northern and California spotted owl subspecies are parapatric (i.e., adjacent to each other in space; 

see fig. 2-1). This latter distribution pattern would enhance the likelihood of genetic 

introgression (exchange of genes between the subspecies). Indeed, introgression 

between northern and California spotted owls occurs and there is a cline of overlap 
13 

The California Spotted Owl: Current State of Knowledge

Figure 2-1—Range of the northern and California spotted owls (Strix occidentalis caurina and S. o. occidentalis, 

respectively) in the Sierra Nevada and their zone of overlap in northeastern California. 
14 

GENERAL TECHNICAL REPORT PSW-GTR-254

in northeastern California near the Pit River (Barrowclough et al. 2011; fig. 2-1). For 

purposes of owl management and conservation, the Pit River is recommended as 

the management dividing line between the northern and the California subspecies 

(Gutiérrez and Barrowclough 2005). Thus, the Hat Creek Ranger District of the 

Lassen National Forest is that unit of U.S. Forest Service managed land where the 

transition of the northern and California subspecies occurs (fig. 2-1). 

Of relevance to this assessment is the systematic relationship of California 

spotted owls occupying various mountain ranges in southern and central coastal 

California. This relationship was unknown at the time of CASPO (Verner et al. 

1992a). Although the “island” populations in southern California were traditionally classified as California spotted owls, those in central coastal California were 

thought to be either California or northern spotted owls owing to potential connectivity with populations in the south or proximity to birds in the north (i.e., northern 

spotted owls). Recent mtDNA analyses demonstrates that owls found in the Santa 

Lucia Mountains (i.e., the most northerly population of spotted owls on the central 

California coast) and birds from several southern California populations belong to 

the California spotted owl taxon (Barrowclough et al. 2005). 

Genetic studies reveal that California spotted owls have low genetic variation. 

Barrowclough et al. (1999) proposed three hypotheses for this low genetic variation. 

The first was that there was a demographic population bottleneck (the population declined for unknown reasons, which led to loss of genetic variability). The 

second was that there was a selective sweep of a superior genotype. The third was 

that there was a founder event; a few founder owls immigrated to the Sierra and 

established a population, which provided a limited genepool for the population. At 

this time, these hypotheses have not been tested explicitly, but some could be tested 

using other genetic markers and tools (see chapter 9). 

Ecology 

The extensive research on spotted owls has allowed scientists to develop substantial insight about the life history strategy of the spotted owl. It is clear from this 

research that the spotted owl is a K-selected species, meaning that natural selection 

has favored the evolution of long lifespans and low reproductive rates as mechanisms to mediate the negative effects of unpredictable environmental conditions 

(in terms of weather variability, disturbance frequency, and other random events 

in nature). This life history strategy has led some scientists to suggest that they 

likely exhibit a “bet-hedging” life history strategy (Franklin et al. 2000, 2004). 

Understanding the spotted owl’s basic ecology is essential for developing predictions about the effects of disturbance. The effect of fire, logging, fuels treatments, 
15 

The California Spotted Owl: Current State of Knowledge

and drought stress on habitat (and perhaps individuals) related to climate change 

on different life history parameters (survival, reproduction, dispersal) and social 

structure and processes needs to be evaluated within the context of the owl’s evolutionary history. For example, disturbance at the nest may cause nest desertion, and 

disturbance that lowers habitat quality may precipitate either territory abandonment 

or divorce (e.g., breaking of pair bonds) (Gutiérrez et al. 2011). The bet-hedging 

strategy predicts a species can overcome short-term negative factors but will have 

more difficulty overcoming the relatively longer term impact of reduction in habitat 

quality. Underpinning its evolutionary strategy is the nature of the animal itself— 

how it behaves, its social system, and how those relate to its reproductive ecology, 

survival, and dispersal. 

Behavior 

Vocalizations— 

Spotted owls communicate using a variety of hoot, whistle, chitter, and “bark” 

vocalizations (Gutiérrez et al. 1995). They use a four-note hoot and a series (a long 

series of hoots based on a foundational four-note hoot) call when defending their 

territories. These two vocalizations are likely also used for pair bond maintenance 

and expressing excitement, respectively (Gutiérrez et al. 1995). VanGelder (2003) 

reported that the vocalizations of California spotted owls from the Sierra Nevada 

appeared to be adapted to forests having higher vegetation complexity than is found 

in forests occupied by Mexican spotted owls because vocalizations attenuated less 

when experimentally broadcast into foliage. Moreover, the structure of vegetation 

where spotted owls were found was a good predictor of song structure. These results indicated that vegetation structure exerted selection pressure on the structure 

of owl vocalizations. 

Social system and territoriality— 

Spotted owls have a monogamous mating system, with pairs forming relatively 

long-term pair bonds (Gutiérrez et al. 1995). However, spotted owls sometimes 

break pair bonds (i.e., “divorce”) after failing to produce young; birds that break 

pair bonds or whose mate has died will form new pair bonds with other birds 

(Blakesley et al. 2006; Gutiérrez et al. 1995, 2011). Coincident with their mating 

system and territoriality, California spotted owls show strong site fidelity (Berigan 

et al. 2012). Thus, frequent breeding dispersal (indicating lower site or mate fidelity) 

could be indicative of disruption of their social system. 

Spotted owls are territorial, which means they defend an area by excluding other pairs or individuals from the core of their home range (Gutiérrez et al. 

1995). For this reason, owls are dispersed rather than clumped within landscapes. 
16 

GENERAL TECHNICAL REPORT PSW-GTR-254

Moreover, core areas of California spotted owls tend to be spatially static over time 

(Berigan et al. 2012). For example, in the Sierra Nevada, spotted owl territories are 

more dispersed than expected by chance, and sites having similar occupancy rates 

are dispersed rather than being clumped (Seamans and Gutiérrez 2006). Unbiased 

estimates of adult survival derived from even very small sampling areas indicate 

that territorial owls generally do not shift territories or undergo breeding dispersal 

from an established territory (Blakesley et al. 2010, Gutiérrez et al. 2011, Zimmerman et al. 2007). 

The risk of divorce or leaving a territory when a mate dies is significant. First, 

if a bird leaves its territory as a result of divorce or mate death, it will be unfamiliar 

with the landscape in a new territory, which places it at a disadvantage (e.g., no 

knowledge of locations of good foraging patches) (Hirons 1985). Second, divorce 

incurs other risks such as failing to find a new mate or finding a new mate that 

is not as high a quality as the one divorced. For example, Gutiérrez et al. (2011) 

reported that birds that lost mates because of a mate’s presumed death (mate never 

detected again on the study area) tended to improve their reproductive success, 

whereas it was not clear that birds who divorced also improved their reproductive 

success. Thus, understanding the reasons why birds divorce may have important 

management implications if forest management activities that lead to disruption of 

pair bonds negatively affect demographic processes. 

Not all spotted owls are territorial, and these nonterritorial individuals are 

called “floaters” (Franklin 1992). Some floaters are younger birds in search of 

their first territory, but others seem to be birds that have left a territory and become 

nonterritorial for unknown reasons. These floaters can occur within or outside the 

home ranges of territorial birds, but it is unknown whether they are tolerated by 

resident birds or simply not detected by resident pairs because they do not attempt 

to defend an area using vocalizations. Although floaters do not contribute to the 

reproductive output of a population, they can influence population dynamics 

because they provide a pool of birds that could colonize vacant territories or pair 

with single birds (Franklin 1992). 

The territory (the area that is actively defended by birds) of a pair is likely 

smaller than their home range although no one has precisely estimated the size of 

the areas they will actively defend (i.e., their true territories). Rather, from a management perspective, the concept of territory has been “approximated” by various 

derivations of a “core area” such as a protected activity center or PAC (an area of 

about 121 ha [300 ac]) that was created to anticipate the likely essential areas used 

for nesting and roosting, but not for foraging or even territorial defense (Verner 
17 

The California Spotted Owl: Current State of Knowledge

et al. 1992b). Other researchers have attempted to estimate the core area (the area 

of concentrated use) of both northern and California spotted owls (Bingham and 

Noon 1997) and to examine how well PACs accommodate long-term use by spotted 

owls for nesting and roosting (Berigan et al. 2012). Based on their assessment of 

the Lassen demographic study population, Bingham and Noon (1997) suggested 

that the core area for California spotted owls was about 813 ha (2,009 ac), which 

was substantially larger than the designated size of a PAC (about 121 ha [300 ac]). 

One reason for this disparity was that Bingham and Noon (1997) estimated the core 

area based on analysis of radiotelemetry data, whereas PACs were designated by 

contiguous association of preferred habitats at and around nests and primary roosts 

(Verner et al. 1992b). 

Intraspecific interactions— 

Members of a pair of spotted owls divide roles when nesting. Females incubate eggs 

and brood young, while males provision females with food so they can maintain 

incubation with little interruption (Gutiérrez et al. 1995). Males defend the territory 

at this time more predictably than females. Thus, males are often detected first in 

occupancy surveys, while females that are actually present at the same sites may go 

undetected until later surveys. When eggs hatch, the owlets are guarded and fed by 

both parents, but the female tends to continue brooding (Gutiérrez et al. 1995). 

Because spotted owl home ranges are relatively large, it is likely impossible for 

territorial pairs to defend their entire home range from other spotted owls, so home 

ranges of adjacent owls often overlap (Gutiérrez et al. 1995). Because spotted owls 

are central-place foragers, they expand their activities outward from their nests or 

roosts to forage on prey that is patchily distributed and that can be depleted through 

predation within those foraging patches (Carey et al. 1992, Carey and Peeler 1995, 

Ward et al. 1998). Thus, their activity declines within far patches relative to close 

patches as distance from the territory center increases owing to travel time. They 

engage in conspecific interactions with “neighbors” (i.e., adjacent territorial owls) 

and “strangers” (non-neighbors and dispersing owls) through hooting vocalizations, and these hooting bouts intensify as foreign owls encroach on areas near the 

territory center. Spotted owls apparently recognize their neighbors because Waldo 

(2002) experimentally demonstrated the “dear enemy hypothesis” for spotted owls, 

where territorial spotted owls responded more strongly to broadcasts of the vocalizations of a stranger than a known neighbor. The adaptive advantage of such a 

conspecific response is that territorial owls do not have to expend energy defending 

a territory from an individual with whom they have already established a territory 

boundary. 
18 

GENERAL TECHNICAL REPORT PSW-GTR-254

Inbreeding is a conspecific interaction that is generally considered maladaptive 

because of the potential for the expression of deleterious alleles. Inbreeding can 

occur between distant and close relatives (siblings, half-siblings, and parents with 

offspring). A comprehensive analysis of inbreeding has not been done for spotted 

owls, but “incest” (inbreeding of close relatives) has been reported for both northern 

and California spotted owls (Carlson et al. 1998). 

Interspecific interactions— 

Spotted owls in California are not apex predators in their food chain. Great horned 

owls (Bubo virginianus) are sympatric, larger, and can prey on spotted owls 

(Gutiérrez et al. 1995). However, spotted and great horned owls do not usually use 

the same habitats; great horned owls typically occupy more open habitats than spotted owls (Johnson 1992). Moreover, the simulated presence of great horned owls in 

the territories of spotted owls does not suppress calling behavior in spotted owls 

(Crozier et al. 2005), which suggests that great horned owls either are not generally a threat to spotted owls or they are not likely to be in the same habitats. Finally, 

spotted owls will sometimes nest in the same stand as great horned owls even 

though great horned owls are known to prey on spotted owls (Gutiérrez et al. 1995). 

The invasion of the barred owl (Strix varia) in western North America has 

raised concern over the potential for this species to negatively affect spotted owls 

(Gutiérrez et al. 2007, USFWS 1990). Barred owls were first documented in California in 1981, and by 2004, one barred owl has been detected as far south as Kings 

Canyon National Park in the southern Sierra Nevada (Dark et al. 1998, Steger et 

al. 2006). The barred owl invasion has been of substantial concern for spotted owl 

conservation and will be covered in detail in chapters 7 and 9. Spotted owls also 

interact with other species on a daily basis. For example, they are routinely mobbed 

by other bird species (Gutiérrez et al. 1995), and they are victims of kleptoparasitism, when other species steal prey the owls have cached (Hunter et al. 1993). 

Activity patterns— 

Spotted owls are primarily active at night. They hunt, defend, socialize, and conduct 

exploratory movements at night (Gutiérrez et al. 1995). However, they also can be 

active at dusk when they often socialize and begin to hunt, but they will opportunistically prey on species that are active during the day (Gutiérrez et al. 1995, Laymon 

1991). During the day, however, they primarily sleep, conduct self-maintenance, and 

guard young while roosting in complex-structured forests (Gutiérrez et al. 1995). 

Roost stands are often areas used consistently by owls over many years (Berigan et 

al. 2012). The areas around nest sites, together with roost sites, serve as the center of 
19 

The California Spotted Owl: Current State of Knowledge

activity for spotted owls (i.e., a prediction of central-place foraging theory) (Carey 

and Peeler 1995). Their night time foraging locations can be anywhere within the 

home range, both within the core area and well beyond it (Williams et al. 2011). 

Although spotted owls are most active at night, there are periods during the night 

when they are more active than others. For example, activity is highest during the 

periods 1 to 3 hours after sunset to 1 to 3 hours before sunrise (Gutiérrez et al. 1995). 

These general activity periods have been quantified by Delaney et al. (1999), where 

the highest prey delivery rates occurred at these times. Owls progressively increase 

prey delivery rates to the nest as young mature, while females spend less time in 

attendance of young as the brood-rearing period progresses (Delaney et al. 1999). 

Response to human activities— 

The spotted owl has long been recognized for its tame behavior because it often 

tolerates close approach by humans (Gutiérrez et al. 1995). Tameness has often been 

mistaken for adaptability or lack of disturbance effects in animals. However, many 

studies have shown that animals may exhibit no outward signs of stress when they 

are actually having a physiological stress response (Beale 2007). Indeed, a study 

of northern spotted owls suggested that birds had elevated levels of corticosterone, 

a stress hormone whose metabolites can be detected in feces, when living near 

roads or areas that had been logged (Wasser et al. 1997). In contrast, Tempel and 

Gutiérrez (2004) analyzed corticosterone metabolites in feces of spotted owls from 

the central Sierra Nevada and found that corticosterone levels were best explained 

by the breeding state of individuals and how samples were stored in the field, rather 

than by the presence of roads or habitat type. 

In another assessment of spotted owl response to humans, Swarthout and Steidl 

(2003) conducted an experimental study of the behavioral response of Mexican spotted owls to recreational hikers and showed that Mexican spotted owls changed certain 

activity patterns in response to both high levels of recreation use and to the presence 

of observers. They suggested that spotted owls were tolerant to moderate levels of 

disturbance (i.e., noise not within 100 m [328 ft] of roosting birds, <50 hikers per day), 

but high levels of recreational activity could cause disturbance (Swarthout and Steidl 

2003). Swarthout and Steidl (2003) results also suggested that researchers should 

minimize their time and activities when working near spotted owls. 

Reproductive Ecology 

Reproduction is a vital rate that contributes to population growth and will be 

covered in chapter 4. However, it is important to set the context of the spotted 

owl’s reproductive ecology in both relative and evolutionary contexts. Spotted owls 

exhibit the lowest reproductive rate among North American owls (Johnsgard 1988). 
20 

GENERAL TECHNICAL REPORT PSW-GTR-254

Mean reproductive output (number of young produced per female) across years 

has varied among the five long-term demography monitoring areas in the Sierra 

Nevada and southern California (0.555 to 0.988 young/female) (Blakesley et al. 

2010, Franklin et al. 2004; see also chapter 4 for details). Moreover, the range in 

annual reproduction by California spotted owls is extremely variable, ranging from 

no young produced within a demographic study area in a given year to nearly all 

birds nesting (Blakesley et al. 2010, Franklin et al. 2004, Seamans and Gutiérrez 2007b, Tempel et al. 2014). As previously noted, this highly variable rate of 

reproduction coupled with a high survival rate has given rise to the hypothesis 

that California spotted owls, like northern spotted owls, exhibit a bet-hedging life 

history strategy (Franklin et al. 2000, Stearns 1976). In the case of the California 

spotted owl, natural selection has favored the evolution of high survival rates that 

allow the owls to forgo breeding in “bad” years so they can breed in “good” years. 

Bad and good are relative terms here and can mean the difference between years of 

high and low prey or years of inclement or mild weather during the critical nesting 

period. The significance of this bet-hedging strategy is that lack of reproduction at 

a site for a few years does not necessarily mean the site is of low quality, but rather 

it could be related to overall poor environmental conditions leading to absence of 

reproduction (Stoelting et al. 2014). 

There are several environmental or ecological factors that have been examined 

to explain patterns of reproduction in California spotted owls. Reproductive success 

is often correlated with annual variation in weather (LaHaye et al. 1997, MacKenzie 

et al. 2012, North 2002, North et al. 2000, Seamans and Gutiérrez 2007b, Stoelting 

et al. 2014). The focus on the relationship with weather and vital rates is common 

because weather is easily measured and can be used to infer both indirect and direct 

potential effects on owls (e.g., it influences plant growth and thus prey populations, 

which affects energy budgets of owls). The influence of weather on reproduction 

is, however, confounded by the substantial variation in the distribution and abundance of the owl’s prey (Ward et al. 1998). Weather can potentially affect spotted 

owls directly either by increasing energy demands of owls, increasing the risk of 

exposure of eggs when females have to leave the nest, and interfering with hunting 

and indirectly by influencing plant growth, which affects population dynamics of 

owl prey (Franklin et al. 2000, Rockweit et al. 2012, Seamans and Gutiérrez 2007b, 

Weathers et al. 2001). 

A peculiar element of reproductive variability in spotted owls, including California spotted owls, is an alternating pattern of high and low years of reproductive 
21 

The California Spotted Owl: Current State of Knowledge

output (Blakesley et al. 2010, Franklin et al. 2004). This pattern has been labeled 

the even-odd effect of reproductive output in owl population studies (Franklin et al. 

1999, 2004). North (2002: 118) also reports “distinct annual fluctuations” of good 

and bad years of reproduction. Why this pattern occurs is unknown, but a logical 

hypothesis is that there is a “cost of reproduction” borne by owl pairs successfully raising young (Forsman et al. 2011, MacKenzie et al. 2009). That is, birds 

that reproduce successfully in one year are less likely to breed in the subsequent 

year because there is an energetic cost to reproduction that presumably reduces 

the condition of females (MacKenzie et al. 2009, Stoelting et al. 2014). However, 

Stoeling et al. (2014) simulated the even-odd cycle in California spotted owls and 

found that the observed cost of reproduction was insufficient either to generate or 

to maintain this even-odd pattern. Stoelting et al. (2014) hypothesized that some 

external, widespread environmental factor was likely responsible for generating and 

maintaining this cycle (see also North 2002). Interestingly, the even-odd effect in 

northern spotted owls appears to be waning with time (Dugger et al. 2016). 

Other observations about individual costs of reproduction have been linked to 

California spotted owls. California spotted owls typically fledge one to three young 

(Gutiérrez et al. 1995), with three young in a brood being uncommon (Tempel et 

al. 2014). There are records of four-egg clutches or broods, but these are extremely 

rare, having been recorded only twice in southern California (Dunn 1901, LaHaye 

1997). Thus, a question arises about the optimal clutch (brood) size for owls. Peery 

and Gutiérrez (2013) showed that when juveniles fledge from broods of two they 

have a greater survival probability than if they fledge either from a single or triplet 

brood. More importantly, this effect carried over to subadult and adult age classes. 

This occurs even though it should be more difficult for parents to provision two 

chicks than one chick. Finally, Peery and Gutiérrez (2013) reported that indices of 

territory quality, based on chick survival, were positively correlated with indices of 

territory quality based on parental reproductive output. 

Whether owls successfully breed at sites also seemed to influence whether they 

will remain on those sites. For example, Gutiérrez et al. (2011) showed that owls 

having higher reproductive output compared to the entire population were less likely 

to disperse. While their result was confounded by mate quality (i.e., a bird might 

be more likely to stay at a site if they have a “high-quality” mate), they also found 

that owls were less likely to leave sites where owl reproduction was higher than the 

population average, which partially controlled for the effect of mate quality because 

these long-term effects accounted for multiple owls occupying the same sites. 
22 

GENERAL TECHNICAL REPORT PSW-GTR-254

The distribution and abundance of owl prey is another key factor that influences spotted owl reproductive success. Yet, studies of owl prey have lagged behind 

the study of other aspects of owl ecology even though prey are central to understanding the ecology of a predator (Newton 1979). The reason for this lack of study 

is simply that prey studies are costly, rather than failure by scientists to recognize 

this obvious connection (Gutiérrez 1985b). Much of our knowledge about spotted 

owl prey comes either from studies of northern spotted owls or the small mammal 

literature, although there are exceptions (Waters and Zabel 1995, Waters et al. 2000, 

Zabel and Waters 1997). However, there have been several studies relating spotted 

owl habitat to owl reproduction and others enumerating prey in the diet (see below). 

Barrows (1985, 1987) first showed that California spotted owls took relatively 

larger prey when they were nesting, and they shifted their diet from larger to 

smaller prey when they failed in their breeding attempt. Thrailkill and Bias (1989) 

also found that diets of breeding owls contained more large prey by both frequency 

and biomass than nonbreeding owls. These early diet studies were both relatively 

limited either in terms of sample size (number of territories sampled in breeding 

and nonbreeding categories) or pellets collected per territory. These sample size 

limitations were overcome by Smith et al. (1999) who collected over 8,000 pellets 

from 109 territories in the San Bernardino Mountains over several years. They 

found that successful nesting owls had a greater percentage of biomass of large prey 

than small prey in diets relative to nonnesting owls. However, there was no difference in size of prey eaten between successful nesters and those pairs that attempted 

to nest but failed, suggesting that nests failed for reasons other than prey dynamics. 

However, the relationship between large prey in the diet and propensity for breeding 

was not clearly established for owls inhabiting the Sierra National Forest (Munton 

et al. 2002). Although it seems logical that spotted owl prey may be affected by 

abundance of pine seed production and thus affect owls, North (2002) found no 

such relationship in the southern Sierra Nevada. 

Evaluating habitat quality is challenging because owls and sites are confounded 

(see “Management Implications” below). That is, it is difficult to partition the effect 

of “high-quality” individuals from the quality of the habitat (when there are two or 

more potentially explanatory factors whose effects are difficult to distinguish, they 

are said to be confounding). Recognizing these confounding effects, LaHaye et al. 

(1997) found in the San Bernardino Mountains that owls nesting in lower elevation 

oak (Quercus chrysolepis Liebm.)/big cone fir (Pseudotsuga macrocarpa (Vasey) 

Mayr) cover types had higher reproductive output than those nesting in other 

higher elevation cover types (LaHaye et al. 1997). Hunsaker et al. (2002) showed 

that owls occupying sites having canopy cover greater than 50 percent had greater 
23 

The California Spotted Owl: Current State of Knowledge

productivity, and these sites also had higher occupancy rates than sites with lower 

canopy cover. North (2002) reported that owls in certain territories almost always 

produced young, even in bad years. North’s finding could be related either to variation in territory quality, variation in owl quality, or an interaction between the two, 

but North did not have sufficient data to examine these possibilities. However, he 

reported that owls which occupied higher quality territories (weighted by reproduction output) used the same nests repeatedly, whereas owls occupying lower quality 

territories used a nest only once, which suggested local site factors were important 

in addition to broad regional influences. 

Habitat Associations 

California spotted owl habitat associations are provided in detail in chapter 3. However, we here provide some generalities and hypotheses about habitat use and their 

relevant ecological context. Spotted owls are primarily forest inhabitants. Their 

habitat selection and habitat requirements are among the most controversial issues 

related to spotted owl conservation because of the economic implications of habitat 

conservation and the design of forest management plans. However, we present some 

general concepts related to owl habitat and the evolution of habitat selection in 

California spotted owls. 

Theoretically, habitat quality of a species should be reflected in individual 

home range size and, therefore, the spacing and density of individual territories in 

a population. For spotted owls, a specific hypothesis about this theory could be that 

home range sizes should be smaller in areas of higher habitat quality. Yet, because 

of territorial interactions (see next paragraph), there will be a threshold of population density, regardless of relative habitat quality. However, the relationship between 

density thresholds relative to habitat quality in different landscapes is unknown. 

One avenue to assess habitat quality is to observe patterns of animal space use 

as a function of the predictions derived from ecological theory because predictions 

of theory can be tested with empirical data. This has been done as two general 

theories of habitat selection have been investigated in spotted owls—the ideal free 

and ideal despotic theories (Fretwell 1972, Fretwell and Lucas 1969). The ideal 

free theory predicts that if animals have perfect knowledge of their environment 

(i.e., are “ideal”), they will select the best habitat first and sequentially select lower 

quality habitats when higher quality ones are occupied. The ideal despotic theory 

predicts that territorial animals will exclude (i.e., are “despots”) other individuals of 

the same species from occupying the same habitats (i.e., territorial behavior prevents simultaneous space use regardless of habitat quality because of intraspecific 

competitive exclusion). Recent evidence suggests California spotted owls follow an 
24 

GENERAL TECHNICAL REPORT PSW-GTR-254

ideal despotic distribution of habitat selection because occupancy is correlated with 

“territory fitness” (“territory fitness” is a measure of territory quality based on the 

survival and reproduction of all owls that occupy that territory over time) (Zimmerman et al. 2003). In a seemingly counterintuitive observation, territorial behavior 

also seems to influence habitat selection by owls native to an area because owls 

immigrating into a population settle closer to territorial pairs than do owls from that 

population who are switching territories within the population (i.e., immigrant owls 

may be using presence of conspecifics as cues to settle in suitable habitat) (Seamans 

and Gutiérrez 2006). This suggests that both behavioral factors and intrinsic habitat 

quality influence habitat selection by spotted owls. Territorial behavior can constrain the allocation of space (habitat) among individuals of a population (there is an 

upper limit for density) and serve as a cue to naïve owls about territorial quality. 

Much has been discussed about the use of private lands by California spotted 

owls in the Sierra Nevada because in some places, public and private lands occur in 

near-alternating parcels and commercial timberland is abundant (Bias and Gutiérrez 

1992, Gutiérrez 1994, Moen and Gutiérrez 1997). Private and public lands often 

have been managed differently, which may result in different dominant forest types 

and forest structure. Irwin et al. (2007) studied foraging owls on primarily private 

land in the northern Sierra Nevada and found that owls tended to forage near nests, 

small streams, and areas with a diversity of conifer species and hardwoods versus 

areas dominated by ponderosa pine (Pinus ponderosa Lawson & C. Lawson). In an 

area of alternating public-private ownership parcels (i.e., “checkerboard” ownership pattern) in the central Sierra Nevada, spotted owls rarely used private land for 

nesting and roosting (Bias and Gutiérrez 1992, Gutiérrez 1994, Moen and Gutiérrez 

1997). This result has been enigmatic because spotted owls occur elsewhere on 

private land in the Sierra Nevada (Irwin et al. 2007, Roberts 20152). The findings 

from the central Sierra Nevada have also prompted work to determine if this pattern is related to placement of nest sites relative to hard edges (spotted owls placed 

their nests farther from edges than predicted by chance [Phillips et al. 2010]) and 

their use of private land for foraging (Williams et al. 2014). In the latter study, the 

researchers found that owls used private land proportionately less than its availability when foraging, even when controlling for central-place foraging tendencies. 

2 Roberts, K. 2015. Personal communication. Senior wildlife biologist, Sierra Pacific 

Industries, 3950 Carson Road, Camino, CA 95709. 
25 

The California Spotted Owl: Current State of Knowledge

Habitat structure— 

The structure of spotted owl habitat has long been of interest because original research suggested that spotted owls were old-growth dependent (e.g., Carey 1985, 

Forsman et al. 1983). Subsequently, the nature of owl-habitat relationships has been 

expanded to include the concept of forest structure (Solis and Gutiérrez 1990). This 

concept suggests that California spotted owls are habitat specialists because they select an array of forests and forest structures that are different than what is generally 

available on the landscape (Gutiérrez et al. 1992, Verner et al. 1992a). For example, 

they select sites that have larger trees, higher canopy cover, and more coarse woody 

debris than occurs on sites available to them (chapter 3). Research on the structure 

of spotted owl vocalizations also supports this forest-structure relationship because 

it appears California spotted owl vocal structure evolved in response to “complex” 

forest structure (VanGelder 2003). Song is the fundamental way by which most 

birds communicate (defend territories, attract mates, maintain pair bonds), and thus 

there is a large body of research on the adaptive nature of vocalizations relative to 

habitat that a species occupies. 

Several hypotheses have been generated to explain why owls select old/mature 

forests (e.g., nest site selection, ambient temperature moderation, availability 

of prey), and these hypotheses are relevant under the habitat structure selection 

hypothesis (Carey 1985, Forsman et al. 1983, Gutiérrez 1985a). To these, we add 

an additional hypothesis that the owls themselves have evolved in fire-adapted 

forest ecosystems because all landscapes spotted owls now inhabit experience 

some type of historical fire regime. In addition, their resilience to fire of low and 

moderate severity is consistent with this fire-adapted hypothesis. Under either a 

predominantly accepted hypothesis that a frequent, low- to moderate-severity fire 

regime was the historical norm in the Sierra Nevada (chapter 5) and an alternative 

that posits a mixed-severity fire regime prevailed in the West historically (e.g., 

DellaSala and Hanson 2015 and references therein; SNEP 1996), there is uncertainty about the precise distribution and abundance of various forest types and 

structures under historical fire regimes. This uncertainty is underscored by the fact 

that the current sites spotted owls use for nesting and roosting in the Sierra Nevada 

indicate that there was a relatively high density of large trees on the landscape 

in many places. That is, the large, old trees at current owl sites were present in 

presettlement times (Gutiérrez et al. 1992). The importance of this uncertainty is 

that it can lead to different predictions for desired future conditions of forests. To 

this end, we propose that neither of these hypotheses about historical fire regimes 
26 

GENERAL TECHNICAL REPORT PSW-GTR-254

is inconsistent with creating and maintaining suitable spotted owl habitat (e.g., 

under the predominant fire paradigm, the structure of old-growth forests that were 

under a suppression regime returned to a condition likely to be suitable for owls, 

at least in terms of diameter distribution of trees and size of trees, after two cycles 

of return fire) (Lydersen and North 2012). Hence, the forest structure hypothesis 

is important to consider when thinking about appropriate management actions. In 

addition, because fire suppression has led to unnaturally high densities of trees in 

most places and owls often inhabit these areas, we think that describing suitable 

owl sites as “dense” may obfuscate the discussion about spotted owls. For one, the 

word “dense” is a normative term and, therefore, conjures different visualizations 

among people about both tree density and vegetation structure, which then leads to 

concerns about such things as spotted owl habitat being a fire hazard, susceptibility of trees to insects and diseases, and effects of climate change on forests. For 

another, there are many forests that do not have spotted owls because either they 

have such a high density of trees or high foliage volume that it may impede hunting 

by owls, or they have low prey densities (these were among the original hypotheses 

linking owls to old-growth forests). Consequently, as qualitative expressions to aid 

discussion, the terms “multistoried structure” or “complex-structured forest” seem 

more appropriate terms to describe owl habitat than does the term “dense” because 

most dense forest stands without tree age or size structure do not harbor owls. In 

this context, complex-structured forests do not necessarily have to have the density 

of trees that often occurs in stands following long periods of fire suppression. 

Moreover, achieving this type of forest structure does not seem incompatible with 

reintroduction of fire as a natural process (Lyderson and North 2012). 

Disturbance— 

The effect of disturbance on owls and their habitat has been a concern, particularly 

the effect of large, high-severity fires and associated salvage of fire-killed trees. 

These can potentially affect owls negatively (Clark et al. 2011, 2013; Lee et al. 2013, 

2015b; see also chapters 3 and 4). Moreover, climate change will likely increase the 

frequency of these types of fires. Research has shown that low- and moderate-severity fires have little or no effect (acute or chronic) on spotted owls (Bond et al. 2002, 

Roberts 2008, Roberts et al. 2011). This result has not been surprising given that 

spotted owls evolved in forests that experienced fire frequently. One study of radiomarked owls showed that owls will forage in areas that have been burned (Bond et 

al. 2009) while another study showed that owls foraged at the edge of burns (Eyes 

2014). Studies of territory occupancy showed that mixed-severity fires had no effect on owls (Lee et al. 2012, Roberts 2011) while high-severity fires had negative 
27 

The California Spotted Owl: Current State of Knowledge

effects (Clark et al. 2013, Lee et al. 2013). However, the owl territories in studies by 

Clark et al. (2013) and Lee et al. (2013, 2015b) were also affected by salvage logging. Lee and Bond (2015a) reported a high occupancy rate of owl territories in the 

first year following the very large Rim Fire in the southern Sierra Nevada. Recently, 

however, Jones et al. (2016) demonstrated a strong first-year impact of the King

Fire in the central Sierra Nevada on California spotted owls. Almost all territories 

that were occupied prior to the fire, but experienced >50 percent high-severity

fire, became unoccupied following the fire—with several owls moving to the

less severely burned territories. Moreover, GPS-tagged owls exhibited strong 

avoidance of high-severity fire burned patches, particularly those in the main, highseverity patch. Currently, additional research has been undertaken to examine the 

longer term impacts of the Rim Fire on the owl population studied by Lee and Bond 

(2015a; Keane 20153). Differences in the inferred effects of the Rim and King Fire 

studies could be the result of more patchily distributed high-severity-fire burned 

patches in the Rim Fire, or differences in methodology between the two studies 

(e.g., the King Fire study used marked birds, whereas the Rim Fire study did not). 

Ongoing work at both sites will provide more perspective on the relative and longer 

term impacts of these fires and whether there are different outcomes to fire effects, 

as these early studies indicate. 

Similarly, logging, especially when used as a treatment to reduce fire hazard, 

is of particular concern (Verner et al. 1992b) because logging treatments will be a 

mechanism to reduce fuel loading in forests. Logging has long been presumed to 

negatively affect spotted owls through loss of habitat (Gutiérrez et al. 1995). Effects 

of logging have been demonstrated recently both indirectly and directly. Seamans 

and Gutiérrez (2007a) showed that occupancy declined when at least 20 ha of highcanopy, mature forest was lost from a territory, but they did not partition losses 

attributable either to logging or fire. Tempel et al. (2014) showed that the proportion 

of high-canopy, mature forest was the best predictor of occupancy by owls. In their 

study, the owl population had declined by 50 percent over the sampling period, 

so the assumption was that disturbance by various kinds of logging was partially 

responsible for the decline. Finally, Stephens et al. (2014) demonstrated experimentally that logging treatments designed to reduce fire risk resulted in a loss of owls. 

As noted above, there appears to be a compounding effect on occupancy of salvage 

logging following fire in owl territories (Clark et al. 2013; Lee et al. 2013, 2015b). 

3 Keane, J.K. 2015. Personal communication. Research wildlife ecologist, U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station, 1731 Research 

Park Dr., Davis, CA 95618. 
28 

GENERAL TECHNICAL REPORT PSW-GTR-254

Diet 

The general diet of spotted owls has been well described in the literature (Gutiérrez et al. 1995). Spotted owls prey primarily, by biomass, on medium-sized small 

mammals, particularly dusky-footed woodrats (Neotoma spp.) and flying squirrels 

(Glaucomys sabrinus) (Gutiérrez et al. 1995). However, they eat a wide array of 

other small mammals, such as mice and voles, as well as birds, lizards, and insects. 

Flying squirrels are found in closed-canopy forests, usually at higher elevations 

than woodrats. Woodrats also use closed-canopy forests and oak woodlands, earlyseral-stage forests (e.g., clearcuts and fire-disturbed landscapes), and shrub cover 

types. Recent studies have generally confirmed past observations of owl diet, but 

there have been some notable differences recorded (see below). 

Diet analysis has been a mainstay of owl studies because owls regurgitate 

indigestible parts of their prey as consolidated “pellets,” which can be found at roost 

and nest sites (Marti 1987). These pellets contain bones, hair, nails, beaks, feathers, 

scales, and exoskeletons. So it is somewhat surprising that only three reports on diet 

composition based on pellet analyses have been published since CASPO (Munton 

et al. 1997, Munton 2002, Smith et al. 1999). Owls generally swallow small prey 

whole and dismember larger animals to swallow smaller chunks. However, they 

often decapitate prey and when they do this, they swallow the head first and then 

the body, or cache the body to eat later (Gutiérrez et al. 1995). They will also swallow the head of a prey item and give the rest of the prey to young. For these reasons, 

strict protocols have been observed when enumerating prey items to avoid double 

counting individuals in a sample (Marti 1987). 

Munton et al. (1997) identified 664 prey remains from 520 pellets found at 

11 territories in low-elevation (300 to 586 m [984 to 1,923 ft]) oak and riparian 

deciduous cover types in the Sierra and Sequoia National Forests of the southern 

Sierra Nevada. They identified 20 species of prey of which mammals comprised 

96 percent, by biomass, of the diet. Woodrats and pocket gophers (Thomomys 

spp.) comprised 80 and 11 percent of the biomass, respectively. Interestingly, the 

contribution of woodrats, mice, and birds to the diet was lower during the breeding 

season than nonbreeding season, and the contribution of voles and pocket gophers 

to the diet showed the opposite pattern. This could mean either that the diversity of 

prey in the diet increased during the breeding season, the results were a function 

of sampling limitations, or the results were a function of annual variation in diet 

among pairs (Munton et al. 1997). 
29 

The California Spotted Owl: Current State of Knowledge

Munton et al. (2002) expanded their earlier study of lower elevation cover types 

by examining 1,140 pellets collected at territories between 305 and 2316 m (1,000 

and 7,600 ft) on the Sierra National Forest. In addition, they added 1 year of data 

(1998) to their original 6-year study. Their sampled sites in this 2002 study included 

all those sites from the 1997 study except for the low-elevation sites on the Sequoia 

National Forest. They identified 2,038 individual prey items from 1,140 pellets. As 

before, woodrats dominated (74 and 82 percent biomass in diet in nonbreeding and 

breeding seasons, respectively) the diet in oak woodlands and riparian-deciduous 

forests (i.e., low elevation), whereas in conifer forests at higher elevations, northern flying squirrels comprised 77 percent of the prey biomass in the diet. Pocket 

gophers comprised the second most important food by biomass at both low and 

higher elevations. These results were different from earlier studies reported for the 

mid- to high-elevation owl habitats in the central Sierra Nevada (Laymon 1988, 

Thrailkill and Bias 1989) where woodrats tended to dominate in the mid-elevation 

forest types, suggesting that there were differences in the prey community between 

the central and southern Sierra Nevada. 

Smith et al. (1999) reported on a large study of owl diet in the San Bernardino 

Mountains of southern California. They sampled the entire population of territorial 

owls, 109 territories, between 1987 and 1991, and identified 8,441 individual prey 

in pellets. Dusky-footed woodrats (N. fuscipes) were the most important prey by 

both percentage frequency (42) and biomass (74). They also found that the proportion of biomass attributed to woodrats increased as elevation increased, which was 

opposite to other owl diet studies where the proportion of the diet attributable to 

woodrats decreased with increasing elevation (e.g., Verner et al. 1992a). They found 

that their large sample size resulted in reasonably precise estimates of woodrats 

on a territory-by-territory basis but not pocket gophers, which might explain why 

Munton et al. (2002) found a preponderance of pocket gophers in the diet (i.e., a few 

sites with large samples of pocket gophers might have skewed the results). In the 

San Bernardino study, flying squirrels were uncommon in the mountain range and 

constituted only 3 percent of the biomass of the diet. This latter result was likely 

related to flying squirrels being at the southern edge of their range, and their low 

abundance did not provide owls an alternative prey source at high elevations as they 

did in the Sierra Nevada. Diet studies of Mexican spotted owls have revealed that 
30 

GENERAL TECHNICAL REPORT PSW-GTR-254

 

both medium-sized small mammals like woodrats and smaller mammals like mice 

and voles (e.g., Peromyscus spp. and Microtus spp.) can be important for reproduction in owls (Ward 2001). Thus, reproduction can occur when there are high populations of various prey species, and these prey populations can differ among years, 

habitats, and regions. 

Cause-Specific Mortality

Predation— 

Spotted owls are subject to predation by great horned owls, northern goshawks 

(Accipiter gentilis), and red-tailed hawks (Buteo jamaicensis) (Gutiérrez et al. 1995). 

Great horned owls can potentially prey on adults or young, and goshawks will take 

juvenile owls (Gutiérrez et al. 1995). In addition, one likely case of predation of an 

adult spotted owl by a barred owl has been reported (Leskiw and Gutiérrez 1998). 

Accidents— 

Accidents are an additional source of mortality in spotted owls. There are records of 

spotted owls dying from collisions with automobiles and tree branches (Gutiérrez et 

al. 1985, Williams et al. 2011). In addition, there is one record of an owl being electrocuted when perching on a power line or transformer (Gutiérrez et al. 1996). 

Disease and infection— 

The appearance and rapid spread of West Nile virus has been a concern because 

spotted owls, like other owls, are quite susceptible to the disease (Gancz et al. 2004). 

Louse flies (family Hippoboscidae), which are common external parasites of spotted owls (Hunter et al. 1994), were implicated in a West Nile outbreak among North 

American owls, including a spotted owl, in a captive holding facility in Ontario, 

Canada, so an existing vector for the virus is present in most spotted owl populations (Gancz et al. 2004). Hull et al. (2010) conducted a survey of antibody titers of 

California spotted owls and found no evidence of West Nile infection. However, 

because of the virus’ apparent virulence to owls, it is doubtful that one could detect 

it through surveillance monitoring. Bacteria can also cause disease in spotted owls. 

Thomas et al. (2002) reported a fatal spirochetosis (an acute, septicemic disease) in a 

northern spotted owl, which was caused by a bacterium, Borrellia sp. It was unknown whether this disease regularly occurs in spotted owl populations. 

Parasites— 

Several survey screenings for hematozoa (blood parasites) have revealed that spotted owls harbor a variety of hematozoa such as Plasmodium sp., Leucocytozoon sp., 

Haemoproteus sp., Trypanosoma, Atoxoplasma sp., and microfilariae (Gutiérrez 

1989, Ishak et al. 2008, Lewicki et al. 2015, Wood and Herman 1943). California 
31 

The California Spotted Owl: Current State of Knowledge

spotted owl populations were found to have the highest rates of infection when 

compared to northern spotted owls, barred owls, and 387 other species of owls 

(Gutiérrez 1989, Ishak et al. 2008). Therefore, if the hematozoa that infect spotted 

owls has deleterious health consequences (e.g., compromising its immune system), 

then it could be an advantage to invading barred owls because barred owls have 

lower rates of hematozoa infection and hematozoa diversity (Ishak et al. 2008, 

Lewicki et al. 2015). Further, given the high rates of infection of California spotted 

owls, such an effect might be more pronounced in competitive interactions between 

this subspecies and the invading barred owl. Lewicki et al. (2015) tested several hypotheses relative to the consequences of the invasion of barred owls into the spotted 

owl’s range and found support for two hypotheses about the relationship between 

invasive species and their parasites—the Enemy Release Hypothesis (ERH) and the 

Parasite Spillback Hypothesis (PSH). The ERH postulates that host populations of 

invasive species will harbor lower parasite species richness in their invaded ranges 

relative to their original ranges, while PSH postulates that invasive species will 

serve as reservoirs for native parasites, which will increase exposure of native species to native parasites, resulting in higher proportions of infective native species 

within populations (Lewicki et al. 2015: 1714). The ERH was supported by the finding that barred owls from the east coast had higher infection rates than barred owls 

from the west coast, but the PSH was supported by the finding that spotted owls had 

higher probabilities of infection than west coast barred owls (Lewicki et al. 2015). 

Hunter et al. (1994) found seven species of ectoparasites belonging to five 

arthropod families either on live spotted owls or museum skins. The authors 

considered three species (a mite, a tick, and a flea) to be accidental on spotted owls 

because they typically are found on rodents, the normal prey of spotted owls. The 

remaining parasites were chewing lice (Strigiphilus syrnii and Kurodaia magna) 

and louse flies (Icosta americana and Ornithoica vicina). Of these parasites infecting owls as true hosts, I. americana was found on live California spotted owls, 

and an unidentified Strigiphilus sp. was found on a museum skin of an owl from 

Mariposa County in the Sierra Nevada. However, Bequaert (1952) found O. vicina 

on a California spotted owl. No helminth or other endoparasites have been reported 

from the California spotted owl but likely exist because they are found in northern 

spotted owl populations (Gutiérrez et al. 1995). 

Chapter Summary 

While most of the information gathered on spotted owls subsequent to CASPO has 

been devoted to monitoring population trends and spatial distributions (see chapter 

4), much new biological information about California spotted owls has also been 
32 

GENERAL TECHNICAL REPORT PSW-GTR-254

collected (see also chapters 3, 4, and 7). This new knowledge about spotted owl 

natural and life history has application to the management of the owl because it 

illuminates spotted owl requirements and responses of owls to disturbance (particularly logging and fire). 

Studies of physiological stress in spotted owls suggest that many routine forest 

operations (e.g., trail maintenance, brush removal, timber cruising and marking) 

are not likely to affect owls if they occur beyond 100 m from the nest or primary 

roost site (Tempel and Gutiérrez. 2003). However, more intrusive activities like road 

building and timber harvest have greater potential to disturb owls and their seasonal 

restriction near owl nests should be maintained. 

Because spotted owls defend their territory using vocalizations described 

above, they can be detected with high probability during surveys, given sufficient 

survey effort. Moreover, the site fidelity exhibited by a territorial pair and the 

consistency of their spatial location among years (Berigan et al. 2012) suggest that 

monitoring of spotted owls over large areas can be accomplished using call-based 

surveys (Tempel and Gutiérrez 2013). Indeed such call-based surveys have been the 

foundation of long-term demographic monitoring in the Sierra Nevada and southern 

California (Blakesley et al. 2010, Connor et al. 2013, Franklin et al. 2004, LaHaye et 

al. 2004, Tempel et al. 2014). However, the keys to unbiased call-based surveys are 

adequate survey effort (Tempel and Gutiérrez 2013), recognition of owl vocalizations by observers, year-to-year consistency within survey areas, and meeting the 

closure assumption of occupancy estimation (i.e., birds do not move in and out of 

surveyed areas; this is difficult to discern with unmarked birds). These studies have 

shown that spotted owls exhibit strong site fidelity, which has management implications. For example, Berigan et al. (2012) showed that owls used the same core areas, 

which contained a large proportion of the PACs established for their conservation 

for nesting and roosting over long time periods (>20 years). A PAC is designated 

whenever an owl is located on public land. However, the majority of PACs were 

delineated shortly after CASPO designed this concept in 1992. They now constitute essentially the only places where owls are currently found, suggesting that 

PACs are an essential management application. They could also represent the only 

remaining relatively large patches of nearly contiguous nesting/roosting habitat 

in the Sierra Nevada on public land. The PACs are also consistent management 

constructs relative to the hypothesis that spotted owls are central-place foragers. 

Thus, modifying PACs runs the risk of losing the owls within those PACs (see also 

chapter 8). The strong site fidelity of owls has several implications for management 

because birds may persist at sites even when site quality has been lowered because 
33 

The California Spotted Owl: Current State of Knowledge

 

of disturbance(s), which makes it difficult to assess the effect of disturbance on 

spotted owls. Moreover, Seamans and Gutiérrez (2007a) showed a correlation with 

habitat change and territory occupancy, which suggests that maintaining key habitat 

areas is important to these birds. 

Research on reproductive ecology provides numerous potential management 

implications, but there remains uncertainty about these implications. The relationship between brood size and territory quality suggests that territories can be 

ranked in terms of their contribution to the population, but presumed low-quality 

territories might actually be indicative of “low-quality” individuals inhabiting those 

sites. Moreover, territories with high turnover or low occupancy rates (which are 

currently presumed to be low quality) might actually improve as a result of forest succession and tree growth. The relationship between habitat and owl nesting 

success and reproductive output is important to managers because knowing which 

habitats might contribute to greater reproduction can inform management actions. 

Nevertheless, like other relationships involving reproductive output, habitat and 

individual quality are confounded. That is, are owls at a given site consistently 

successful because of the habitat conditions, the owls themselves (quality or experience), or both? This suggests, in addition to predictions from population viability 

theory, that “decommissioning” unoccupied PACs limits future options because 

PACs might be recolonized if they improve in quality with time (see Seamans and 

Gutiérrez 2007a). As another example, if owls exhibit low reproduction and PACs 

are removed for such reason, it will likely negatively affect the population because 

the bet-hedging life history strategy predicts that these owls will breed sometime 

and therefore may actually be important contributors to population demographic 

processes over the long term. 

Franklin et al. (2000) suggested that good habitat may buffer owls against the 

effects of bad weather. That is, while managers cannot control weather, they can 

manage habitat and conserve existing high-quality habitat because such habitat confers survival or reproductive advantages to owls when bad weather occurs relative 

to lower quality habitat (Franklin et al. 2000). These research findings suggest that 

it is prudent, if not necessary, to maintain sufficient amounts of high-quality habitat 

(high canopy, large trees, complex-structured habitat) rather than low-quality 

habitat (i.e., habitats with forest condition metrics on the low end of the observed 

distribution used by owls). 

Studies of diet analysis suggest that different management techniques could 

enhance prey habitat in low- and higher elevation habitats and among habitats within 

similar elevation zones. For example, at higher elevations in the Sierra Nevada, 

closed-canopy forests should be promoted to benefit the primary prey species (flying 
34 

GENERAL TECHNICAL REPORT PSW-GTR-254

 

squirrel), but some amount of chaparral and early-seral stage forest can be maintained to benefit the primary prey species at lower elevations (woodrats). 

In summary, substantial new information on California spotted owls has 

emerged since CASPO. This new information has the potential to inform management. Yet, there continues to be uncertainty about important aspects of the owl’s 

biology, specifically how the owl is affected by disturbance (see also chapter 8). 

Literature Cited 

American Ornithologists’ Union [AOU]. 1957. Check-list of North American 

birds. 4th ed. Washington, DC. 691 p. 

Barrowclough, G.F. 1982. Geographic variation, predictiveness, and subspecies. 

The Auk. 99: 601–603. 

Barrowclough, G.F. 1990. The description of geographic variation in bird 

populations. In: Bell, B.D.; Cossee, R.O. [et al.], eds. Proceedings of Acta XX 

Congressus Internationalis Ornithologici: 495–503. 

Barrowclough, G.F.; Groth, J.G.; Mertz, L.A.; Gutiérrez, R.J. 2005. Genetic 

structure, introgression, and a narrow hybrid zone between northern and 

California spotted owls (Strix occidentalis). Molecular Ecology. 14: 1109–1120. 

Barrowclough, G.F.; Gutiérrez, R.J.; Groth, J.G. 1999. Phylogeography of 

spotted owl (Strix occidentalis) populations based on mitochondrial DNA 

sequences: gene flow, genetic structure, and a novel biogeographic pattern. 

Evolution. 53: 919–931. 

Barrowclough, G.F.; Gutiérrez, R.J.; Groth, J.G.; Lai, J.E.; Rock, D.F. 2011. 

The Hybrid Zone between northern and California spotted owls in the Cascade– 

Sierran Suture Zone. The Condor. 113: 581–589. 

Barrows, C.W. 1985. Fledging success relative to fluctuations in diet for spotted 

owls in California. In: Gutiérrez, R.J.; Carey, A.B., tech. eds. Ecology and 

management of the spotted owl in the Pacific Northwest. Gen. Tech. Rep. PNWGTR-185. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific 

Northwest Forest and Range Experiment Station: 50–54. 

Barrows, C.W. 1987. Diet shifts in breeding and nonbreeding spotted owls. Journal 

of Raptor Research. 21: 95–97. 

Beale, C.M. 2007. The behavioral ecology of disturbance responses. International 

Journal of Comparative Psychology. 20: 111–120. 
35 

The California Spotted Owl: Current State of Knowledge

Bequaert, J.C. 1952. The Hippoboscidae or louse-flies (Diptera) of mammals and 

birds. Part I. Structure, physiology and natural history. Entomologica Americana. 

32: 1–209. 

Berigan, W.J.; Gutiérrez, R.J.; Tempel, D.J. 2012. Evaluating the efficacy of 

protected habitat areas for the California spotted owl using long-term monitoring 

data. Journal of Forestry. 110: 299–303. 

Bias, M.A.; Gutiérrez, R.J. 1992. Habitat associations of California spotted owls 

in the central Sierra Nevada. Journal of Wildlife Management. 56: 584–595. 

Bingham, B.B.; Noon, B.R. 1997. Mitigation of habitat ‘take’: application to 

habitat conservation planning. Conservation Biology. 11: 127–139. 

Blakesley, J.A.; Anderson, D.R.; Noon, B.R. 2006. Breeding dispersal in the 

California spotted owl. Condor. 108: 71–81. 

Blakesley, J.A.; Seamans, M.E.; Conner, M.M.; Franklin, A.B.; White, G.C.; 

Gutiérrez, R.J.; Hines, J.E.; Nichols, J.D.; Munton, T.E.; Shaw, D.W.H.; 

Keane, J.J.; Steger, G.N.; McDonald, T.L. 2010. Population dynamics of 

spotted owls in the Sierra Nevada, California. Wildlife Monographs. 174: 1–36. 

Bond, M.L.; Gutiérrez, R.J.; Franklin, A.B.; Lahaye, W.S.; May, C.A.; 

Seamans, M.E. 2002. Short-term effects of wildfires on spotted owl survival, 

site fidelity, mate fidelity, and reproductive success. Wildlife Society Bulletin.

30: 1022–1028. 

Bond, M.L.; Lee, D.E.; Siegel, R.B.; Ward, J.P. 2009. Habitat use and selection 

by California spotted owls in a postfire landscape. Journal of Wildlife 

Management. 73: 1116–1124. 

Cary, A.B. 1985. A summary of the scientific basis for spotted owl management. 

In: Gutiérrez, R.J.; Carey, A.B., tech. eds. Ecology and management of spotted 

owls in the Pacific Northwest. Gen. Tech. Rep. PNW-GTR-185. Portland, OR: 

U.S. Department of Agriculture, Forest Service, Pacific Northwest Forest and 

Range Experiment Station: 100–114. 

Carey, A.B.; Horton, S.P.; Biswell, B.L. 1992. Northern spotted owls: influence

of prey base and landscape character. Ecological Monographs. 62: 223–250. 

Carey, A.B.; Peeler, K.C. 1995. Spotted owls: resource and space use in mosaic 

landscapes. Journal of Raptor Research. 29: 223–239. 
36 

GENERAL TECHNICAL REPORT PSW-GTR-254

Carlson, P.C.; Lahaye, W.S.; Franklin, A.B. 1998. Incestuous behavior in spotted 

owls. The Wilson Bulletin. 110: 562–564. 

Clark, D.A.; Anthony, R.G.; Andrews, L.S. 2011. Survival rates of northern 

spotted owls in post-fire landscapes of southwest Oregon. Journal of Raptor 

Research. 45: 38–47. 

Clark, D.A.; Anthony, R.G.; Andrews, L.S. 2013. Relationship between wildfire, 

salvage logging, and occupancy of nesting territories by northern spotted owls. 

Journal of Wildlife Management. 77: 672–688. 

Crozier, M.L.; Seamans, M.E.; Gutiérrez, R.J. 2005. The effect of broadcasting 

great horned owl vocalizations on spotted owl vocal responsiveness. Journal of 

Raptor Research. 39: 111–118. 

Crozier, M.L.; Seamans, M.E.; Gutiérrez, R.J.; Loschl, P.J.; Horn, R.B.; 

Sovern, S.G.; Forsman, E.D. 2006. Does the presence of barred owls suppress 

the calling behaviour of spotted owls? Condor. 108: 760–769. 

Dark, S.; Gutiérrez, R.J.; Gould, G.I., Jr. 1998. The barred owl (Strix varia) 

invasion in California. Auk. 115: 50–56. 

Delaney, D.K.; Grubb; T.G.; Beier, P. 1999. Activity patterns of nesting Mexican 

spotted owls. Condor. 101: 42–49. 

DellaSala, D.A.; Hanson, C.T., eds. 2015. The ecological importance of mixedseverity fires: nature’s phoenix. Waltham, MA: Elsevier. 450 p.

Dugger, K.M.; Forsman, E.D.; Franklin, A.B.; Davis, R.J.; White, G.C.; 

Schwarz, C.J.; Burnham, K.P.; Nichols, J.D.; Hines, J.E.; Yackulic, C.B.; 

Doherty, P.F., Jr.; Bailey, L.; Clark, D.A.; Ackers, S.H.; Andrews, L.S.; 

Augustine, B.; Biswell, B.L.; Blakesley, J.; Carlson, P.C.; Clement, M.J.; 

Diller, L.V.; Glenn, E.M.; Green, A.; Gremel, S.A.; Herter, D.R.; Higley, 

J.M.; Hobson, J.; Horn, R.B.; Huyvaert, K.P.; McCafferty, C.; McDonald, 

T.; McDonnell, K.; Olson, G.S.; Reid, J.A.; Rockweit, J.; Ruiz, V.; Saenz, J.; 

Sovern, S.G. 2016. The effects of habitat, climate, and barred owls on long-term 

demography of northern spotted owls. Condor. 118: 57–116. 

Dunn, H.H. 1901. The spotted owl (Syrnium occidentale). Oologist. 18: 165–167. 
37 

The California Spotted Owl: Current State of Knowledge

Forsman, E.D.; Anthony, R.G.; Dugger, K.M.; Glenn, E.M.; Franklin, A.B.; 

White, G.C.; Schwarz, C.J.; Burnham, K.P.; Anderson, D.R.; Nichols, J.D.; 

Hines, J.E.; Lint, J.B.; Davis, R.J.; Ackers, S.H.; Andrews, L.S.; Biswell, 

B.L.; Carlson, P.C.; Diller, L.V.; Gremel, S.A.; Herter, D.R.; Higley, J.M.; 

Horn, R.B.; Reid, J.A.; Rockweit, J.; Schaberl, J.P.; Snetsinger, T.J.; Sovern, 

S.G. 2011. Population demography of northern spotted owls. Studies in avian 

biology. No. 40. Berkeley, CA: University of California Press. 106 p. 

Franklin, A.B. 1992. Population regulation in northern spotted owls: theoretical 

implications for management. In: McCullough, D.R.; Barrett, R.H., eds. Wildlife 

2001: populations. New York: Elsevier Press: 815–827. 

Franklin, A.B.; Anderson, D.R.; Gutiérrez, R.J.; Burnham, K.P. 2000. Climate, 

habitat quality, and fitness in northern spotted owl populations in northwestern 

California. Ecological Monographs. 70: 539–590. 

Franklin, A.B.; Burnham, K.P.; White, G.C.; Anthony, R.J.; Forsman, E.D.; 

Schwarz, C.; Nichols, J.D.; Hines, J. 1999. Range-wide status and trends in 

northern spotted owl populations. Unpublished report. On file with: Colorado 

Cooperative Fish and Wildlife Research Unit, Calorado State University, 201 

JVK Wagar Bldg., 1484 Campus Delivery, Fort Collins, CO 80523-1484. 

Franklin, A.B.; Gutiérrez, R.J.; Nichols, J.D.; Seamans, M.E.; White, G.C.; 

Zimmerman, G.S.; Hines, J.E.; Munton, T.E.; LaHaye, W.S.; Blakesley, 

J.A.; Steger, G.N.; Noon, B.R.; Shaw, D.W.H.; Keane, J.J.; McDonald, T.R.; 

Britting, S. 2004. Population dynamics of the California spotted owl (Strix 

occidentalis occidentalis): a meta-analysis. Ornithological Monographs. 54: 

1–54. 

Fretwell, S.D. 1972. Populations in a seasonal environment. Monographs in 

population biology 5. Princeton, NJ: Princeton University Press. 224 p. 

Fretwell, S.D.; Lucas, H.L., Jr. 1969. On territorial behavior and other factors 

influencing habitat distribution in birds. I. Theoretical development. Acta 

Biotheoretica. 19: 16–36. 

Gancz, A.Y.; Barker, I.K.; Lindsay, R.; Dibernardo, A.; McKeever, K.; Hunter, 

B. 2004. West Nile virus outbreak in North American owls, Ontario, 2002. 

Emerging Infectious Diseases. 10: 2135–2142. 
38 

GENERAL TECHNICAL REPORT PSW-GTR-254

Gutiérrez, R.J. 1985a. An overview of recent research on the spotted owl. In: 

Gutiérrez, R.J.; Carey, A.B., tech. eds. Ecology and management of spotted owls 

in the Pacific Northwest. Gen. Tech. Rep. PNW-GTR-185. Portland, OR: U.S. 

Department of Agriculture, Forest Service, Pacific Northwest Forest and Range 

Experiment Station: 39–49. 

Gutiérrez, R.J. 1985b. Information and research needs for spotted owl 

management. In: Gutiérrez, R.J.; Carey, A.B., tech. eds. Ecology and 

management of the spotted owl in the Pacific Northwest. Gen. Tech. Rep. PNWGTR-185. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific 

Northwest Forest and Range Experiment Station: 115–118. 

Gutiérrez, R.J. 1989. Hematozoa from the spotted owl. Journal of Wildlife 

Diseases. 25: 614–618. 

Gutiérrez, R.J. 1994. Changes in the distribution and abundance of spotted owls 

during the past century. Studies in Avian Biology. 15: 293–300. 

Gutiérrez, R.J. 1996. Biology of the spotted owl. Studies in Avian Biology. 

17: 2–5. 

Gutiérrez, R.J. 2004. Science and the conservation of the California spotted owl. 

In: Chancellor, R.D.; Meyburg, B.-U., eds. Raptors worldwide. Berlin, Germany: 

The Worlk Working Group on Birds of Prey and Owls. Proceedings of the 6th 

world conference on birds of prey and owls: 317–327. 

Gutiérrez, R.J. 2008. Spotted owl research: a quarter century of contributions 

to education, ornithology, ecology, and wildlife management. Condor. 110: 

792–798. 

Gutiérrez, R.J.; Barrowclough, G.F. 2005. Redefining the distributional 

boundaries of the northern and California spotted owls: implications for 

conservation. Condor. 107: 182–187. 

Gutiérrez, R.J.; Carey, A.B., tech. eds. 1985. Ecology and management of the 

spotted owl in the Pacific Northwest. Gen. Tech. Rep. PNW-GTR-185. Portland, 

OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Forest 

and Range Experiment Station. 119 p. 
39 

The California Spotted Owl: Current State of Knowledge

Gutiérrez, R.J.; Cheng, A.S.; Becker, D.R.; Cashen, S.; Ganz, D.; Gunn, J.; 

Liquori, M.; Merrill, A.; Saah, D.S.; Price, W. 2015. Legislated collaboration 

in a conservation conflict: a case study of the Quincy Library Group, California. 

In: Redpath, S.M.; Gutiérrez, R.J.; Wood, K.A.; Young, J.C., eds. Conflicts 

in conservation: navigating towards solutions. Cambridge, United Kingdom: 

Cambridge University Press: 271–283. 

Gutiérrez, R.J.; Franklin, A.B.; LaHaye, W.S. 1995. Spotted owl (Strix 

occidentalis). In: Poole, A.; Gill, F., eds. The birds of North America No. 179: 

life histories for the twenty-first century. Washington, DC: The Philadelphia 

Academy of Sciences and The American Ornithologists’ Union. 28 p. 

Gutiérrez, R.J.; Franklin, A.B.; LaHaye, W.; Meretsky, V.J.; Ward, J.P. 1985. 

Juvenile spotted owl dispersal in northwestern California: Preliminary results. 

In: Gutiérrez, R.J.; Carey, A.B., tech. eds. Ecology and management of the 

spotted owl in the Pacific Northwest. Gen. Tech. Rep. PNW-GTR-185. Portland, 

OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Forest 

and Range Experiment Station: 60–65. 

Gutiérrez, R.J.; LaHaye, W.S.; Zimmerman, G.S. 2011. Breeding dispersal in 

an isolated population of spotted owls Strix occidentalis: evidence for improved 

reproductive output. Ibis. 153: 592–600. 

Gutiérrez, R.J.; Seamans, M.E.; Peery, M.Z. 1996. Intermountain movement 

by Mexican spotted owls (Strix occidentalis lucida). Great Basin Naturalist. 

56: 87–89. 

Gutiérrez, R.J.; Verner, J.; McKelvey, K.S.; Noon, B.R.; Steger, G.S.; Call, 

D.R.; LaHaye, W.S.; Bingham, B.B.; Senser, J.S. 1992. Habitat relations of the 

California spotted owl. In: Verner, J.; McKelvey, K.S.; Noon, B.R.; Gutiérrez, 

R.J.; Gould, G.I., Jr.; Beck, T.W., eds. The California spotted owl: a technical 

assessment of its current status. Gen. Tech. Rep. PSW-GTR-133. Albany, CA: 

U.S. Department of Agriculture, Forest Service, Pacific Southwest Research 

Station: 79–147. 

Haig, S.M.; Mullins, T.D.; Forsman, E.D. 2004. Subspecific relationships and 

genetic structure in the spotted owl. Conservation Genetics. 5: 683–705. 

Hirons, G.J. 2009. The effects of territorial behaviour on the stability and 

dispersion of tawny owl (Strix aluco) populations. Journal of Zoology. 1: 21–48. 
40 

GENERAL TECHNICAL REPORT PSW-GTR-254

Hull, J.M.; Keane, J.J.; Tell, L.; Ernest, H.B. 2010. West Nile virus antibody 

surveillance in three Sierra Nevada raptors of conservation concern. Condor. 

112: 168–172. 

Hunsaker, C.T.; Boroski, B.B.; Steger, G.N. 2002. Relations between canopy 

cover and the occurrence and productivity of California spotted owls. In: Scott, 

M.J.; Heglund, P.J.; Morrison, M.L.; Haufler, J.B.; Raphael, M.G.; Wall, W.A.; 

Samson, F.B., eds. Predicting species occurrences: issues of accuracy and scale. 

Covelo, CA: Island Press: 687–700. 

Hunter, J.E.; Gutiérrez, R.J.; Franklin, A.B.; Olson, D. 1994. Ectoparasites of 

the spotted owl. Journal of Raptor Research. 28: 232–235. 

Hunter, J.E.; Jewett, C.; Padilla, A. 1993. Kleptoparasitism of northern spotted 

owls. Northwestern Naturalist. 74: 28–29. 

Irwin, L.L.; Clark, L.A.; Rock, D.C.; Rock, S.L. 2007. Modeling foraging 

habitat of California spotted owls. Journal of Wildlife Management. 71: 

1183–1191. 

Ishak, H.D.; Dumbacher, J.P.; Anderson, N.L.; Keane, J.J.; Valkiūnas, G.; 

Haig, S.M.; Tell, L.A.; Sehgal, R.N.M. 2008. Blood parasites in owls with 

conservation implications for the spotted owl (Strix occidentalis). PLoS ONE. 

3(5): e2304. doi:10.1371/journal.pone.0002304. 

Johnsgard, P.A. 1988. North American owls: biology and natural history. 

Washington, DC: Smithsonian Institution Press. 295 p. 

Johnson, D.H. 1992. Spotted owls, great horned owls, and forest fragmentation 

in the central Oregon cascades. Corvallis, OR: Oregon State University. 125 p. 

M.S. thesis. 

Jones, G.; Gutiérrez, R.J.; Tempel, D.; Whitmore, S.A.; Berigan, W.J.; Peery, 

M.Z. 2016. Megafires: an emerging threat to old-forest species. Frontiers in 

Ecology and the Environment. 14: 300–306. 

Keane, J.J. 2014. California spotted owl: scientific considerations for forest 

planning. In: Long, J.W.; Quinn-Davidson, L.; Skinner, C.N., eds. Science 

synthesis to support socioecological resilience in the Sierra Nevada and southern 

Cascade Range. Gen. Tech. Rep. PSW-GTR-247. Albany, CA: U.S. Department 

of Agriculture, Forest Service, Pacific Southwest Research Station: 437–467. 

LaHaye, W.S. 1997. Four young fledged by a pair of California spotted owls. 

Western Birds. 28: 108–109. 
41 

The California Spotted Owl: Current State of Knowledge

 

LaHaye, W.S.; Gutiérrez, R.J.; Call, D.R. 1997. Nest-site selection and 

reproductive success of California spotted owls. Wilson Bulletin. 109: 42–51. 

LaHaye, W.S.; Zimmerman, G.S.; Gutiérrez, R.J. 2004. Temporal variation 

in the vital rates of an insular population of spotted owls (Strix occidentalis 

occidentalis): contrasting effects of weather. Auk. 121: 1056–1069. 

Laymon, S.A. 1988. The ecology of the spotted owl in the central Sierra Nevada, 

California. Berkeley, CA. University of California. 285 p. Ph.D. dissertation. 

Laymon, S.A. 1991. Diurnal foraging by spotted owls. Wilson Bulletin. 103: 

138–140. 

Lee, D.E.; Bond, M.L. 2015a. Occupancy of California spotted owl sites following 

a large fire in the Sierra Nevada, California. Condor. 117: 228–236.

Lee, D.E.; Bond, M.L. 2015b. Previous year’s reproductive state affects spotted 

owl site occupancy and reproduction responses to natural and anthropogenic 

disturbances. Condor. 117: 307–319. 

Lee, D.E.; Bond, M.L.; Borchert, M.I.; Tanner, R. 2013. Influence of fire and 

salvage logging on site occupancy of spotted owls in the San Bernardino and 

San Jacinto Mountains of Southern California. Journal of Wildlife Management. 

77: 1327–1341. 

Lee, D.E.; Bond, M.L.; Siegel, R.B. 2012. Dynamics of breeding-season site 

occupancy of the California spotted owl in burned forests. Condor. 114: 792–802. 

Leskiw, T.; Gutiérrez, R.J. 1998. Possible predation of a spotted owl by a barred 

owl. Western Birds. 29: 225–226. 

Lewicki, K.E.; Huyvaert, K.P.; Piaggio, A.J.; Diller, L.V.; Franklin, A.B. 2015. 

Effects of barred owl (Strix varia) range expansion on Haemoproteus parasite 

assemblage dynamics and transmission in barred and northern spotted owls 

(Strix occidentalis caurina). Biological Invasions. 17: 1713–1727. 

Lõhmus, A. 2004. Raptor habitat studies—the state of the art. In: Chancellor, R.D.; 

Meyburg, B.-U., eds. Raptors worldwide. Berlin, Germany: The World Working 

Group on Birds of Pray and Owls. Proceedings of the 6th world conference on 

birds of prey and owls: 279–296. 

Lydersen, J.; North, M. 2012. Topographic variation in active-fire forest structure 

under current climate conditions. Ecosystems. 15: 1134–1146. 
42 

GENERAL TECHNICAL REPORT PSW-GTR-254

 

Mackenzie, D.I.; Nichols, J.D.; Seamans, M.E.; Gutiérrez, R.J. 2009. Modeling 

species occurrence dynamics with multiple states and imperfect detection. 

Ecology. 90: 823–835. 

MacKenzie, D.I.; Seamans, M.E.; Gutiérrez, R.J.; Nichols, J.D. 2012. 

Investigating the population dynamics of California spotted owls without marked 

individuals. Journal of Ornithology. 152 (Supplement 2): S597–S604. 

Marti, C.D. 1987. Raptor food habits studies. In: Giron Pendleton, B.A.; Millsap, 

B.A.; Cline, K.W.; Bird, D.M., eds. Raptor management techniques manual. 

Washington, DC: National Wildlife Federation: 67–80. 

Mayr, E. 1982. Of what use are subspecies? Auk. 99: 593–595. 

Moen, C.A.; Gutiérrez, R.J. 1997. California spotted owl habitat selection in 

the central Sierra Nevada. 61: 1281–1287. 

Munton, T.E.; Johnson, K.D.; Steger, G.N.; Eberlein, G.P. 1997. The diet of 

California spotted owls in riparian deciduous and oak habitats of the southern 

Sierra Nevada. In: Pillsbury, N.H.; Verner, J.; Tietje, W.D., tech. coords. 

Proceedings of a symposium on oak woodlands: ecology, management, and 

urban interface issues. Gen. Tech. Rep. PSW-GTR-160. Albany, CA: U.S. 

Department of Agriculture, Forest Service, Pacific Southwest Research Station: 

683–687. 

Munton, T.E.; Johnson, K.D.; Steger, G.N.; Eberlein, G.P. 2002. Diets of 

California spotted owls in the Sierra National Forest. In: Verner, J., tech. ed. 

Proceedings of a symposium on the Kings River Sustainable Forest Ecosystems 

Project: progress and current status. Gen. Tech. Rep. PSW-GTR-183. Albany, 

CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research 

Station: 99–105. 

Newton, I. 1979. Population ecology of raptors. Vermillion, SD: Buteo Books. 

432 p. 

North, M.P. 2002. Environmental factors associated with spotted owl reproduction. 

In: Verner, J., tech. ed. Proceedings of a symposium on the Kings River 

Sustainable Forest Ecosystems Project: progress and current status. Gen. Tech. 

Rep. PSW-GTR-183. Albany, CA: U.S. Department of Agriculture, Forest 

Service, Pacific Southwest Research Station: 117–119. 

North, M.; Steger, G.; Denton, R.; Eberlein, G.; Munton, T.; Johnson, K. 2000. 

Association of weather and nest-site structure with reproductive success in 

California spotted owls. Journal of Wildlife Management. 64: 797–807. 
43 

The California Spotted Owl: Current State of Knowledge

Peery, M.Z.; Gutiérrez, R.J. 2013. Life-History tradeoffs in spotted owls (Strix 

occidentalis): implications for assessment of territory quality. Auk. 130: 132–140. 

Phillips, C.E.; Tempel, D.J.; Gutiérrez, R.J. 2010. Do California spotted owls 

select nest trees close to forest edges? Journal of Raptor Research. 44: 311–314. 

Redpath, S.M.; Young, J.; Evely, A.; Adams, W.M.; Sutherland, W.J.; 

Whitehouse, A.; Amar, A.; Lambert, R.A.; Linnell, J.D.C.; Watt, A.; 

Gutiérrez, R.J. 2013. Understanding and managing conservation conflicts. 

Trends in Ecology and Evolution. 28: 100–109. 

Roberts, S.L. 2008. Effects of fire on California spotted owls and their mammalian 

prey in the central Sierra Nevada, California. Davis, CA: University of 

California. 122 p. Ph.D. dissertation. 

Roberts, S.; North, M. 2012. California spotted owls. In: North, M., tech. ed. 

Managing Sierra Nevada forests. Gen. Tech. Rep. PSW-GTR-237. Albany, CA: 

U.S. Department of Agriculture, Forest Service, Pacific Southwest Research 

Station: 61–71. 

Roberts, S.L.; van Wagtendonk, J.W.; Miles, A.K.; Kelt, D.A. 2011. Effects 

of fire on spotted owl site occupancy in a late-successional forest. Biological 

Conservation. 144: 610–619. 

Rockweit, J.T.; Franklin, A.B.; Bakken, G.S.; Gutiérrez, R.J. 2012. Potential 

influences of climate and nest structure on spotted owl reproductive success: a 

biophysical approach. PLoS ONE. 7: e41498. doi:10.1371/journal.pone.0041498. 

Seamans, M.E.; Gutiérrez, R.J. 2006. Spatial dispersion of spotted owl sites and 

the role of conspecific attraction on settlement patterns. Ethology Ecology & 

Evolution. 18: 99–111. 

Seamans, M.E.; Gutiérrez, R.J. 2007a. Habitat selection in a changing 

environment: The relationship between habitat alteration and spotted owl 

territory occupancy and breeding dispersal. Condor. 109: 566–576. 

Seamans, M.E.; Gutiérrez, R.J. 2007b. Sources of variability in spotted owl 

population growth rate: testing predictions using long-term mark–recapture 

data. Oecologia. 152: 57–70. 

Seamans, M.E.; Gutiérrez, R.J.; Moen, C.A.; Peery, M.Z. 2001. Spotted owl 

demography in the central Sierra Nevada. Journal of Wildlife Management. 

65: 425–431. 
44 

GENERAL TECHNICAL REPORT PSW-GTR-254

Sierra Nevada Ecosystem Project [SNEP]. 1996. Fire and fuels. In: Volume 1: 

Final report to Congress, Sierra Nevada Ecosystem Project Science Team and 

Special Consultants, eds. Wildland Resources Center Report 36. Davis, CA: 

Centers for Water and Wildland Resources, University of California– Davis: 

62–71. 

Smith, R.B.; Peery, M.Z.; Gutiérrez, R.J.; LaHaye, W.S. 1999. The relationship 

between spotted owl diet and reproductive success in the San Bernardino 

Mountains, California. Wilson Bulletin. 111: 22–29. 

Stearns, S.C. 1976. Life-history tactics: a review of the ideas. Quarterly Review 

of Biology. 51: 3–47. 

Steger, G.N.; Werner, L.R.; Munton, T.E. 2006. First documented record of the 

barred owl in the southern Sierra Nevada. Western Birds. 37: 106–109. 

Stoelting, R.E.; Gutiérrez, R.J.; Kendall, W.L.; Peery, M.Z. 2015. Life-history 

tradeoffs and reproductive cycles in spotted owls. Auk. 132: 46–64. 

Swarthout, E.C.H.; Steidl, R.J. 2003. Experimental effects of hiking on breeding 

Mexican spotted owls. Conservation Biology. 17: 307–315. 

Tempel, D.J.; Gutiérrez, R.J. 2003. Fecal corticosterone levels in California 

spotted owls exposed to low-intensity chainsaw sound. Wildlife Society Bulletin. 

31: 698–702. 

Tempel, D.J.; Gutiérrez, R.J. 2004. Factors related to fecal corticosterone 

levels in California spotted owls: implications for assessing chronic stress. 

Conservation Biology. 18: 538–547. 

Tempel, D.J.; Gutiérrez, R.J. 2013. Relation between occupancy and abundance 

for a territorial species, the California spotted owl. Conservation Biology. 27: 

1087–1095. 

Tempel, D.J.; Gutiérrez, R.J.; Whitmore, S.A.; Reetz, M.J.; Stoelting, 

R.E.; Berigan, W.J.; Seamans, M.E.; Peery, M.Z. 2014. Effects of forest 

management on California spotted owls: implications for reducing wildfire risk 

in fire-prone forests. Ecological Applications. 24: 2089–2106.

Tempel, D.J.; Peery, M.Z.; Gutiérrez, R.J. 2014. Using integrated population 

models to improve conservation monitoring: California spotted owls as a case 

study. Ecological Modelling. 289: 86–95. 
45 

The California Spotted Owl: Current State of Knowledge

Thomas, N.J.; Bunikis, A.J.; Barbour, A.G.; Wolcott, M.J. 2002. Fatal 

spirochetosis due to a relapsing fever-like Borrelia sp. in a northern spotted owl. 

Journal of Wildlife Diseases. 38: 187–193. 

Thrailkill, J.; Bias, M.A. 1989. Diets of breeding and nonbreeding California 

spotted owls. Journal of Raptor Research. 23: 39–41. 

U.S. Department of the Interior, Fish and Wildlife Service [USFWS]. 1990. 

Endangered and threatened wildlife and plants: determination of threatened 

status for the northern spotted owl. Federal Register. 55: 26114–26194. 

U.S. Department of the Interior, Fish and Wildlife Service [USFWS]. 1993. 

Endangered and threatened wildlife and plants; final rule to list the Mexican 

spotted owl as a threatened species. Federal Register. 58: 14248–14271. 

U.S. Department of the Interior, Fish and Wildlife Service [USFWS]. 2003. 

Endangered and threatened wildlife and plants; 12-month finding for a petition to 

list the California spotted owl (Strix occidentalis occidentalis). Federal Register. 

68: 7580−7608.

U.S. Department of the Interior, Fish and Wildlife Service [USFWS]. 2006. 

Endangered and threatened wildlife and plants; 12-month finding for a petition to 

list the California spotted owl (Strix occidentalis occidentalis). Federal Register. 

71: 29886−29908.

VanGelder, J.J. 2003. Variation in four-note location calls of male spotted owls 

(Strix occidentalis). Arcata, CA: Humboldt State University. 69 p. M.S. thesis. 

Verner, J.; Gutiérrez, R.J.; Gould, G.I., Jr. 1992a. The California spotted owl: 

general biology and ecological relations. In: Verner, J.; McKelvey, K.S.; Noon, 

B.R.; Gutiérrez, R.J.; Gould, G.I., Jr.; Beck, T.W., eds. The California spotted 

owl: a technical assessment of its current status. Gen. Tech. Rep. PSW-GTR-133. 

Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest 

Research Station: 55–77. 

Verner, J.; McKelvey, K.S.; Noon, B.R.; Gutiérrez, R.J.; Gould, G.I., Jr.; 

Beck, T.W. 1992b. The California spotted owl: a technical assessment of its 

current status. Gen. Tech. Rep. PSW-GTR-133. Albany, CA: U.S. Department of 

Agriculture, Forest Service, Pacific Southwest Research Station. 285 p.

Waldo, S.L. 2002. Song discrimination of neighbors and strangers by male 

territorial northern spotted owls (Strix occidentalis caurina). Arcata, CA: 

Humboldt State University. 49 p. M.S. thesis. 
46 

GENERAL TECHNICAL REPORT PSW-GTR-254

Ward, J.P., Jr. 2001. Ecological responses by Mexican spotted owls to environmental variation in the Sacramento Mountains, New Mexico. Fort Collins, CO: 

Colorado State University. 409 p. Ph.D. dissertation. 

Ward, J.P.; Gutiérrez, R.J.; Noon, B.R. 1998. Habitat selection by northern 

spotted owls: the consequences of prey selection and distribution. Condor. 100: 

79–92. 

Wasser, S.K.; Bevis, K.; King, G.; Hanson, E. 1997. Noninvasive physiological 

measures of disturbance in the northern spotted owl. Conservation Biology. 11: 

1019–1022. 

Waters, J.R.; McKelvey, K.S.; Zabel, C.J.; Luoma, D. 2000. Northern flying 

squirrel mycophagy and truffle production in fir forests in northeastern 

California. In: Powers, R.F.; Hauxell, D.L.; Nakamura, G.M., tech. eds. 

Proceedings of the California Forest Soils Council conference on forest soils 

biology and forest management. Gen. Tech. Rep. PSW-GTR-178. Albany, CA: 

U.S. Department of Agriculture, Forest Service, Pacific Southwest Research 

Station: 73–97. 

Waters, J.R.; Zabel, C.J. 1995. Flying squirrel densities in fir forests of 

northeastern California. Journal of Wildlife Management. 59: 858–866. 

Weathers, W.W.; Hodum, P.J.; Blakesley, J.A. 2001. Thermal ecology and 

ecological energetics of California spotted owls. Condor. 103: 678–690. 

Williams, P.J.; Gutiérrez, R.J.; Whitmore, S.A. 2011. Home range and habitat 

selection of spotted owls in the central Sierra Nevada. Journal of Wildlife 

Management. 75: 333–343. 

Williams, P.J.; Whitmore, S.A.; Gutiérrez, R.J. 2014. Use of private lands 

for foraging by California spotted owls in the central Sierra Nevada. Wildlife 

Society Bulletin. 38: 705–709. 

Wood, S.F.; Herman, C.M. 1943. The occurrence of blood parasites in birds from 

Southwestern United States. Journal of Parasitology. 29: 187–196. 

Zabel, C.J.; Waters, J.R. 1997. Food preferences of captive northern flying 

squirrels from the Lassen National Forest in northeastern California. Northwest 

Science. 71: 103–107. 

Zimmerman, G.S.; Gutiérrez, R.J.; LaHaye, W.S. 2007. Finite study areas and 

vital rates: sampling effects on estimates of spotted owl survival and population 

trends. Journal of Applied Ecology. 44: 963–971. 
47 

The California Spotted Owl: Current State of Knowledge

Zimmerman, G.S.; LaHaye, W.S.; Gutiérrez, R.J. 2003. Empirical support for a 

despotic distribution in a California spotted owl population. Behavioral Ecology. 

14: 433–437. 

Zink, R.M. 2004. The role of subspecies in obscuring avian biological diversity 

and misleading conservation policy. Proceedings Royal Society of London. B 

271: 561–564. 
48 

GENERAL TECHNICAL REPORT PSW-GTR-254
49 

The California Spotted Owl: Current State of Knowledge

Chapter 3: California Spotted Owl Habitat 

Characteristics and Use 

Susan L. Roberts1 

Introduction 

California spotted owls (Strix occidentalis occidentalis) establish large home 

ranges averaging about 1279 ha (3,160 ac) (table 3-1), and within these home ranges 

individual owls select habitat at different scales, depending on their activity. At the 

smallest spatial scale, the nest tree, it appears there is very limited flexibility in 

the requirements. However, as owls select habitat at larger scales and for different 

activities, from nest stand to core area to foraging habitat, there is greater variability in the habitat characteristics, which suggests greater flexibility in selection. 

Currently, researchers have not established definitions of the size of a nest stand 

or core area, nor have they reached consensus on how to calculate these aspects of 

owl habitat. This is at least partially because each researcher uses a certain method 

to estimate the nest stand or core area that is relevant only to the particular question they are investigating, and as those questions differ between research projects, 

the methods and definitions for those terms also differ. This chapter presents the 

current research describing spotted owl habitat characteristics and is organized 

by spatial scale, starting with the nest tree, followed by the nest stand, core area, 

foraging habitat, prey habitat, and finally the home range. Next is a brief assessment 

of the current research on the effects of fire on spotted owl habitat, and followed by 

relevant management implications. 

Habitat Characteristics 

Nest and Nest Tree Characteristics 

California spotted owls are habitat specialists that are strongly associated with 

older, closed-canopy forests with multiple layers in the mid and upper canopies. 

All research shows they use large, old trees and snags as structures for nest and 

roost sites, embedded in a forest stand that has complex structure (Blakesley et al. 

2005, Gutiérrez et al. 1992, Verner et al. 1992a). Owls nest in cavities, broken tree 

tops, and occasionally on platforms such as old nests or mistletoe brooms located in 

large conifers, oaks, and snags (Verner et al. 1992a). Often, these are the largest and 

oldest trees in the stand and many have structural defects, such as a broken or split 

tops that have multiple terminal leaders (North et al. 2000). In mixed-conifer forests 

1

 Susan L. Roberts is a wildlife ecologist and private consultant, P.O. Box 2163, 

Wawona, CA 95389. 
50 

GENERAL TECHNICAL REPORT PSW-GTR-254

Table 3-1—Estimates of individual California spotted owl home ranges in 

mixed-conifer forests for the breeding season from various telemetry studies 

using the 100 percent minimum convex polygon estimation methoda 

Mean home Home range Sample 

Study authors range size standard error Study areaa size 

- - - - - -Hectares (acres) - - - - - - 

Zabel et al. 1992 2195 (5,423) 701 (1,731) Lassen NF 9 

Gallagher 2010 1653 (4,085) 336 (830) Plumas NF 9 

Call et al. 1992 1520 (3,756) Not reported Tahoe NF 5 

Williams et al. 2011 946 (2,338) Not reported El Dorado NF, 14 

Tahoe NF 

Eyes 2014 634 (1,567) 200 (494) Yosemite NP 14 

Zabel et al. 1992 728 (1,799) 65 (160) Sierra NF 24 

NF = national forest, NP = national park. 

a Study results are organized by latitude of the study area from north to south. 

of the Sierra Nevada, across 141 spotted owl nests, the owls show no preference for 

any particular tree species, and the average nest tree is 124 cm (49 in) in diameter 

at breast height (d.b.h.) and 31 m (103 ft) tall with an average nest height of 23 m 

(74 ft) (Gutiérrez et al. 1992, Roberts et al. 2011). Owls using nests with an overhead 

canopy of “high foliage volume” have higher reproductive success than owls using 

sites with low foliage volume (North et al. 2000). In hardwood forests, of the 13 

nests observed, nests were typically in live hardwood tree species with an average 

nest height of 12 m (38 ft), and an average nest tree d.b.h. and total height of 76 cm 

(30 in) and 17 m (55 ft), respectively (Gutiérrez et al. 1992). Occasionally, owls nest 

in giant sequoia (Sequoiadendron giganteum (Lindl.) J. Buchholz) or Coulter pine 

(Pinus coulteri D. Don). 

Nest Stand Characteristics 

Nest stands of California spotted owls typically have high canopy closure and cover 

(≥75 percent for both) [Note: when citing studies, we use terminology consistent 

with Jennings et al. (1999); however, many studies fail to accurately distinguish 

between canopy closure and cover (see chapter 5 for clarification)], an abundance 

of large (>61 cm [24 in] d.b.h.) trees, and multiple canopy layers comprising trees of 

different sizes, but numerically dominated by medium-sized trees (30 to 61 cm [12 

to 24 in]) (Bias and Gutiérrez 1992, Blakesley et al. 2005, Chatfield 2005, Moen and 

Gutiérrez 1997, North et al. 2000, Roberts et al. 2011, Seamans 2005) (fig. 3-1). 

There is no definitive estimate of the size of nest stands as each researcher 

used a stand size that was relevant to the question(s) they were investigating and 
51 

The California Spotted Owl: Current State of Knowledge J. Kane 

Figure 3-1—Light detection and ranging (LiDAR) point cloud data of 1 ha (2.47 ac) illustrating 

multidimensional forest structure at a California spotted owl nest in a forest that burned at low to 

moderate severity 6 years prior to this LiDAR collection in Yosemite National Park, California. Tree 

heights are represented along a continuous color gradient with warmer colors (yellow to red) showing 

increasing crown height and bright blue showing ground level. The nest tree (50 m [167 ft] tall and 

172 cm [68 in] diameter at breast height), is the tallest tree in the stand and located just northwest of 

center (see white arrow). 

the methods they applied. Gutiérrez et al. (1992) reported that compared to random 

stands, nest stands had greater basal area of live trees and snags (42 to 80 m2/ha 

[185 to 350 ft2/ac] and 4 to 7 m2/ha [19 to 31 ft2/ac], respectively) and often had an 

abundance of large coarse woody debris (i.e., logs and large limbs on the ground). 

The association of large trees and snags and high canopy cover and closure were 

consistent regardless of the amount of area measured at the nest stand, which varied 

among studies (e.g., 0.04 ha [0.1 ac] in Moen and Gutiérrez 1997; 0.2 ha [0.5 ac] in 

North et al. 2000, Blakesley et al. 2005, and Roberts et al. 2011; or 40 ha [99 ac] in 

Chatfield 2005). Importantly, numerous studies showed that owl site occupancy and 
52 

GENERAL TECHNICAL REPORT PSW-GTR-254

adult survivorship increased with a greater proportion of area of the nest stand containing these critical nest stand characteristics (e.g., high canopy cover or closure 

and basal area) (Blakesley et al. 2005, Chatfield 2005, Franklin et al. 2000, Roberts 

et al. 2011, Seamans and Gutiérrez 2007, Tempel et al. 2014). 

Specific nest stand characteristics are highly correlated with juvenile spotted 

owl habitat selection. During the postfledging rearing period (after fledging and 

before dispersal), juveniles roosted within 800 m (875 yd) of the nest and in areas 

with high canopy closure (≥70 percent) and snag density (Whitmore 2009). Whitmore (2009) also estimated the mean area encompassing juvenile roosts was 125 ha 

(308 ac) suggesting this area around the nest provides critical habitat during a sensitive time (i.e., juvenile rearing). The complex vertical structure in late-successional 

forests (e.g., multiple layers in the mid- and upper canopy) provides deeper shading 

and protects juvenile and adult owls from overheating in areas that frequently reach 

38 °C (100 °F) in summer (Barrows 1981, Weathers et al. 2001). This complex 

vertical canopy structure may also protect owls from predation. Phillips et al. 2010 

showed owls select nest sites that are farther from high-contrast edges (i.e., mature 

forest patches that abruptly change to shrub-dominated or early-seral patches) than 

expected by chance despite other researchers observing owls foraging in those edge 

habitats. 

Core Area Habitat Characteristics 

As central-place foragers, spotted owls concentrate their activities around nests 

and roosts, with foraging activity reduced the farther they get from their nest or 

roost (Carey et al. 1992, Ward et al. 1998). This concentrated use area is commonly 

referred to as the “core area,” which is the amount of habitat a territorial owl or pair 

and young use consistently, including the nesting, roosting, and foraging habitat that 

contains vital habitat characteristics essential to each pair’s survival and reproductive success (Bingham and Noon 1997, Blakesley et al. 2005, Rosenberg and 

McKelvey 1999, Swindle et al. 1999, Williams et al. 2011). The core area is smaller 

than a home range, which is all of the area used by an individual owl. 

Researchers have applied various criteria to identify and represent owl core 

use areas for the purpose of habitat analysis. Commonly, to delineate an area for 

habitat analysis that would be used by a territorial pair (by reducing spatial overlap 

between neighboring pairs), researchers apply either half of the minimum (0.8 km 

[0.5 mi]; Blakesley et al. 2005), or the average (1.1 km [0.7 mi]; Seamans 

and Gutiérrez 2007, Tempel et al. 2014) distance between adjacent nests (i.e., 

nearest-neighbor distance) as the radius to define their core area. These two 

examples define core areas of 203 ha (500 ac) and 400 ha (988 ac), respectively. 
53 

The California Spotted Owl: Current State of Knowledge

If radiotelemetry data is available, researchers can refine their core area sizes by 

using actual owl location data rather than estimating core use areas via distances 

between nests (e.g., Bingham and Noon 1997). Alternatively, Berigan et al. (2012) 

used 95 percent of each owl’s locations to delineate a core area and averaged across 

all 38 of their radiotagged owls to define an average core area of 140 ha (347 ac) for 

their study. 

Regardless of the amount of area different researchers use to define owl core 

area, the results of habitat analyses based on these defined areas demonstrate consistency in habitat characteristics of owl core areas. Occupancy, site colonization, 

adult survival, and reproductive success are positively associated with the proportion of the core area containing structurally complex conifer forest with large trees 

and high canopy cover (Blakesley et al. 2005, Seamans and Gutiérrez 2007, Tempel 

et al. 2014). Further, as the proportion of forest types that are not used for nesting 

(e.g., homogeneous forests consisting of only smaller, similar-aged young trees) 

increases in the core area, owl occupancy and reproductive success decline (Blakesley et al. 2005). However, the variation in the habitat classes available was relatively 

low (i.e., homogeneous habitat) where the non-nesting habitat mostly consisted of 

pole-sized stands, and there were not many other habitat types represented in their 

study area. This lack of variation in non-nesting habitat types could have potentially 

masked the influence of structural heterogeneity in core areas on owl occupancy 

and reproduction. Several other studies suggest that core areas of spotted owls 

have greater structural heterogeneity (e.g., increased edge between forest structure 

classes) than the nest stand and often include areas of lower canopy cover (e.g., 40 

to 70 percent, Call et al. 1992; 30 to 50 percent, Tempel et al. 2014) and a wider 

range of forest structure classes, including shrub/sapling patches and especially 

habitat patch edges (Eyes 2014, Tempel et al. 2014). This habitat heterogeneity can 

promote increased prey diversity, abundance, and population stability throughout 

the long owl breeding and juvenile dependency period (March through September) 

(Roberts et al. 2015). Studies of northern spotted owls suggest reproductive success is positively associated with foraging habitat quality, and fledging success 

improves with increasing prey abundance (Carey et al. 1992, Rosenberg et al. 2003). 

However, it is difficult to determine a threshold of heterogeneity and find a balance 

between habitat heterogeneity and minimal fragmentation. California spotted owl 

reproductive success is negatively correlated with the proportion of nonforested 

areas and forest types not used for nesting or foraging within the 203-ha (500-ac) 

core areas (Blakesley et al. 2005). Spotted owls may need a connected matrix of 

high canopy cover/closure throughout their core area to maintain protection from 

predators because they have to return to their nest or roost after foraging. Having 
54 

GENERAL TECHNICAL REPORT PSW-GTR-254

to cross large, open areas could expose them to predation, especially if those open 

areas are connected to areas inhabited by great horned owls (Bubo virginianus), 

known predators of spotted owls (Verner et al. 1992b). 

Foraging Habitat Characteristics 

Spotted owl foraging habitat is characterized by a mosaic of vegetation types and 

seral stages infused within mature forest (fig. 3-2). This juxtaposition of mature 

closed-canopy forest and open-canopy patches may promote higher prey diversity 

and abundance by increasing habitat diversity across the forest landscape (Franklin 

et al. 2000, Tempel et al. 2014, Ward et al. 1998, Zabel et al. 1995). This habitat 

mosaic is correlated with higher reproductive output and survival in northern spotted 

owls (Strix occidentalis caurina) (Franklin et al. 2000). Northern and California 

J. Kane 

Figure 3-2—Light detection and ranging (LiDAR) data illustrats canopy height modeling of 

an area equivalent to a spotted owl “Protected Activity Center” 121 ha (300 ac) in Yosemite 

National Park, California. The legend displays the modeled tree size classes in diameter at breast 

height for individual trees. The “cropped” corners are due to the confinement of the LiDAR data 

collection (the collection footprint) and have nothing to do with habitat structure or connectivity. 
55 

The California Spotted Owl: Current State of Knowledge

spotted owls forage in high-contrast edges more often than in interior patches (i.e., 

non-edges) characterized by greater structural homogeneity (Clark 2007, Eyes 2014, 

Folliard et al. 2000, Ward et al. 1998). In the Sierra Nevada, California spotted owls 

select edge habitat for foraging (Eyes 2014, Williams et al. 2011) suggesting that 

foraging owls exploit a heterogeneous forest matrix when foraging. These results 

are consistent with prey studies in the Sierra Nevada, suggesting small mammal 

diversity is enhanced by increased structural heterogeneity at large spatial scales and 

greater development of mature forest structure (Kelt et al. 2014, Roberts et al. 2015). 

Within the larger mosaic of vegetation types, contiguous patches of mature 

closed-canopy forests are an important characteristic of spotted owl foraging 

habitat. Williams et al. (2011) found foraging owls selected mature forests with 

higher canopy cover (≥40 percent) in greater proportion relative to its availability 

in the landscape. Mature forests with an abundance of large trees and patches of 

greater canopy cover and closure (generally >50 percent) provide both important 

roosting habitat for spotted owls and foraging habitat for northern flying squirrels 

(Glaucomys sabrinus), a principal prey species of spotted owls in Sierra Nevada forests (Meyer et al. 2007a, 2007b; Roberts et al. 2011, 2015; Waters and Zabel 1995). 

The inclusion of larger California black oaks (Quercus kelloggii Newberry) in these 

forests may also benefit dusky-footed woodrats (Neotoma fuscipes) (Innes et al. 

2007), another important spotted owl prey species. 

The enhancement of habitat heterogeneity without fragmenting existing mature 

closed-canopy forest represents a significant challenge in forest management 

(Stephens et al. 2010, 2014). One approach, based on retrospective analysis of fire 

effects, suggests creation of a dynamic mosaic of tree clumps and openings (≥0.3 ha 

[0.7 ac]) of variable sizes, shapes, spatial configurations, and seral stages (Kane et 

al. 2013). This approach can enhance forest resilience to fire and other disturbances 

and protect existing stands of mature, multicanopied forest that is preferred spotted 

owl habitat. However, fuel and restoration treatments designed to increase ecological resilience should strive to balance the short-term impacts of fuel reduction 

on habitat quality with the long-term benefits of these treatments (Stephens et al. 

2010, 2014). Of the number of forest treatments executed within owl foraging areas 

to reduce fuels, Gallagher (2010) showed foraging spotted owls avoided recently 

treated Defensible Fuel Profile Zones where the mechanical treatments create 

stands with widely and regularly spaced trees to reduce fire spread. Gallagher’s 

results were less clear for other fuel treatments (e.g., understory thinning), possibly 

due to a lack of statistical power to detect a treatment effect. These and other fuel 

treatments may fragment spotted owl habitat, especially when applied uniformly 
56 

GENERAL TECHNICAL REPORT PSW-GTR-254

across the forest landscape or in sensitive habitat areas (e.g., nest sites). Nest stands 

and owl core areas are especially important because California spotted owls forage 

close to the nest or roost (Eyes 2014, Gallagher 2010, Irwin et al. 2007). Moreover, 

Stephens et al. (2014) showed that landscape-level strategy of applying fuels treatments reduced the number of owl territories. Therefore, improving or maintaining 

forest structure in nest stands and core areas for both survival and reproduction 

(e.g., unfragmented, high canopy cover with some structural heterogeneity) could 

greatly benefit California spotted owls. Forest openings and habitat edges created 

by mechanical treatments or fire may enhance oak (Quercus spp.) and pine (Pinus 

spp.) regeneration and growth (Bigelow et al. 2011, York and Battles 2008). These 

forest openings are also associated with increased densities of woodrats, a largebodied prey species, and other spotted owl prey species (Innes et al. 2007, Kelt et 

al. 2014, Roberts et al. 2015), and owl fitness may be positively linked to woodrat 

abundance (Smith et al. 1999). Clearly, there is a key uncertainty in Sierra Nevada 

spotted owl biology concerning a balance of connectivity between forest patches 

with high canopy cover and adjacent forest openings and habitat edges. 

Prey Habitat Characteristics 

Habitat characteristics of most spotted owl prey remains largely unstudied in the 

Sierra Nevada, with limited additional information published since Williams et al. 

(1992). However, several recent studies have contributed to a better understanding 

of prey habitat characteristics, especially for northern flying squirrels, duskyfooted and big-eared (N. macrotis) woodrats, and deer (Peromyscus maniculatus) 

and brush mice (P. boylii). These combined species represent the primary prey of 

California spotted owls in the Sierra Nevada and elsewhere (e.g., southern California) (Williams et al. 1992). 

In the mid-elevation forests of the Sierra Nevada, northern flying squirrels 

are associated with mature forest stands with patches of moderate to high canopy 

closure (often exceeding 70 percent), large (>75 cm [30 in] d.b.h.) live or dead trees, 

thick (≥3 cm [1 in]) and extensively distributed litter layers, and sparsely distributed 

coarse woody debris or understory cover (e.g., shrubs and tall herbaceous plants) 

(Kelt et al. 2014; Meyer et al. 2005a, 2007; Pyare and Longland 2002; Roberts et 

al. 2015; Waters and Zabel 1995). Northern flying squirrels may select nesting or 

foraging sites in proximity to riparian habitat (Meyer et al. 2005a, 2007a, 2007b) 

or in moist mixed-conifer stands (Meyer et al. 2005a, Wilson et al. 2008). Riparian 

habitat is also associated with increased truffle (i.e., the fruiting bodies of ectomychorrizal fungi) (Meyer and North 2005) and tree hair lichen (Bryoria fremontii) 

(Rambo 2010) abundance, which compose the primary diet of northern flying 
57 

The California Spotted Owl: Current State of Knowledge

squirrels (Meyer et al. 2005b, Smith et al. 2007). Truffle diversity is also positively 

associated with proximity to riparian areas, which are generally characterized 

by wetter soils with denser vegetation (Meyer and North 2005). Although flying 

squirrel foraging habitat may be associated with coarse woody debris cover in many 

parts of its geographic range (Smith 2007), most studies in the Sierra Nevada find 

either no association (e.g., Meyer et al. 2007a, Pyare and Longland 2002) or a weak 

association between flying squirrel occurrence and coarse woody debris abundance 

(e.g., Kelt et al. 2014). Excessive or widespread woody debris and understory 

vegetation (e.g., saplings) may hinder movements of this volant species during 

foraging bouts or predator evasion (Kelt et al. 2014, Roberts et al. 2015), but sparse 

and spatially variable patches of woody debris (within the natural range of variation) may benefit flying squirrels by providing protective cover or foraging cues for 

truffles (e.g., Pyare and Longland 2001). Fire that occurs under the natural range of 

variation for the region will remove rotten down woody material, but much of the 

large, sound logs will remain after fire, providing sparse, spatially variable patches 

of woody debris (Knapp et al. 2005). 

In lower elevation forests, woodlands, and shrublands of the west-side Sierra 

Nevada, the dusky-footed woodrat (located in the northern Sierra Nevada), bigeared woodrat (located in the central and southern Sierra Nevada), and brush mouse 

are positively associated with oak cover or large oak (>33 cm [13 in] d.b.h.) density 

(Innes et al. 2007, Kelt et al. 2014, Roberts et al. 2008). Oaks (especially, California black oak) provide woodrats and brush mice with valuable food resources, 

especially acorns (Carraway and Verts 1991, Innes et al. 2007). Brush mice also 

tend to favor sites with greater herbaceous plant or shrub cover (Kelt et al. 2014, 

Laudenslayer and Fargo 2002) and may also be associated with riparian areas or 

dense clumps of tanoak (Lithocarpus densiflorus) (Amacher et al. 2008). Duskyfooted woodrats and brush mice exhibit moderate avoidance of areas with greater 

canopy cover, tree basal area, and large snag densities, especially at broader spatial 

scales; although woodrats may favor these habitat features at finer scales (Kelt et 

al. 2014) as well as logs and steep slopes (Innes et al. 2007). These scale-dependent 

habitat features emphasize the importance of promoting broad-scale structural 

heterogeneity and habitat complexity for small-mammal communities (Kelt et al. 

2014, Roberts et al. 2008). 

The deer mouse occupies a diverse array of habitats in lower and upper montane forest, woodland, and shrubland habitats of the Sierra Nevada (Verner and 

Boss 1980). This habitat generalist species is also one of the most numerous and 

widespread of all small mammals in North America with highly variable habitat 

associations across the Sierra Nevada (e.g., Amacher et al. 2008; Coppeto et al. 
58 

GENERAL TECHNICAL REPORT PSW-GTR-254

2006; Kelt et al. 2014; Monroe and Converse 2006; Roberts et al. 2008, 2015). 

Studies of the short-term effects of mechanical thinning or fire on deer mice are 

also varied in the Sierra Nevada, with posttreatment responses ranging from positive to negative to inconsequential. However, most studies agree that the effects of 

mechanical and prescribed fire treatments on deer mouse populations are either 

negligible or short lived, both in the Sierra Nevada (Stephens et al. 2014) and across 

the larger United States (Converse et al. 2006). 

A few recent studies provide insights in the habitat use patterns of flying 

squirrels and deer mice in burned landscapes of the Sierra Nevada. Roberts et al. 

(2015) found unburned refugia (i.e., unburned patches within fire perimeters) were 

positively associated with northern flying squirrels in mid-elevation forests of 

Yosemite National Park. Unburned patches and low- to moderate-severity fire may 

also promote truffle diversity across these forest landscapes in Yosemite (Meyer 

et al. 2008). In contrast, greater fire severity (and mechanical thinning intensity) 

eliminates suitable habitat for flying squirrels by removing tree canopy cover, 

overall biomass, and litter depth below thresholds generally suitable for this species 

(e.g., ≤55 percent canopy cover) (Lehmkuhl et al. 2006, Meyer et al. 2007a, Roberts 

et al. 2015). In contrast to flying squirrels, deer mice occupy a variety of burned 

and unburned habitats in lower and upper montane habitats of the Sierra Nevada, 

but respond negatively to increased fire severity in mid-elevation forests of Yosemite (Roberts et al. 2008, 2015). Information pertaining to fire effects on woodrats 

is currently lacking in the Sierra Nevada, although Lee and Tietje (2005) found 

virtually no effect of prescribed fire on dusky-footed woodrat demography in the 

Central Coast Range of California. 

Home Range Characteristics 

A home range is defined as the area used by an individual to meet its requirements 

for survival and reproduction (to distinguish from “territory” see chapter 2) and 

understanding home range requirements is essential for the conservation of a species. Theoretically, smaller home ranges should be of greater habitat quality because 

individuals expend less energy to satisfy their needs (McNab 1963). For higher level 

trophic predators such as spotted owls, large home ranges are typical for a variety 

of reasons (see chapter 2 for details). 

California spotted owls establish and defend large, year-round home ranges that 

contain higher habitat diversity than their northern subspecies (Forsman et al. 1984, 

Gutiérrez et al. 1992, Moen and Gutiérrez 1997, Verner et al. 1992b). Home range 

size estimates vary among studies (634 to 2195 ha [1,567 to 5,423 ac]), study area 

(latitude), and individual owls (table 3-1). Generally, California spotted owl home 
59 

The California Spotted Owl: Current State of Knowledge

ranges are largest in the northern Sierra Nevada and smallest in the southern Sierra 

Nevada. In the southern Sierra Nevada, specifically Sierra National Forest, where 

oaks are the dominant tree, owl home ranges are significantly smaller (Zabel et al. 

1992). Home range size is similar between years, sexes (Eyes 2014, Gallagher 2010, 

Williams et al. 2011, Zabel et al. 1992), and seasons, but there are often seasonal 

shifts in territorial delineations among neighboring pairs (Zabel et al. 1992). Owl 

home ranges frequently include heterogeneity and habitat edges; however, increases 

in heterogeneity lead to increases in home range size, suggesting a negative correlation of too much heterogeneity on habitat quality (Eyes 2014, Williams et al. 2011). 

Consistently across studies and study areas, owl home ranges contain a greater 

abundance of large trees and greater proportion of mature forest than is randomly 

available across the landscape (Call et al. 1992, Moen and Gutiérrez 1997, Williams 

et al. 2011). Owls will forage in patches of smaller sized trees (“pole-sized” 15 to 

28 cm [6 to 11 in] d.b.h.), but the presence of residual, large (super-canopy) trees 

greatly influenced owl use (Bias and Gutiérrez 1992, Moen and Gutiérrez 1997, 

Williams et al. 2011). Although there is substantial variation among individual owls, 

Williams et al. (2011) found that the average home range in their study was comprised of patches of low canopy cover (11.8 percent), hardwood forest (3.5 percent), 

pole-size conifer forest with ≥40 percent canopy cover (6.3 percent), mediumsized (28.1 to 61 cm [11.1 to 24 in] d.b.h.) conifer forest with >70 percent canopy 

cover (47.1 percent), mature (>61 cm d.b.h.) forest with >70 percent canopy cover 

(10.7 percent) and mature forest with 40 to 70 percent canopy cover (1.6 percent). 

However, their study reflects an area with limited availability of patches of mature 

forest >30 ha (74 ac) owing to timber harvesting, and this forest type may have been 

underrepresented in terms of owl selection (Williams et al. 2011). Further, when 

investigating the habitat type composition of owl home ranges in heavily managed 

forests, the results are confounded by what habitat types are available to the owl and 

do not truly reflect spotted owl preferences. 

Delineating the proportions and configuration of habitat patches in owl home 

ranges is nearly impossible using ground-based data because of the large-scale, 

landscape-level habitat metrics necessary for the analyses. Therefore, researchers 

typically use remotely sensed data, most commonly derived from satellites (see 

chapter 6 for details on remote sensing). However, vegetation maps available at this 

scale are often inaccurate, especially for residual trees (Moen and Gutiérrez 1997, 

Williams et al. 2011). Further research is needed to determine the size, composition, 

and configuration of habitat patches contained in an owl’s average home range. The 

use of light detection and range (LiDAR) technology can greatly assist this research 
60 

GENERAL TECHNICAL REPORT PSW-GTR-254

(see chapter 6). For example, important forest characteristics such as canopy cover 

and tree heights (fig. 3-1) can be quantified within spotted owl home ranges (e.g., 

fig. 3-2). 

Effects of Fire on Spotted Owl Habitat 

Fire is a dynamic ecological process in Sierra Nevada forests that varies greatly 

over space and time (Sugihara et al. 2006, van Wagtendonk and Lutz 2007). The 

effects of fire on spotted owl habitat are complex because fire burns heterogeneously across the landscape, resulting in a mosaic of variable fire severities (please 

refer to chapter 5 for more details on the regime and natural range of variation for 

fire frequency and severity for the Sierra Nevada). In low-fire-severity patches, fire 

consumed the surface fuels (e.g., low vegetation, coarse woody debris, and litter) 

and many shrubs and some small trees, but in these patches, nearly all canopy trees 

survived (Key and Benson 2005). In moderate-severity patches, fire consumed 

most of the surface fuels and small trees, as well as removed up to 75 percent of 

the canopy trees. In high-severity patches, all of the surface fuels were consumed 

by fire as well as nearly all mature plants, including >75 percent of canopy trees 

as determined from ground-based measurements (Composite Burn Index) (Key 

and Benson 2005) or >95-percent reduction in tree basal area or canopy cover as 

determined from remotely sensed data (Relative Differenced Normalized Burn 

Ratio) (Miller et al. 2009). In Yosemite National Park (central Sierra Nevada), 

where forests have a very minimal history of mechanical treatments, managers 

have allowed fires (prescribed and wild) to burn since the 1970s. Under the natural 

fire regime for mixed-conifer forests in Yosemite, with fires burning every 2 to 14 

years that resulted in a mosaic of low to moderate fire severities, fire had no effect 

on spotted owl occupancy (Roberts et al. 2011). Further, although their study did 

not differentiate the fire-severity proportions within their burned areas, Bond et al. 

(2002) found that fire did not negatively affect spotted owl pair bonds, site fidelity, 

or reproductive success. High-severity patches, however, affected colonization on 

two territories in another area in the central Sierra Nevada, but did not affect territory extinction (Tempel et al. 2014), although it is unknown how their results may 

or may not be confounded by postfire salvage logging of their study area. Fires that 

result in large patches of high-severity fire significantly reduce owl colonization, 

occupancy, and use of these forest types (Eyes 2014, Roberts et al. 2011, Tempel et 

al. 2014). In southern California, Lee et al. (2013) found that owl extinction probability increased as high-fire-severity patches exceeded 50 ha (123.5 ac). In Yosemite National Park, the largest high-severity patch size foraging owls used more than 
61 

The California Spotted Owl: Current State of Knowledge

once was 36.0 ha (89.0 ac), and the mean high-severity patch size used by foraging 

owls was 6.5 ha (16.1 ac) (SE = 10.5 ha [25.9 ac]) (Eyes 2104). Ideally, fire-resilient 

landscapes that contain contiguous patches of closed-canopy mature forest embedded with smaller forest openings and variable forest structure and composition (e.g., 

presence of large oaks) may sustain long-term foraging opportunities for spotted 

owls. A landscape with this forest structure would be largely consistent with the 

currently understood forest structure under a natural fire regime for this region 

(van Wagtendonk and Lutz 2007). Indeed, fires that burn within the natural range 

of variation for the Sierra Nevada, such as frequent low to moderate fires, tend to 

maintain habitat characteristics (e.g., retention of large trees and higher canopy 

closure) essential for spotted owl occupancy (Roberts et al. 2011). 

Restoring and maintaining forest resilience to fire is currently a major concern 

for forest managers, especially when considering the needs of sensitive species 

such as the California spotted owl. The closed-canopy forests that are important 

to spotted owl occupancy and nesting, tend to have spatially contiguous high fuel 

volumes that increase the vulnerability of these forests to uncharacteristically large 

and severe fires (Agee and Skinner 2005, Agee et al. 2000, Weatherspoon et al. 

1992). The impacts of climate change, longer fire seasons, and extended droughts, 

compounded by a century of fire suppression, have led to larger and more severe 

fires across the range of the California spotted owl, most notably in mixed-conifer 

forests (Mallek et al. 2013, Miller and Safford 2012). These trends are critical, 

because while California and northern spotted owls will forage throughout burned 

forests, they tend to avoid large high-severity patches (Clark 2007, Eyes 2014). 

Additionally, the abundances of many owl prey species (e.g., northern flying squirrel, deer mouse) are negatively correlated with fire severity (Roberts et al. 2008, 

2015). In contrast, Bond et al. (2009) reported that owls frequently used highseverity patches for foraging, but based their conclusion on a limited owl sample 

size and a single year (4 years after the fire) of postfire data, which may fail to 

account for potential time-lag responses of a territorial species with high site 

fidelity. Since the completion of their brief study, anecdotal observations indicate 

that at least one of their four study owls abandoned their territory within the burn, 

switched mates, and shifted their habitat use away from high-severity patches.2 

However, while owls may be avoiding the interior of these high-severity patches, 

they will forage in the high-contrast edges created by high-severity fire (Eyes 2014), 

further suggesting that habitat heterogeneity may be important to owls. The 

2 Galloway, R. 2015. Personal communication. Wildlife biologist, Sequoia National Forest, 

1839 S Newcomb St., Porterville, CA 93257. 
62 

GENERAL TECHNICAL REPORT PSW-GTR-254

balance between enough habitat heterogeneity for successful owl foraging and too 

much heterogeneity leading to owl habitat fragmentation remains elusive. Importantly, there may need to be an essential connection between the juxtaposition of 

those edges to forest with dense canopy for spotted owls to avoid depredation. The 

only two cases of observed spotted owl depredation in Yosemite National Park 

occurred along high-contrast edges created by recent (<5 years) high-severity fire 

(Roberts pers. obs.). 

New Findings Relative to Management Guidelines 

The current standards and guidelines used by the U.S. Forest Service (USFS) to 

manage California spotted owl (USDA FS 2004) are founded on the core area 

concept, as described in the California spotted owl report (Verner et al. 1992b). 

Based on the recommendations of Verner et al. (1992b), the Sierra Nevada Forest 

Plan Amendment (i.e., Sierra Framework; USDA FS 2004) formally established the 

121-ha (300-ac) protected activity centers (PACs) that USFS biologists delineate 

around a spotted owl activity center, such as a nest (fig. 3-2). These PACs were 

designed to include either the observed or the suspected nest stands and the best 

available habitat in a contiguous and compact arrangement. These designated areas 

are managed to contain: 

• ≥ two layers of tree canopy

• ≥ 60 percent canopy cover

• An average d.b.h. ≥61 cm (24 in) for the dominant and codominant trees

• Some snags ≥114 cm (45 in) d.b.h.

• Higher than average volume of snags and down woody debris 

Biologists designate a home range core area (HRCA) around each PAC, and the 

sizes of HRCAs are based on the average breeding pair home range of spotted owls 

(USDA FS 2004). Because spotted owl home range sizes increase with latitude, 

managers vary sizes of HRCAs as follows: 243 ha (600 ac) on the Sequoia and 

Sierra National Forests; 405 ha (1,000 ac) on the Modoc, Inyo, Humboldt-Toiyabe, 

Plumas, Tahoe, Eldorado, and Stanislaus National Forests, and the southern district 

of the Lassen National Forest; and 971 ha (2,400 ac) on the northern two districts 

of the Lassen National Forest. Managers attempt to maintain or develop desired 

conditions within HCRAs using five criteria: 

• ≥ two layers of tree canopy

• ≥50 percent canopy cover

• ≥61 cm (24 in) d.b.h. for the dominant and codominant trees

• A “number” of live trees >114 cm (45 in) d.b.h. 

• Higher than average volume of snags and down woody debris 
63 

The California Spotted Owl: Current State of Knowledge

These are the areas USFS managers must consider, as defined by the existing 

forest plan standards and guidelines, when developing forest treatment prescriptions, especially for mechanical treatments. Unless exempted for specific reasons, 

the USFS generally avoids mechanical treatments inside PACs, but prescribed fire 

can be used inside a PAC, and any management activity (though typically limited) 

can occur in HRCAs. 

Research investigating the efficacy of USFS spotted owl PACs in protecting 

essential habitat around owl activity centers (i.e., nests or roosts) is limited. Berigan 

et al. (2012) found that PACs, as estimated and updated by USFS staff following the 

directives established in the Sierra Framework (USFS 2004), protected essential 

high-use habitat for California spotted owls. They showed that the mean PAC area 

(116.3 ± 3.4 ha [287.5 ± 8.4 ac]) for 29 owls was similar to the mean size of their 

estimated core areas actually used by those same 29 owls (135.4 ± 31.9 ha [334.7 ± 

78.8 ac]) over 24 years of observations. They also found 70 percent spatial overlap 

between delineated PACs and observed use areas using 90 percent of the locations 

for each individual of a pair. 

Research has yet to provide an estimate of the threshold value for the amount 

of mature or late-successional conifer forests that is required to support a pair of 

spotted owls. However, habitat alteration (e.g., mechanical tree removal) involving 

≥20 ha (49 ac) of a 121-ha PAC was negatively correlated with site colonization 

and occupancy (Seamans and Gutiérrez 2007). Seamans and Gutiérrez (2007) 

also suggested that this human-caused habitat alteration was correlated with either 

decreased owl survival or increased emigration from their study population. These 

researchers did not use radiotelemetry to follow their study owls, thereby making 

it difficult to know the fate (i.e., survival) of an owl that abandoned its territory. 

Regardless of their true fate, it is concerning when owls disappear from their longestablished territories after mechanical treatments of ≥20 ha (49 ac) occurred within 

their PAC. 

Chapter Summary 

• Fuel and forest restoration treatments, including the use of fire, could 

attempt to balance the short-term impacts of these treatments on habitat 

quality with the long-term benefits to the ecosystem. 

• Although one study showed that the current size for spotted owl PACs (121 

ha [300 ac]) may be adequate to protect current core use areas, there is insufficient evidence (i.e., large-scale experimental research) to ascertain whether 

PACs provide long-term spotted owl persistence on national forest lands. 
64 

GENERAL TECHNICAL REPORT PSW-GTR-254

• All of the research strongly indicates that large, old trees are important 

aspects of spotted owl habitat, providing complex vertical structure and 

canopy layering as well as potential nesting cavities. Although the presence 

of large trees alone is insufficient for the persistence of spotted owls, restoration treatments that prioritize the retention of large and old trees, even 

in marginal habitat, can form the foundation for future high-quality habitat 

where the site potential is adequate. 

• Conservation efforts would be enhanced by prioritizing areas on the landscape that may enable the protection of spotted owl habitat from standreplacing fire. This could include the strategic identification of areas 

targeted for (1) fuel treatments to reduce wildfire risk to occupied forest landscapes and (2) protection objectives during incident management 

to minimize the impacts of wildfire and fire management operations to 

critical habitat. To begin this landscape prioritization, there is a need for 

accurate, landscape-level vegetation maps and a better understanding of 

the importance of vegetation types (and their patch sizes) to spotted owl 

occupancy, reproduction, and long-term population persistence and viability. Using accurate vegetation maps to identify important habitat needs to 

be coupled with our understanding of fire behavior across the landscape. 

It may be important to incorporate in our forest restoration planning how 

topography will affect fire behavior and how fire and topography will interact with the vegetation to influence the fire effects in an area. There are 

tools available (e.g., ArcFuels; http://www.arcfuels.org/) that could act as a 

place to start for managers to assess wildfire risk and aid in fuels management planning. 

• Forest restoration treatments may increase the abundance of spotted owl 

prey by promoting late-seral forest conditions, vegetation heterogeneity, 

and shrub and oak patches. In addition, managing fires for a mosaic of 

burn severities (dominated by low- and moderate-severity patches), including contiguous patches of unburned refugia, promotes suitable habitat for 

diverse small-mammal assemblages including northern flying squirrels, 

deer mice, and woodrats. 

• Wildland fires (prescribed fire and wildfire) that burn primarily at low to 

moderate severity (including unburned patches) likely maintain spotted owl 

occupancy while increasing resilience of the forest landscape in the long 

term. Although high-severity (i.e., stand-replacing) fires may also benefit 

spotted owls in smaller patches and proportions more consistent with the 

natural range of variation, large high-severity-burn patches may significantly 
65 

The California Spotted Owl: Current State of Knowledge

curtail habitat use and occupancy and long-term persistence of suitable 

nesting and roosting habitat. There is insufficient information available to 

allow a determination of the potential threshold responses of spotted owls 

to high-severity fire. 

• Managers focusing forest treatments on enhancing spotted owl habitat may 

wish to juxtapose nesting or roosting habitat structures in some stands (or 

larger habitat patches) and foraging habitat in others, keeping in mind that 

it is important to maintain a balance to minimize habitat fragmentation. 

Consider using the biophysical environment (e.g., topography, soils, and 

climate water deficit) as well as fire behavior and crew safety to guide the 

treatment placement and prescriptions. 

• For stands where the enhancement of nesting or roosting habitat is the 

objective, the research reviewed above suggests increasing or maintaining 

the abundance of large live trees and snags and canopy cover with complex 

layering. In stands where the promotion of foraging habitat is the objective, the research reviewed above suggests facilitating shrub or hardwood 

patches, large oaks, and small canopy gaps that provide sufficient edge 

habitat and foraging opportunities. Forest landscapes that contain a greater 

proportion of mature forest with old and large trees will provide more suitable habitat for spotted owls. 

Literature Cited 

Agee, J.K.; Bahro, B.; Finney, M.A.; Omi, P.N.; Sapsis, D.B.; Skinner, C.N.; 

van Wagtendonk, J.W.; Weatherspoon, C.P. 2000. The use of fuel breaks in 

landscape fire management. Forest Ecology and Management. 127: 55–66.

Agee, J.K.; Skinner, C.N. 2005. Basic principles of forest fuel reduction 

treatments. Forest Ecology and Management. 211: 83–96. 

Amacher, A.J.; Barrett, R.H.; Moghaddas, J.J.; Stephens, S.L. 2008. 

Preliminary effects of fire and mechanical fuel treatments on the abundance of 

small mammals in the mixed-conifer forest of the Sierra Nevada. Forest Ecology 

and Management. 255: 3193–3202. 

Barrows, C.W. 1981. Roost selection by spotted owls: an adaptation to heat stress. 

Condor. 83: 302–309. 

Berigan, W.J.; Gutiérrez, R.J.; Tempel, D.J. 2012. Evaluating the efficacy of 

protected habitat areas for the California spotted owl using long-term monitoring 

data. Journal of Forestry. 110: 299–303. 
66 

GENERAL TECHNICAL REPORT PSW-GTR-254

Bias, M.A.; Gutiérrez, R.J. 1992. Habitat association of California spotted owls in 

the central Sierra Nevada. Journal of Wildlife Management. 56: 584–595. 

Bigelow, S.; North, M.; Salk, C. 2011. Using light to predict fuels-reduction and 

group-selection effects on succession in Sierran mixed-conifer forest. Canadian 

Journal of Forest Research. 41: 2051–2063. 

Bingham, B.B.; Noon, B.R. 1997. Mitigation of habitat “take”: application to 

conservation planning. Conservation Biology. 11: 127–139. 

Blakesley, J.A.; Noon, B.R.; Anderson, D.R. 2005. Site occupancy, apparent 

survival, and reproduction of California spotted owls in relation to forest stand 

characteristics. Journal of Wildlife Management. 69: 1554–1654. 

Bond, M.L.; Gutiérrez, R.J.; Franklin, A.B.; LaHaye, W.S.; May, C.A.; 

Seamans, M.E. 2002. Short-term effects of wildfires on spotted owl survival, 

site fidelity, mate fidelity, and reproductive success. Wildlife Society Bulletin.

30: 1022–1028. 

Bond, M.L.; Lee, D.E.; Siegel, R.B.; Ward, J.P. 2009. Habitat use and selection 

by California spotted owls in a postfire landscape. Journal of Wildlife Management. 73: 1116–1124. 

Call, D.R.; Gutiérrez, R.J.; Verner, J. 1992. Foraging habitat and home-range 

characteristics of California spotted owls in the Sierra Nevada. Condor. 

94: 880–888. 

Carey, A.B.; Horton, S.P.; Biswell, B.L. 1992. Northern spotted owls: influence

of prey base and landscape character. Ecological Monographs. 62: 223–250. 

Carraway, L.N.; Verts, B.J. 1991. Neotoma fuscipes. Mammalian Species. 

386: 1–10. 

Chatfield, A.H. 2005. Habitat selection by a California spotted owl population: 

a landscape scale analysis using resource selection functions. St. Paul, MN: 

University of Minnesota. 66 p. M.S. thesis. 

Clark, D.A. 2007. Demography and habitat selection of northern spotted owls 

in post-fire landscapes of southwestern Oregon. Corvallis, OR: Oregon State 

University. 218 p. M.S. thesis. 

Coppeto, S.A.; Kelt, D.A.; Van Vuren, D.H.; Wilson, J.A.; Bigelow, S. 2006. 

Habitat associations of small mammals at two spatial scales in the northern 

Sierra Nevada. Journal of Mammalogy. 87: 402–413. 
67 

The California Spotted Owl: Current State of Knowledge

Eyes, S.A. 2014. The effects of fire severity on California spotted owl habitat use 

patterns. Arcata, CA: Humboldt State University. 68 p. M.S. thesis. 

Folliard, L.B.; Reese, K.; Diller, L.V. 2000. Landscape characteristics of northern 

spotted owl nest sites in northwestern California. Journal of Raptor Research. 34: 

75–84. 

Franklin, A.B.; Gutiérrez, R.J.; Burnham, K.P. 2000. Climate, habitat 

quality, and fitness in northern spotted owl population in northwest California. 

Ecological Monographs. 70: 539–590. 

Gallagher, C.V. 2010. Spotted owl home range and foraging patterns following 

fuels-reduction treatments in the northern Sierra Nevada, California. Davis, CA: 

University of California. 58 p. M.S. thesis. 

Gutiérrez, R.J.; Verner, J.; McKelvey, K.S.; Noon, B.R.; Steger, G.N.; Call, 

D.R.; LaHaye, W.S.; Bingham, B.B.; Senser, J.S. 1992. Habitat relations of the 

California spotted owl. In: Verner, J.; McKelvey, K.S.; Noon, B.R.; Gutiérrez, 

R.J.; Gould, G.I., Jr.; Beck, T.W., tech. coords. The California spotted owl: 

a technical assessment of its current status. Gen. Tech. Rep. PSW-GTR-133. 

Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest 

Research Station: 79–98. 

Innes, R.J.; Van Vuren, D.H.; Kelt, D.A.; Johnson, M.L.; Wilson, J.A.; Stine, 

P.A. 2007. Habitat associations of dusky-footed woodrats (Neotonia fuscipes) in 

mixed-conifer forest of the northern Sierra Nevada. Journal of Mammalogy. 88: 

1523–1531. 

Irwin, L.L.; Clark, L.A.; Rock, D.C.; Rock, S.L. 2007. Modeling foraging 

habitat of California spotted owls. Journal of Wildlife Management. 71: 

1183–1191. 

Jennings, S.B.; Brown, N.D.; Sheil, D. 1999. Assessing forest canopies and 

understory illumination: canopy closure, canopy cover and other measures. 

Forestry. 72: 59–73. 

Kane, V.R.; North, M.P.; Lutz, J.A.; Churchill, D.J.; Roberts, S.L.; Smith, 

D.F.; McGaughey, R.J.; Kane, J.T.; Brooks, M.L. 2013. Assessing fire effects 

on forest spatial structure using a fusion of landsat and airborne LiDAR data in 

Yosemite National Park. Remote Sensing of the Environment. 151: 89–101. 
68 

GENERAL TECHNICAL REPORT PSW-GTR-254

Kelt, D.A.; Van Vuren, D.H.; Kelt, D.A.; Johnson, M.L.; Wilson, J.A.; Innes, 

R.J.; Jesmer, B.R.; Ingram, K.P.; Smith, J.R.; Bigelow, S.W.; Burnett, R.D.; 

Stine, P.A. 2014. Small mammals exhibit limited spatiotemporal structure in 

Sierra Nevada forests. Journal of Mammalogy. 94: 1197–1213. 

Key, C.H.; Benson, N.C. 2005. Landscape assessment (LA): sampling and 

analysis methods. In: Lutes, D.C., tech. ed. FIREMON: fire effects monitoring 

and inventory system. Gen. Tech. Rep. RMRS-GTR-164-CD. Ogden, UT: U.S. 

Department of Agriculture, Forest Service, Rocky Mountain Research Station: 

LA-1–LA-51 

Knapp, E.E.; Keeley, J.E.; Ballenger, E.A.; Brennan, T.J. 2005. Fuel 

reduction and coarse woody debris dynamics with early season and late 

season prescribed fire in a Sierra Nevada mixed conifer forest. Forest Ecology 

and Management. 208: 383–397. 

Laudenslayer, W.F., Jr.; Fargo, R.J. 2002. Small mammal populations and 

ecology in the Kings River sustainable forest ecosystems project area. In: Verner, 

J., ed. Proceedings of a symposium on the Kings River Sustainable Forest 

Ecosystem Project: progress and current status. Gen. Tech. Rep. PSW-GTR-183. 

Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest 

Research Station: 133–142. 

Lee, D.E.; Bond, M.L.; Borchert, M.I.; Tanner, R. 2013. Influence of fire and 

salvage logging on site occupancy of spotted owls in the San Bernardino and San 

Jacinto Mountains of southern California: fire and spotted owl dynamics. Journal 

of Wildlife Management. 77: 1327–1341. 

Lee, D.E.; Tietje, W.D. 2005. Dusky-footed woodrat demography and prescribed 

fire in a California oak woodland. Journal of Wildlife Management. 69:

1211–1220. 

Lehmkuhl, J.F.; Kistler, K.D.; Begley, J.S.; Boulanger, J. 2006. Demography 

of northern flying squirrels informs ecosystem management of western interior 

forests. Ecological Applications. 16: 584–600. 

Mallek, C.; Safford, H.; Viers, J.; Miller, J. 2013. Modern departures in fire 

severity and area vary by forest type, Sierra Nevada and southern Cascades, 

California, USA. Ecosphere. 4: 1–28. 

McNab, B.K. 1963. Bioenergetics and the determination of home-range size. 

American Naturalist. 97: 133–140. 
69 

The California Spotted Owl: Current State of Knowledge

Meyer, M.D.; Kelt, D.A.; North, M.P. 2005a. Fungi in the diets of northern flying 

squirrels and lodgepole chipmunks in the Sierra Nevada. Canadian Journal of 

Zoology. 83: 1581–1589. 

Meyer, M.D.; Kelt, D.A.; North, M.P. 2005b. Nest trees of northern flying 

squirrels in the Sierra Nevada. Journal of Mammalogy. 86: 275–280. 

Meyer, M.D.; Kelt, D.A.; North, M.P. 2007a. Microhabitat associations of 

northern flying squirrels in burned and thinned forest stands of the Sierra 

Nevada. American Midland Naturalist. 83: 1581–1589. 

Meyer, M.D.; North, M.P. 2005. Truffle abundance in riparian and upland mixedconifer forest of California’s southern Sierra Nevada. Canadian Journal of 

Botany. 83: 1015–1020. 

Meyer, M.D.; North, M.P.; Kelt, D.A. 2007b. Nest trees of northern flying 

squirrels in Yosemite National Park, California. Southwestern Naturalist. 

52: 157–161. 

Meyer, M.D.; North, M.P.; Roberts, S.L. 2008. Truffle abundance in recently 

prescribed burned and unburned forests in Yosemite National Park: implications 

for mycophagous mammals. Fire Ecology. 4: 105–114. 

Miller, J.D.; Knapp, E.E.; Key, C.H.; Skinner, C.N.; Isbell, C.J.; Creasy, R.M.; 

Sherlock, J.W. 2009. Calibration and validation of the relative differenced 

normalized burn ratio (RdNBR) to three measures of fire severity in the Sierra 

Nevada and Klamath Mountains, California, USA. Remote Sensing of the 

Environment. 113: 645–656. 

Miller, J.D.; Safford, H. 2012. Trends in wildfire severity: 1984 to 2010 in the 

Sierra Nevada, Modoc Plateau and southern Cascades, California, USA. Fire 

Ecology. 8: 41–57. 

Moen, C.A.; Gutiérrez, R.J. 1997. California spotted owl habitat selection in the 

central Sierra Nevada. Journal of Wildlife Management. 61: 1281–1287. 

Monroe, M.E.; Converse, S.J. 2006. The effects of early season and late season 

prescribed fires on small mammals in a Sierra Nevada mixed conifer forest. 

Forest Ecology and Management. 236: 229–240. 

North, M.P.; Steger, G.N.; Denton, R.; Eberlein, G.; Munton, T.E.; Johnson, K. 

2000. Association of weather and nest-site structure with reproductive success in 

California spotted owls. Journal of Wildlife Management. 64: 797–807. 
70 

GENERAL TECHNICAL REPORT PSW-GTR-254

 

Phillips, C.E.; Tempel, D.J.; Gutiérrez, R.J. 2010. Do California spotted owls 

selected nest tress close to forest edges? BioOne. 44: 311–314. 

Pyare, S.; Longland, W. 2001. Patterns of ectomycorrhizal fungi consumption by 

small mammals in remnant old growth forests of the Sierra Nevada. Journal of 

Mammalogy. 82: 681–689. 

Pyare, S.; Longland, W. 2002. Interrelationships among northern flying squirrels, 

truffles, and microhabitat structure in Sierra Nevada old-growth habitat. 

Canadian Journal of Forest Research. 32: 1016–1024. 

Rambo, T.R. 2010. Structure and composition of corticolous epiphyte communities 

in a Sierra Nevada old-growth mixed-conifer forest. The Bryologist. 113: 55–71. 

Roberts, S.L.; North, M.P. 2012. California spotted owls. In: North, M.P., ed. 

Managing Sierra Nevada forests. Gen. Tech. Rep. PSW-GTR-237. Albany, CA: 

U.S. Department of Agriculture, Forest Service, Pacific Southwest Research 

Station: 61–71. 

Roberts, S.L.; van Wagtendonk, J.W.; Miles, A.K.; Kelt, D.A. 2011. Effects 

of fire on spotted owl site occupancy in a late-successional forest. Biological 

Conservation. 144: 610–619. 

Roberts, S.L.; van Wagtendonk, J.W.; Miles, A.K.; Kelt, D.A. 2015. Effects of 

fire on small mammal communities in frequent-fire forests in California. Journal 

of Mammalogy. 96: 107–119. 

Roberts, S.L.; van Wagtendonk, J.W.; Miles, A.K.; Kelt, D.A.; Lutz, J.A. 2008. 

Modeling the effects of fire severity and spatial complexity on small mammals in 

Yosemite National Park, California. Fire Ecology. 4: 83–104. 

Rosenberg, D.K.; McKelvey, K.S. 1999. Estimation of habitat selection for 

central-place foraging animals. Journal of Wildlife Management. 63: 1028–1038. 

Seamans, M.E. 2005. Population biology of the California spotted owl in the 

central Sierra Nevada. St. Paul, MN: University of Minnesota. 141 p. Ph.D. 

dissertation. 

Seamans, M.E.; Gutiérrez, R.J. 2007. Habitat selection in a changing 

environment: the relationship between habitat alteration and spotted owl territory 

occupancy and breeding dispersal. Condor. 109: 566–576. 
71 

The California Spotted Owl: Current State of Knowledge

Smith, W.P. 2007. Ecology of Glaucomys sabrinus: habitat, demography, and 

community relations. Journal of Mammalogy. 88: 862–881. 

Smith, R.B.; Peery, M.Z.; Gutiérrez, R.J.; Lahay, W.S. 1999. The relationship 

between spotted owl diet and reproductive success in the San Bernardino 

Mountains, California. Wilson Bulletin. 111: 22–29. 

Stephens, S.L.; Bigelow, S.W.; Burnett, R.D.; Collins, B.M.; Gallagher, C.V.; 

Keane, J.; Kelt, D.A.; North, M.P.; Roberts, L.J.; Stine, P.A.; Van Vuren, 

D.H. 2014. California spotted owl, songbird, and small mammal responses to 

landscape fuel treatments. Bioscience. 64: 893–906. 

Stephens, S.L.; McIver, J.D.; Boerner, R.E.; Fettig, C.J.; Fontane, J.B.; 

Hartsough, B.R.; Kennedy, P.L.; Schwilk, D.W. 2010. The effects of forest 

fuel-reduction treatments in the United States. Bioscience. 62: 549–560. 

Sugihara, N.G.; van Wagtendonk, J.W.; Fites-Kaufman, J. 2006. Fire as an 

ecological process. In: Sugihara, N.G.; van Wagtendonk, J.W.; Shaffer, K.E.; 

Fites-Kaufman, J.; Thode, A.E., eds. Fire in California’s ecosystems. Berkeley, 

CA: University of California Press: 58–74. 

Swindle, K.A.; Ripple, W.J.; Meslow, C.E.; Schafer, D. 1999. Old-forest 

distribution around spotted owl nests in the central Cascade Mountains, Oregon. 

Journal of Wildlife Management. 63: 1212–1221. 

Tempel, D.J.; Gutiérrez, R.J.; Whitemore, S.A.; Reetz, M.J.; Stoelting, 

R.E.; Berigan, W.J.; Seamans, M.E.; Peery, M.Z. 2014. Effects of forest 

management on California spotted owls: implications for reducing wildfire risk 

in fire-prone forests. Ecological Applications. 24: 2089–2106. 

U.S. Department of Agriculture, Forest Service [USDA FS]. 2004. Sierra 

Nevada Forest Plan Amendment: final supplemental environmental impact 

statement. San Francisco, CA: Pacific Southwest Region. 

van Wagtendonk, J.W.; Lutz, J.A. 2007. Fire regime attributes of wildland fires in 

Yosemite National Park, USA. Fire Ecology. 3: 34–52. 

Verner, J.; Boss, A.S. 1980. California wildlife and their habitats: western Sierra 

Nevada. Gen. Tech. Rep. PSW-GTR-37. Berkeley, CA: U.S. Department of 

Agriculture, Forest Service, Pacific Southwest Forest and Range Experimental 

Station. 439 p. 
72 

GENERAL TECHNICAL REPORT PSW-GTR-254

Verner, J.; Gutiérrez, R.J.; Gould, G.I., Jr. 1992a. The California spotted owl: 

general biology and ecological relations. In: Verner, J.; McKelvey, K.S.; Noon, 

B.R.; Gutiérrez, R.J.; Gould, G.I., Jr.; Beck, T.W., tech. coords. The California 

spotted owl: a technical assessment of its current status. Gen. Tech. Rep. PSWGTR-133. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific 

Southwest Research Station: 55–77. 

Verner, J.; McKelvey, K.S.; Noon, B.R.; Gutiérrez, R.J.; Gould G.I., Jr.; Beck, 

T.W. 1992b. Assessment of the current status of the California spotted owl, with 

recommendations for management. In: Verner, J.; McKelvey, K.S.; Noon, B.R.; 

Gutiérrez, R.J.; Gould, G.I., Jr.; Beck, T.W., tech. coords. The California spotted 

owl: a technical assessment of its current status. Gen. Tech. Rep. PSW-GTR-133. 

Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest 

Research Station: 3–26. 

Ward, J.P., Jr.; Gutiérrez, R.J.; Noon, B. R. 1998. Habitat selection by northern 

spotted owls: the consequences of prey selection and distribution. Condor. 100: 

79–92. 

Waters, J.R.; Zabel, C.J. 1995. Northern flying squirrel densities in fir forests of 

northeastern California. Journal of Wildlife Management. 59: 858–866. 

Weathers, W.W.; Hodum, P.J.; Blakesley, J.A. 2001. Thermal ecology and 

ecological energetics of California spotted owls. Condor. 103: 678–690. 

Weatherspoon, C.P.; Husari, S.J.; van Wagtendonk, J.W. 1992. Fire and fuels 

management in relation to owl habitat in forests of the Sierra Nevada and 

southern California. In: Verner, J.; McKelvey, K.S.; Noon, B.R.; Gutiérrez, R.J.; 

Gould, G.I., Jr.; Beck, T.W., tech. coords. The California spotted owl: a technical 

assessment of its current status. Gen. Tech. Rep. PSW-GTR-133. Albany, CA: 

U.S. Department of Agriculture, Forest Service, Pacific Southwest Research 

Station: 247–260. 

Whitmore, S.A. 2009. Habitat use of juvenile California spotted owls (Strix 

occidentalis occidentalis) during the post-fledging dependency period in 

northeastern California. Chico, CA: California State University. 72 p. 

M.S. thesis. 
73 

The California Spotted Owl: Current State of Knowledge

Williams, D.F.; Verner, J.; Sakai, H.F.; Waters, J.R. 1992. General biology 

of major prey species of the California spotted owl. In: Verner, J.; McKelvey, 

K.S.; Noon, B.R.; Gutiérrez, R.J.; Gould, G.I., Jr.; Beck, T.W., tech. coords. 

The California spotted owl: a technical assessment of its current status. Gen. 

Tech. Rep. PSW-GTR-133. Albany, CA: U.S. Department of Agriculture, Forest 

Service, Pacific Southwest Research Station: 207–221. 

Williams, P.J.; Gutiérrez, R.J.; Whitemore, S.A. 2011. Home range and habitat 

selection of spotted owls in the central Sierra Nevada. Journal of Wildlife 

Management. 75: 333–343. 

Wilson, J.A.; Kelt, D.A.; Van Vuren, D.H. 2008. Home range and activity 

of northern flying squirrels (Glaucomys sabrinus) in the Sierra Nevada. 

Southwestern Naturalist. 53: 21–28. 

York, R.; Battles, J. 2008. Growth response of mature trees versus seedlings 

to gaps associated with group selection management in the Sierra Nevada, 

California. Western Journal of Applied Forestry. 23: 94–98. 

Zabel, C.J.; McKelvey, K.S.; Ward, J.P., Jr. 1995. Influence of primary prey 

on home-range size and habitat-use patterns of northern spotted owls (Strix 

occidentalis caurina). Canadian Journal of Zoology. 73: 433–439. 
74 

GENERAL TECHNICAL REPORT PSW-GTR-254
75 

The California Spotted Owl: Current State of Knowledge

Chapter 4: Population Distribution and Trends 

of California Spotted Owls 

Douglas J. Tempel, R.J. Gutiérrez, and M. Zachariah Peery1 

Distribution 

Geographic Range 

Following Verner et al.’s (1992) technical assessment of the California spotted owl 

(CASPO), we divided the range of the California spotted owl (Strix occidentalis 

occidentalis) into two major physiographic provinces: the Sierra Nevada and the 

mountains of southern California (Tehachapi Pass was the demarcation between 

the regions). Verner et al. (1992) noted that these provinces are geographically 

distinct and that movement of owls between them is probably rare, which remains 

true today (see “Population and Conservation Genetics of California Spotted Owls” 

section below). The California spotted owl is also found in the coastal mountains 

north to Monterey Bay, but much less is known about owl numbers and locations 

along the coast (see figs. 4-1 and 4-2). That portion of the southern Cascade Range 

that abuts the Sierra Nevada has been considered to encompass the range of the 

California spotted owl on the east side of California (see chapter 2). Where the 

ranges of the northern (S. o. caurina) and California spotted owls meet, a hybrid 

zone occurs in the area of contact near the Pit River (Barrowclough et al. 2011; see 

chapter 2). Hereafter, we refer to owls occurring south of the Pit River as belonging 

to the Sierra Nevada population of California spotted owls. 

Within the Sierra Nevada population, the distribution of owls is relatively 

contiguous. The majority of owls occur within the mid-elevation, mixed-conifer 

forests on the west slope of the Sierra Nevada. Some owls also occur at lower elevations in the oak woodlands of the western foothills in the southern Sierra Nevada, 

at higher elevations in red-fir forests, and in conifer forests on the eastern slope 

of the mountains (Verner et al. 1992). In contrast, the owl population in central 

and southern California is more fragmented because owls inhabit major mountain 

ranges and mountain complexes that are isolated to varying degrees, which limits 

movement of individuals among these mountain ranges. In this chapter, we focus 

almost solely on the Sierra Nevada population of owls while deferring discussion of 

southern California to chapter 9. However, when discussing general properties of 

1

 Douglas J. Tempel is a postdoctoral research associate, Department of Forest and 

Wildlife Ecology, University of Wisconsin, 1630 Linden Dr., Madison, WI 53706; R. J. 

Gutiérrez is a professor and Gordon Gullion Endowed Chair Emeritus, University of 

Minnesota, 2003 Upper Buford Circle, St. Paul, MN 55108; M. Zachariah Peery is an 

associate professor, Department of Forest and Wildlife Ecology, University of Wisconsin, 

1630 Linden Dr., Madison, WI 53706. 
76 

GENERAL TECHNICAL REPORT PSW-GTR-254

Figure 4-1—Unique detections of California spotted owls from 1900 through 1992 using databases provided by the California Department of Fish and Wildlife and Pacific Southwest Region of the U.S. Forest Service. 
77 

The California Spotted Owl: Current State of Knowledge

Figure 4-2—Unique detections of California spotted owls from 1993 through 2013 using databases provided by the California Department of Fish and Wildlife and Pacific Southwest Region of the U.S. Forest Service. 
78 

GENERAL TECHNICAL REPORT PSW-GTR-254

spotted owl population dynamics, we may also refer to other subspecies as well as 

southern California owl populations. 

Distribution of Owls and Gaps in Distribution 

Verner et al. (1992) noted that unlike the northern spotted owl, there were no obvious gaps in the distribution of the California spotted owl. This observation led them 

to recommend a conservation strategy based upon identification of habitat, protection of key habitat areas or activity centers around nests and roosts (i.e., protected 

activity centers PACs), and specific guidelines for timber harvest (restrictions on 

size of trees harvested, standards for tree basal area retention, and restrictions on 

canopy cover reductions; see chapter 1). To evaluate the CASPO premise of no gaps 

in the distribution, we obtained the California spotted owl databases from the California Department of Fish and Wildlife (CDFW) and the Pacific Southwest Region 

of the U.S. Forest Service (USFS). Both databases primarily included information 

for owl detections (i.e., mark-recapture or reproductive data were not consistently 

recorded), and many physical locations were represented by a large number of 

detections obtained over many years. Because we could not establish individual 

identities for most of the detections or, in many cases, even reliably assign detections to a specific owl territory, these databases cannot be used to infer trends in 

population size. However, they do provide a general, rangewide distribution of 

California spotted owls and some indication of the proportion of owls found on 

public versus private lands. Further examination of these databases showed that the 

CDFW database was missing many owl detections on USFS land, particularly after 

1993. Therefore, we combined the databases and attempted to eliminate duplicate 

detections (i.e., detections in the same geographic location on the same date). 

We estimated that there were 15,322 spotted owl detections prior to 1992 

(CASPO) and 34,365 detections from 1993 through 2013 (post-CASPO) (figs. 4-1 

and 4-2). The increase in the number of detections after 1992 was largely due to 

increased survey effort on national forest lands. The overall distribution of owls 

was largely similar for the two time periods (pre-1993 and 1993–2013), but there 

were noticeably fewer detections after 1992 within the Transverse Range north of 

Santa Barbara on the Los Padres National Forest. As noted above, there appeared 

to be a significant gap in the owl’s distribution between the Sierra Nevada and the 

mountains of southern and central California. In addition, there appeared to be gaps 

in the owl’s distribution between the major mountain ranges of southern and central 

California, particularly along the central coast. Most spotted owl detections were 

on public lands (88 percent prior to 1993, 87 percent from 1993 through 2013), and 

for both time periods >90 percent of the detections on public lands were within U.S. 
79 

The California Spotted Owl: Current State of Knowledge

national forests. Although there were clearly more California spotted owls on public 

lands, we could not determine how much of the observed difference in detections 

on public versus private lands was due to greater survey effort on public lands, 

particularly around proposed timber sales within U.S. national forests. Private lands 

may constitute an important component of California spotted owl habitat throughout its range, and owl conservation would benefit from the effective management of 

habitat on private lands. 

Demographic Rates 

History of Demographic Research in the Sierra Nevada 

Spotted owls exhibit high adult survival rates with low temporal variation, whereas 

their reproductive rates are low and vary greatly from year to year (Franklin et al. 

2000, Seamans and Gutiérrez 2007). Franklin et al. (2000) invoked these patterns 

as a “bet-hedging” life history strategy (Stearns 1976) where natural selection has 

favored the evolution of long lifespans to increase the likelihood that individuals 

will experience years that are favorable for reproduction (see also chapter 2). 

Data collected on five long-term California spotted owl study areas have provided substantial empirical data on demographic rates and population trends subsequent to CASPO (Verner et al. 1992). Of these five study areas, four were in the 

Sierra Nevada (see fig. 4-3)—three on national forests (Lassen, Eldorado, and Sierra) 

and one within Sequoia and Kings Canyon National Parks. Data collection began in 

1986 on the Eldorado and in 1990 on the other three study areas; all of these studies 

continued through 2014. The fifth study area was located on the San Bernardino 

National Forest in southern California where data were collected from 1987 through 

2000 (see fig. 4-3). Two meta-analysis workshops have been conducted to analyze 

demographic rates and population trends on the Sierra Nevada study areas (Blakesley et al. 2010, Franklin et al. 2004), but more recent studies have provided updated 

analyses that included additional data collected after the second meta-analysis 

(Conner et al. 2013, Tempel and Gutiérrez 2013, Tempel et al. 2014b). 

Additionally, Sierra Pacific Industries (SPI) recently initiated systematic surveys on five study areas throughout the Sierra Nevada where the company owned 

significant amounts of land (proportion of land owned by SPI ranged from 34 to 69 

percent).2

 Although Roberts et al. (see footnote 2) concluded that populations on 

2 Roberts, K.; Hall, W.E.; Shufelberger, A.J.; Reno, M.A.; Schroeder, M.M. 2015. The 

occurrence and occupancy status of the California spotted owl on Sierra Pacific Industries’ 

lands in the Sierra Nevada of California. 11 p. Unpublished document: On file with: Sierra 

Pacific Industries, 3950 Carson Rd., Camino, CA 95709. 
80 

GENERAL TECHNICAL REPORT PSW-GTR-254

Figure 4-3—Locations of California spotted owl demography studies in relation to forested habitat 

(shaded gray) throughout California. (Franklin et al. 2004; reproduced with permission of © American Ornithological Union). 
81 

The California Spotted Owl: Current State of Knowledge

their study areas were stable or increasing, we contend that their survey data are 

currently insufficient to assess population trends for several reasons: 

• Detection probabilities were not modeled. 

• Surveys were conducted over a limited number of years (2012–2014), 

whereas trends on the study areas discussed above took more than 10 years 

to detect because spotted owls show high site fidelity and are long lived. 

• Survey effort increased over time. 

• There typically is a “learning curve” associated with initiation of occupancy studies that yields an increase of occupied sites solely related to 

accumulated knowledge of field technicians. 

• Most of the owls were unmarked and thus could not be individually 

identified. 

Roberts et al. (2015) also reported higher owl densities on their study areas than 

the Lassen and Eldorado National Forest study areas. However, we caution that 

density is not always a reliable indicator of habitat quality because large numbers of 

owls may be maintained in “sink” habitats (i.e., within-habitat reproduction is insufficient to balance local mortality) by continued immigration from more productive, 

nearby areas of “source” habitat (Pulliam 1988). Moreover, they sampled relatively 

small study areas, and there is often an “edge effect” associated with areas that are 

small relative to the home range size of the species being monitored. Although it 

is possible that the areas surveyed by Roberts et al. (see footnote 2) contain stable 

populations, additional years of data, including data on individual identification, 

reproduction, and survival, would be needed to make this determination. 

Reproduction 

Blakesley et al. (2010) reported substantial variation in reproductive rates (number 

of young fledged per territorial female for which reproduction was assessed) among 

the four Sierra Nevada study areas, ranging from 0.48 on the Sierra to 0.99 on the 

Eldorado. Because different studies sometimes use different units of measurement, we have used caution when comparing reproductive rates among studies. 

For example, Franklin et al. (2004) used the number of female young fledged per 

territorial female (assuming a 50:50 sex ratio among offspring), whereas Blakesley et al. (2010) used the total number of young fledged per territorial female. 

In addition, Seamans (2005) found that differences in field protocols used by 

researchers on different study areas affected estimates of annual reproductive rates, 

particularly whether one or two nonreproduction protocols were needed to infer 

nonreproduction. Therefore, in contrast to Franklin et al. (2004), Blakesley et al. 
82 

GENERAL TECHNICAL REPORT PSW-GTR-254

(2010) standardized field protocols among the four study areas such that the criteria 

for inferring nonreproduction had to be met on at least two surveys in a given year. 

However, this stricter requirement (i.e., two surveys vs. one) eliminated many data 

observations from the Eldorado because there were many instances when only one 

nonreproduction protocol was available; in most of these instances, it was likely 

that owls did not reproduce. This removal of observations where reproduction 

likely equaled zero could bias reproductive analyses that incorporate covariates for 

territories or individual owls. MacKenzie et al. (2009) recommended analyses using 

multistate occupancy models that distinguished between surveys where reproduction was detected or not detected to make more efficient use of reproductive data. 

Blakesley et al. (2010) reported that reproduction declined over time on the 

Eldorado National Forest but was relatively constant on the other study areas. 

Furthermore, they found support for an even-odd (EO) year effect on reproductive 

rates for all four study areas (see chapter 2 for a discussion of this even-odd pattern) 

with the strongest pattern occurring for the Eldorado and Lassen National Forests; 

this pattern has also been reported for northern spotted owls (e.g., Forsman et al. 

2011). Thus, spotted owl reproduction in much of the Sierra Nevada appeared to follow an alternating pattern where years of relatively high reproduction were followed 

by years of relatively low reproduction, although there remained much variation 

not explained by the even-odd pattern. In addition, Stoelting et al. (2015) could not 

simulate the even-odd cycle in California spotted owls using a cost of reproduction 

estimated for the Eldorado (see chapter 2). 

Adult female California spotted owls (≥3 years old) have higher reproductive 

rates than subadult females (1 or 2 years old). For example, Blakesley et al. (2010) 

found that the annual proportion of subadult females among all territorial females 

had a strong negative correlation with reproductive rates on the Eldorado and Sierra 

National Forests. In addition, reproductive rates for adult females were much higher 

than those for subadult females on the Eldorado (Seamans and Gutiérrez 2007, 

Seamans et al. 2001, Tempel et al. 2014a) and Lassen Nationa Forests (Blakesley et 

al. 2001) in analyses that were independent of the two California spotted owl metaanalyses (Blakesley et al. 2010, Franklin et al. 2004). 

Reproductive rates have been correlated with climatic conditions, either during the previous winter or the early nesting period. Seamans and Gutiérrez (2007) 

reported that reproductive rates on the Eldorado were negatively correlated with El 

Niño events, which in California typically result in winters with greater precipitation and warmer temperatures than average. Additionally, they found that reproduction was negatively correlated with colder temperatures and greater precipitation 
83 

The California Spotted Owl: Current State of Knowledge

during incubation (April). Similarly, North et al. (2000) reported that colder temperatures and greater precipitation during the early breeding season (March to May) 

on the Sierra National Forest and Sequoia and Kings Canyon National Parks were 

negatively correlated with reproduction. Similar patterns have been observed for 

northern spotted owls (Franklin et al. 2000). These results have led to the hypothesis that colder temperatures and increased precipitation during the early nesting 

season negatively affect reproduction either by increasing the energetic requirements of owls, increasing the risk of egg exposure during incubation, or interfering 

with foraging (Franklin et al. 2000, Rockweit et al. 2012). 

Finally, reproductive rates have been correlated with habitat characteristics, 

both within owl territories and at nest sites. When assessing the relationship 

between demographic rates (e.g., reproduction, survival, or occupancy) and habitat, 

scientists have considered various spatial scales as reference points. For example, at 

least four spatial scales have been used: 

• The home range, which has been estimated from radiotelemetry locations 

• The territory (the area actively defended by resident owls), which has typically been assumed to be approximately half the mean nearest neighbor 

distance between territory centers 

• The core area of use within an animal’s home range, which is an area that 

receives concentrated use and is thought to encompass critical components 

such as nest sites, refugia, and foraging areas (Samuel et al. 1985) 

• The area immediately surrounding the nest site 

On the Lassen National Forest, Blakesley et al. (2005) assessed the relationship 

between reproductive output and the surrounding habitat within owl territories 

(estimated as 203 ha [508 ac] using half the mean nearest neighbor distance, which 

they referred to as the “nest area”). They found that reproduction was negatively 

correlated with the amount of nonforest or forests dominated by small trees (<30 

cm [12 in] diameter at breast height [d.b.h.]) and positively correlated with the 

amount of nesting habitat, which were forests dominated by medium (30 to 61 cm 

[12 to 24 in] d.b.h.) or large trees (>61 cm [24 in] d.b.h.) and having high canopy 

cover (>70 percent). Two different studies on the Eldorado National Forest found 

strong support for a negative correlation between reproduction and the amount of 

oak woodlands within owl territories (estimated as 150 ha [400 ac] using half the 

mean nearest neighbor distance) (Seamans 2005, Tempel et al. 2014a). On the Sierra 

National Forest, Hunsaker et al. (2002) reported a positive correlation between 

“productivity” and forests with >50 percent canopy cover at each of three different 

spatial scales (72 ha [178 ac], 168 ha [415 ac], and 430 ha [1,063 ac]) that roughly 
84 

GENERAL TECHNICAL REPORT PSW-GTR-254

corresponded to the home range, territory, and core area of use as defined above. 

The authors defined productivity as an index of reproductive output where productivity values at a territory ranged from zero to nine (0 = no owls present, 9 = nesting 

pair that produced three fledglings; see Hunsaker et al. [2002] for more details). At 

the spatial scale of the immediate nest area (0.05 ha [0.12 ac]), North et al. (2000) 

reported that reproduction was positively correlated with the foliage volume above 

the nest site. 

Survival 

Blakesley et al. (2010) reported high apparent survival of adult California spotted 

owls on the four Sierra Nevada study areas, ranging from 0.810 to 0.891. They also 

found that adults had higher annual survival rates than first- or second-year subadults and males have slightly higher survival rates than females. Higher survival 

rates for males (Seamans 2005, Seamans and Gutiérrez 2007, Tempel et al. 2014a) 

and adults (Tempel et al. 2014a) were also reported for the Eldorado National Forest 

in analyses independent from Blakesley et al. (2010). Data analyses for the Sierra 

Nevada studies have generally avoided estimation of juvenile survival rates because 

of potentially significant biases caused by undetected emigration of juveniles 

from the study areas (Burnham et al. 1996, Zimmerman et al. 2007). Estimates 

of nonjuvenile spotted owl survival have also been criticized as potentially biased 

because of undetected emigration of nonjuveniles (Boyce et al. 2005, Loehle et al. 

2005), but this bias has been shown to be negligible because nonjuvenile owls (in 

contrast to juveniles) rarely disperse from study areas as large as those in the Sierra 

Nevada (Zimmerman et al. 2007). LaHaye et al. (2004) estimated that apparent 

juvenile survival rates in an insular population in the San Bernardino Mountains 

(i.e., juvenile emigration rates from this mountain range were negligible) was 0.368, 

which was similar to that reported on the Lassen National Forest (0.333) (Blakesley 

et al. 2001). Of note was that Blakesley et al. (2001) designed their study to improve 

estimation of juvenile survival. 

Like reproduction, apparent survival has been correlated with habitat conditions within an owl territory. Blakesley et al. (2005) found that nonjuvenile survival 

was positively correlated with the amount of nesting habitat (see above) on the 

Lassen National Forest. In addition, Seamans (2005) and Tempel et al. (2014a) 

both reported that nonjuvenile survival rates on the Eldorado National Forest were 

positively correlated with the amount of forest dominated by medium (30 to 61 cm 

[12 to 24 in] d.b.h.) or large trees (>61 cm [24 in] d.b.h.) and having high canopy 
85 

The California Spotted Owl: Current State of Knowledge

cover (≥70 percent). Tempel et al. (2014a) also found a positive correlation between 

survival and the amount of edge between shrubs/saplings and forest, but the 95 

percent confidence interval on the beta coefficient overlapped zero. 

Seamans and Gutiérrez (2007) conducted the only study that assessed climate 

effects on survival of California spotted owls in the Sierra Nevada. They found that 

survival was positively correlated with snow depth, which was opposite of their a 

priori prediction. Their results further suggested a quadratic relationship between 

survival and the Southern Oscillation Index, such that survival was greatest in 

years that were not dominated by either El Niño or La Niña weather patterns. The 

Southern Oscillation Index is a measure of atmospheric pressure differences in the 

southern Pacific Ocean that provides an indication of the development and intensity 

of El Niño or La Niña events. In the Sierra Nevada, El Niño events typically result 

in warmer, wetter winters and La Niña events typically result in colder winters; 

thus the quadratic relationship suggested that survival was highest when winters 

were not too wet or too cold. Furthermore, their weather models explained less 

temporal variation in survival than they did in reproduction (60 vs. 84 percent); 

reproduction also exhibited much greater temporal variation than survival. 

Population Size and Trends 

Population Size 

To our knowledge, there has never been a formal attempt to estimate rangewide 

population sizes of the California spotted owl. We have provided summaries of the 

number of known California spotted owl sites obtained from the CDFW and the 

USFS (see above; figs. 4-1 and 4-2), but these data were not collected as part of a 

scientifically rigorous sampling scheme throughout the owl’s geographic range. 

Therefore, to assess whether the overall population is declining, we must rely upon 

population trends estimated from individual, long-term study populations. Fortunately, the four study areas in the Sierra Nevada from which estimates have been 

derived were large and spanned the extent of the mountain range, and thus likely 

provided a representative estimate of trends throughout the Sierra Nevada. 

Population Trends 

Population trends of spotted owls are typically reported as the annual rate of 

population change (λ ) where λ indicates the population size in year t + 1 relative t t 

to the population size in year t. Thus, λ = 1.0 for a stationary population, λ > 1.0 t 

for an increasing population, and λ < 1.0 for a declining population. Furthermore, t 

the overall change in population size during a defined period of time is expressed 

as realized population change (Δ ) where Δ indicates the population size in year t t t 
86 

GENERAL TECHNICAL REPORT PSW-GTR-254

 

relative to the population size at the beginning of the study period (Franklin et al. 

2004). The realized population change is equivalent to the product of the annual 

rates of population change over the study period (1 × λ1 × λ2 × λ3 × ... λt-1). When 

assessing population trends, the processes affecting population change depend 

upon the scale of the population under consideration. Within the overall, rangewide 

population, changes in population size are due to a combination of reproduction and 

survival. However, within finite study areas, changes in population size are due to a 

combination of reproduction, survival, immigration, and emigration. Therefore, the 

estimates we report below for finite sampling areas will incorporate immigration 

and emigration of owls across study area boundaries, although the immigration and 

emigration rates are typically unknown. 

Estimated population trends for spotted owls have benefitted from advances 

in analytical methods since the first northern spotted owl meta-analysis in 1993 

(Gutiérrez 2008). Researchers first used stage-based population projection matrices 

and estimates of demographic rates to determine changes in abundance within 

specified age classes during annual time increments (Blakesley et al. 2001, LaHaye 

et al. 2004, Noon et al. 1992, Seamans et al. 2001). Using this approach, the annual 

rate of population change was obtained by finding the dominant eigenvalue for a 

defined population matrix. From the perspective of spotted owl studies, the “rate 

of population change” provided by projection matrices may be biased low because 

the estimated juvenile survival rates implicitly incorporate emigration (i.e., juvenile 

dispersal) from a study area, but the matrices do not account for immigration onto a 

study area. To accommodate this fact, Seamans and Gutiérrez (2007) and Seamans 

et al. (2001) used the estimated juvenile survival rate derived from an analysis of 

an insular spotted owl population in the San Bernardino Mountains as a surrogate 

for juvenile survival in the Eldorado National Forest under the assumption that the 

values would be similar between the Eldorado and southern California. From 1990 

through 1999, population trends estimated using projection matrices suggested that 

both the Lassen ( = 0.910, SE = 0.025; Blakesley et al. 2001) and Eldorado National 

Forests ( = 0.948, SE = 0.026; Seamans et al. 2001) populations experienced significant declines. 

However, Pradel (1996) developed a new method to estimate λ using mark- t 

recapture data, which was motivated by a desire to obtain unbiased estimates of 

λ for northern spotted owl study areas (Gutiérrez 2008). This statistical method, t 

referred to as a temporal symmetry model, estimated recruitment, nonjuvenile 

survival, and population change directly from the mark-recapture data. This 

approach implicitly incorporated both emigration and immigration because new 

recruits can be individuals that were either born on or immigrated onto a study 

area and apparent survival rates reflected either true mortality or emigration off a 
87 

The California Spotted Owl: Current State of Knowledge

study area. The first two and the last estimates of λt were not used in any analysis 

because the first and last estimates were confounded with recapture probability 

and the second estimate had a potential bias from “trap response” or a “learning 

curve” for field crews at the beginning of studies (Hines and Nichols 2002). “Trap 

responses” have occurred when observers preferentially sampled known owl sites or 

when owls either avoided or preferentially responded to human presence by virtue 

of behavioral conditioning. “Learning curves” have been a function of personnel 

becoming familiar with a new study area and accomplishing work objectives more 

efficiently as they gained experience (i.e., if the same number of observers detected 

more owls because of greater experience, the population could falsely be assumed 

to be growing when it is not). The Pradel method was used in the two California 

spotted owl meta-analyses (Blakesley et al. 2010, Franklin et al. 2004). Franklin et 

al. (2004) reported that the Pradel estimates of mean λt from 1992 through 1999 for 

the Sierra Nevada studies (except the Eldorado) were < 1.0, but all of the 95 percent 

confidence intervals overlapped 1.0, which meant that it was uncertain if declines 

had actually occurred. Subsequently, Blakesley et al. 2010 reported that the Pradel 

estimates of mean λt from 1992 through 2002 were <1.0 for the Sierra and Lassen 

National Forest and slightly >1.0 for the Eldorado National Forest and Sequoia and 

Kings Canyon National Parks, but again all of the 95 percent confidence intervals 

overlapped 1.0. However, the estimate of Δ for the Lassen National Forest sug- t 

gested that this population declined over the study period. 

Population trends have been recently reanalyzed for all four study areas using 

new statistical techniques and incorporating additional data collected after the 

second meta-analysis (Conner et al. 2013, Tempel and Gutiérrez 2013, Tempel et al. 

2014b). Conner et al. (2013) used the Pradel model within both maximum-likelihood 

and Bayesian frameworks to conclude that the Lassen and Sierra study populations had median λt

 less than 1.0. In addition, their Bayesian analysis showed that 

the Lassen and Sierra study areas had 0.69 and 0.40 probabilities, respectively, of 

declining by ≥15 percent over the study period. In contrast, the Sequoia and Kings

Canyon National Parks study population had a median λ >1.0 and only a 0.04 t 

probability of a ≥15-percent decline. The authors recently updated their analyses to 

include additional data collected in 2012 and 2013, which suggests it is even more 

likely that the Lassen and Sierra National Forests study populations have declined 

(fig. 4). Bayesian methods will allow generation of a posterior distribution for Δt, 

which allows the estimation of probabilities of specified declines of interest rather 

than the classic statistical approach of rejecting or accepting the null hypothesis 

that = 1.0 at a specified probability level (typically p = 0.05). Thus, Conner et 

al. (2013) suggested that Bayesian methods were more informative for managing 

species of conservation interest than traditional statistical methods. 
88 

GENERAL TECHNICAL REPORT PSW-GTR-254

Figure 4-4—Estimated posterior distributions of overall realized population change (Δ ) of California t spotted owls based on posterior distributions of λ from 10,000 Markov chain Monte Carlo simula- t tions. Data are from three Sierra Nevada study areas (Lassen [LAS], Sierra [SIE], and Sequoia and 

Kings Canyon [SKC]), 1990−2013 (used with permission of John Keane).

Tempel and Gutiérrez (2013) used the Pradel model to estimate = 0.725 (95 

percent confidence interval = 0.445 to 1.004) for the Eldorado study population 

from 1993 through 2010; this result closely matched the estimated trends in territory occupancy. They also noted that the Eldorado “density” study area was not 

surveyed entirely prior to 1993 because of funding constraints, which resulted in 

a gradual expansion of their study area size from 1990 through 1993 until funding 
89 

The California Spotted Owl: Current State of Knowledge

was adequate to survey the entire study area, and that the initial λt estimates would 

have been biased had they included mark-recapture data collected prior to 1993. 

Tempel et al. (2014b) then developed an integrated population model (IPM) for 

the Eldorado National Forest study population that used all data collected on the 

Eldorado (occupancy, reproductive, and mark-recapture histories for juveniles and 

nonjuveniles) in a unified analysis. They first used a multistate occupancy model 

that accounted for imperfect detection to obtain annual counts of the number of 

young produced and the number of nonjuvenile territorial birds. These counts were 

then used as input data to the IPM, along with the mark-recapture histories. The 

IPMs offer several advantages over the traditional analysis of individual datasets, 

including greater precision in parameter estimates and the ability to estimate demographic parameters (e.g., immigration rates) for which no explicit data are available. 

They found that mean λ was <1.0 ( = 0.969, 95 percent credible interval = 0.957 to 

0.980), which resulted in a 50 percent decline in population size from 1990 through 

2012 ( = 0.501, 95 percent CRI 0.383 to 0.641; see fig. 4-5). Tempel et al. (2014b) 

were able to use data from 1990 through 1992 because the multistate occupancy 

model imputed the count values at territories that were not surveyed in a given year 

and the mark-recapture data were used solely to estimate apparent survival rates. 

110 

100 

90 

80 

70 

60 

50 

40 

30 

1990 1995 2000 2005 2010 

Number of adults 

Year 

Adults (integrated population model) 

Adults (occupancy) 

Figure 4-5—Posterior means (95 percent CRI [credible interval]) of realized population change 

from a Bayesian integrated population model for California spotted owls in the central Sierra 

Nevada, 1990–2012 (Tempel et al. 2014a; reproduced with permission of Elsevier Inc.©). 
90 

GENERAL TECHNICAL REPORT PSW-GTR-254

They attributed the larger observed decline (cf. Tempel and Gutiérrez [2013]) to the 

use of additional data and an increase in the number of territories occupied by single 

owls during the study. They also found that changes in λ were more highly correlated

with immigration rate than any other demographic rate (reproductive rate, juvenile 

survival, and nonjuvenile survival), which suggested that changes in population size 

were also influenced by processes occurring outside of the study area. 

Blakesley et al. (2001) and Seamans and Gutiérrez (2007) performed sensitivity 

analyses to assess which demographic rates had the most influence on changes in λ. 

Blakesley et al. (2001) reported that λ was most sensitive to changes in adult female 

survival on the Lassen National Forest, but fecundity contributed more to observed 

variation in λ because fecundity varied more than survival. Similarly, Seamans and 

Gutiérrez (2007) found that λ was most sensitive to changes in survival of nonjuvenile owls on the Eldorado National Forest, but that reproductive output and survival 

made similar contributions to changes in λ because reproductive output varied more 

than survival. The authors of both of these studies, as well as Tempel et al. (2014b), 

observed that juvenile survival made the least contribution of any demographic rate 

to changes in λ.

Thus far, despite its obvious relevance, researchers have attempted only one 

comprehensive assessment of how changes in habitat conditions within California 

spotted owl territories are correlated with changes in λ. Tempel et al. (2014a) created annual vegetation maps for owl territories on the Eldorado National Forest that 

differed over time because of timber harvest, wildfire, and forest succession. They 

found that reproduction was negatively correlated with medium-intensity timber 

harvests and the amount of hardwood forest within territories, where “mediumintensity harvests” encompassed a range of harvest types (group selection, singletree selection, thinning for hazardous fuels reduction, fuel break, commercial thin). 

In addition, they found that nonjuvenile survival was positively correlated with the 

amount of high canopy cover (≥70 percent) forest dominated by medium or large 

trees (see above). However, life-stage simulations showed that changes in λ at the 

territory scale were more correlated with changes in the amount of high canopy 

cover forest than with the other variables they measured (R2

 = 0.74 for a logarithmic 

regression; see fig. 4-6). 

Site Occupancy 

A growing number of studies have assessed site occupancy for California spotted 

owls, where the site has typically been defined as a unique owl territory based on 

the presence of roosting or nesting owls. Occupancy studies can be one or more 

years in length, but only multiseason studies provide information on changes in 
The California Spotted Owl: Current State of Knowledge

A B 

1.0 1.0 

Area of high-canopy-cover forest Amount of habitat edge 

0.9 (R2

 = 0.74) 0.9 (R2 = 0.32) 

0.8 0.8 

0.7 0.7 

0.6 0.6 

0.5 0.5 

0.4 0.4 

0 50 100 150 200 250 300 350 0 10 20 30 

Hectares Kilometers 

C D 

1.0 

Area of medium-intensity harvests Area of hardwood forest 

1.0 

0.9 (R2

 = 0.01) 0.9 (R2 = 0.02) 

0.8 0.8

0.7 0.7

0.6 0.6 

0.5 0.5

0.4 

0 25 50 75 100 125 150 0 25 50 75 100 125 

0.4 

Hectares Hectares 

Figure 4-6—Results of a life-stage simulation analysis to assess the sensitivity of annual population growth rate (λ) of 

California spotted owls to changes in forest vegetation conditions within owl territories. One thousand values of λ were 

generated by drawing the following habitat variables from a uniform distribution: (a) area (ha) of high canopy cover (≥70 

percent) forest dominated by trees ≥30.5 cm (≥12 in) diameter at breast height; (b) amount (km) of habitat edge; (c) area (ha) 

of medium-intensity timber harvests; and (d) area (ha) of hardwood forest (Tempel et al. 2014b; reproduced with permission 

of 2014 Ecological Society of America ©). 

occupancy status over time and what factors (e.g., habitat conditions, timber harvest, wildfire) are correlated with these changes. Changes in occupancy status are 

the combined effect of two processes: 

• Local colonization, which is the probability that a previously unoccupied 

site becomes occupied. 

• Local extinction, which is the probability that a previously occupied site 

becomes unoccupied (MacKenzie et al. 2003). 

91 
92 

GENERAL TECHNICAL REPORT PSW-GTR-254

Refer to chapter 3 for a review of how habitat and habitat disturbances such as 

wildfire and logging affect spotted owl territory occupancy dynamics. 

Because spotted owl territories can only be occupied by one or two individuals, occupancy-based studies may be a cost-effective alternative to mark-recapture 

studies for assessing owl population trends. Indeed, as we previously noted, Tempel 

and Gutiérrez (2013) found that trends in territory occupancy on the Eldorado 

National Forest from 1993 through 2010 closely matched population trends estimated from mark-recapture data using the Pradel model. However, Tempel et al. 

(2014a) analyzed data on the Eldorado from 1990 through 2012 using an integrated 

population model and found a larger population decline than Tempel and Gutiérrez 

(2013), partly owing to an increase over time in the number of territories occupied 

by single owls. Thus, multistate occupancy models that distinguish between territories occupied by single owls from those occupied by owl pairs may be preferable 

when inferring demographic trends from spotted owl occupancy data. Furthermore, 

occupancy studies are ideally suited to assess owl responses to management activities (e.g., timber harvest or prescribed burns), wildfire, climate, and other factors. 

Population and Conservation Genetics of California 

Spotted Owls 

Genetic methods and principles can provide valuable insights into the population 

status and management of species of conservation concern in many ways (Allendorf 

and Luikart 2007). These include (1) identifying conservation units, (2) estimating connectivity (i.e., gene flow and dispersal) among fragmented populations, 

(3) quantifying the level of genetic variation present within populations, and (4) 

characterizing demographic history. Genetic information can be applied to several 

other aspects of species conservation, but we limit our review to the four topics 

listed above, which in our opinion are the most relevant to the conservation of 

California spotted owls. We also note that issues associated with spotted owl-barred 

owl (Strix varia) hybridization are discussed in detail in chapter 6. For each of the 

conservation genetic issues we discuss, we first provide a brief overview of relevant 

population genetics principles to help interpret previous genetic studies on spotted 

owls and inform future management and research directions. 

Conservation Units

 A conservation unit is typically defined as a group of individuals that merits 

conservation attention independent of other such groups (Ryder 1986). Conservation units have been defined in several ways and applied to species with a variety 

of objectives. Typically, “evolutionary significant units” (ESUs) refer to populations 
93 

The California Spotted Owl: Current State of Knowledge

that are reproductively isolated from other populations and, as a result, have evolved 

unique adaptations through natural selection (Moritz 1994, Ryder 1986). The ESUs 

are conserved because the adaptations that differentiate them from other groups of 

conspecifics may be important for the persistence of the entire species in light of 

rapid environmental change. In contrast, a “management unit” (MU) generally represents a demographically independent population that receives little immigration 

from other populations (Moritz 1994, Palsbøll et al. 2007). The MUs are managed 

independently of other units because they are not expected to be “rescued” via 

immigration. From a legal perspective, two roughly parallel types of conservation 

units can be listed as “threatened” or “endangered” under the Endangered Species 

Act (ESA 1973): a taxon (species or subspecies) and a distinct population segment 

(DPS). The subspecies concept was defined in chapter 2, and as described therein 

can be listed under the ESA with the objective of preserving the evolutionary potential of the species, similar to the rationale behind delineating ESUs. The DPS status 

can be assigned to a population based on its evolutionary, ecological, or geopolitical 

discreteness, significance to the entire range of the species, and conservation status. 

In practice, the delineation of all types of conservation units is frequently informed 

using genetic data given the genetic underpinnings of adaptive traits (Crandall et al. 

2000, Moritz 1994, Palsbøll et al. 2007). 

As described in chapter 2, spotted owls are clearly divided into three welldelineated subspecies based on phylogeographic patterns in the mitochondrial 

DNA (Barrowclough et al. 1999, 2005; Haig et al. 2004) as well as differences in 

microsatellite allele frequencies among populations (Funk et al. 2008a). With the 

exception of a small number of California spotted owl haplotypes detected within 

the geographic range of northern spotted owls, the three subspecies appear to 

be “reciprocally monophyletic” based on the control region of the mitochondrial 

genome (i.e., all sampled haplotypes were more closely related to other haplotypes in 

the same subspecies than to those of the other subspecies) (Haig et al. 2004). Thus, 

these three groups could be also be considered as discrete ESUs, each of which is 

important to conserve in order to maintain the evolutionary potential of the species, 

at least according to one commonly used definition for ESUs (Moritz 1994). 

Within the range of the California subspecies, genetic data appears to support 

the designation of multiple MUs. Owls in the Sierra Nevada are clearly genetically distinct from owls in the mountains of southern California (Barrowclough 

et al 1999, 2005; Funk et al. 2008a; Haig 2004). Large areas of unsuitable lowland 

habitat between the Sierra Nevada and southern California mountains certainly 

impede dispersal to the point that owls in these two regions are demographically 

independent. Indeed, Barrowclough et al. (2005) estimated that approximately zero 
94 

GENERAL TECHNICAL REPORT PSW-GTR-254

to one female migrant was exchanged between these two regions per generation. 

Some mountain ranges within southern California may also be demographically 

independent, and thus could be treated as discrete MUs, given limited gene flow 

(Barrowclough et al. 2005; see also below) and that little dispersal by marked individuals has been observed as part of mark-recapture studies (LaHaye et al. 2001). 

However, additional genetic analyses that include nuclear DNA would be needed to 

define MUs within southern California. 

Connectivity in Fragmented Populations 

Understanding the level of connectivity among populations that are isolated to some 

degree by habitat fragmentation or physiographic barriers (e.g., nonforested habitat 

between mountain ranges) has several important implications for the conservation 

of species, where connectivity can refer to gene flow (the movement of genes) or 

dispersal (movement of individuals). The isolation of formerly contiguously distributed populations into remnant habitat patches can impede gene flow and dispersal, 

thereby hastening extinction through a variety of genetic and demographic processes symptomatic of small populations (Keller and Waller 2002). As a result, a 

daunting array of genetic approaches has been developed to quantify gene flow and 

dispersal and assess whether habitat fragmentation has affected these processes to 

the point where management intervention is required (Lowe and Allendorf 2010). 

It is important to recognize that different genetic methods often yield inferences 

about gene flow and dispersal that apply to different time scales (i.e., a single 

generation to thousands of generations) and have a range of limitations, including 

the assumption of genetic drift-migration equilibrium (stable gene flow and effective population size) and difficulty at estimating dispersal when it is high enough to 

affect local population dynamics (Paetkau et al. 2004, Palsbøll et al. 2007, Peery et 

al. 2008). Moreover, maintaining the distinction between gene flow and individual 

dispersal is important because the dispersal of individuals does not necessarily 

translate to gene flow (Nosil et al. 2005, Peery et al. 2010). 

Although spotted owl habitat within the Sierra Nevada has been extensively 

modified over the past approximately 150 years, we expect relatively little effect of 

historical habitat fragmentation on gene flow or dispersal in California spotted owls 

in this region. Spotted owls and their habitat remain reasonably well-distributed 

across the Sierra Nevada and the species is a strong disperser (Forsman et al. 2002). 

In the only genetic-based study of connectivity within the Sierra Nevada, Barrowclough et al. (2005) estimated gene flow among spotted owl populations sampled 

in the northern, central, and southern Sierra Nevada using coalescent approaches 

applied to patterns of mitochondrial sequence variation. Their estimates of the 
95 

The California Spotted Owl: Current State of Knowledge

effective number of female migrants ranged from zero (northern to central) to 25 

(central to northern) per generation. An estimate of 25 migrants per generation is 

considered reasonably high from a population genetics perspective, whereas zero is 

clearly low. Gene flow estimates using this method, however, represent long-term 

averages (i.e., over evolutionary time scales) and do not necessarily reflect current 

rates of gene flow (or dispersal). Further study using nuclear genetic markers (e.g., 

microsatellites) and landscape-scale sampling could provide additional insights into 

current rates of gene flow and dispersal within the Sierra Nevada, as well as the 

environmental factors that influence these processes. Funk et al. (2008b) conducted 

such a “landscape genetics” study for northern spotted owls using 10 microsatellites 

to understand how landscape-scale topographic features such as major mountain ranges, valleys, and rivers impeded gene flow. In this study, gene flow was 

impeded by natural barriers such as mountain ranges without suitable owl habitat at 

higher elevations and, paradoxically, smaller and relatively undeveloped valleys, but 

not the large and extensively modified Willamette Valley. 

As described above, Barrowclough et al. (2005) estimated low levels of gene 

flow among California spotted owl populations occurring in the mountains of 

southern California. Clearly, the natural isolation of these mountain ranges by 

unsuitable habitats such as deserts has acted as a barrier to gene flow over long 

time scales. However, urbanization and habitat development over the past century 

could have increased the isolation of these populations and further reduced gene 

flow and dispersal (LaHaye and Gutiérrez 2005, Verner et al. 1992). Additional 

genetic-based studies of connectivity using nuclear genetic markers could provide 

insight into the extent to which habitat fragmentation currently threatens southern 

California populations. 

Genetic Variation Within Populations 

Conserving adaptive genetic variation within populations is important for maintaining the evolutionary potential of species (Frankel and Soulé 1981). Loss of alleles 

occurs at a relatively rapid rate in small (e.g., bottlenecked) populations because of 

the enhanced effects of genetic drift, and the resultant loss of alleles with adaptive 

significance can compromise the ability of the species to adapt to future environmental change (Frankham et al. 1999, Lande and Shannon 1996, O’Brien and 

Evermann 1988). Inbreeding, the mating of close kin, is also more likely to occur 

in bottlenecked populations because remaining individuals tend to be related. Rare 

deleterious alleles are more likely to be expressed in inbred populations owing to 

high levels of homozygosity, which can result in declines in individual fitness (i.e., 

inbreeding depression) and increase the likelihood of extinction (Keller and Waller 
96 

GENERAL TECHNICAL REPORT PSW-GTR-254

2002). Thresholds for effective population sizes below which species will become 

vulnerable to the loss of adaptive genetic variation and inbreeding have been the 

subjective debate among scientists and are likely species-specific traits. Nevertheless, consensus exists that preventing the loss of adaptive genetic variation and 

inbreeding depression is best accomplished by maintaining large, well-connected 

populations. 

California spotted owls exhibit relatively little sequence variation in the mitochondrial DNA control region compared to northern and Mexican (S. o. lucida) 

spotted owls (Barrowclough et al. 1999, 2005; Haig et al. 2004). Because the control 

region is a nonprotein coding sequence, it likely reflects the evolutionary history 

of mitochondrial genes given that the mitochondrial DNA represents a single 

nonrecombining genome. Indeed, nucleotide diversity, which represents the average 

number of nucleotide differences per site across pairs of randomly selected DNA 

sequences, is several times lower in California spotted owls sampled in the Sierra 

Nevada than in populations of the other two subspecies (Barrowclough et al. 1999, 

2005). Moreover, no sequence variation was detected in the mitochondrial control 

region of owls sampled in southern California, either in an initial screening of 10 

individuals in the San Bernardino and San Jacinto Mountains (Barrowclough et al. 

1999), or in an expanded sample of 38 individuals that included owls from Mount 

Palomar (Barrowclough et al. 2005). Barrowclough et al. (1999, 2005) offered three 

possible explanations for relatively low observed mitochondrial diversity in California spotted owls: 

• Small historical and current effective population sizes 

• Historical population expansion (i.e., a colonization event or in situ 

recovery from a historical bottleneck) 

• A beneficial mutation followed by a “selective sweep.” 

Of these possibilities, clearly a population bottleneck would have the most 

detrimental impacts on the evolutionary potential of California spotted owls. 

Genetic variation has also been assessed in a rangewide study of spotted owls 

using a panel of 10 microsatellite loci (tandemly repeating nuclear DNA sequences; 

Funk et al. 2008a). Although Funk et al. (2008a) did not directly report heterozygosity or allelic diversity for each sampled population, they stated that minimum 

expected heterozygosity across sampling sites was 0.685, which included two localities in the Sierra Nevada. This level of heterozygosity is typical of wild populations 

and, at face value, does not seem symptomatic of a severe population bottleneck. 

Caution, however, should be exercised when interpreting levels of genetic variation 

present in microsatellite markers because of “ascertainment bias,” which results 
97 

The California Spotted Owl: Current State of Knowledge

from researchers selecting the most polymorphic loci from a larger panel of candidate loci for use in population genetic studies. Highly polymorphic loci are useful 

for characterizing population genetic structure, understanding introgression, and 

estimating dispersal, but may be subject to high mutation rates and, therefore, yield 

an optimistic perspective of effective population size. Clearly, additional work is 

needed to fully characterize understanding of the demographic and microevolutionary factors that have shaped present-day genetic variation in spotted owls (see 

“Characterizing Demographic History” section below). 

To date, tests of inbreeding or inbreeding depression have not been conducted 

for California spotted owls. However, several lines of evidence suggest that 

inbreeding does not currently threaten California spotted owls in the Sierra Nevada. 

First, Funk et al. (2008a) reported that observed heterozygosity did not deviate 

from expected heterozygosity for any of their sampled populations (inbred populations are expected to have lower observed than expected heterozygosity) across 

their panel of microsatellite loci. Second, natal dispersal is strong in spotted owls 

(Forsman et al. 2002) and, as a result, incestuous matings are rarely observed in 

this species (Carlson et al. 1998, Forsman et al. 2002). Third, in the Sierra Nevada, 

California spotted owls remain well-distributed and occur at higher abundances 

than typically observed in populations experiencing noticeable impacts of inbreeding depression. By contrast, spotted owls in the mountains of southern California 

are distributed among relatively small and insular populations that are likely connected by low levels of gene flow and are likely to be more susceptible to inbreeding. Nevertheless, even in the Sierra Nevada, future reductions in owl habitat from 

timber harvesting, fire, and climate change could result in smaller, more isolated 

owl populations that are more susceptible to the detrimental effects of inbreeding. 

Characterizing Demographic History 

Changes in effective population size, such as bottlenecks, often register signals in 

the DNA of the individuals that make up the population. Thus, the demographic 

history of a population of interest can be studied by examining relevant aspects of 

genetic variability in present-day populations. Genetic methods provide an appealing means for understanding changes in effective population because they only 

require a population sample taken at a single point in time, as opposed to long-term 

population monitoring (although historical samples can strengthen inferences). 

Consequently, many population genetic methods have been developed that can be 

used to characterize the demographic history of species of conservation concern 

such as California spotted owls (e.g., Beaumont 1999, 2003; Cornuet and Luikart 
98 

GENERAL TECHNICAL REPORT PSW-GTR-254

1996; Garza and Williamson 2001). However, as with genetic estimators of connectivity, caution must be exercised when interpreting the results of genetic-based 

assessments of demographic history as they often require making assumptions (e.g., 

about the way genes mutate) that are difficult to test, and results can be sensitive to 

violations of assumptions (Peery et al. 2012). 

Applications of genetic data to questions of demographic history in California 

spotted owls are few, but as discussed above, this subspecies has depauperate mitochondrial DNA variation compared to northern and Mexican spotted owls (Barrowclough 1999, 2005; Haig 2004). Demographic explanations for relatively low genetic 

variation in California spotted owls, both in the Sierra Nevada and southern California, are uncertain but include persistently small populations, population bottlenecks, 

and recent colonization followed by population expansion (Barrowclough et al. 

1999, 2005). These three competing hypotheses could be tested using coalescent 

methods applied to a panel of nuclear markers (e.g., microsatellites) or mitochondrial 

DNA (Beaumont 1999, 2003; Drummond and Rambaut 2007; Wu and Drummond 

2011), as well as based on differences in microsatellite diversity statistics (Cornuet 

and Luikart 1996, Garza and Williamson 2001, Luikart and Cornuet 1998). Indeed, 

Funk et al. (2010) used genetic bottleneck tests based on microsatellite diversity 

statistics to test for declines in effective population size in northern spotted owls and 

demonstrated that bottlenecks were generally apparent in populations that demographic studies indicated were declining. Again, although such methods are sensitive to several potentially important assumptions, careful application of bottleneck 

tests and associated methods could provide important and novel insights into the 

demographic history of California spotted owls. Moreover, these genetic methods 

have the potential for reconstructing demographic history on longer time scales 

than spanned by California spotted owl demography studies (about 25 years), which 

could provide insights into how historical changes in forest extent and structure and 

climate have affected this subspecies. Finally, emerging genomic methods now provide increasing opportunities for more detailed reconstructions of the demographic 

history of California spotted owls (Hung et al. 2014). 

Chapter Summary 

Population data gathered subsequent to CASPO demonstrates that owl populations 

have declined over the past 20 years on three of the four long-term demographic 

study areas in the Sierra Nevada, which removes one of the key uncertainties 

of CASPO. Because these study areas were not selected at random, it cannot be 

inferred unequivocally that they represent the status of spotted owls in the entire 

Sierra Nevada. However, these study areas are large, span the entire length of the 
99 

The California Spotted Owl: Current State of Knowledge

Sierra Nevada, occur primarily in the mid-elevation forests that have the highest densities of owls, and exhibit no obvious special selection criteria that would 

likely result in the bias of derived information. Therefore, we infer that spotted owl 

populations in the Sierra Nevada are declining on most landscapes. We note that 

the populations that have declined are all located on national forests, and the only 

stationary population is located within Sequoia and Kings Canyon National Parks. 

The differences among study areas may have been related to differences in forest 

management, the presence of giant sequoia (Sequoiadendron giganteum (Lindl.) 

J. Buchholz) groves in Sequoia and Kings Canyon National Parks, differences in 

the proportion of oak woodlands, or some combination of these factors (Blakesley 

et al. 2010). We further note that whereas barred owls have negatively affected 

northern spotted owl populations, barred owls have appeared on the study areas in 

the Sierra Nevada only within the past 10 years and are either uncommon (Lassen) 

or extremely rare (Eldorado and Sierra National Forests, Sequoia and Kings Canyon 

National Parks). Thus, the observed population declines on the Eldorado and Sierra 

cannot be attributed to barred owls, and these declines may intensify if barred owls 

continue their southern range expansion in the future. Finally, these studies highlight the importance of long-term monitoring studies of long-lived species where 

small annual population declines are difficult to detect but result in large cumulative 

declines over long time periods. 

Reproductive output and nonjuvenile survival contribute more to variation in 

spotted owl population size than juvenile survival. It is now well established that 

California spotted owl survival (and reproduction, to a lesser degree) is dependent 

upon having a sufficient amount of high canopy cover forest containing larger trees 

within breeding territories. This forest cover type has been positively correlated 

with survival rates at all four of the long-term demographic study areas and with 

reproductive rates at two of the study areas (Lassen and Sierra). Furthermore, 

Tempel et al. (2014a) noted that population growth rate (λ, which is determined 

by reproduction and survival) at the territory scale was strongly dependent upon 

the amount of high canopy cover (≥70 percent) forest within owl territories. Their 

results also suggested that maintaining between 100 and 150 ha (247 and 370 ac) 

of high canopy cover forest within owl territories would be sound conservation 

practice because small changes in annual population growth rate can translate into 

large changes in realized population size over extended periods of time, and populations in the Sierra Nevada have already declined by as much as 50 percent over the 

past two decades. In addition, 100 to 150 ha is a more realistic target than managing 

for amounts of high canopy cover forest (e.g., 200 ha) that maximized population 

growth in Tempel et al. (2014a). This amount of habitat also coincides with the size 
100 

GENERAL TECHNICAL REPORT PSW-GTR-254

of spotted owl PACs, which have been consistently used for nesting and roosting 

over a 24-year period on the Eldorado (Berigan et al. 2012). Finally, some evidence 

exists that northern and California spotted owls may benefit from some habitat 

heterogeneity and edge between forest types (Franklin et al. 2000, Tempel et al. 

2014a), but the best available data indicate that sufficient high canopy cover forest is 

needed within owl territories. 

A key remaining uncertainty is the degree to which changes in demographic 

rates and population abundance are related to various types of habitat disturbance 

such as high-severity wildfire and timber harvest. Thus far, the evidence for how 

disturbance may affect spotted owls has been mixed. Tempel et al. (2014a) reported 

that local colonization was negatively correlated with wildfire, but that timber 

harvest had relatively minor effects on reproduction, nonjuvenile survival, and 

territory occupancy. However, they found that territory fitness and occupancy were 

highly correlated with the amount of high canopy cover forest within owl territories, so disturbances that reduce this cover type could negatively affect spotted owl 

populations. Other studies that focused specifically on site occupancy and wildfire 

suggested that owls were resilient to low- and moderate-severity fire, but vacated 

territories when large areas were burned at high severity. Because of the remaining 

uncertainty on how timber harvest and wildfire affect spotted owls, the apparent 

benefits that closed-canopy forests provide owls, and the substantial recent population declines in some regions, landscape-scale fuel treatments implemented to 

reduce fire risk within owl habitat cannot be adequately assessed for their efficacy 

without an accompanying rigorous monitoring program. 

Ideally, future research would be conducted within an experimental context, but 

experimental studies are likely to be impractical because of logistical difficulties 

and the large home ranges of spotted owls. Therefore, researchers and managers will 

likely need to continue to rely on correlative, quasi-experimental approaches that 

account for logging and wildfire effects in a rigorous manner. In addition, simulation 

modeling of owl populations at larger spatial scales where the model parameter values are based on empirical results from smaller study areas may provide insights into 

regional population dynamics. For example, recent modeling for the northern spotted 

owl suggested that the demographic performances of regional metapopulations were 

more affected by complex source-sink dynamics among the metapopulations than by 

metapopulation specific habitat values (Schumaker et al 2014). 
101 

The California Spotted Owl: Current State of Knowledge

Genetic investigations support treating California spotted owls in the Sierra 

Nevada and southern California, collectively, as a discrete ESU in light of their 

genetic divergence from the northern and Mexican subspecies. Further, genetic 

exchange and dispersal between southern California and Sierra Nevada owl populations appears to be very low, suggesting that owls in these two regions should 

be treated as independent management units. Genetic variation is low and may 

constrain the ability of California spotted owls to adapt to inevitable future environmental change, and further population declines could result in even lower levels 

of genetic variation and greater constraints on evolutionary potential. Evidence for 

historical (long-term) population declines from genetic data is equivocal but could 

be tested with additional analyses. In general, rapidly emerging technologies and 

analytical frameworks within the field of “conservation genomics” provide exciting 

new opportunities for characterizing the population structure and demographic 

history of California spotted owls. 

Literature Cited 

Allendorf, F.W.; Luikart, G. 2007. Conservation and the genetics of populations. 

Malden, MA: Blackwell Publishing. 642 p. 

Barrowclough, G.F.; Groth, J.G.; Mertz, L.A.; Gutiérrez, R.J. 2005. 

Genetic structure, introgression, and a narrow hybrid zone between northern 

and California spotted owls (Strix occidentalis). Molecular Ecology. 14(4): 

1109–1120. 

Barrowclough, G.F.; Gutiérrez, R.J.; Groth, J.G. 1999. Phylogeography of 

spotted owl (Strix occidentalis) populations based on mitochondrial DNA 

sequences: gene flow, genetic structure, and a novel biogeographic pattern. 

Evolution. 53(3): 919–931. 

Barrowclough, G.F.; Gutierréz, R.J.; Groth, J.G.; Lai, J.E.; Rock, D.F. 2011. 

The hybrid zone between northern and California spotted owls in the CascadeSierran suture zone. Condor. 113(3): 1–9. 

Beaumont, M.A. 1999. Detecting population expansion and decline using 

microsatellites. Genetics. 153(4): 2013–2029. 

Beaumont, M.A. 2003. Estimation of population growth or decline in genetically 

monitored populations. Genetics. 164(3): 1139–1160. 
102 

GENERAL TECHNICAL REPORT PSW-GTR-254

Berigan, W.J.; Gutiérrez, R.J.; Tempel, D.J. 2012. Evaluating the efficacy of 

protected habitat areas for the California spotted owl using long-term monitoring 

data. Journal of Forestry. 110(6): 299–303. 

Blakesley, J.A.; Noon, B.R.; Anderson, D.R. 2005. Site occupancy, apparent 

survival, and reproduction of California spotted owls in relation to forest stand 

characteristics. Journal of Wildlife Management. 69(4): 1554–1564. 

Blakesley, J.A.; Noon, B.R.; Shaw, D.W.H. 2001. Demography of the California 

spotted owl in northeastern California. Condor. 103(4): 667–677. 

Blakesley, J.A.; Seamans, M.E.; Conner, M.M.; Franklin, A.B.; White, G.C.; 

Gutiérrez, R.J.; Hines, J.E.; Nichols, J.D.; Shaw, D.W.H.; Munton, T.E.; 

Keane, J.J.; Steger, G.N.; Noon, B.R.; McDonald, T.L.; Britting, S. 2010. 

Population dynamics of spotted owls in the Sierra Nevada, California. Wildlife 

Monographs. 174: 1–36. 

Boyce, M.S.; Irwin, L.L.; Barker, R. 2005. Demographic meta-analysis: 

synthesizing vital rates for spotted owls. Journal of Applied Ecology. 42(1): 

38–49. 

Burnham, K.P.; Anderson, D.R.; White, G.C. 1996. Meta-analysis of vital rates 

of the northern spotted owl. In: Forsman, E.D.; DeStefano, S.; Raphael, M.G.; 

Gutiérrez, R.J., eds. Demography of the northern spotted owl: studies in avian 

biology No. 17. Lawrence, KS: Allen Press: 92–101. 

Carlson, P.C.; LaHaye, W.S.; Franklin, A.B. 1998. Incestuous behavior in 

spotted owls. Wilson Bulletin. 110(4): 562–564. 

Conner, M.M.; Keane, J.J.; Gallagher, C.V.; Jehle, G.; Munton, T.E.; 

Shaklee, P.A.; Gerrard, R.A. 2013. Realized population change for long-term 

monitoring: California spotted owl case study. Journal of Wildlife Management. 

77(7): 1449–1458. 

Cornuet, J.M.; Luikart, G. 1996. Description and power analysis of two tests for 

detecting recent population bottlenecks from allele frequency data. Genetics. 

144(4): 2001–2014. 

Crandall, K.A.; Bininda-Emonds, O.R.P.; Mace, G.M.; Wayne, R.K. 2000. 

Considering evolutionary processes in conservation biology. Trends in Ecology 

and Evolution. 15(7): 290–295. 

Drummond, A.J.; Rambaut, A. 2007. BEAST: Bayesian evolutionary analysis by 

sampling trees. BMC Evolutionary Biology. 7: 214. 
103 

The California Spotted Owl: Current State of Knowledge

Endangered Species Act of 1973 [ESA]; 16 U.S.C. 1531–1536, 1538–1540. 

Forsman, E.D.; Anthony, R.G.; Dugger, K.M.; Glenn, E.M.; Franklin, A.B.; 

White, G.C.; Schwarz, C.J.; Burnham, K.P.; Anderson, D.R.; Nichols, J.D.; 

Hines, J.E.; Lint, J.B.; Davis, R.J.; Ackers, S.H.; Andrews, L.S.; Biswell, 

B.L.; Carlson, P.C.; Diller, L.V.; Gremel, S.A.; Herter, D.R.; Higley, J.M.; 

Horn, R.B.; Reid, J.A.; Rockweit, J.; Schaberl, J.P.; Snetsinger, T.J.; Sovern, 

S.G. 2011. Population demography of northern spotted owls: studies in avian 

biology no. 40. Berkeley, CA: University of California Press. 106 p. 

Forsman, E.D.; Anthony, R.G.; Reid, J.A.; Loschl, P.J.; Sovern, S.G.; Taylor, 

M.; Bisswell, B.L.; Ellingson, A.; Melsow, E.C.; Miller, G.S.; Swindle, 

K.A.; Thrailkill, J.A.; Wagner, F.F.; Seaman, D.E. 2002. Natal and breeding 

dispersal of northern spotted owls. Wildlife Monographs. 149: 1–35. 

Frankel, O.H.; Soulé, M.E. 1981. Conservation and evolution. Cambridge, 

United Kingdom: Cambridge University Press. 327 p. 

Frankham, R.; Lees, K.; Montgomery, M.E.; England, M.E.; Lowe, P.R.; 

Briscoe, D.A. 1999. Do population size bottlenecks reduce evolutionary 

potential? Animal Conservation. 2(4): 255–260. 

Franklin, A.B.; Anderson, D.R.; Gutiérrez, R.J.; Burnham, K.P. 2000. Climate, 

habitat quality, and fitness in northern spotted owl populations in northwestern 

California. Ecological Monographs. 70(4): 539–590. 

Franklin, A.B.; Gutiérrez, R.J.; Nichols, J.D.; Seamans, M.E.; White, G.C.; 

Zimmerman, G.S.; Hines, J.E.; Munton, T.E.; LaHaye, W.S.; Blakesley, 

J.A.; Steger, G.N.; Noon, B.R.; Shaw, D.W.H.; Keane, J.J.; McDonald, T.L.; 

Britting, S. 2004. Population dynamics of the California spotted owl (Strix 

occidentalis occidentalis): a meta-analysis. Ornithological Monographs. 54: 

1–54. 

Funk, W.C.; Forsman, E.D.; Johnson, M.; Mullins, T.D.; Haig, S.M. 2010. 

Evidence for recent population bottlenecks in northern spotted owls (Strix 

occidentalis caurina). Conservation Genetics. 11(3): 1013–1021. 

Funk, W.C.; Forsman, E.D.; Mullins, T.D.; Haig, S.M. 2008a. Introgression 

and dispersal among spotted owl (Strix occidentalis) subspecies. Evolutionary 

Applications. 1(1): 161–171. 
104 

GENERAL TECHNICAL REPORT PSW-GTR-254

Funk, W.C.; Forsman, E.D.; Mullins, T.D.; Haig, S.M. 2008b. Landscape 

features shape genetic structure in threatened northern spotted owls. OpenFile Report 2008-1240. Reston, VI: U.S. Department of the Interior, Geological 

Survey. 12 p. 

Garza, J.C.; Williamson, E.G. 2001. Detection of reduction in population size 

using data from microsatellite loci. Molecular Ecology. 10(2): 305–318. 

Gutiérrez, R.J. 2008. Spotted owl research: a quarter century of contributions 

to education, ornithology, ecology, and wildlife management. Condor. 110(4): 

792–798. 

Haig, S.M.; Mullins, T.D.; Forsman, E.D. 2004. Subspecific relationships and 

genetic structure in the spotted owl. Conservation Genetics. 5(5): 683–705. 

Hines, J.E.; Nichols, J.D. 2002. Investigations of potential bias in the estimation 

of λ using Pradel’s (1996) model for capture-recapture data. Journal of Applied 

Statistics. 29(1–4): 573–587. 

Hung, C.M.; Shaner, P.J.L.; Zink, R.M.; Liu, W.C.; Chu, T.C.; Huang, W.S.; 

Li, S.H. 2014. Drastic population fluctuations explain the rapid extinction of 

the passenger pigeon. Proceedings of the National Academy of Sciences of the 

United States of America. 111(29): 10636–10641. 

Hunsaker, C.T.; Boroski, B.B.; Steger, G.N. 2002. Relations between canopy 

cover and the occurrence and productivity of California spotted owls. In: Scott, 

J.M.; Hegland, P.J.; Morrison, M.L., eds. Predicting species occurrences: issues 

of accuracy and scale. Washington, DC: Island Press: 687–700. 

Keller, L.F.; Waller, D.M. 2002. Inbreeding effects in wild populations. Trends in 

Ecology and Evolution. 17(5): 230–241. 

LaHaye, W.S.; Gutiérrez, R.J. 2005. The spotted owl in southern California: 

ecology and special concerns for maintaining a forest-dwelling species in a 

human-dominated desert landscape. In: Kus, B.E.; Beyers, J.L., eds. Planning 

for biodiversity: bringing research and management together. Proceedings of 

a symposium for the south coast ecoregion. Gen. Tech. Rep. PSW-GTR-195. 

Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest 

Research Station: 199–209. 

LaHaye, W.S.; Gutiérrez, R.J.; Dunk, J.R. 2001. Natal dispersal of the spotted 

owl in southern California: dispersal profile of an insular population. Condor. 

103(4): 691–700. 
105 

The California Spotted Owl: Current State of Knowledge

 

LaHaye, W.S.; Zimmerman, G.S.; Gutiérrez, R.J. 2004. Temporal variation 

in the vital rates of an insular population of spotted owls (Strix occidentalis 

occidentalis): contrasting effects of weather. Auk. 121(4): 1056–1069. 

Lande, R.; Shannon, S. 1996. The role of genetic variation in adaptation and 

population persistence in a changing environment. Evolution. 50(1): 434–437. 

Loehle, C.; Irwin, L.; Rock, D.; Rock, S. 2005. Are survival rates for northern 

spotted owls biased? Canadian Journal of Zoology. 83(10): 1386–1390. 

Lowe, W.H.; Allendorf, F.W. 2010. What can genetics tell us about population 

connectivity? Molecular Ecology. 19(15): 3038–3051. 

Luikart, G.; Cornuet, J.M. 1998. Empirical evaluation of a test for identifying 

recently bottlenecked populations from allele frequency data. Conservation 

Biology. 12(1): 228–237. 

MacKenzie, D.I.; Nichols, J.D.; Hines, J.E.; Knutson, M.G.; Franklin, A.B. 

2003. Estimating site occupancy, colonization, and local extinction when a 

species is detected imperfectly. Ecology. 84(8): 2200–2207. 

MacKenzie, D.I.; Nichols, J.D.; Seamans, M.E.; Gutiérrez, R.J. 2009. Modeling 

species occurrence dynamics with multiple states and imperfect detection. 

Ecology. 90(3): 823–835. 

Moritz, C. 1994. Defining ‘evolutionarily significant units’ for conservation. 

Trends in Ecology and Evolution. 9(10): 373–375. 

Noon, B.R.; McKelvey, K.S.; Lutz, D.W.; LaHaye, W.S.; Gutierrez, R.J.; 

Moen, C.A. 1992. Estimates of demographic parameters and rates of population 

change. In: Verner, J.; McKelvey, K.S.; Noon, B.R.; Gutiérrez, R.J.; Gould, G.I., 

Jr.; Beck., T.W., eds. The California spotted owl: a technical assessment of its 

current status. Gen. Tech. Rep. PSW-GTR-133. Albany, CA: U.S. Department of 

Agriculture, Forest Service, Pacific Southwest Research Station: 175–186.

North, M.; Steger, G.; Denton, R.; Eberlein, G.; Munton, T.; Johnson, K. 2000. 

Association of weather and nest-site structure with reproductive success in 

California spotted owls. Journal of Wildlife Management. 64(3): 797–807. 

Nosil, P.; Vines, T.H.; Funk, D.J. 2005. Reproductive isolation caused by natural 

selection against immigrants from divergent habitats. Evolution. 59(4): 705–719. 

O’Brien S.J.; Evermann, J.F. 1988. Interactive influence of infectious disease and 

genetic diversity in natural populations. Trends in Ecology and Evolution. 3(10): 

254–259. 
106 

GENERAL TECHNICAL REPORT PSW-GTR-254

Paetkau, D.; Slade, R.; Burden, M.; Estoup, A. 2004. Genetic assignment 

methods for the direct, real-time estimation of migration rate: a simulationbased exploration of accuracy and power. Molecular Ecology. 13(1): 55–65. 

Palsbøll, P.J.; Bérubé, M.; Allendorf, F.W. 2007. Identification of management 

units using population genetic data. Trends in Ecology and Evolution. 22(1): 

11–16. 

Peery, M.Z.; Beissinger, S.R.; House, R.F.; Bérubé, M.; Hall, L.A.; Sellas, A.; 

Palsbøll, P.J. 2008. Characterizing source-sink dynamics with genetic parentage 

assignments. Ecology. 89(10): 2746–2759. 

Peery, M.Z.; Hall, L.A.; Sellas, A.; Beissinger, S.R.; Moritz, C.; Bérubé, M.; 

Raphael, M.G.; Nelson, S.K.; Golightly, R.T.; McFarlane-Tranquilla, L.; 

Newman, S.; Palsbøll, P.J. 2010. Genetic analyses of historic and modern 

marbled murrelets suggest decoupling of migration and gene flow after habitat 

fragmentation. Proceedings of the Royal Society B-Biological Series. 277(1682): 

697–706. 

Peery, M.Z.; Kirby, R.; Reid, B.N.; Stoelting, R.; Doucet-Bëer, E.; Robinson, 

S.J.; Vasquez-Carrillo, J.N.; Palsbøll, P.J. 2012. Reliability of genetic 

bottleneck tests for detecting recent population declines. Molecular Ecology. 

21(14): 3403–3418. 

Pradel, R. 1996. Utilization of capture-mark-recapture for the study of recruitment 

and population growth rate. Biometrics. 52(2): 703–709. 

Pulliam, H.R. 1988. Sources, sinks, and population regulation. American 

Naturalist. 132(5): 652–661. 

Rockweit, J.T.; Franklin, A.B.; Bakken, G.S.; Gutiérrez, R.J. 2012. Potential 

influences of climate and nest structure on spotted owl reproductive success: 

a biophysical approach. PLoS ONE. 7(7): e41498. doi:10.1371/journal. 

pone.0041498. 

Ryder, O.A. 1986. Species conservation and systematics: The dilemma of 

subspecies. Trends in Ecology and Evolution. 1(1): 9–10. 

Samuel, M.D.; Pierce, D.J.; Garton, E.O. 1985. Identifying areas of concentrated 

use within the home range. Journal of Animal Ecology. 54(3): 711–719. 

Schumaker, N.H.; Brookes, A.; Dunk, J.R.; Woodbridge, B.; Heinrichs, J.A.; 

Lawler, J.J.; Caroll, C.; LaPlante, D. 2014. Mapping sources, sinks, and 

connectivity using a simulation model of northern spotted owls. Landscape 

Ecology. 29(4): 579–592. 
107 

The California Spotted Owl: Current State of Knowledge

Seamans, M.E. 2005. Population biology of the California spotted owl in the 

central Sierra Nevada. Saint Paul, MN: University of Minnesota. 141 p. Ph.D. 

dissertation. 

Seamans, M.E.; Gutiérrez, R.J. 2007. Sources of variability in spotted owl 

population growth rate: testing predictions using long-term mark-recapture data. 

Oecologia 152(1): 57–70. 

Seamans, M.E.; Gutiérrez, R.J.; Moen, C.A.; Peery, M.Z. 2001. Spotted owl 

demography in the central Sierra Nevada. Journal of Wildlife Management. 

65(3): 425–431. 

Stearns, S.C. 1976. Life-history tactics: a review of the ideas. Quarterly Review 

of Biology 51(1): 3–47. 

Stoelting, R.E.; Gutiérrez, R.J.; Kendall, W.L.; Peery, M.Z. 2015. Life history 

trade-offs and reproductive cycles in spotted owls. Auk. 132(1): 46–64. 

Tempel, D.J.; Gutiérrez, R.J. 2013. Relation between occupancy and abundance 

for a territorial Species, the California spotted owl. Conservation Biology. 27(5): 

1087–1095. 

Tempel, D.J.; Gutiérrez, R.J.; Whitmore, S.A.; Reetz, M.J.; Stoelting, 

R.E.; Berigan, W.J.; Seamans, M.E.; Peery, M.Z. 2014a. Effects of forest 

management on California spotted owls: implications for reducing wildfire risk 

in fire-prone forests. Ecological Applications. 24(8): 2089–2106.

Tempel, D.J.; Peery, M.Z.; Gutiérrez, R.J. 2014b. Using integrated population 

models to improve conservation monitoring: California spotted owls as a case 

study. Ecological Modelling. 289: 86–95. 

Verner, J.; McKelvey, K.S.; Noon, B.R.; Gutiérrez, R.J.; Gould, G.I., Jr.; 

Beck, T.W., eds. 1992. The California spotted owl: a technical assessment of its 

current status. Gen. Tech. Rep. PSW-GTR-133. Albany, CA: U.S. Department of 

Agriculture, Forest Service, Pacific Southwest Research Station. 285 p.

Wu, C.H.; Drummond, A.J. 2011. Joint inference of microsatellite mutation 

models, population history and genealogies using transdimensional Markov 

Chain Monte Carlo. Genetics. 188(1): 151–164. 

Zimmerman, G.S.; Gutiérrez, R.J.; LaHaye, W.S. 2007. Finite study areas and 

vital rates: sampling effects on estimates of spotted owl survival and population 

trends. Journal of Applied Ecology. 44(5): 963–971. 
108 

GENERAL TECHNICAL REPORT PSW-GTR-254
109 

The California Spotted Owl: Current State of Knowledge

Chapter 5: Current and Projected Condition of 

Mid-Elevation Sierra Nevada Forests 

Malcolm P. North, Mark W. Schwartz, Brandon M. Collins, 

and John J. Keane1 

Introduction 

Most of the California spotted owl’s (Strix occidentalis occidentalis) habitat is 

concentrated in mid-elevation forests of the Sierra Nevada (see chapter 9 for 

a discussion of southern California spotted owls and their habitat), which are 

made up primarily of ponderosa pine (Pinus ponderosa Lawson & C. Lawson), 

mixed-conifer, white fir (Abies concolor (Gord. & Glend.) Lindl. ex Hildebr.), and 

mixed-evergreen forest types. These forests have undergone substantial change 

since the arrival of Europeans and are projected to dynamically respond to ongoing factors affecting ecosystem conditions. In this chapter, we summarize some of 

the historical changes in mid-elevation forests that have most extensively altered 

ecosystem conditions. We also explore sources and spatial distribution of the more 

extant changes in forest condition. We then discuss likely trends in forest response 

to projected stressors, particularly climate change, drought, and fire. Finally, we 

examine recent research and resulting changes in management practices that might 

affect future forest conditions in an effort to increase ecosystem resilience. 

Forest Management 

Management practices, including fire suppression, over the past century have 

largely shaped current forest conditions in the Sierra Nevada. These conditions 

significantly vary with land ownership because owners have different incentives 

and constraints that influence their management practices. We examine the three 

main ownerships in the Sierra Nevada, their historical management practices, and 

current conditions of the different forests. 

Ownerships 

The U.S. Forest Service (USFS) is the largest steward of public lands in the Sierra 

Nevada. About 2.93 million ha (7.24 million ac; 47 percent of the 6.24 million ha 

1 Malcolm P. North is a research forest ecologist, Mark W. Schwartz is a professor in 

the Department of Environmental Science and Policy, University of California–Davis, 

Davis, CA 95616, Brandon Collins is a research scientist at the Center for Fire Research 

and Outreach, University of California Berkeley, Berkeley, CA 94598, John J. Keane 

is a research wildlife ecologist U.S. Department of Agriculture, Forest Service, Pacific 

Southwest Research Station, 1731 Research Park Dr., Davis, CA 95618. 
110 

GENERAL TECHNICAL REPORT PSW-GTR-254

[15.42 million ac]) in the Sierra Nevada bioregion are under USFS management 

(Davis and Stoms 1996). The USFS has a broad mandate of managing national 

forests for multiple use and providing sustainable ecosystem services (NFMA 

1976). About 2.34 million ha (5.77 million ac; about 37 percent) of conifer forests 

in the Sierra Nevada are in private ownership (Davis and Stoms 1996), and their 

management is governed by California’s Forest Practice Regulations, which promotes “achiev[ing] a balance between growth and harvest over time consistent with 

the harvesting methods within the rules of the Board, maintain functional wildlife 

habitat..., retain or recruit late and diverse seral stage habitat components..., and 

maintain growing stock, genetic diversity, and soil productivity” (CA FPR Section 

897, USDA FS 2012). There are five national parks in the Sierra Nevada and southern Cascades: Devil’s Postpile, Lassen, Sequoia and Kings Canyon, and Yosemite 

covering 696 000 ha (1.73 million ac). The National Park Service (NPS) serves as 

steward of these parks and is under a mandate to provide recreational opportunities 

for people, and to protect and showcase natural resources without exploitation. 

Historical Management Practices 

Until about 1990, similar management objectives and silvicultural prescriptions 

were used on both public and private lands (McKelvey and Johnston 1992). Therefore, we combined our synopsis of management practices during this period for 

both ownerships. 

Early logging prior to 1900 occurred mainly near mining operations and associated communities at low elevations in the southern and central Sierra Nevada, with 

most logging occurring below national forest lands. Logging extended to mid and 

high elevations in the northern Sierra Nevada to support mining at higher elevations 

and lands adjacent to the Southern Pacific Railroad line (McKelvey and Johnston 

1992). The Lake Tahoe and Truckee River basins were exceptions to these general 

patterns as they were extensively logged to support the Comstock silver mines in 

western Nevada. Away from railroad lines, log removal was limited to wagons and 

short-haul skidding with animals and steam “donkeys.” Because of these transportation limitations, most logging consisted of high-grading of large and valuable trees. 

With improvements in transportation, timber harvest increased steadily after 1900, 

although it declined for about a decade during the Great Depression. Timber harvest 

in the Sierra Nevada peaked in the post-World War II years, and then stabilized 

generally ranging between about 1.3 and 1.7 million board feet (mmbf) (1960 through 

1990), with a short decline during the 1980s recession (McKelvey and Johnston 1992). 
111 

The California Spotted Owl: Current State of Knowledge

Prior to the 1980s, most silvicultural prescriptions were selection harvests of 

commercially valuable trees, leaving those with marginal value standing. Clearcutting prescriptions were incorporated in the 1970s, and clearcuts accounted for most 

of the volume in the mid-1980s. In the late 1980s, most volume was harvested using 

salvage prescriptions, following mortality principally from fire and insect events. 

In general, similar harvest prescriptions tended to be implemented on public and 

private lands prior to 1990 (McKelvey and Johnston 1992). 

A legacy of management practices during this period is a reduction in “defect” 

trees (Bouldin 1999). Over many decades, stand improvement practices often 

involved removing “defects” such as trees with broken tops, missing limbs, rot and 

large cavities (see Walsh and North 2012 for examples). Such defect trees typically 

require many years to develop and thus decades of this practice have probably 

resulted in a significant decline in these structures often used by wildlife for nesting, resting, and roosting habitat (Bull et al. 1997, Carey 2002, Carey et al. 1997, 

Cockle et al. 2011, Hunter and Bond 2001, Wiebe 2011). 

McKelvey and Johnston (1992) summarized four key changes in forest conditions that occurred from 1850 through 1992: (1) the loss of old, large-diameter trees 

and associated large downed logs; (2) a shift in species composition toward shadetolerant, fire-sensitive tree species (i.e., from pines to fir and cedar); (3) increases in 

fuel loads associated with the mortality of small-diameter trees; and (4) the presence of fuel ladders that facilitate crown fire. Further, they indicated that management direction identified in land management plans (LMPs) for Sierra Nevada 

national forests current at that time would likely not alleviate these concerns and 

trends in forest dynamics. The LMPs projected that national forest lands in the 

western Sierra Nevada would be converted to even-age systems using clearcut, seed 

tree, and shelterwood prescriptions at a rate of 91 600 ha (226,350 ac) per decade 

and that selection logging would occur on 32 000 ha (80,000 ac) per decade. This 

management direction provided no guarantees that old, large trees and their derivatives (e.g., large snags and logs) would be maintained. Rather, it suggested large 

proportions of future forest would trend toward areas of even-aged plantations with 

stands of dense, smaller diameter trees (McKelvey and Johnston 1992). 

Forest Management Since 1990 

Concern for the conservation of California spotted owls began in the mid-1980s 

with awareness first raised over the status of the related subspecies, the northern 

spotted owl (Strix occidentalis caurina). With the adoption of the California spotted 

owl guidelines following the California spotted owl technical assessment (CASPO) 

in 1992 (Verner et al. 1992), national forest and private ownership management 
112 

GENERAL TECHNICAL REPORT PSW-GTR-254

practices significantly diverged in the mid-1990s. Timber harvest dramatically 

decreased on USFS lands. Overall, 83.4 percent of the timber volume harvested 

between 1994 and 2013 was generated from private lands with public lands contributing from 10 to 24 percent (fig. 5-1). 

National Forest System lands— 

About 490 000 ha (1.2 million ac) of treatments occurred on National Forest System 

(NFS) lands in the Sierra Nevada between 1990 and 2014 (table 5-1; see appendix p. 

155 for more detailed information). The number of treated acres has declined over 

time, from highs of 40 000 to 48 000 ha (100,000 to 120,000 ac) per year in 1990– 

1992 to a low of around 8000 ha (20,000 ac) per year in 2011–2013 (table 5-1, fig. 

5-2). The highest proportion of total treated acres from 1990 through 2014 occurred 

on the Lassen (113 966 ha [284,916 ac]) and Plumas National Forests (101 764 ha 

[254,411 ac]), with intermediate amounts on the Stanislaus (63 802 ha [159,804 ac]), 

Tahoe (62 683 ha [156,708 ac]), and Eldorado (58 098 ha [145,244 ac]) National 

Forests (fig. 5-3). 

Concurrent with a decline in the number of acres treated has been a change 

in the predominant silvicultural prescriptions used on NFS lands (table 5-1, fig. 

5-2). From 1990 through 1994, the predominant silvicultural prescriptions were 

sanitation and salvage cuts, followed by lower amounts of clearcuts and overstory 

removal. Adoption of CASPO guidelines in 1993 led to an increase in commercial 

thinning following CASPO guidelines that maintained all trees >30 in diameter at 

breast height (d.b.h.), maintained overstory canopy cover >40 percent, and removed 

0 

200,000 

400,000 

600,000 

800,000 

1,000,000 

1,200,000 

1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 

Volume (thousand board feet)

Public land 

Private land 

Year 

Figure 5-1—Annual timber volume harvested (thousand board feet [mmbf]) by year on public and private 

lands from counties in the Sierra Nevada 1994–2013. See text for further details. Source: Timber Yield 

Tax program, California State Board of Equalization. 
113 

The California Spotted Owl: Current State of Knowledge

Table 5-1— Treatment hectares accomplished on National Forest in the Sierra Nevada by silvicultural prescription and year during 1990–2014 

Overstory Single tree 

Clear- Commercial Group removal Salvage Sanita- Seed selection 

Year cut thin selection Other cut cut tion cut tree cut cut Total 

1990 5,390 4,113 109 134 4,023 16,052 15,017 970 1,625 47,434 1991 4,000 1,746 49 67 2,086 14,004 14,831 999 1,951 39,733 1992 3,770 855 33 16 2,121 11,714 29,853 341 929 49,631 1993 2,425 2,052 90 22 1,799 10,620 18,011 552 686 36,258 1994 2,063 1,131 36 56 1,506 3,278 8,924 283 152 17,428 1995 3,120 5,082 33 143 2,636 674 7,529 369 437 20,022 1996 7,389 7,905 148 79 582 576 9,764 511 2,581 29,535 

1997 2,191 10,199 120 207 135 688 9,815 146 658 24,159 1998 689 12,573 25 142 559 2,335 12,910 74 620 29,927 

1999 346 13,617 449 423 45 2,622 948 21 826 19,296 

2000 31 10,940 93 929 0 183 1,728 17 248 14,169 

2001 28 7,635 280 632 8 126 1,094 48 96 9,947 

2002 263 8,864 400 370 43 160 256 0 330 10,686 

2003 2,451 7,611 100 186 0 9,893 1,010 0 91 21,342 2004 13 13,611 273 0 0 1,414 1,402 5 1,250 17,969 

2005 3 5,067 108 0 1 669 1,245 2 900 7,995 

2006 1,714 12,755 410 29 3 3,648 617 0 454 19,630 2007 201 11,331 252 17 0 297 388 0 31 12,516 

2008 0 3,743 26 8 0 1,047 315 0 77 5,216 

2009 103 6,668 157 42 0 6,632 452 0 307 14,360 

2010 238 7,546 399 97 0 1,046 176 0 320 9,822 

2011 110 6,086 184 96 0 690 24 0 1 7,190 

2012 0 7,924 203 44 0 216 0 0 90 8,477 

2013 0 5,046 39 1 0 3,675 84 0 0 8,844 

2014 194 1,931 45 3 0 5,437 0 0 91 7,701 

Total 36,732 176,028 4,063 3,740 15,547 97,694 136,393 4,339 14,751 489,287 
114 

GENERAL TECHNICAL REPORT PSW-GTR-254

Figure 5-2—Treatment acres accomplished on national forests in the Sierra Nevada by silvicultural 

prescription and year, 1990–2014. Source: Taken from USFS Forest Activities Tracking System 

courtesy of Joe Sherlock (Pacific Southwest Research Region silviculturist). 
115 

The California Spotted Owl: Current State of Knowledge

Figure 5-3—Treatment acres accomplished on national forests in the Sierra Nevada by silvicultural 

prescription and national forest, 1990–2014. LTBMU = Lake Tahoe Basin Management Unit. Source: 

Taken from USFS Forest Activities Tracking System courtesy of Joe Sherlock (Pacific Southwest 

Research Region silviculturist). 

small trees to an upper diameter limit. The proportion of sanitation cuts dropped in 

1999 as existing contracts established before CASPO were completed and CASPO 

prescriptions became the predominant silvicultural prescription. Commercial thinning associated with CASPO guidelines, a focus on forest thinning to meet fuels 

reduction objectives, and postfire salvage logging have been the dominant prescriptions on NFS lands in the Sierra Nevada between 1999 and 2014 (fig. 5-2). 

About 255 143 ha [665,357 ac] of silvicultural treatments occurred within the 

range of the California spotted owl in the Sierra Nevada (see appendix p. 155 for 

details), of which about 199 600 ha (299,000 ac; 45 percent) were treated from 2002 

through 2014 when NFS spatial data on treatments was complete (table 5-2, fig. 5-4). 

Sanitation cuts were the predominant silvicultural prescription used during 1990– 

1994. Commercial thin was the predominant prescription used during 1996–2013, 

followed by episodic salvage events and smaller amounts of clearcutting (table 5.2). 
116 

GENERAL TECHNICAL REPORT PSW-GTR-254 Table 5-2—Treatment hectares accomplished on National Forest lands within the range of the California spotted owl in the Sierra Nevada by silvicultural prescription and year during 1990–2014 Overstory Single Tree Clear Commercial Group Removal Salvage Sanitation Seed-tree Selection Year Cut Thin Selection Other Cut Cut Cut Cut Cut Total 1990 4,123 1,246 109 121 3,213 1,467 10,348 899 1,577 23,104 1991 3,262 1,486 49 67 1,864 5,803 6,599 986 1,936 22,051 1992 3,488 609 11 16 973 4,888 7,257 305 834 18,380 1993 1,790 1,228 89 22 417 4,268 733 424 641 9,613 1994 1,746 851 36 56 551 166 4,164 253 152 7,975 1995 1,341 2,852 11 143 559 24 747 348 137 6,163 

1996 742 3,655 78 40 89 0 5,316 139 286 10,344 1997 2,146 4,396 66 180 135 47 746 146 459 8,322 1998 418 8,151 20 90 29 2,298 1,088 57 83 12,235 1999 250 8,555 282 286 45 2,469 670 6 811 13,375 

2000 4 6,837 43 470 0 168 1,340 17 214 9,094 2001 28 5,925 127 632 8 111 439 48 94 7,412 2002 263 4,847 28 370 43 93 79 0 329 6,051 2003 2,450 5,032 3 186 0 9,480 814 0 45 18,010 2004 13 9,255 94 0 0 824 1,284 5 643 12,119 2005 3 4,359 50 0 1 576 650 2 209 5,850 2006 1,724 11,012 408 29 3 2,187 568 0 454 16,386 2007 115 9,448 231 7 0 281 319 0 28 10,428 2008 0 3,203 8 8 0 1,041 313 0 66 4,639 2009 4 5,295 96 38 0 6,343 411 0 43 12,230 2010 7 7,971 343 89 0 1,045 174 0 314 9,942 2011 12 5,431 172 77 0 201 20 0 0 5,912 

2012 0 6,734 183 37 0 75 0 0 90 7,119 2013 0 5,632 14 1 0 3,574 82 0 0 9,303 2014 0 760 14 0 0 2,339 0 0 91 3,203 Total 23,928 124,768 2,565 2,964 7,929 49,770 44,162 3,637 9,538 269,261 
117 

The California Spotted Owl: Current State of Knowledge

Figure 5-4—Treatment acres accomplished on national forest lands within the range of the California spotted owl in the Sierra Nevada by silvicultural prescription and national forest, 1990–2014. 

Sources: Taken from USFS Forest Activities Tracking System courtesy of Joe Sherlock (Pacific 

Southwest Research Region silviculturist); owl range from California Department of Fish and 

Wildlife. 
118 

GENERAL TECHNICAL REPORT PSW-GTR-254

Current USFS practices often focus on two metrics when implementing management treatments; maximum tree diameter removed (“diameter limits”) and 

residual canopy cover. Although trees up to 75 cm (30 in) d.b.h can be marked for 

removal, in many forests that have been previously thinned, the maximum diameter 

limit is set to a lower size because removing larger trees would drop the residual 

canopy cover below the target. Canopy cover is usually indirectly estimated using 

the Forest Vegetation Simulator or FVS model based upon the number, size, and 

species of the leave trees. As an indirect estimate, FVS assumes a certain amount of 

crown overlap (Crookston and Stage 1999) and does not account for spatial variability in tree locations (Christopher and Goodburn 2008). Nor does the FVS-generated 

canopy cover target consider canopy closure patterns or distinguish between 

clumped or regular distributions, differences that appear to be important functional 

and structural attributes of fire-adapted forests (Churchill et al. 2013, Larson and 

Churchill 2012, Lydersen et al. 2013). 

Canopy cover targets are a featured objective in recent management guidance 

documents (e.g., Sierra Nevada Forest Plan Amendments of 2001 and 2004; USDA 

FS 2001, 2004) and are set to be no lower than an average of 40 percent in the larger 

“home range core area” (HRCA), and no lower than an average of 50 percent in the 

“protected activity center” (PAC). Treatment in owl PACs is intended to be limited 

(see the 2004 Sierra Nevada Forest Plan Amendment; USDA FS 2004), but canopy 

cover targets are still widely used when fuels reduction treatments are implemented 

within the HRCA on NFS lands. The cumulative area of PACs and HRCAs affects 

a fairly large proportion of a landscape. See chapter 3 for more details on these 

management designations and their detailed definitions. 

On national forests, some aspects of spotted owl habitat have likely improved 

since the 1992 release of the CASPO guidelines. Average tree diameter in many 

forests has increased because of growth and the removal of smaller trees in treated 

stands while retaining all trees >75 cm (30 in) d.b.h. In general, the amount of forest 

dominated by large trees is probably gradually increasing, although some studies 

suggest climate change or drought mortality may be disproportionately higher in 

larger than smaller trees (Lutz et al. 2009, van Mantgem et al. 2009). Likewise, 

forest growth increases canopy cover and, even in treated stands, cover is retained 

at 40 percent or greater. 

However, in three of the four owl demographic areas, populations are declining. It is uncertain to what degree some of this decline is due to legacy effects 

(e.g., loss of large tree and defect structure removal and reduction in canopy cover) 

before CASPO guidelines took hold after 1992. Compounding the uncertainty is 
119 

The California Spotted Owl: Current State of Knowledge

the increased role of high-severity wildfire in changing forest conditions. More owl 

habitat is now affected by wildfire than by mechanical treatment each year (North 

et al. 2012), and its effects on habitat conditions likely vary with severity and patch 

size effects of fire behavior. 

Private industrial forest lands— 

About 1.2 million ha (2.9 million ac) of silvicultural treatments were approved or 

completed on private industrial forestlands between 1990 and 2013 (table 5-3; see 

appendix p. 155 for detailed information). Of the majority of acres attributed with 

a specific silvicultural prescription, the predominant treatments were selection cuts 

(322 652 ha [806,630 ac]), shelterwood cuts (201 622 ha [504,054 ac]), commercial 

thins (114 460 ha [286,152 ac]), clearcuts (105 493 ha [263,733 ac]), and sanitation salvage cuts (82 541 ha [206,352 ac]) across the 1990-2013 assessment period 

(table 5-3). The highest numbers of treated acres were recorded for Shasta, Lassen, 

Plumas and Tehama Counties (table 5-4). At least 403 876 ha (998,000 ac) of treatment are recorded to have occurred within the range of the California spotted owl 

in the Sierra Nevada during 1997–2013 (table 5-5). 

On average, forests on private land are younger (71 years) than those on public 

land (104 to 115 years) (Stewart et al. 2016) and often lack the stand structural 

features associated with old forests such as “defect” trees and large snags and logs. 

Most commercial harvest is concentrated on the large ownerships predominantly 

in the southern Cascades and northern Sierra Nevada. Almost 60 percent of commercial harvest on private lands comes from five northern California counties 

(Humboldt, Shasta, Siskiyou, Mendocino, and Plumas), and collectively, private 

ownership forests produce about 85 percent of California forests’ lumber, pulp and 

bioenergy products (Morgan et al. 2012). 

National parks— 

The NPS maintenance of mid-elevation forest conditions faces three challenges. 

A primary constraint to NPS resource management is that much of these parks 

is within federally designated wilderness areas, and mechanical manipulation is 

restricted in these areas. The NPS does not generally mechanically manipulate 

vegetation but will for human safety or park infrastructure. Hence, managing tree 

density can only be accomplished with fire; both prescribed fire and managed wildland fire. The NPS is further constrained by a limited capacity to deploy prescribed 

fire. Limited staffing and air quality restrictions generally result in a relatively small 

fraction of the national parks being treated with prescribed fire (North et al. 2012). 

The prescribed fire that has been deployed is typically limited to areas of high 
120 

GENERAL TECHNICAL REPORT PSW-GTR-254 Table 5-3—Treatment hectares completed or approved in timber harvest plans on private industrial forest lands in the Sierra Nevada by silvicultural prescription and year during 1990–2013 Damaged Seed- Clear Commercial timber- Fuel Group Sanitation tree Selection Shelter- Year cut thin Conversion land break selection Other Rehabilitation salvage cut cut wood cut Transition Total 1990 1,595 1,265 0 0 0 0 21,271 679 1,811 2,024 6,643 8,673 8,026 51,986 1991 3,852 6,909 0 0 0 0 11,041 229 2,827 1,171 6,206 12,483 9,014 53,732 1992 478 4,677 0 0 0 0 9,180 568 1,382 795 7,620 4,622 9,909 39,231 1993 956 1,945 6 0 0 0 10,056 826 1,813 1,061 12,410 7,303 7,129 43,504 1994 998 3,826 16 0 0 0 8,105 634 2,044 1,659 7,708 3,498 9,750 38,236 1995 1,075 4,689 8 0 0 0 9,710 623 2,446 1,205 12,767 5,354 8,864 46,741 1996 1,132 6,789 177 0 0 5 12,130 585 4,567 1,395 13,860 7,073 10,374 58,087 1997 711 5,887 0 0 0 96 7,513 773 3,309 833 20,285 3,369 5,021 47,798 

1998 735 5,763 220 0 0 18 6,869 1,957 11,368 419 10,976 3,809 4,024 46,159 1999 1,977 6,480 2 0 32 407 6,384 1,635 2,781 864 27,269 5,931 4,041 57,803 2000 2,099 7,733 600 0 193 1,404 1,412 559 4,214 1,162 19,386 10,665 713 50,139 

2001 4,274 9,477 27 2 274 1,316 2,819 1,252 3,272 2,519 11,973 12,582 348 50,136 2002 2,618 6,679 164 17 380 2,463 307 806 2,691 1,019 14,041 10,079 91 41,356 

2003 4,325 4,920 334 75 254 1,239 876 1,581 4,991 2,951 17,619 10,361 465 49,990 2004 4,730 6,692 149 104 902 1,294 344 743 4,015 921 14,841 11,297 459 46,490 

2005 5,990 3,997 436 107 243 2,053 22 688 2,852 1,861 11,713 14,582 166 44,709 

2006 4,901 1,663 382 15 160 2,110 45 933 1,120 1,076 12,160 5,095 59 29,719 

2007 4,066 4,460 576 0 203 3,086 47 703 3,639 2,965 9,821 14,357 170 44,092 

2008 6,768 4,813 107 3 1,049 3,185 133 402 2,649 1,013 16,403 8,625 90 45,240 

2009 4,963 1,591 85 192 181 1,962 0 327 767 454 13,150 10,238 49 33,959 

2010 3,674 328 46 239 179 2,772 0 122 1,049 196 4,614 4,210 0 17,428 

2011 3,755 881 193 405 1,068 4,509 0 158 639 187 7,093 3,122 27 22,038 

2012 7,678 2,311 139 377 433 2,566 0 104 4,443 347 9,625 5,567 93 33,684 

2013 6,552 1,124 238 1,390 408 10,996 0 153 436 194 5,058 4,317 186 31,053 

Approved (no 26,829 10,900 794 1,803 2,505 42,383 1,716 1,181 12,384 2,091 33,189 16,773 817 153,366 

completiondate)

Total 106,729 115,801 4,699 4,730 8,464 83,866 109,980 18,221 83,508 30,380 326,432 203,983 79,884 1,176,677 
121 

The California Spotted Owl: Current State of Knowledge

Table 5-4—Treatment hectares completed or approved in timber harvest plans on private industrial forest lands in the Sierra Nevada by silvicultural prescription and county during 1990–2013 

Damaged SeedClear- Commercial timber- Fuel Group Sanitation tree Selection ShelterCounty cut thin Conversion land break selection Other Rehabilitation salvage cut cut wood cut Transition Total 

Alpine 55 44 8 0 0 0 246 0 181 25 92 38 0 688 

Amador 2,262 449 341 0 873 630 5,021 737 312 1,535 5,684 3,575 3,209 24,629 

Butte 9,384 6,254 35 656 258 2,362 7,598 3,909 2,724 2,910 13,516 13,282 2,056 64,945 

Calaveras 5,788 3,391 84 0 1,192 1,721 9,709 1,237 747 2,071 16,909 3,991 4,659 51,500 

El Dorado 12,968 6,170 237 0 619 1,321 13,020 1,732 7,975 5,962 9,651 19,576 6,937 86,169 

Fresno 117 490 326 0 0 457 1,153 157 214 116 20,655 15 352 24,053 

Kern 0 12 0 0 0 0 258 97 652 0 3,383 0 0 4,403 

Lassen 12,610 20,689 37 55 457 15,660 10,313 1,891 12,162 167 66,747 18,695 8,200 167,682 

Madera 0 0 0 0 0 0 165 81 77 7 905 151 466 1,852 

Mariposa 38 81 0 0 124 343 682 642 208 330 3,766 399 812 7,426 

Nevada 2,897 2,545 1,021 5 321 8,107 3,017 704 5,377 1,386 11,709 13,926 3,886 54,902 

Placer 2,831 2,688 1,023 0 203 3,815 10,000 1,325 8,454 1,729 10,019 17,115 4,075 63,276 

Plumas 6,003 13,664 1,267 2,266 1,438 15,356 3,780 732 5,761 2,021 45,186 19,998 12,413 129,887 

Shasta 34,379 40,782 283 1,410 1,785 19,590 29,586 2,434 28,402 4,692 63,212 47,773 15,069 289,396 

Sierra 1,406 4,902 2 0 42 6,794 1,522 202 3,498 719 7,546 6,886 1,067 34,586 

Tehama 9,674 8,639 7 297 565 3,994 9,255 873 3,910 3,259 22,219 29,974 8,559 101,224 

Tulare 0 91 0 0 122 2 334 42 453 40 2,111 0 164 3,359 

Tuolumne 4,222 3,860 7 26 404 133 1,932 551 1,368 2,709 20,139 1,479 6,663 43,493 

Yuba 2,095 1,050 21 15 61 3,583 2,391 874 1,031 703 2,983 7,108 1,294 23,209 

Total 106,729 115,801 4,699 4,730 8,464 83,866 109,980 18,221 83,508 30,380 326,432 203,983 79,884 1,176,677 
122 

GENERAL TECHNICAL REPORT PSW-GTR-254 Table 5-5—Treatment hectares completed in timber harvest plans on private industrial forest lands within the range of the California spotted owl in the Sierra Nevada by silvicultural prescription during 1997–2013 Silvicultural Prescription 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 Totals Clearcut 14 55 449 896 1,873 1,901 3,321 3,467 4,642 2,762 2,748 5,195 3,240 2,252 1,768 4,423 3,112 42,118 

Commercial thin 42 22 295 416 1,115 1,606 2,770 2,646 2,360 696 1,066 3,478 1,418 265 805 1,221 1,020 21,241 Conversion 0 1 2 31 1 17 201 93 29 188 388 89 59 46 177 119 236 1,679 

Fuelbreak/defensible space 0 0 32 173 165 304 250 889 141 143 203 583 181 105 954 387 408 4,917 Group selection 7 8 394 601 514 1,895 1,096 1,167 2,038 2,022 2,886 3,101 1,724 1,381 4,381 1,861 8,900 33,976 Rehabilitation— 113 30 32 124 215 165 912 354 450 286 382 197 233 74 11 46 116 3,742 

understocking

Sanitation salvage 44 21 313 163 594 1,235 1,715 2,296 2,402 821 2,089 2,431 640 622 246 2,633 318 18,582 Seed tree removal cut 157 21 212 694 173 300 958 136 264 437 489 97 28 40 55 172 92 4,326 Seed tree seed cut 30 3 22 36 157 297 603 467 681 483 2,023 430 426 147 132 159 94 6,189 Selection 127 547 3,047 7,253 2,647 6,415 12,218 9,617 8,749 7,029 7,321 13,049 11,803 4,133 3,257 4,401 3,741 105,355 Shelterwood prep cut 0 0 0 0 0 94 71 1 752 58 53 0 0 13 0 5 0 1,047 Shelterwood rem/ 0 93 0 0 5 239 0 358 829 32 935 874 505 668 60 1,091 0 5,688 

commercial thin

Shelterwood removal cut 123 461 1,316 1,595 2,718 2,688 5,425 5,885 7,184 3,498 6,230 5,826 4,523 1,598 1,641 2,769 2,342 55,821 Substantially damaged tim- 0 0 0 0 0 17 31 103 0 15 0 3 192 1 405 135 511 1,413 

berland

Transition 87 48 38 225 50 45 83 87 58 59 101 86 49 0 26 52 44 1,137 

Totals 743 1,311 6,152 12,207 10,228 17,219 29,653 27,566 30,581 18,528 26,914 35,440 25,020 11,345 13,917 19,474 20,934 307,230 
123 

The California Spotted Owl: Current State of Knowledge

human use (e.g., sequoia groves, Yosemite Valley). Consequently, the NPS uses 

wildfire to the extent possible to accomplish forest management objectives (van 

Wagtendonk and Lutz 2007). Managing to retain and restore resilient forest ecosystems has been more aggressive on NPS lands than other areas because NPS policy 

enables wildfires in appropriate locations to run their course when feasible.

There has been a recent untethering of the NPS resource stewardship from 

directives of striving for historical representation (USDI NPS 2012) with growing 

recognition that this is an unattainable and undesirable goal (USDI NPS 2012). In 

response, NPS has taken on management planning to build ecosystem resilience for 

coping with changing climates. “National Park Natural Condition Assessments” are 

designed to identify key indicators of natural condition (http://www.nature.nps.gov/ 

water/nrca/). “Resource Stewardship Strategies” (http://www.nature.nps.gov/water/ 

planning/resourcestewardshipstrategies.cfm) are attempts to plan for future management, including climate change (http://www.nps.gov/subjects/climatechange/ 

response.htm). Parks units are now compelled to consider climate change adaptation 

and how to manage for climate-resilient forests. This new management directive is 

likely to include incentives to foster forested ecosystems that are resilient to a range 

of future stressors. 

Current Status of Forests With Potential California Spotted 

Owl Habitat 

Focusing solely on lands included within the California wildlife habitat relations 

(CWHR)-defined California spotted owl range map for the Sierra Nevada, existing 

vegetation classification and mapping (EVEG) estimates that there are about 1.98 

million ha (4.9 million ac) of CWHR class 4M or greater habitat (4M, 4D, 5M, 5D, 

6) (>30 cm [12in] d.b.h., >40 percent canopy cover), with approximately 75, 7, and 

18 percent occurring on NFS, NPS, and private/other government (POG) lands, 

respectively (table 5-6). About 53 percent of the 4M and greater classes are classified as Sierra Nevada mixed conifer (SMC), with the majority of SMC occurring 

on NFS lands. About 1.2 million ha (2.9 million ac) of 4D and greater classes (4D, 

5D, 6) (>30 cm [12 in] d.b.h., >60 percent canopy cover) are estimated to be present, 

with 73, 9, and 18 percent distributed across NFS, NPS, and POG lands, respectively (table 5-7). The 4D and greater class habitat is predominantly classified as 

white fir (53 percent). For CWHR class 5M and above (5M, 5D, 6) (>60 cm [24 in] 

d.b.h., >40 percent canopy cover), about 607 029 ha (1.5 million ac) are estimated 

with 80, 10, and 10 percent distributed on NFS, NPS, and POG lands, respectively 

(table 5-8). The 5M and above class is classified primarily as SMC (63 percent). 
124 

GENERAL TECHNICAL REPORT PSW-GTR-254 Table 5-6—Distribution (hectares) of California wildlife habitat relationships class 4M or greater (4M, 4D, 5M, 5D, 6) by vegetation type and land ownership within the range of the California spotted owl in the Sierra Nevada Montane Sierran Blue oak Douglas- Eastside Jeffrey Lodgepole hardwood/ Montane Ponderosa mixed/ White 

Agency woodland fir pine pine pine conifer hardwood pine Red fir conifer fir Other Total 

National forest:

Eldorado 76 0 3 1,147 1,371 5,361 6,836 12,492 31,045 110,115 9,708 545 178,698 

Inyo 0 0 128 130 1,760 14 108 0 581 1,943 134 1,648 6,448 

Lassen 61 0 2,443 3,053 3,014 2,101 1,628 7,276 9,733 109,522 26,607 395 165,834 

LTBMU 0 0 14 5,985 2,041 8 0 0 3,928 13,671 2,083 925 28,655 

Plumas 63 10,817 5,736 277 218 19,422 7,505 6,450 7,725 189,595 40,043 172 288,024 

Sequoia 3,014 0 918 8,259 3,818 11,508 20,575 9,744 27,643 71,187 950 3,282 160,898 

Sierra 1,317 0 0 2,311 8,833 21,699 26,239 25,945 36,208 89,457 934 21,977 234,921 

Stanislaus 798 0 0 2,475 1,263 14,242 17,660 21,667 25,933 108,734 6,086 569 199,427 

Tahoe 262 20,594 1,004 446 2,689 11,322 7,865 5,411 22,943 130,935 17,331 488 221,291 

National park:

Lassen Volcanic 0 0 0 45 91 83 0 0 1,475 5,772 1,464 162 9,092 

National Park

Sequoia and Kings 0 0 0 4,605 1,299 3 707 1 10,185 29,100 3,876 3,204 52,980 

Canyon National Parks

Yosemite National Park 51 124 0 6,379 3,577 151 2,560 2,594 15,873 51,009 2,055 2,417 86,791 

Private/other government 7,676 32,841 5,453 266 433 50,934 44,103 58,134 357 146,686 2,961 3,090 352,934 

Total 13,319 64,376 15,699 35,381 30,407 136,851 135,786 149,714 193,629 1,057,726 114,232 38,873 1,985,993 
125 

The California Spotted Owl: Current State of Knowledge

Table 5-7—Distribution (hectares) of California wildlife habitat relationships class 4D or greater (4D, 5D, 6) by vegetation type and land ownership within the range of the California spotted owl in the Sierra Nevada 

Montane Sierran 

Blue oak Douglas- Eastside Jeffrey Lodgepole hardwood/ Montane Ponderosa mixed/ 

Agency woodland fir pine pine pine conifer hardwood pine Red fir conifer White fir Other Total 

National forest:

Eldorado 43 0 1 276 279 4,559 6,733 171 10,348 7,191 78,489 4,506 112,597 Inyo 0 0 9 16 552 10 72 210 0 98 285 14 1,267 Lassen 18 0 367 1,221 801 1,558 1,335 188 3,262 4,568 41,096 10,967 65,381 LTBMU 0 0 0 255 184 6 0 22 0 95 540 289 1,391 Plumas 22 9,520 943 56 4 15,993 5,160 164 4,164 2,038 82,963 12,766 133,793 Sequoia 728 0 27 1,347 573 9,011 19,073 1,105 7,657 8,982 40,907 249 89,658 Sierra 796 0 0 719 5,505 14,989 23,848 9,956 23,198 21,436 65,164 707 166,318 Stanislaus 461 0 0 329 320 12,539 16,982 155 16,148 10,150 80,245 4,823 142,153 Tahoe 127 16,881 239 189 1,644 7,434 5,426 182 2,704 8,826 97,631 8,270 149,552 

National park:

Lassen Volcanic 0 0 0 41 2 50 0 21 0 124 1,920 282 2,440 

National Park

Sequoia and Kings 0 0 0 1,590 517 3 615 848 1 8,150 24,686 3,106 39,516 

Canyon National Parks

Yosemite National Park 4 118 0 1,386 3,178 59 1,557 1,794 623 13,786 41,346 1,990 65,841 Private/other 3,274 25,002 881 41 228 41,461 31,520 1,612 39,692 112 74,863 1,148 219,833 

government

Total 5,473 51,520 2,468 7,465 13,786 107,674 112,320 16,426 107,797 85,555 630,137 49,117 1,189,740 
126 

GENERAL TECHNICAL REPORT PSW-GTR-254 Table 5-8—Distribution (hectares) of California wildlife habitat relationships class 5M or greater (5M, 5D, 6) by vegetation type and land ownership within the range of the California spotted owl in the Sierra Nevada Montane Sierran Blue oak Douglas- Eastside Jeffrey Lodgepole hardwood/ Montane Ponderosa mixed/ 

Agency woodland fir pine pine pine conifer hardwood pine Red fir conifer White fir Other Total 

National forest:

Eldorado 0 0 1 113 247 53 10 356 3 494 19 255 1 862 45 25 436 

Inyo 0 0 1 4 170 2 0 0 156 74 10 81 497 Lassen 12 0 39 196 96 468 56 978 1 754 26 684 5 423 96 35 803 LTBMU 0 0 0 931 467 2 0 0 774 1 865 46 260 4 346 Plumas 18 7 349 767 57 25 7 588 1 332 2 915 2 823 86 033 14 335 3 123 244 Sequoia 186 0 211 4 563 798 1 575 1 880 2 033 6 064 25 406 329 565 43 611 

Sierra 188 0 0 41 1 953 2 902 1 607 9 108 12 169 40 341 490 2 542 71 340 Stanislaus 12 0 0 352 273 1 035 64 1 155 7 298 26 106 2 738 86 39 120 Tahoe 89 15 545 378 279 1 661 6 383 757 1 661 16 704 95 909 12 072 290 151 729 

National park:

Lassen Volcanic 0 0 0 36 18 6 0 0 478 2 997 445 104 4 085 

National Park

Sequoia and Kings

Canyon National

Parks 0 0 0 0 0 2 0 0 1 4 397 0 0 4 400 

Yosemite National 7 46 0 1 389 1 686 20 82 1 074 10 750 33 991 1 708 984 51 737 

Park

Private/other 456 15 202 416 16 5 4 505 1 115 13 596 67 27 765 349 355 63 847 

government

Total 968 38 141 1 813 7 978 7 402 24 542 6 902 32 876 62 532 390 825 39 807 5 412 619 196 

LTBMU = Lake Tahoe Basin Management Unit. 
127 

The California Spotted Owl: Current State of Knowledge

Most acres of important California spotted owl habitat classes occur on NFS 

lands. Between about 133 547 and 166 326 ha (330,000 and 411,000 ac) of 4D and 

greater habitat is estimated to occur on the Sierra, Tahoe, Stanislaus, and Plumas 

National Forests, while between 65 155 and 112 503 ha (161,000 and 278,000 ac) 

are estimated to occur on the Eldorado, Sequoia, and Lassen National Forests 

(tables 5-6 to 5-8). The Inyo National Forest and Lake Tahoe Basin Management 

Unit support fewer habitat acres, as the Inyo overlaps minimally with the range of 

the California spotted owl in the Sierra Nevada, while habitat is generally limited to 

the western half of the Lake Tahoe basin. Although inferences about amounts and 

distributional patterns of California spotted owl habitat may be tempered given the 

uncertainty regarding the accuracy and consistency of the base vegetation maps, 

results highlight the importance of NFS lands for providing spotted owl habitat in 

the Sierra Nevada. About 73 to 80 percent of the CWHR habitat classes most often 

used by owls are estimated to currently occur on NFS lands. 

Historical Fire Effects on Mid-Elevation Forests 

Fire is a critical ecosystem process throughout Sierra Nevada mid-elevation forests. 

This is particularly the case for yellow and Jeffrey pine (P. jeffreyi Balf.) and mixedconifer forest types within the Sierra Nevada, where fire historically (i.e., pre-EuroAmerican settlement) occurred frequently, with generally low- to moderate-severity 

effects (Skinner and Taylor 2006, van Wagtendonk and Fites-Kaufman 2006). 

Numerous studies demonstrate that this fire frequency (5 to 15 years) maintained 

low-density stands across much of the landscape, composed of primarily large, 

fire-resistant trees. Reconstructed conifer densities (trees >15 cm [6 in] d.b.h.) in 

these forest types ranged from 60 to 82 trees/ha (24 to 41 trees/ac) (Collins et al. 

2011; Scholl and Taylor 2010; Taylor 2004, 2010). Collins et al. (2011) estimated the 

average canopy cover for historical forest conditions was 22 percent, with a range 

of 8 to 37 percent. Interestingly, these canopy cover estimates are similar to those 

measured in a contemporary Jeffrey pine-mixed-conifer forest that has a more intact 

disturbance regime (i.e., no timber harvesting and limited fire suppression) in the 

Sierra San Pedro Martir, Baja, California (Stephens and Gill 2005). However, stand 

density, structure, and composition likely varied depending on topographic and 

edaphic conditions, as well as a result of the stochastic patchiness of fire effects. 

The preponderance of evidence in the scientific literature currently supports the 

notion that contemporary forests that have not been subject to recent forest management (i.e., tree removal) are generally considerably denser than forests found prior 

to 100+ years of fire exclusion and selective logging. However, a few recent studies 

conducted in the Sierra Nevada challenge the prevailing understanding of historical 
128 

GENERAL TECHNICAL REPORT PSW-GTR-254

forest structure and fire patterns (see Baker 2014, Odion et al. 2014). They indicate 

that stand-replacing fire effects were a greater component of historical fire regimes 

than the predominant body of research suggests, and that resulting tree densities 

were greater than those reported in previous studies (e.g., Ansley and Battles 1998, 

Bouldin 1999, Collins et al. 2011, Knapp et al. 2013, McKelvey and Johnson 1992, 

North et al. 2007, Parson and Debenedetti 1979, Scholl and Taylor 2010, Taylor 

2004, Taylor et al. 2014, Vankat and Major 1978). Odion et al. (2014) used stand 

age estimates from Forest Inventory and Analysis data to infer past proportions of 

stand-replacing fire. From this they concluded that current “reference” conditions 

underrepresent early successional plant communities created by stand-replacing 

fire. Baker (2014) used historical tree data from land survey markers to reconstruct 

historical proportions and patch sizes of stand-replacing fire across large landscapes. He concluded that historical forests in the Sierra Nevada were generally 

much denser, hence supported much greater amounts of stand-replacing fire than 

other historical forest reconstructions have reported. The significance of his conclusions, and their applicability to restoration of mixed-conifer forests in the Sierra 

Nevada, merit careful consideration and vetting through the scientific community 

to reconcile the foundation of the discrepancies with existing published literature. 

Concerns about the source of the observed discrepancies include: 

• Potential bias in plot/tree selections. Baker used General Land Office 

survey witness trees that have been shown to be biased toward trees that 

were less likely to be harvested—smaller trees or less commercially valuable species, hence higher likelihood that the trees would persist as markers for locating survey points (see Bouldin 2008, Manies and Mladenoff 

2000). Odion et al. 2014 only included plot data from wilderness areas and 

national parks, which in the Sierra Nevada tend to be in higher elevations, 

hence a greater proportion of upper montane forest types. Upper montane 

forests are associated with longer intervals between fire and greater proportions of high-severity relative to the pine-mixed-conifer forests in the lower 

montane zone (Van de Water and Safford 2011). This limits the applicability 

of the study across the pine-mixed-conifer zone. 

• Limited density of tree samples. Baker (2014) relied on sampling densities 

that are less than 1 tree per (80 ac) 32.3 ha. 

• Misinterpretation of tree data. Odion et al. (2014) used composite stand-age 

estimates as evidence of postfire cohort initiation dates. These composite estimates have a high degree of error in capturing actual tree initiation 

dates, and as a result, are a poor representation of the time since last standreplacing disturbance (Stevens et al. 2016). 
129 

The California Spotted Owl: Current State of Knowledge

These limitations and others (see Fulé et al. 2013) call into question the robustness of these studies and their applicability toward forest restoration efforts. 

Several studies have demonstrated a high degree of spatial complexity across 

historical landscapes, which consisted of early seral vegetation (e.g., dense conifer 

regeneration, shrubs) and denser mature forest stands (e.g., Beaty and Taylor 2001, 

Collins et al. 2015, Nagel and Taylor 2005, Stephens et al. 2015, Taylor 2000), 

within a matrix of generally low-density stands. This complexity was likely a 

product of differential fire effects and timing, including some stand-replacing fire, 

driven by variability in multiple factors: vegetation/fuels, topography, site productivity/moisture availability, and climate. Estimates of historical stand-replacing 

fire in mixed-conifer and yellow pine forests range from 5 to 10 percent of the area 

within a burn at any given time (Mallek et al. 2013), which was likely aggregated 

in small patches (usually <2 ha [5 ac]) distributed across the landscape (Collins and 

Stephens 2010, Show and Kotok 1924). Drainage bottoms associated with larger 

perennial streams may have experienced less frequent fire than more upslope 

locations and thus were able to sustain more consistently dense and multilayered 

canopies (Collins and Skinner 2014). Another attribute associated with frequent fire 

in these forests is a complex spatial pattern of trees, consisting of isolated individuals, multiple tree clumps, and openings (Churchill et al. 2013, Fry et al. 2014, Knapp 

et al. 2012, Lydersen et al. 2013). This complexity was also most likely driven by 

fine-scale patchiness in fire effects and was yet another source of heterogeneity in 

historical forest conditions (Show and Kotok 1924). 

Drivers of Forest Change 

Current and Projected Fire Effects 

Irrespective of any uncertainty about the historical role of fire in the Sierra Nevada, 

contemporary fire patterns in the Sierra Nevada differ from those that occurred 

historically. The differences are in both overall proportion and patch sizes of standreplacing fire, which are in many cases greater for contemporary fires (Mallek et al. 

2013; Stephens et al. 2013, 2014). The proportion of stand-replacing fires and burn 

patch sizes also have been increasing in the Sierra Nevada from 1984 through 2010 

(Miller et al. 2009, Miller and Safford 2012, Steel et al. 2015). These changes in 

fire characteristics are driven by (1) fire suppression, which tends to constrain fire 

occurrence to burning primarily under the most extreme fire weather conditions 

because these are the conditions when a small minority of fires escape initial suppression efforts (Finney et al. 2011), allows an increase in surface and ladder fuels 

to accumulate, and fosters increased connectivity and homogeneity of vegetation 
130 

GENERAL TECHNICAL REPORT PSW-GTR-254

patterns (Collins et al. 2011, Hessburg et al. 2005, Parsons and Debenedetti 1979, 

Taylor et al. 2014); and (2) climate change, which has and will increase the length 

of the dry season, which increases both the risk and scale for high-severity fires 

(Collins 2014, Westerling et al. 2011). Further, projected increases in temperature 

and decreases in snowpack for the Sierra Nevada (Safford et al. 2012) are likely to 

result not only in a continued increasing trend in both patch size and proportion of 

landscape with stand-replacing fire but also an increasing potential for repeated 

stand-replacing fire, which can lead to vegetation type conversion (Stephens et al. 

2013). Current trajectories of fire size and impact, along with predicted doubling of 

predicted future fire likelihoods, suggest a future in which proportions of standreplacing fire in the Sierra Nevada exceed levels interpreted from historical data, 

regardless of sources. 

Postfire Forest Management

A recent assessment of land cover change in California demonstrated that fire now 

accounts for a greater proportion of live tree mortality or “loss” than any other 

activity (e.g., timber harvesting, development) (Sleeter et al. 2011). Recent research 

has also demonstrated an increasing proportion of stand-replacing fires and fire 

patch sizes since 1984 (Miller and Safford 2012, Miller et al. 2009), which has 

raised concerns about what type of forest, if any, will be reestablished following 

stand-replacing fire. Recent studies from the northern Sierra Nevada and southern 

Cascade Range found very low natural conifer regeneration in areas affected by 

stand-replacing fire up to 11 years following the burn (Collins and Roller 2013, 

Crotteau et al. 2013). The low conifer regeneration has been attributed mainly to the 

lack of direct mechanisms for seed persistence or dispersal into large stand-replacing patches (Barton 2002, Goforth and Minnich 2008, Keeley 2012). This suggests 

that frequent fire intervals for high-intensity fires result in slow and uncertain 

reforestation of conifer forests, which is particularly evident for pine species (Collins and Roller 2013). If the desired condition for mixed-conifer forests affected by 

stand-replacing fire is to have mixed-conifer forests return within several decades, 

then some management intervention may be necessary, particularly by planting pine 

species, to ensure greater future fire resilience. 

Harvest of fire-killed trees (salvage) commonly accompanies reforestation 

efforts in burned areas. Salvage can have a range of ecological effects depending on 

the extent of burn area harvested and the removal method (i.e., whole tree harvest, 

cut to length, etc.). Management objectives for salvage-harvesting include recovering economic value of timber (Sessions et al. 2004), increasing personnel safety for 
131 

The California Spotted Owl: Current State of Knowledge

 

reforestation efforts, and reducing large woody surface fuel accumulation (Peterson 

et al. 2015, Ritchie et al. 2013, Zhang et al. 2008), which can increase fire resilience. 

Although salvage-harvesting generally achieves these objectives, it has fewer shortterm (<10 years) ecological benefits (Long et al. 2014, Peterson et al. 2009, Ritchie 

and Knapp 2014). In particular, there can be negative impacts on habitat, including 

the removal of snags, which also ultimately reduces coarse wood on the forest 

floor (Swanson et al. 2011). Over the long term (>30 years), the tradeoffs of salvage 

harvesting versus leaving fire-killed stands unaltered are less clear. A salvageharvested and reforested area may return to mature conifer forest more quickly 

than an unaltered burned area, but could lead to a loss of habitat diversity over the 

landscape if large areas are planted using conventional techniques (i.e., equal spacing among planted trees). 

However, stand-replacing fire facilitates development of alternate vegetation 

types (e.g., montane chaparral or California black oak forests [Quercus kelloggii 

Newberry]), which may be underrepresented in many contemporary landscapes 

(Cocking et al. 2012, 2014; Nagel and Taylor 2005). Spatial scale is a critical consideration when balancing these tradeoffs. For example, if patches of stand-replacing 

fire are large (e.g., >200 ha [500 ac]) and left unaltered, the potential for colonization by montane chaparral across the entire patch is high (Collins and Roller 2013, 

Conrad and Radosevich 1982, Crotteau et al. 2013, Goforth and Minnich 2008), 

resulting in a homogenization of landscape vegetation rather than increasing vegetation diversity. 

Climate Change 

General climate change model projections for the Sierra Nevada have temperatures 

increasing 3 to 6 °C (5.2 to 10.4 °F) during the 21st century (Cayan et al. 2013). 

Precipitation models differ, with some predicting increases and others decreases 

in net precipitation (Cayan et al. 2013). These models, however, mask consistent 

predictions of decreased winter snowpack and increased ecosystem moisture stress 

(Cayan et al. 2013), accompanied by an increase in the frequency of extreme climatic events (droughts as well as flooding) (Gershunov et al. 2013). These climate 

change models consistently suggest that by the late 21st century, the Sierra Nevada 

will experience (1) a decreasing fraction of its annual precipitation as snow, and 

hence loss of snowpack; (2) increasing temperatures that will increase dry season 

soil moisture stress (climate water deficit [CWD]); (3) a higher fraction of annual 

precipitation in fewer storm events; (4) an increased frequency of drought, and (5) 

a lengthening of the fire season because of earlier onset and later ending of warm, 

dry conditions. 
132 

GENERAL TECHNICAL REPORT PSW-GTR-254

There are several ways to project the potential consequences of a changing 

climate on the distribution of Sierra Nevada vegetation types. One approach to 

projecting future ecosystem composition as a consequence of climate change is to 

project the future distribution of forest types. Ecosystem models, such as the MC1 

model (Lenihan et al. 2008), are used to project the distribution of ecosystems into 

the future. The results of these models suggest upward shifts in most vegetation 

types, loss of subalpine forests, and massive forest conversion from types that now 

dominate to those characteristic of warmer and drier environments. 

Another approach uses simple climatic envelope modeling to identify locations 

where current forest cover is projected to fall outside historical climatic parameters 

for that forest type (Schwartz unpublished data). These models also predict significant reorganization of forested ecosystems during the next century as warmer and 

drier conditions prevail, driving upslope expansion of grassland, savannah, and 

shrub-dominated ecosystems. This approach identifies the climatic attributes that 

describe present occurrences of each ecosystem type, and then overlays climate 

projections for different periods into the future (e.g., 2040–2070) onto sites to 

identify when and where instances of an ecosystem type are projected to no longer 

be within a suitable climate space for that ecosystem. 

Changing climates are relevant to risk of fire, and all projections of future fire 

conditions that consider climate models predict a near doubling of fire likelihoods 

(e.g., Westerling et al. 2011). With fire extent, severity, and frequency already 

increasing in many places (Miller and Safford 2012, Miller et al. 2009), fire is 

likely to influence changes in forest cover types. Site type change from repeated 

high-severity fire is already occurring (Stephens et al. 2013). Future changes may 

be driven by voluntary recruitment or as an active adaptation strategy by planting 

different species in an effort to create more resilient forests. Vegetation models 

suggest that many portions of the mid-elevation conifer zone will be vulnerable to 

such changes. 

Upper montane forests will likely also undergo significant changes (North et 

al. 2016). Modeling of predicted conditions in the Lake Tahoe basin suggests that 

forested areas that would not have benefited greatly from fuels treatments in the 

20th century owing to low fire activity may need significant fuels treatments by the 

end of this century because of projected increase in fire activity (Loudermilk et 

al. 2013, 2014). Modeling also suggests that fire activity will increase significantly 

because of longer fire seasons that will allow more widespread fire ignitions from 

lightning (Yang et al. 2015). An analysis of trends in the upper elevation of burn 

areas over the past several decades suggests that wildfires may already be increasing in frequency in upper montane forests (Schwartz et al. 2015). 
133 

The California Spotted Owl: Current State of Knowledge

Increasing frequency and intensity of drought result in increased tree stress and 

have been implicated in widespread increases in large tree mortality (Dolanc et al. 

2014, McIntyre et al. 2015, van Mantgem et al. 2009). Climate change projections of 

forest ecosystems (Lenihan et al. 2003), forest communities (Schwartz, unpublished 

report) and tree species (McKenzie 2010) all suggest that existing mid-elevation 

coniferous forests are poised for conversion to other forest types over much of their 

current distribution, given drivers such as wildfire. Pests and pathogens as drivers 

of forest change may also be increasing (e.g., Smith et al. 2005). Collectively, these 

trends strongly suggest that, under current management practices, all mid-elevation 

coniferous forests are threatened with conversion to vegetation characteristic 

of warmer, drier, and more frequently burned types such as montane chaparral, 

mixed-hardwood forests, and even grasslands (Lenihan et al. 2008). 

Putting these predictions into context, however, requires understanding of the 

spatial resolution of climate projections. Projecting future climate is done using 

one or more “general circulation models” (GCMs) (IPCC 2013). Although the list 

of GCMs continues to grow (>15), each GCM is a complex multivariate simulation 

of future climate on a global scale (IPCC 2013). The global nature of these models 

is such that they might not capture local processes well, even after downscaling 

(Gershunov et al. 2013). Although multiple models provide the opportunity to 

estimate variance in outcomes, they are likely to underestimate the true uncertainty 

with respect to climate futures. The variation among interrelated and nonindependent global models does not allow capturing the range of variability that might be 

expected in future climates. Further, microscale variation projections (e.g., cold 

air drainages) are locally downscaled under the general assumption that current 

patterns of local variation will be the same in the future. Hence, cold air drainages remain cold air drainages. Finally, we have a relatively poor understanding of 

forest soils in the Sierra Nevada and an equally poor understanding of the way that 

soils modify the extent to which changing climate will be expressed by changing 

forest composition, structure, and function. The consequence of this fine-scale 

uncertainty is that despite strong predictions of major forest changes in response 

to climate predictions at large spatial scales, there are likely to be refugia where 

cooler, moister forest types may persist. Identifying and conserving forests in these 

refugia might help provide long-term owl habitat even under accelerating changes 

in climate conditions. 

Projections of forest change suggest that under warmer and drier future climate 

scenarios, all Sierra Nevada forest types are at risk of conversion to some other 

plant community over the majority of their current distributions. This includes 

the mid-elevation coniferous forests upon which California spotted owls currently 

depend. Many currently forested regions of the Sierra Nevada are predicted to be 
134 

GENERAL TECHNICAL REPORT PSW-GTR-254

shrub or grassland dominated in the future. Models of late 21st century climate also 

suggest a future replete with unique combinations of species pools, climate, and 

disturbance regimes on the complex mixes of Sierra Nevada geologic substrates. 

The result is many regions may experience conditions that have no strict analogs 

in the past. This reduces our capacity to predict how they may respond. Forests in 

some geographic locations (e.g., drainage bottoms) may persist; others (e.g., southfacing slopes) may undergo pronounced shifts in environmental conditions and thus 

be more likely to change in structure and composition. 

The forests of the Sierra Nevada are complex in composition, structure, and 

function. This complexity reflects wide variation in environmental conditions at 

both local and regional scales, rich floristic diversity, and a highly varied history of 

natural and human disturbances (Franklin and Fites-Kaufmann 1996). The role of 

geological and climatic diversity in creating this complex mosaic of vegetation is 

prominent. It is this very complexity that may provide an opportunity to ameliorate 

the potential for total conversion through forest management. 

Future Management of Mid-Elevation Forests 

If owl habitat has improved as a result of fire suppression, such improvement may well be illusory and short-lived. Fire is inevitable in these forests, 

and the probability of catastrophic fire—certainly one of the greatest 

threats to owl habitat—increases as surface fuels and ladder fuels continue 

to accumulate. Overly dense stands are subject to extensive mortality from 

drought and insects, including loss of the most desirable large, old trees—a 

management policy characterized as ‘hands-off plus fire exclusion’ (allow 

forest succession to proceed uninterrupted by periodic natural disturbances) 

would likely lead to degraded and depauperate, rather than healthy and 

biologically diverse, ecosystems (Weatherspoon et al. 1992: 253). 

Currently, mid-elevation forests in the Sierra Nevada are prone to high-severity 

fire, drought stress and loss of large trees, and climatically driven vegetation 

changes. Hands-off management is likely to perpetuate the compromised resilience 

of mid-elevation forests. Active management that decreases fuel loads and stand 

density can help reduce wildfire severity, water competition, and slow vegetation change. These active management choices may also affect forest conditions, 

particularly in dense stands with high canopy cover, that have been associated 

with preferred spotted owl habitat. New management practices are needed that can 

accommodate the multitude of management objectives that include fuels reduction, forest resilience, and some high canopy cover forest conditions (McKelvey 
135 

The California Spotted Owl: Current State of Knowledge

and Weatherspoon 1992). Some studies have suggested this can be accomplished 

by increasing structural heterogeneity associated with ecosystem resilience in 

fire-dependent forests (Churchill et al. 2013, Lydersen and North 2012, North 

et al. 2009, Stephens and Gill 2005, Stephens et al. 2007). New management 

practices now attempt to realign forest conditions with their historical variability 

using existing stand structure (“what you’ve got to work with”) and topography to 

structure management actions. Topography is used because it is closely tied to two 

key processes that seem to strongly influence forest conditions: local productivity 

(associated with water availability) and fire regime. 

Creating Forest Heterogeneity 

Forest heterogeneity at the landscape level in the Sierra Nevada is strongly influenced by water availability (Tague et al. 2009) as measured by CWD (the difference 

between potential and actual plant evapotranspiration). Stephenson (1998) first 

proposed that topographic differences in plant water availability (actual evaporative 

transpiration [AET) and CWD determined forest type and productivity. Subsequent 

modeling found general agreement between predicted and actual forest conditions 

in the southern Sierra Nevada using just AET and CWD (Miller and Urban 1999a, 

1999b). For example, fir-dominated forests are usually most abundant where water 

availability is high (such as on deep soils with their high water-holding capacities); 

whereas pine-dominated forests are most abundant where water availability is low 

(such as on shallow soils or in rain shadows) (Fites-Kaufman et al. 2007, Meyer 

et al. 2007, Stephenson 1998). Slope steepness and slope position (e.g., ridgetop, 

midslope, valley bottom) are also important factors, as they affect the reception and 

retention of both meteoric waters and water flowing above, within, and beneath the 

soil. Recent large-scale analysis of forests in Yosemite National Park using light 

detection and ranging found CWD to be the best predictor of forest conditions, 

including canopy cover (Kane et al. 2013, 2014, 2015a, 2015b). 

Although overstory forest patterns seem to be associated with CWD, understory conditions are strongly shaped by fire. Lydersen and North (2012) assessed 

a wide topographic distribution of forests with restored fire regimes. They found 

that fire history had the strongest influence on understory stand structure. Smalltree density decreased and shrub cover increased with the increased fire severity 

and frequency that tend to occur on upper slope and ridgetop locations (Lydersen 

and North 2012). Consistent with other studies, they found that overstory forest 

conditions were associated with topographic differences in CWD (Lutz et al. 2010). 

The greatest densities of large, overstory trees, high total basal area and canopy 

cover, and an abundance of large snags and logs were in more mesic, productive 

sites such as lower slopes and riparian areas, which have lower CWD. This high 
136 

GENERAL TECHNICAL REPORT PSW-GTR-254

biomass forest structure existed in these topographic positions regardless of fire 

history. These findings suggest that CWD and fire intensity strongly influence 

forest overstory and understory conditions, respectively. Topography’s influence 

on these two factors appears to produce the heterogeneity characteristic of montane forest landscapes (Lydersen and North 2012, Taylor and Skinner 2003). It 

also provides a means to estimate which areas in the landscape had the high stem 

density and canopy closure conditions that might support species associated with 

these conditions (Taylor and Skinner 1998). Underwood et al. (2010) tested this idea 

using fisher and California spotted owl radiotelemetry locations. They found higher 

than expected use of topographic areas associated with higher productivity, forest 

biomass, and canopy cover such as found in canyon bottoms, lower slopes, and 

northeast aspect positions. 

Heterogeneity within frequent-fire forest types across the Western United 

States has recently been examined using a meta-analysis of historical forest structure (Larson and Churchill 2012). The within-stand structure has been characterized as containing three main conditions: individual trees, clumps of trees, and 

openings or gaps (ICO) (Abella and Denton 2009, Churchill et al. 2013, Larson and 

Churchill 2012, Larson et al. 2012, Sánchez Meador et al. 2011). In this pattern, 

openings may inhibit crown-fire spread under most (less than severe) weather conditions (Agee et al. 2000, Agee and Skinner 2005, Stephens and Moghaddas 2005) 

and may be as effective as fuel breaks with regularly spaced trees with wide crown 

separations (Kennedy and Johnson 2014, Ritchie et al. 2007). The variable microclimate and vegetation conditions between the three conditions may also provide 

greater habitat diversity for both plants and animals (Roberts et al. 2015). Recent 

work in the Sierra Nevada using a rare stem map from 1929 has quantified an ICO 

pattern in mixed-conifer forest (Lydersen et al. 2013). This work provides measures 

of the relative proportions, sizes, and compositions of each of the three conditions, 

individual trees, clumps of trees, and openings within active-fire forests. Because 

stand conditions with an active fire regime vary with topography (Lydersen and 

North 2012) and different forest types, this single study with a small sample size 

might be used with caution until more research has been completed. 

The openings in an ICO pattern may also increase forest drought resilience. 

Models suggest that openings could increase soil moisture (Bales et al. 2011) 

because more snow reaches the forest floor, melting into the soil instead of being 

intercepted in tree crowns where some of the snow directly sublimates back into 

the atmosphere (Molotch et al. 2007). Although montane forests are adapted to 

annual drought stress characteristic of Mediterranean climates, periods of multiple, 

consecutive dry years can have major impacts (e.g., Guarin and Taylor 2005). For 
137 

The California Spotted Owl: Current State of Knowledge

example, there was substantial mortality of conifer trees in the San Bernardino 

Mountains after the drought of the late 1990s and early 2000s. In the absence of frequent fire, increases in forest density result in greater competition for scarce water 

(Dolph et al. 1995, Innes 1992). A major concern is potential increases in older tree 

mortality because large trees are often more prone to drought-induced mortality 

(Allen et al. 2010). Some studies have found higher than expected mortality rates in 

large trees (Dolph et al. 1995, Lutz et al. 2009, Ritchie et al. 2008 Smith et al. 2005). 

Research has not yet been conducted about whether ICOs reduce drought stress in 

adjacent tree groups. However, current large-tree mortality rates (van Mantgem and 

Stephenson 2007, van Mantgem et al. 2009) suggest that a ”leave-it-alone” forest 

management policy that does not reduce stand density could contribute to the loss 

of old-growth trees (Fettig et al. 2008, 2010a, 2010b; Ritchie et al. 2008). 

There are many areas in the Sierra Nevada where mechanical treatment is 

currently infeasible (e.g., steep slopes, wilderness, roadless areas, etc.) (North et al. 

2015). An alternative is the use of managed fire, which is one of the most effective 

and efficient means of promoting forest resilience (Collins et al. 2009, North et al. 

2012). Although first-entry burns may actually increase fire hazard because of tree 

mortality and vigorous shrub regrowth (Schmidt et al. 2008, Skinner 2005), subsequent low-intensity burns can often produce greater heterogeneity and are more 

effective at reducing surface fuels than mechanical treatments. However, using fire 

in forests that have imbedded human development has significant risks. These risks 

include potential impacts to people and property from smoke production, reduced 

recreation opportunities, inadequate personnel to conduct and monitor fires, 

liability for fire escapes, and risk-adverse policies and institutions. Many of the 

issues relating to fuel treatment intensity and fire use are inherently social in nature 

(McCaffrey and Olsen 2012). In the future, managed fire may be more widely used 

but will probably be relegated to more remote areas where potential effects on rural 

communities are greatly reduced. 

Chapter Summary 

The processes that influence the distribution and dynamics of forests in the Sierra 

Nevada occur across large landscapes and multiple land ownerships. Yet, public 

land agencies struggle to coordinate management strategies and actions across management units, as well as ownership boundaries. A regional strategy to manage for 

the long-term viability of mid-elevation coniferous forests that accounts for climate 

change and fire-resilient forest ecosystems would be an important and valuable step 

toward these desired outcomes. 
138 

GENERAL TECHNICAL REPORT PSW-GTR-254

The mid-elevation coniferous forests of the Sierra Nevada, in their entirety, are 

highly threatened with conversion to warmer, drier adapted vegetation types. The 

drivers of this forecasted change are the synergy of warming and drying climate, 

unsustainable and unprecedented densities of trees, ensuing drought-induced stress, 

and increasingly severe wildfires. Large fires such as the Rim Fire in 2013 (100 000 

ha [250,000 ac]) and the King Fire in 2014 (40 000 ha [100,000 ac]) have resulted in 

dead tree swaths (i.e., at or close to 100 percent mortality) of unprecedented size in 

the mid-elevation zone in just the past two years. Even larger fires have occurred in 

the Western United States in recent decades and are plausible for the Sierra Nevada. 

With climate change models predicting significant increases in fire probabilities 

(as much as double current probabilities) during this century, and increasing fuel 

loads, the prospect of large-scale, stand-replacing fire effects that affect significant 

portions of the lower and middle elevations of the Sierra Nevada over the next few 

decades is an increasing possibility. These conditions pose significant challenges 

to land managers because efforts to maintain current forest conditions are likely 

to fail. This represents a severe threat to sustaining old-growth habitat conditions 

associated with the spotted owl. 

Our survey of forest change from historical to current conditions, and discussion 

of drivers of change, suggest there are significant management challenges in maintaining a well-connected network of closed-canopy mid-elevation conifer stands. 

We focus on five fundamental conclusions regarding the response of mid-elevation 

coniferous forests to contemporary and anticipated future drivers of change in the 

Sierra Nevada. First, based on our collective knowledge of pre-European forest 

structure and composition, the heterogeneity of historical forests likely provided a 

variety of conditions, including patches of forest vegetation that were suitable for 

species requiring high densities of large trees. However, the size and connectivity of 

high-density patches of medium to larger trees (i.e., 27 to 60 cm [11 to 24 in] d.b.h 

and >60 cm [24 in] d.b.h) in the Sierra Nevada under an active fire regime was likely 

much smaller than it is currently. These largely second-growth trees have grown and 

expanded on the landscape after most of the very large trees (i.e., >100 cm [40 in] 

d.b.h) were removed and fire suppression reduced young tree mortality. 

Second, changing climate and increasing severity of wildfires threaten to 

decrease the current extent and connectivity of mature, dense stands. Third, management decisions predicated on reducing proximate threats to ecosystems (e.g., 

large-scale stand-replacing fire) by reducing fuels and tree density will result in 

some decreases in the concentration of high-density, mature-tree patches. Current 
139 

The California Spotted Owl: Current State of Knowledge

management on NFS lands predominantly consists of protecting remaining highpriority pockets of suitable habitat while reducing fuels over broader landscapes. 

These fuel treatments have been applied on only small portions of the landscape 

(North et al. 2012, 2015) and have been inadequate in preventing large patches of 

stand-replacing fire. In contrast, the strategic and careful reduction of continuous 

high fuel loads in portions of high-density, mature forests by mechanical thinning 

and prescribed fire may reduce the risk of stand-replacing fire and forest type 

conversion. This fuel reduction effort would sustain larger forested landscapes 

that include suitable nesting, roosting, and foraging habitat. Ecosystem response 

models to changing climates suggest that stand-replacing fire will result in conversion of significant amounts of mid-elevation mixed-conifer forests to hardwood, 

scrub, and grassland vegetation. Based on modeling, conservation strategies for the 

fisher (Martes pennanti), another threatened species in the southern Sierra Nevada, 

project similar habitat loss because of climate- and disturbance-driven changes in 

forest conditions (Scheller et al. 2011, Spencer et al. 2010, Syphard et al. 2011). A 

calculated response to restore resiliency at a landscape scale is necessary to maintain a network of mature, closed-canopy coniferous forests in the Sierra Nevada. 

Fourth, owing to different management priorities on private lands and constraints on mechanical thinning in national parks, the opportunities for meaningful 

long-term ecosystem management experiments may be largely limited to lands 

managed by the USFS. Evaluation of forest-restoration approaches will depend 

upon actually using adaptive management strategies and incorporating scientific 

support needed to monitor management effectiveness and inform changes to 

improve success (Gutiérrez et al. 2015). Further, all federal land managers are 

faced with demanding management objectives (e.g., clean air, water provisioning 

to lowlands, minimizing human risk, maintaining species diversity and ecosystem 

integrity) such that ecosystem-driven objectives that reduce specific attention to 

any individual species are favored. 

Fifth, there is inadequate understanding of the degree to which California 

spotted owls would be affected by the predominant ecosystem-based approaches to 

managing for fire and adapting to climate change. A silvicultural strategy that creates a mosaic of different density patches (e.g., North et al. 2009) is currently viewed 

by some as the best opportunity to preserve some intact old-growth, legacy forests 

in the Sierra Nevada. An ecosystem-based forest restoration strategy that prioritizes 

resilience to fire and changing climates appears to offer a defensible approach to the 

dilemma that western coniferous forests face in the coming decades. 
140 

GENERAL TECHNICAL REPORT PSW-GTR-254

Literature Cited 

Abella, S.R.; Denton, C.W. 2009. Spatial variation in reference conditions: 

historical tree density and pattern on a Pinus ponderosa landscape. Canadian 

Journal of Forest Research. 39: 2391–2403. 

Agee, J.K.; Bahro, B.; Finney, M.A.; Omi, P.N.; Sapsis, D.B.; Skinner, C.N.; 

van Wagtendonk, J.W.; Weatherspoon, C.P. 2000. The use of shaded fuel 

breaks in landscape fire management. Forest Ecology and Management. 127: 

55–66. 

Agee, J.K.; Skinner, C.N. 2005. Basic principles of forest fuel reduction 

treatments. Forest Ecology and Management. 211: 83–96. 

Allen, C.D.; Alison, K.; Macalady, H.C.; Bachelet, D.; McDowell, N.G.; 

Vennetier, M.; Kitzberger, T.; Rigling, A.; Breshears, D.D.; Hogg, E.H.; 

Gonzalez, P.; Fensham, R.J.; Zhang, Z.; Castro, J.; Demidova, N.; Lim, 

J.H.; Gillian, A.; Running, S.W.; Semerci, A.; Cobb, N. 2010. A global 

overview of drought and heat-induced tree mortality reveals emerging climate 

change risks for forests. Forest Ecology and Management. 259: 660–684. 

Ansley, J.A.S.; Battles, J.J. 1998. Forest composition, structure, and change in an 

old-growth mixed conifer forest in the northern Sierra Nevada. Journal of the 

Torrey Botanical Society. 125: 297–308. 

Baker, W.L. 2014. Historical forest structure and fire in Sierran mixed-conifer 

forests reconstructed from General Land Office survey data. Ecosphere. 5(7): 79.

Bales, R.C.; Hopmans, J.W.; O’Green, A.T.; Meadows, M.; Hartsough, P.C.; 

Kirchner, P.; Hunsaker, C.T.; Beaudette, D. 2011. Soil moisture response to 

snowmelt and rainfall in a Sierra Nevada mixed-conifer forest. Vadose Zone 

Journal. 10: 786–799. 

Barton, A.M. 2002. Intense wildfire in southeastern Arizona: transformation of a 

Madrean oak-pine forest to oak woodland. Forest Ecology and Management. 165: 

205–212. 

Beaty, R.M.; Taylor, A.H. 2001. Spatial and temporal variation of fire regimes in a 

mixed conifer forest landscape, southern Cascades, California, USA. Journal of 

Biogeography. 28: 955–966. 

Bouldin, J. 1999. Twentieth-century changes in forests of the Sierra Nevada, 

California. Davis, CA: University of California–Davis. 222 p. Ph.D. dissertation. 
141 

The California Spotted Owl: Current State of Knowledge

Bouldin, J. 2008. Some problems and solutions in density estimation from bearing 

tree data: a review and synthesis. Journal of Biogeography. 35: 2000–2011. 

Bull, E.L.; Parks, C.G.; Torgersen, T.R. 1997. Trees and logs important to 

wildlife in the interior Columbia River basin. Gen. Tech. Rep. PNW-GTR-391. 

Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest 

Research Station. 55 p. 

California Forestry Practices Regulations. [CA-FPR ]. 2012. 

Carey, A.B. 2002. Response of northern flying squirrels to supplementary dens. 

Wildlife Society Bulletin. 30: 547–556. 

Carey, A.B.; Wilson, T.M.; Maguire, C.C.; Biswell, B.L. 1997. Dens of northern 

flying squirrels in the Pacific Northwest. Journal of Wildlife Management. 61: 

684–699. 

Cayan, D.R.; Tyree, M.; Kunkel, K.E.; Castro, C.; Gershunov, A.; Barsugli, 

J.; Ray, A.J.; Overpeck, J.; Anderson, M.; Russell, J.; Rajagopalan, B.; 

Rangwala, I.; Duffy, P. 2013. Future climate: projected average. In: Garfin, 

G.; Jardine, A.; Merideth, R.; Black, M.; LeRoy, S., eds. Assessment of climate 

change in the Southwest United States: a report prepared for the national climate 

assessment. A report by the Southwest Climate Alliance. Washington, DC: 

Island Press: 101–125. 

Christopher, T.A.; Goodburn, J.M. 2008. The effects of spatial patterns on the 

accuracy of Forest Vegetation Simulator (FVS) estimates of forest canopy cover. 

Western Journal of Applied Forestry. 23: 5–11. 

Churchill, D.J.; Larson, A.J.; Dahlgreen, M.C.; Franklin, J.F.; Hessburg, P.F.; 

Lutz, J.A. 2013. Restoring forest resilience: from reference spatial patterns to 

silvicultural prescriptions and monitoring. Forest Ecology and Management. 

291: 442–457. 

Cocking, M.I.; Varner, J.M.; Knapp, E.E. 2014. Long-term effects of fire 

severity on oak–conifer dynamics in the southern Cascades. Ecological 

Applications. 24: 94–107. 

Cocking, M.I.; Varner, J.M.; Sherriff, R.L. 2012. California black oak responses 

to fire severity and native conifer encroachment in the Klamath Mountains. 

Forest Ecology and Management. 270: 25–34. 

Cockle, K.L.; Martin, K.; Wesolowski, T. 2011. Woodpeckers, decay, and the 

future of cavity-nesting vertebrate communities worldwide. Frontiers in Ecology 

and the Environment. 9: 377–382. 
142 

GENERAL TECHNICAL REPORT PSW-GTR-254

Collins, B.M. 2014. Fire weather and large fire potential in the northern Sierra 

Nevada. Agricultural and Forest Meteorology. 189–190: 30–35. 

Collins, B.M.; Everett, R.G.; Stephens, S.L. 2011. Impacts of fire exclusion and 

managed fire on forest structure in an old growth Sierra Nevada mixed-conifer 

forest. Ecosphere. 2: 51. 

Collins, B.M.; Lydersen, J.M.; Everett, R.G.; Fry, D.L.; Stephens, S.L. 2015. 

Novel characterization of landscape-level variability in historical vegetation 

structure. Ecological Applications. 25: 1167–1174. 

Collins, B.M.; Miller, J.D.; Thode, A.E.; Kelly, M.; van Wagtendonk, J.W.; 

Stephens, S.L. 2009. Interactions among wildland fires in a long-established 

Sierra Nevada natural fire area. Ecosystems. 12: 114–128.

Collins, B.M.; Roller, G.B. 2013. Early forest dynamics in stand-replacing fire 

patches in the northern Sierra Nevada, California, USA. Landscape Ecology. 

28: 1801–1813. 

Collins, B.M.; Skinner, C. 2014. Fire and fuels. In: Long, J.; Skinner, C.; QuinnDavidson, L., eds. Science synthesis to promote resilience of social-ecological 

systems in the Sierra Nevada and southern Cascade Range. Gen. Tech. Rep. 

PSW-GTR-247. Albany, CA: U.S. Department of Agriculture, Forest Service, 

Pacific Southwest Research Station. 723 p. Chapter 4.1.

Collins, B.M.; Stephens, S.L. 2010. Stand-replacing patches within a ‘mixed 

severity’ fire regime: quantitative characterization using recent fires in a longestablished natural fire area. Landscape Ecology. 25: 927–939.

Crookston, N.L.; Stage, A.R. 1999. Percent canopy cover and stand structure 

statistics from the Forest Vegetation Simulator. Gen. Tech. Rep. RMRS-GTR-24. 

Fort. Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky 

Mountain Research Station. 11 p. 

Crotteau, J.S.; Varner, J.M.; Ritchie, M.W. 2013. Post-fire regeneration across a 

fire severity gradient in the southern Cascades. Forest Ecology and Management. 

287: 103–112. 

Davis, F.W.; Stoms, D.M. 1996. Sierran vegetation: a gap analysis. In: Sierra 

Nevada Ecosystem Project: final report to Congress. Vol. II. Assessments and 

scientific basis for management options. Wildland Resources Center Report 

No. 37. Davis, CA: University of California, Centers for Water and Wildlands 

Resources: 671–690. 
143 

The California Spotted Owl: Current State of Knowledge

Dolanc, C.R.; Safford, H.D.; Dobrowski, S.Z.; Thorne, J.H. 2014. Twentieth 

century shifts in abundance and composition of vegetation types of the Sierra 

Nevada, CA, US. Applied Vegetation Science. 17: 442–455. 

Dolph, K.L.; Mori, S.R.; Oliver, W.W. 1995. Long-term response of old-growth 

stands to varying levels of partial cutting in the eastside pine type. Western 

Journal of Applied Forestry. 10(3): 101–108. 

Fettig, C.J.; Borys, R.; Dabney, C. 2010a. Effects of fire and fire surrogate 

treatments on bark beetle-caused tree mortality in the Southern Cascades, 

California. Forest Science. 56: 60–73. 

Fettig, C.J.; Borys, R.R.; McKelvey, S.R.; Dabney, C.P. 2008. Blacks Mountain 

Experimental Forest: bark beetle responses to differences in forest structure and 

the application of prescribed fire in interior ponderosa pine. Canadian Journal of 

Forest Research. 38: 924–935. 

Fettig, C.J.; McKelvey, S.R. 2010. Bark beetle responses to stand structure and 

prescribed fire at Blacks Mountain Experimental Forest, California: 5-year data. 

Fire Ecology. 6(2): 26–42. 

Fettig, C.J.; McKelvey, S.R.; Cluck, D.R.; Smith, S.L. 2010b. Effects of 

prescribed fire and season of burn on direct and indirect levels of tree mortality 

in ponderosa and Jeffrey pine forests in California. Forest Ecology and 

Management. 260: 207–218. 

Finney, M.A.; Grenfell, I.C.; McHugh, C.W.; Seli, R.C.; Tretheway, D.; 

Stratton, R.D.; Brittain, S. 2011. A method for ensemble wildland fire 

simulation. Environmental Modeling and Assessment. 16: 153–167. 

Fites-Kaufman, J.A.; Rundel, P.; Stephenson, N.L.; Weixelman D.A. 2007. 

Montane and subalpine vegetation of the Sierra Nevada and Cascade Ranges. 

In: Barbour, M.G.; Keeler-Wolf, T.; Schoenherr, A.A., eds. Terrestrial vegetation 

of California: Los Angeles, CA: University of California Press: 456–501. 

Franklin, J.F.; Fites-Kaufmann, J.A. 1996. Assessment of late-successional 

forests of the Sierra Nevada: In: Sierra Nevada Ecosystem Project: final report to 

Congress. Volume II. Assessments and scientific basis for management options. 

Wildland Resources Center Report No. 37. Davis, CA: University of California– 

Davis, Centers for Water and Wildland Resources. 627–662. Vol. 2. Chapter 21. 
144 

GENERAL TECHNICAL REPORT PSW-GTR-254

Fry, D.L.; Stephens, S.L.; Collins, B.M.; North, M.P.; Franco-Vizcaino, E.; 

Gill, S.J. 2014. Contrasting spatial patterns in active-fire and fire-suppressed 

Mediterranean climate old-growth, mixed conifer forests. PLoS ONE. 

doi:10.1371/journal.pone.0088985. 

Fulé, P.Z.; Swetnam, T.W.; Brown, P.W.; Falk, D.A.; Peterson, D.L.; Allen, 

C.D.; Aplet, G.A.; Battaglia, M.A.; Binkley, D.; Farris, C.; Keane, R.E.; 

Margolis, E.Q.; Grissino-Mayer, H.; Miller, C.; Sieg, C.H.; Skinner, C.; 

Stephens, S.L.; Taylor, A. 2013. Unsupported inferences of high-severity fire 

in historical dry forests of the western United States: response to Williams and 

Baker. Global Ecology and Biogeography. 23(7): 825–830. 

Gershunov, A.; Rajagopalan, B.; Overpeck, J.; Guirguis, K.; Cayan, D.; 

Hughes, M.; Dettinger, M.; Castro, C.; Schwartz, R.E.; Anderson, M.; Ray, 

A.J.; Barsugli, J.; Cavazos, T.; Alexander, M. 2013. Future climate: projected 

extremes. In: Garfin, G.; Jardine, A.; Merideth, R.; Black, M.; LeRoy, S., eds. 

Assessment of climate change in the United States: a report prepared for the 

national climate assessment. Washington, DC: Island Press: 126–147. 

Goforth, B.R.; Minnich, R.A. 2008. Densification, stand-replacement wildfire, 

and extirpation of mixed conifer forest in Cuyamaca Rancho State Park, southern 

California. Forest Ecology and Management. 256: 36–45. 

Guarin, A.; Taylor, A.H. 2005. Drought triggered tree mortality in mixed conifer 

forests in Yosemite National Park, California. Forest Ecology and Management. 

218: 229–244. 

Gutiérrez, R.J.; Cheng, A.S.; Becker, D.R.; Cashen, S.; Ganz, D.; Gunn, J.; 

Liquori, M.; Merrill, A.; Saah, D.S.; Price, W. 2015. Legislated collaboration 

in a conservation conflict: a case study of the Quincy Library Group, California: 

In: Redpath, S.; Gutiérrez, R.J.; Wood, K.; Young, J.C., eds. Cambridge, United 

Kingdom: Cambridge University Press: 271–283. 

Hessburg, P.F.; Agee, J.K.; Franklin, J.F. 2005. Dry forests and wildland fires 

of the inland Northwest USA: contrasting the landscape ecology of the presettlement and modern eras. Forest Ecology and Management. 211: 117–139. 

Hunter, J.E.; Bond, M.L. 2001. Residual trees: wildlife associations and 

recommendations. Wildlife Society Bulletin. 29: 995–999. 

Innes, J.L. 1992. Forest decline. Progress in Physical Geography. 16: 1–64. 
145 

The California Spotted Owl: Current State of Knowledge

Intergovernmental Panel on Climate Change [IPCC]. 2013. Climate change 

2013: the physical science basis. Contribution of working group I to the fifth 

assessment report of the Intergovernmental Panel on Climate Change: In: 

Stocker, T.F.; Qin, D.; Plattner, G.-K.; Tignor, M.; Allen, S.K.; Boschung, J.; 

Nauels, A.; Xia, Y.; Bex, V.; Midgley, P.M., eds. Cambridge, United Kingdom 

and New York: Cambridge University Press. 1535 p. 

Kane, V.R.; Cansler, C.A.; Povak, N.A.; Kane, J.T.; McGaughey, R.J.; Lutz, 

J.A.; Churchill, D.J.; North, M.P. 2015a. Mixed severity fire effects within 

the Rim Fire: relative importance of local climate, fire weather, topography and 

forest structure. Forest Ecology and Management. 358: 62–79. 

Kane, V.R.; Lutz, J.A.; Cansler, C.A.; Povak, N.A.; Churchill, D.; Smith, D.F.; 

Kane, J.T.; North, M.P. 2015b. Water balance and topography predict fire and 

forest structure patterns. Forest Ecology and Management. 338: 1–13. 

Kane, V.R.; Lutz, J.A.; Roberts, S.L.; Smith, D.F.; McGaughey, R.J.; Povak, 

N.A.; Brooks, M.L. 2013. Landscape-scale effects of fire severity on mixedconifer and red fir forest structure in Yosemite National Park. Forest Ecology

and Management. 287: 17–31. 

Kane, V.R.; North, M.; Lutz, J.A.; Churchill, D.; Roberts, S.L.; Smith, D.F.; 

McGaughey, R.J.; Kane, J.T.; Brooks, M.L. 2014. Assessing fire-mediated 

change to forest spatial structure using a fusion of Landsat and airborne LiDAR 

data in Yosemite National Park. Remote Sensing of Environment. 151: 89–101. 

Keeley, J.E. 2012. Ecology and evolution of pine life histories. Annual Review of 

Forest Science. 69: 445–453. 

Kennedy, M.C.; Johnson, M.C. 2014. Fuel treatment prescriptions alter spatial 

patterns of fire severity around the wildland-urban interface during the Wallow 

Fire, Arizona. Forest Ecology and Management. 318: 122–132. 

Knapp, E.; North, M.; Benech, M.; Estes, B. 2012. The variable-density thinning 

study at Stanislaus-Tuolumne Experimental Forest. In: North, M., ed. Managing 

Sierra Nevada forests. Gen. Tech. Rep. PSW-GTR-237. Albany, CA: U.S. 

Department of Agriculture, Forest Service, Pacific Southwest Research Station: 

127–140. 

Knapp, E.E.; Skinner, C.N.; North, M.P.; Estes, B.L. 2013. Long-term overstory 

and understory change following logging and fire exclusion in a Sierra Nevada 

mixed-conifer forest. Forest Ecology and Management. 310: 903–914. 
146 

GENERAL TECHNICAL REPORT PSW-GTR-254

Larson, A.J.; Churchill, D. 2012. Tree spatial patterns in fire-frequent forests 

of western North America, including mechanisms of pattern formation and 

implications for designing fuel reduction and restoration treatments. Forest 

Ecology and Management. 267: 74–92. 

Larson, A.J.; Stover, K.C.; Keyes, C.R. 2012. Effects of restoration thinning 

on spatial heterogeneity in mixed-conifer forest. Canadian Journal of Forest 

Research. 42: 1505–1517. 

Lenihan, J.M.; Bachelet, D.; Neilson, R.P.; Drapek, R. 2008. Response of 

vegetation distribution, ecosystem productivity, and fire to climate change 

scenarios for California. Climatic Change. 87: S215–S230. 

Lenihan, J.M.; Drapek, R.; Bachelet, D.; Neilson, R.P. 2003. Climate change 

effects on vegetation distribution, carbon, and fire in California. Ecological 

Applications. 13: 1667–1681. 

Long, J.; Skinner, C.; Charnley, S.; Hubbert, K.R.; Quinn-Davidson, L.; 

Meyer, M.D. 2014. Post-wildfire management. In: Long, J.W.; Quinn-Davidson, 

L.; Skinner, C. N., eds. Science synthesis to support socioecological resilience in 

the Sierra Nevada and southern Cascade Range. Gen. Tech. Rep. PSW-GTR-247. 

Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest 

Research Station: 187–220. Chapter 4.3. 

Loudermilk, E.L.; Scheller, R.M.; Weisberg, P.J.; Yang, J.; Dilts, T.E.; Karam, 

S.; Skinner, C.N. 2013. Carbon dynamics in the future forest: the importance 

of long-term successional legacy and climate-fire interactions. Global Change 

Biology. 19: 3502–3515. 

Loudermilk, E.L.; Stanton, A.; Scheller, R.M.; Dilts, T.E.; Weisberg, P.J.; 

Skinner, C.; Yang, J. 2014. Effectiveness of fuel treatments for mitigating 

wildfire risk and sequestering forest carbon: a case study in the Lake Tahoe 

Basin. Forest Ecology and Management. 323: 114–125. 

Lutz, J.A.; van Wagtendonk, J.A.; Franklin, J.F. 2009. Twentieth-century 

decline of large-diameter trees in Yosemite National Park, California. Forest 

Ecology and Management. 257: 2296–2307. 

Lutz, J.A.; van Wagtendonk, J.A.; Franklin, J.F. 2010. Climatic water deficit, 

tree species ranges, and climate change in Yosemite National Park. Journal of 

Biogeography. 37: 936–950. 

Lydersen, J.; North, M. 2012. Topographic variation in active-fire forest structure 

under current climate conditions. Ecosystems. 15: 1134–1146. 
147 

The California Spotted Owl: Current State of Knowledge

Lydersen, J.M.; North, M.P.; Knapp, E.E.; Collins, B.M. 2013. Quantifying 

spatial patterns of tree groups and gaps in mixed-conifer forests: reference 

conditions and long-term changes following fire suppression and logging. Forest 

Ecology and Management. 304: 370–382. 

Mallek, C.; Safford, H.; Viers, J.; Miller, J. 2013. Modern departures in fire 

severity and area vary by forest type, Sierra Nevada and southern Cascades, 

California. Ecosphere. 4: 153. 

Manies, K.L.; Mladenoff, D.J. 2000. Testing methods to produce landscapescale presettlement vegetation maps from the U.S. public land survey records. 

Landscape Ecology. 15: 741–754. 

McCaffrey, S.M.; Olsen, C.S. 2012. Research perspectives on the public and fire 

management: a synthesis of current social science on eight essential questions. 

Gen. Tech. Rep. NRS-GTR-104. Newtown Square, PA: U.S. Department of 

Agriculture, Forest Service, Northern Research Station. 40 p. 

McIntyre, P.J.; Thorne, J.H.; Dolanc, C.R.; Flint, A.L.; Flint, L.E.; Kelly, M.; 

Ackerly, D.D. 2015. Twentieth-century shifts in forest structure in California: 

denser forests, smaller trees, and increased dominance of oaks. Proceedings of 

the National Academy of Sciences of the United States of America. doi: 10.1073/ 

pnas.1410186112. 

McKelvey, K.S.; Johnston, J.D. 1992. Historical perspectives on forest of the 

Sierra Nevada and the Transverse Range of southern California: forest conditions 

at the turn of the century. In: Verner, J.; McKelvey, K.S.; Noon, B.R.; Gutierrez, 

R.J.; Gould, G.I., Jr.; Beck, T.W., eds. The California spotted owl: a technical 

assessment of its current status. Gen. Tech. Rep. PSW-GTR-133. Albany, CA: 

U.S. Department of Agriculture, Forest Service, Pacific Southwest Research 

Station: 225–246. Chapter 11. 

McKenzie, D. 2010. The effects of climatic change and wildland fires on air quality 

in national parks and wilderness areas. Fire Management Today. 70: 26–28. 

Meyer, M.; North, M.; Gray, A.; Zald, H. 2007. Influence of soil thickness on 

stand characteristics in a Sierra Nevada mixed-conifer forest. Plant and Soil. 

294: 113–123. 

Miller, C.; Urban, D.L. 1999a. Forest pattern, fire, and climatic change in the 

Sierra Nevada. Ecosystems. 2: 76–87. 

Miller, C.; Urban, D.L. 1999b. A model of surface fire, climate and forest pattern 

in the Sierra Nevada, California. Ecological Modelling. 114: 113–135. 
148 

GENERAL TECHNICAL REPORT PSW-GTR-254

Miller, J.D.; Safford, H.D. 2012. Trends in wildfire severity 1984–2010 in the 

Sierra Nevada, Modoc Plateau and southern Cascades, California. Fire Ecology. 

8: 41–57. 

Miller, J.D.; Safford, H.D.; Crimmins, M.; Thode, A.E. 2009. Quantitative 

evidence for increasing forest fire severity in the Sierra Nevada and southern 

Cascade Mountains, California and Nevada. Ecosystems. 12: 16–32. 

Molotch, N.P.; Blanken, P.D.; Williams, M.W. Turnipseed, A.A.; Monson, 

R.K.; Margulis, S.A. 2007. Estimating sublimation of intercepted and subcanopy snow using eddy covariance systems. Hydrological Processes. 21: 

1567–1575. 

Morgan, T.A.; Brandt, J.P.; Songster, K.E.; Keegan, C.E., III; Christensen, 

G.A. 2012. California’s forest products industry and timber harvest, 2006. Gen. 

Tech. Rep. PNW-GTR-866. Portland, OR: U.S. Department of Agriculture, 

Forest Service, Pacific Northwest Research Station. 48 p.

Nagel, T.A.; Taylor A.H. 2005. Fire and persistence of montane chaparral in 

mixed conifer forest landscapes in the northern Sierra Nevada, Lake Tahoe 

Basin, California. Journal of the Torrey Botanical Society. 132: 442–457. 

National Forest Management Act [NFMA]. 1976. 16 U.S.C. 1600 (note). 

North, M.; Brough, A.; Long, J.; Collins, B.; Bowden, P.; Yasuda, D.; Miller, 

J.; Suighara, N. 2015. Constraints on mechanized treatment significantly limit 

mechanical fuels reduction extent in the Sierra Nevada. Journal of Forestry. 113: 

40–48. 

North, M.; Collins, B.; Safford, H.; Stephenson, S. 2016. Montane Forests. In: 

Mooney, H.; Zavelta, E., eds. Ecosystems of California. Berkeley, CA: University 

of California Press: 553–578. 

North, M.; Collins, B.M.; Stephens, S. 2012. Using fire to increase the scale, 

benefits, and future maintenance of fuels treatments. Journal of Forestry. 110: 

392–401. 

North, M.; Innes, J.; Zald, H. 2007. Comparison of thinning and prescribed fire 

restoration treatments to Sierran mixed-conifer historic conditions. Canadian 

Journal of Forest Research. 37: 331–342. 

North, M.; Stine, P.; O’Hara, K.; Zielinski, W.; Stephens, S. 2009. An 

ecosystem management strategy for Sierran mixed-conifer forests. Gen. Tech. 

Rep. PSW-GTR-220. Albany, CA: U.S. Department of Agriculture, Forest 

Service, Pacific Southwest Research Station. 184 p.
149 

The California Spotted Owl: Current State of Knowledge

Odion, D.C.; Hanson, C.T.; Arsenault, A.; Baker, W.L.; DellaSala, D.A.; Hutto, 

R.L.; Klenner, W.; Moritz, M.A.; Sherriff, R.L.; Veblen, T.T.; Williams, 

M.A. 2014. Examining historical and current mixed-severity fire regimes in 

ponderosa pine and mixed-conifer forests of western North America. PLoS 

ONE. 9:e87852. 

Parsons, D.J.; Debenedetti, S.H. 1979. Impact of fire suppression on a mixedconifer forest. Forest Ecology and Management. 2: 21–33. 

Peterson, D.L.; Agee, J.K.; Aplet, G.H.; Dykstra, D.P.; Graham, R.T.; 

Lehmkuhl, J.F.; Pilliod, D.S.; Potts, D.F.; Powers, R.F.; Stuart, J.D. 2009. 

Effects of timber harvest following wildfire in western North America. Gen. 

Tech. Rep. PNW-GTR-776. Portland, OR: U.S. Department of Agriculture, 

Forest Service, Pacific Northwest Research Station. 51 p.

Peterson, D.W.; Dodson, E.K.; Harrod, RJ. 2015. Post-fire logging reduces 

surface woody fuels up to four decades following wildfire. Forest Ecology and 

Management. 338: 84–91. 

Ritchie, M.W.; Knapp, E.E. 2014. Establishment of a long-term fire salvage study 

in an interior ponderosa pine forest. Journal of Forestry. 112: 395–400. 

Ritchie, M.W.; Knapp, E.E.; Skinner, C.N. 2013. Snag longevity and surface fuel 

accumulation following post-fire logging in a ponderosa pine dominated forest. 

Forest Ecology and Management. 287: 113–122. 

Ritchie, M.W.; Skinner, C.N.; Hamilton, T.A. 2007. Probability of wildfireinduced tree mortality in an interior pine forest: effects of thinning and 

prescribed fire. Forest Ecology and Management. 247: 200–208.

Ritchie, M.W.; Wing, B.M.; Hamilton, T.A. 2008. Stability of the large tree 

component in treated and untreated late-seral interior ponderosa pine stands. 

Canadian Journal of Forest Research. 38: 919–923. 

Roberts, S.L.; Kelt, D.A.; van Wagtendonk, J.W.; Miles, A.K.; Meyer, M.D. 

2015. Effects of fire on small mammal communities in frequent-fire forests in 

California. Journal of Mammalogy. 96: 107–119. 

Safford, H.D.; North, M.; Meyer, M.D. 2012. Climate change and the relevance of 

historical forest conditions. In: North, M., ed. Managing Sierra Nevada forests. 

Gen. Tech. Rep. PSW-GTR-237. Albany, CA: U.S. Department of Agriculture, 

Forest Service, Pacific Southwest Research Station: 23–46. Chapter 3.
150 

GENERAL TECHNICAL REPORT PSW-GTR-254

Scheller, R.M.; Spencer, W.D.; Rustigian-Romsos, H.; Syphard, A.D. 2011. 

Using stochastic simulation to evaluate competing risks of wildfires and fuels 

management on an isolated forest carnivore. Landscape Ecology. 26: 1491–1504. 

Schmidt, D.A.; Taylor, A.H.; Skinner, C.N. 2008. The influence of fuels 

treatment and landscape arrangement on simulated fire behavior, southern 

Cascade Range, California. Forest Ecology and Management. 255: 3170–3184. 

Scholl, A.E.; Taylor, A.H. 2010. Fire regimes, forest change, and self-organization 

in an old-growth mixed-conifer forest, Yosemite National Park. Ecological 

Applications. 20: 362–380. 

Schwartz, M.W.; Butt, N.; Dolanc, C.R.; Holguin, A.; Moritz, M.A.; North, 

M.P.; Safford, H.D.; Stephenson, N.L.; Thorne, J.T.; van Mantgem, P.J. 

2015. Increasing elevation of fire in the Sierra Nevada and implications for forest 

change. Ecosphere. 6(7): art121. 

Sessions, J.; Bettinger, P.; Buckman, R.; Newton, M.; Hamann, A.J. 2004. 

Hastening the return of complex forests following fire: the consequences of 

delay. Journal of Forestry. 102: 38–45. 

Show, S.B.; Kotok, E.I. 1924. The role of fire in the California pine forests. 

Bulletin No. 1294. Washington, DC: U.S. Department of Agriculture, 

Government Printing Office. 80 p. 

Skinner, C.N. 2005. Reintroducing fire into the Blacks Mountain Research Natural 

Area: effects on fire hazard. In: Ritchie, M.W.; Maguire, D.A.; Youngblood, A., 

tech. coords. Proceedings of the symposium on ponderosa pine: issues, trends, 

and management. Gen. Tech. Rep. PSW-GTR-198. Albany, CA: U.S. Department 

of Agriculture, Forest Service, Pacific Southwest Research Station: 245–257.

Skinner, C.N.; Taylor, A.H. 2006. Southern Cascade bioregion. In: Sugihara, 

N.G.; van Wagtendonk, W.; Fites-Kaufman, J.W.J.; Shaffer, K.E.; Thode, A.E., 

eds. Fire in California’s ecosystems. Berkeley, CA: University of California 

Press: 195–224. 

Sleeter, B.M.; Wilson, T.S.; Soulard, C.E.; Liu, J. 2011. Estimation of late 

twentieth century land-cover change in California. Environmental Monitoring 

and Assessment. 173: 251–266. 

Smith, T.F.; Rizzo, D.M.; North, M. 2005. Patterns of mortality in an old-growth 

mixed-conifer forest of the southern Sierra Nevada, California. Forest Science. 

51: 266–275. 
151 

The California Spotted Owl: Current State of Knowledge

Spencer, W.D.; Rustigian-Romsos, H.; Strittholt, J.; Scheller, R.; Zielinski, 

W.J.; Truex, R.L. 2010. Using occupancy and population models to assess 

habitat conservation opportunities for an isolated carnivore population. 

Biological Conservation. 144: 788–803. 

Steel, Z.L.; Safford, H.D.; Viers, J.H. 2015. The fire frequency-severity 

relationship and the legacy of fire suppression in California forests.

Ecosphere. 6: 8. 

Stephens, S.L.; Agee, J.K.; Fulé, P.Z.; North, M.P.; Romme, W.H.; Swetnam, 

T.W.; Turner, M.G. 2013. Managing forests and fire in changing climates. 

Science. 342: 41–42. 

Stephens, S.L.; Bigelow, S.W.; Burnett, R.D.; Collins, B.M.; Gallagher, C.V.; 

Keane, J.; Kelt, D.A.; North, M.P.; Roberts, S.L.; Stine, P.; Van Vuren, 

D.H. 2014. California spotted owl, songbird, and small mammal responses to 

landscape-scale fuel treatments. Bioscience. 64: 893–906. 

Stephens, S.L.; Gill, S.J. 2005. Forest structure and mortality in an old-growth 

Jeffrey pine-mixed conifer forest in north-western Mexico. Forest Ecology and 

Management. 205: 15–28. 

Stephens, S.L.; Lydersen, J.M.; Collins, B.M.; Fry, D.L.; Meyer, M.D. 2015. 

Historical and current landscape-scale ponderosa pine and mixed conifer forest 

structure in the Southern Sierra Nevada. Ecosphere. 6: art79. 

Stephens, S.L.; Martin, R.E.; Clinton, N.E. 2007. Prehistoric fire area and 

emissions from California’s forests, woodlands, shrublands, and grasslands. 

Forest Ecology and Management. 251: 205–216. 

Stephens, S.L.; Moghaddas, J.J. 2005. Experimental fuel treatment impacts 

on forest structure, potential fire behavior, and predicted tree mortality in a 

California mixed conifer forest. Forest Ecology and Management. 215: 21–36. 

Stephenson, N.L. 1998. Actual evapotranspiration and deficit: biologically 

meaningful correlates of vegetation distribution across spatial scales. Journal of 

Biogeography. 25: 855–870. 

Stevens, J.T.; Safford, H.D.; North, M.P.; Fried, J.S.; Gray, A.N.; Brown, 

P.M.; Dolanc, C.R.; Dobrowski, S.Z.; Falk, D.A.; Farris, C.A.; Franklin, 

J.F.; Fulé, P.Z.; Hagmann, R.K.; Knapp, E.E.; Miller, J.A.; Smith, D.F.; 

Swetnam, T.W.; Taylor, A.H. 2016. Average stand age from forest inventory 

plots does not describe historical fire regimes in ponderosa pine and mixedconifer forests of western North America. PLoS One. 11(5): e0147688. 
152 

GENERAL TECHNICAL REPORT PSW-GTR-254

Stewart, W.; Sharma, B.; York, R.; Diller, L.; Hamey, N.; Powell, R.; Swiers, 

R. 2016. Forestry. In: Mooney, H.; Zavaleta, E., eds. Ecosystems of California. 

Oakland, CA: University of California: 817–834. 

Swanson, M.E.; Franklin, J.F.; Beschta, R.L.; Crisafulli, C.M.; Dellasala, D.; 

Hutto, R.L.; Lindenmayer, R.B.; Swanson, F.J. 2011. The forgotten stage 

of forest succession: early-successional ecosystems on forest sites. Frontiers in 

Ecology and the Environment. 9: 117–125. 

Syphard, A.D.; Scheller, R.M.; Ward, B.C.; Spencer, W.D. 2011. Simulating 

landscape-scale effects of fuels treatments in the Sierra Nevada, California. 

International Journal of Wildland Fire. 20: 364–383. 

Tague, C.; Heyn, K.; Christensen, L. 2009. Topographic controls on spatial 

patterns of conifer transpiration and net primary productivity under climate 

warming in mountain ecosystems. Ecohydrology. 2: 541–554. 

Taylor, A.H. 2000. Fire regimes and forest changes in mid and upper montane 

forests of the southern Cascades, Lassen Volcanic National Park, California. 

Journal of Biogeography. 27: 87–104. 

Taylor, A.H. 2004. Identifying forest reference conditions on early cut-over lands, 

Lake Tahoe Basin. Ecological Applications. 14: 1903–1920. 

Taylor, A.H. 2010. Fire disturbance and forest structure in an old-growth Pinus 

ponderosa forest, southern Cascades. Journal of Vegetation Science. 21: 561–572. 

Taylor, A.H.; Skinner, C.N. 1998. Fire history and landscape dynamics in a latesuccessional reserve in the Klamath Mountains, California. Forest Ecology and 

Management. 111: 285–301. 

Taylor, A.H.; Skinner, C.N. 2003. Spatial patterns and controls on historical fire 

regimes and forest structure in the Klamath Mountains. Ecological Applications. 

13: 704–719. 

Taylor, A.H.; Vandervlugt, A.M.; Maxwell, R.S.; Beaty, R.M.; Airey, C.; 

Skinner, C.N. 2014. Changes in forest structure, fuels and potential fire 

behaviour since 1873 in the Lake Tahoe Basin. Applied Vegetation Science. 

17: 17–31. 

Underwood, E.C.; Viers, J.H.; Quinn, J.F.; North, M. 2010. Using topography 

to meet wildlife and fuels treatment objectives in fire-suppressed landscapes. 

Journal of Environmental Management. 46: 809–819. 
153 

The California Spotted Owl: Current State of Knowledge

 

U.S. Department of Agriculture, Forest Service [USDA FS]. 2001. Sierra 

Nevada forest plan amendment: final environmental impact statement. 

Vallejo, CA: Pacific Southwest Region. http://www.sierraforestlegacy.org/

Resources/Conservation/LawsPoliciesRegulation/KeyForestServicePolicy/ 

SierraNevadaFramework/Framework-FSROD01.pdf. 

U.S. Department of Agriculture, Forest Service [USDA FS]. 2004. Sierra 

Nevada forest plan amendment: final supplemental environmental impact 

statement. Vallejo, CA: Pacific Southwest Region. Final programmatic 

environmental impact statement: National Forest System land management 

planning. http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/fsbdev3_046095. 

pdf USFS. 2012. 

U.S. Department of Agriculture, Forest Service [USDA FS]. 2012. National 

forest system land management planning. Federal Register. 77. No. 68: 

21162–21276. 

U.S. Department of the Interiors, National Park Service [USDI NPS]. 2012. 

National Park System Advisory Board Science Committee. Revisiting Leopold: 

resource stewardship in the National Park System. Washington, DC: National 

Park System Advisory Board Science Committee 23 p. http://www.nps.gov/ 

calltoaction/pdf/LeopoldReport_2012.pdf. 

Vankat, J.L.; Major, J. 1978. Vegetation changes in Sequoia National Park, 

California. Journal of Biogeography. 5: 377–402. 

Van de Water, K.M.; Safford, H.D. 2011. A summary of fire frequency estimates 

for California vegetation before Euro-American settlement. Fire Ecology. 7: 

26–58. 

van Mantgem, P.J.; Stephenson, N.L. 2007. Apparent climatically induced 

increase of tree mortality rates in a temperate forest. Ecology Letters. 10: 

909–916. 

van Mantgem, P.J.; Stephenson, N.L.; Byrne, J.C.; Daniels, L.D.; Franklin, 

J.F.; Fule, P.Z.; Harmon, M.E.; Larson, A.J.; Smith, J.M.; Taylor, A.H.; 

Veblen, T.T. 2009. Widespread increase of tree mortality rates in the Western 

United States. Science. 323: 521–524. 

van Wagtendonk, J.W.; Fites-Kaufman, J.A. 2006. Sierra Nevada bioregion. 

In: Sugihara, N.G.; van Wagtendonk, J.W.; Shaffer, K.E.; Fites-Kaufman J.A.; 

Thode, A.E., eds. Fire in California’s ecosystems. Berkeley, CA: University of 

California Press: 264–294. 
154 

GENERAL TECHNICAL REPORT PSW-GTR-254

van Wagtendonk, J.W.; Lutz, J.A. 2007. Fire regime attributes of wildland fires in 

Yosemite National Park. Fire Ecology. 3: 34–52. 

Verner, J.; McKelvey, K.S.; Noon, B.R.; Gutiérrez, R.J.; Gould, G.I., Jr.; Beck, 

T.W., tech. coords. 1992. The California spotted owl: a technical assessment of 

its current status. Gen. Tech. Rep. PSW-GTR-133. Albany, CA: U.S. Department 

of Agriculture, Forest Service, Pacific Southwest Research Station. 285 p. 

Walsh, D.; North, M. 2012. Appendix: examples of forest structures that may 

provide wildlife habitat. In: North, M., ed. Managing Sierra Nevada forests. Gen. 

Tech. Rep. PSW-GTR-237. Albany, CA: U.S. Department of Agriculture, Forest 

Service, Pacific Southwest Research Station: 177–184.

Weatherspoon, C.P.; Husari, S.J.; van Wagtenkonk, J.W. 1992. Fire and fuels 

management in relation to owl habitat in forests of the Sierra Nevada and 

southern California: In: Verner, J.; McKelvey, K.S.; Noon, B.R.; Gutiérrez, R.J.; 

Gould, G.I., Jr.; Beck, T.W., tech. coords. 1992. The California spotted owl: 

a technical assessment of its current status. Gen. Tech. Rep. PSW-GTR-133. 

Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest 

Research Station: 2470–260. 

Westerling, A.; Bryant, B.; Preisler, H.; Holmes, T.; Hidalgo, H.; Das, T.; 

Shrestha, S. 2011. Climate change and growth scenarios for California wildfire. 

Climatic Change. 109: 445–463. 

Wiebe, K.L. 2011. Nest sites as limiting resources for cavity-nesting birds in 

mature forest ecosystems: a review of the evidence. Journal of Field Ornithology. 

82: 239–248. 

Yang, J.; Weisberg, P.J.; Dilts, T.E.; Loudermilk, E.L.; Scheller, R.M.; 

Stanton, A.; Skinner, C. 2015. Predicting wildfire occurrence distribution with 

spatial point process models and its uncertainty assessment: a case study in the 

Lake Tahoe Basin. International Journal of Wildland Fire. 24: 380–390. 

Zhang, J.; Webster, J.; Powers, R.F.; Mills, J. 2008. Reforestation after the 

Fountain Fire in northern California: an untold success story. Journal of Forestry. 

106: 425–430. 
155 

The California Spotted Owl: Current State of Knowledge

Appendix 5-1: Information on Source and Data Quality 

Issues for Timber Harvest Volume, Silvicultural 

Prescriptions, and Habitat Data 

Introduction 

Assessing past trends and current status of timber harvest volume, number of 

treated acres, and predominant silvicultural prescriptions used, and the distribution 

and amounts of important California spotted owl (Strix occidentalis occidentalis) 

habitat types across the Sierra Nevada are a fundamental component for evaluating 

conservation status. This appendix identifies the data sources for the information 

summarized in chapter 5 for the above metrics. As described in the following sections, each data set has strengths and limitations as to completeness or accuracy of 

the data that must be considered when drawing inferences. Nevertheless, these data 

provide the sole sources of currently available data that provide valuable insight and 

information on trends and status of forest management treatments and habitat status. 

Trends in Timber Volume Harvested From the Sierra Nevada: 

1994–2013 

Annual summaries of timber volume harvested from public and private lands by 

county in California are available from the California State Board of Equalization, Timber Tax Program, 2014. Annual summaries are available for 1994–2013, 

consisting of nonspatial, tabular data reporting annual timber volume harvested in 

thousands of board feet by county. Counties were filtered to include only those that 

intersect any portion of the California spotted owl range in the Sierra Nevada as 

determined using the species distribution map maintained by the California Wildlife Habitat Relationships Program, California Department of Fish and Wildlife. 

Because the timber volume data are nonspatial, some portion of the volume was 

harvested from county areas outside of the range of the California spotted owl. 

Patterns in Silvicultural Prescriptions on National Forest Lands: 

1990–2014 

Prior to 2002, forest management treatments on national forest lands were tracked 

using the Stand Record (SRF) system. The SRF was a nonspatial, tabular database 

that recorded acres treated by silvicultural prescription by U.S. Department of 

Agriculture Forest Service (USFS) management unit (national forest and ranger 

district). Beginning in 2002, the USFS switched to use of the Forest Activity C 

Tracking System (FACTS) system for recording forest management treatments 
156 

GENERAL TECHNICAL REPORT PSW-GTR-254

and activities. The FACTS is a spatial database that records the footprint of timber 

management activities as well as acres treated by silvicultural prescription by 

management unit. Efforts have been made to generate and incorporate spatial data 

for forest treatments conducted prior to 2002, but not all projects have been entered 

into the database, and some unquantified proportion of the total area treated prior 

to 2002 is not spatially mapped. Thus, available information on USFS treatments 

consists of a complete tabular, nonspatial summary of activities from 1990 through 

2014 by national forest and ranger district (though October 2014). Spatial data on 

treatment type, amount, and location are complete from 2002 through 2014. Spatial 

data are incomplete for 1990–2001 and include some unquantified proportion of the 

actual activities.1 The nonspatial data provides insight into the acres treated by silvicultural prescription and trends in the use of different silvicultural prescriptions over 

time during 1990–2014 on national forests that intersect any portion of the range of 

the California spotted owl in the Sierra Nevada (Sequoia, Sierra, Stanislaus, Inyo, 

Eldorado, Tahoe, Plumas, and Lassen National Forests, and the Lake Tahoe Basin 

Management Unit). Treated acres on some national forests are located outside of the 

range of the spotted owl so not all treatments occurred within the range of the owl. 

Numbers reported consist of the number of acres accomplished. The spatial data 

provide opportunity to assess treatment acres and silvicultural prescriptions used 

within the range of the California spotted owl in the Sierra Nevada as determined 

by using the species distribution map maintained by the California Wildlife Habitat 

Relationships Program, California Department of Fish and Wildlife. However, the 

spatial data are incomplete for 1990–2002, and thus summaries based on the spatial 

data do not include all acres treated during the 1990–2002 period. 

Patterns in Silvicultural Prescriptions on Private Industrial Forest 

Lands: 1990–2013 

Information on acres treated by silvicultural method on private industrial timberland and nonindustrial private lands is available in the CALFIRE Forest Practice 

Database managed by the California Department of Forestry and Fire Protection. 

Nonspatial, tabular data are available to assess acres by silvicultural prescription by 

county for the 1990–2013 time period. Counties were filtered to include only those 

that intersect any portion of the California spotted owl range in the Sierra Nevada 

as determined using the species distribution map maintained by the California 

1 Sherlock, J. 2015. Personal communication. regional silviculturalist, USDA Forest 

Service, Pacific Southwest Research Region. 
157 

The California Spotted Owl: Current State of Knowledge

Wildlife Habitat Relationships Program, California Department of Fish and Wildlife. Because the timber volume data are nonspatial, some portion of the volume 

was harvested from county areas outside of the range of the California spotted owl. 

Spatial data on private industrial forest land silvicultural treatments are available 

for the 1997–2013 period. This database includes all acres approved and completed 

for treatment under all timber harvest plans (THPs) approved beginning in 1997 

and extending through 2013. However, this database does not include acres that 

were approved in THPs prior to 1997, yet the actual on-the-ground projects were 

conducted after 1997. A review of the 1997–2013 database indicates that most 

treatments are completed 4 to 6 years after approval, but that many acres are not 

reported as completed until 6 to 12 years after approval. Thus, the spatial data 

include all acres approved/or completed for 1997–2013 THPs, but more acres were 

actually treated then are shown because of pre-1997 THP acres not being included 

in the database. 

Status and Trends in California Spotted Owl Habitat 

in the Sierra Nevada 

The only source of information on the current distribution and abundance of California spotted owl habitat across the owl’s range in the Sierra Nevada is provided 

by the existing vegetation classification and mapping (EVEG) map maintained by 

the Remote Sensing Laboratory, Pacific Southwest Region, USFS. The EVEG map 

stiches together map products developed using different imagery and methods at the 

national forest and national park scale to provide a bioregional-scale map product of 

habitat across the Sierra Nevada. No formal accuracy assessments have been conducted to validate the map across the bioregion or to resolve differences in habitat 

classifications resulting from different mapping approaches using different imagery 

at different spatial and temporal scales. Thus, inferences about habitat amounts and 

distributions should be tempered until formal accuracy assessments are completed 

to validate map accuracy and consistency across the Sierra Nevada. Nevertheless, 

these data provide the sole source of information on current amounts of California 

spotted owl habitat across the Sierra Nevada. 
158 

GENERAL TECHNICAL REPORT PSW-GTR-254
159 

The California Spotted Owl: Current State of Knowledge

Chapter 6: Mapping Forest Conditions: Past, 

Present, and Future 

Maggi Kelly1 

Introduction 

Mapping and mapped data have always been critical to public land managers and 

researchers for identifying and characterizing wildlife habitat across scales, monitoring species and habitat change, and predicting and planning future scenarios. 

Maps and mapping protocols are often incorporated into wildlife and habitat 

management plans, as is the case with the California spotted owl (Strix occidentalis 

occidentalis), a subspecies of management concern. Current spotted owl managers on all Sierra Nevada national forests use canopy cover and tree size guidelines 

designed to provide habitat for sensitive species (Chopping et al. 2012, Moghaddas 

et al. 2010) and to estimate accurately these important habitat metrics across scales 

from nest trees and the area surrounding them to broader scale characterization of 

core foraging and home ranges. These mapping tasks can be challenging in California forests, particularly in the Sierra Nevada because they exhibit great variability 

in composition, cover, and topography, and complex legacies of fire and logging 

(Hyde et al. 2005). 

In this chapter, I have focused on mapping technology that can be used in 

the analysis of owl use of forested habitat. I reviewed and summarized 18 peerreviewed papers published from 1992 through 2013 that described the use of remote 

sensing, aerial imagery, or other mapped products to assess forest structure used 

by California spotted owls across scales and that also were specific about mapping 

protocols. Because many of the newer papers used new remote sensing technologies such as light detection and ranging (LiDAR), I have presented a retrospective 

of mapping methods before the detailed summary of the literature on California 

spotted owl. 

Owl Habitat Mapping Methods, Strengths, and 

Weaknesses 

Historical Mapping Technology 

Approaches to mapping wildlife habitat have been varied. They have included a 

range of remote sensing products and methods, manual delimitation and automated 

classifications, and mapping at many scales (Gottschalk et al. 2005, McDermid et al. 

1 Maggi Kelly is a geographer and professor, Department of Environmental Science, 

Policy, and Management, and Cooperative Extension Specialist, University of California– 

Berkeley, 130 Mulford Hall, Berkeley, CA 94720. 
160 

GENERAL TECHNICAL REPORT PSW-GTR-254

2005). Data describing owl habitat have been gathered from field surveys (e.g., Bond 

et al. 2004), black and white or color air photos (e.g., Dugger et al. 2011, Ripple et 

al. 1997), or digital aerial imagery (e.g., Seamans and Gutiérrez 2007); and existing 

mapped products such as timber survey maps (e.g., Blakesley et al. 1992, Call et al. 

1992), Landsat-derived vegetation maps (e.g., Bond et al. 2009, Hines et al. 2005), 

and fire-severity maps (e.g., Roberts et al. 2011). Remotely sensed imagery at fine 

spatial resolution (e.g., 1 m [3.3 ft]) and moderate resolution (e.g., 30 m [99 ft]) has 

also been used. Table 6-1 summarizes the types of remote sensing and mapping 

products commonly used for the mapping of spotted owl habitat. 

Aerial Photography 

Aerial photographs provide spatially detailed records of landscapes (Morgan et 

al. 2010). Despite the increase in the number and types of digital sensors available 

to managers and scientists, aerial photography remains a valuable tool for habitat 

Table 6-1—Map products typically used to understand California spotted owl habitat 

Type Product Data scale/resolution Example reference 

Aerial photography Black and white imagery 1:12,000 to 1:40,000 Ripple et al. 1997 

Aerial photography Color photography 1:12,000 to 1:40,000 Blakesley et al. 2005, Dugger 

et al. 2011 

Aerial photography Color infrared photography 1:12,000 to 1:20,000; 1 m Lee et al. 2013 

Aerial photography Digital orthophoto quadrangles 1:20,000 to 1:24,000; 1 m Seamans and Gutiérrez 2007 

Optical remote sensing NAIP 1 m Lee et al. 2013, Williams 

et al. 2011 

Optical remote sensing IKONOS (Satellite) 1 to 4 m Moghaddas et al. 2010 

Optical remote sensing QuickBird 0.6 to 2.5 m Chopping et al. 2012 

Optical remote sensing Landsat-5 Thematic Mapper 30 m Hunter et al. 1995, Moen 

and Gutiérrez 1997 

Optical remote sensing Relative differenced 30 m Roberts et al. 2011 

normalized burn ratio 

Optical remote sensing USFS EVEG 30 m Bond et al. 2009, 

Hines et al. 2005 

LiDAR Airborne discrete return 10- to 50-cm footprint García-Feced et al. 2011, 

Hyde et al. 2005 

LiDAR Airborne waveform 25- to 50-m footprint Chopping et al. 2013 

Existing mapped products Timber strata maps 1:20,000; misc. Blakesley et al. 1992, Irwin 

et al. 2007 

Existing mapped products FRAP fire perimeter maps Bond et al. 2002 

NAIP = National Agriculture Imagery, LiDAR = light detection and ranging, USFS EVEG = U.S. Forest Service existing vegetation, FRAP = Fire 

Resources and Assessment Program. 
161 

The California Spotted Owl: Current State of Knowledge

mapping for several reasons. First, aerial photographs predate satellite imagery; in 

California, imagery archives include images from the 1930s onward (Morgan et 

al. 2010). Second, the spatial detail provided by aerial photography is high, even 

when analog photographs are digitized. For example, a 1:20,000-scale photograph 

scanned at 200 dots per inch (dpi) will provide a digital image of 2.54-m (8.38-ft) 

resolution, and at 600 dpi yields 0.85-m (2.8-ft) resolution (Jensen 2000). This 

compares favorably to Landsat pixels, which are 30-m resolution and are similar to 

current high-resolution sensors such the QuickBird sensor. Third, when digitized, 

aerial photographs can be analyzed with powerful image analysis techniques. 

Although many of these techniques were originally developed for satellite imagery, 

they have also expanded upon the range of analysis techniques now available for 

aerial photographs (Cohen et al. 1996, Morgan et al. 2010). 

The spatial scale of aerial photography influences how it is used. Large-scale 

(1:2,400 to 1:1,200) photographs can be used to map individual trees, stream 

reaches, and fine-scale habitat photographs at 1:20,000-to 1:4,800-scale are 

used to map forest stand polygons, vegetation communities, and habitat patches. 

Photographs of 1:40,000-scale-and-smaller are useful for general land cover with 

minimum mapping units (MMUs) of 2 to 4 ha (5 to 10 ac) (Wulder 1998). Aerial 

photographs are captured most commonly as panchromatic (black and white 

visible), color, or false-color infrared (CIR). These can be analyzed manually, with 

a trained analyst tracing boundaries between land cover patches (e.g., Chatfield 

2005), and in more automated fashion, using similar algorithms pioneered in remote 

sensing (Cohen et al. 1996). 

A standard format for digital aerial photographs is the digital orthophoto quadrangle (DOQ), which uses a standard image rectification procedure that aligns the 

image with longitude and latitude or other coordinate system. The U.S. Geological 

Survey (USGS) provides the largest catalog of DOQs, which may exist as far back 

as the early part of the 20th century. Typical spatial resolutions for DOQs are 1 m 

and less. More recently (since 2005 in California), the National Agriculture Imagery 

Program (NAIP) has been providing free periodic (usually every 5 years) digital 

CIR aerial imagery at 1-m resolution during the agricultural growing seasons in the 

continental United States. These images have proved useful for forest and habitat 

mapping (Cleve et al. 2008, Jakubowski et al. 2013a). 

Landsat 

The launch of the Earth Resources Technology Satellite 1, or ERTS-1 (ERTS-1) 

(later renamed Landsat-1) in 1972 (Lauer et al. 1997, Melesse et al. 2007) permanently changed the way remote sensing served resource management, although 
162 

GENERAL TECHNICAL REPORT PSW-GTR-254

not immediately. From 1980 to 2000, there was nearly 20 years of increasing use 

of Landsat imagery by land managers and scientists for mapping forest vegetation 

(Franklin et al. 2000), particularly in California. Landsat-5 was launched in 1984 

with the Thematic Mapper (TM) moderate resolution (30-m [99-ft]), six-band 

multispectral (typically broad spectral information in the visible to near-infrared 

light) sensor on board, and became the workhorse for remote sensing of land cover 

(Cohen and Goward 2004, Wulder et al. 2012). Throughout the 1980s and 1990s, 

the USDA Forest Service (USFS) and the California Department of Forestry and 

Fire Protection collaborated in California to produce a statewide Land Cover 

Mapping and Monitoring Program (LCMMP) to improve the quality and capability of monitoring data, and to minimize costs for statewide land cover monitoring 

(Levien et al. 2002). The mapping project aimed to support resource inventory, 

fire management, and habitat conservation goals, and an initial goal was to update 

these maps to quantify land cover changes every 6 years with the collaboration of 

the California Division of Forestry and Fire Protection (Franklin et al. 2000). Their 

initial method involved image segmentation into forest polygons (stands) using 

spectral and textural inputs, and either unsupervised classification or linear spectral 

mixed analysis. Results were calibrated with Forest Inventory and Analysis (FIA) 

data. Map attributes include a vegetation life-form class (e.g., conifer, hardwood, 

chaparral), vegetation type from the Classification and Assessment with Landsat of 

Visible Ecological Groupings (CALVEG) classification scheme, and canopy cover 

and size class estimates for forest stands. A Kauth Thomas algorithm (a transformation of spectral data to brightness, greenness, and wetness) applied to multitemporal 

Landsat imagery provided information for magnitude and direction of land cover 

change (Rogan et al. 2003). 

The mapping protocol has evolved over time and been updated by the USFS 

when needed, and now forms the basis of EVEG (“existing vegetation”). EVEG is 

a Landsat-derived product that captures vegetation characteristics using automated, 

systematic procedures that map large areas of California and is supplemented with 

onsite field visits. The current map attributes consist of vegetation types using the 

CALVEG classification system and forest structural characteristics such as tree and 

shrub canopy cover and tree stem diameters. Current map product characteristics 

include a 1-ha (2.5-ac) MMU for most vegetation conditions (there is no MMU for 

lakes and conifer plantations); life form (conifer, mix, hardwood, shrub, grass, barren, agriculture, urban, ice/snow, water), within-life-form classes that are “crosswalked” to state and regional vegetation mapping standards, information on canopy 
163 

The California Spotted Owl: Current State of Knowledge

closure of conifer and hardwood forests, mapped as a function of canopy closure in 

10 percent classes, and size of overstory tree as interpreted from aerial photography 

and satellite imagery. 

Vegetation maps derived from Landsat data have been used widely to study 

California spotted owl habitat (Bond et al. 2004, Hunter et al. 1995, Moen and 

Gutiérrez 1997). Landsat imagery, as well as the statewide vegetation map product 

derived from Landsat (i.e., EVEG), has been used since the 1990s for mapping 

wildlife habitat (Gottschalk et al. 2005) and is being used increasingly in sophisticated species distribution models that map habitat suitability for important wildlife 

species in California. The broad coverage and spectral detail of the Landsat sensors 

are useful for large-coverage mapping of species and canopy cover, but this imagery 

is not able to detect the residual tree component of forests dominated by mediumsized trees that is a critical component driving use by owls in these younger forests 

(García-Feced et al. 2011, Moen and Gutiérrez 1997). Residual trees are large trees 

within younger forests that may possibly serve as nest trees and influence forest 

stand temperature. These detailed aspects of forest structure are now better able to 

be mapped using a range of “active” remote sensing methods, such as LiDAR. 

High Spatial Resolution Imagery 

There have been a number of launches of satellites carrying high spatial resolution 

(approximately 5 m [16.5 ft] or less) multispectral sensors that have been used to 

map forests. The first of these was IKONOS, which was launched in 1999 with a 3- 

to 5-day return interval and imaged in panchromatic (1-m [3.3-ft]) and multispectral 

(4-m [13.2-ft]) modes. The QuickBird satellite (panchromatic band = 60 cm [24 in], 

multispectral bands = 2.5 m [8.3 ft]) was launched in 2001, and 2003 saw the launch 

of the OrbView satellite, which acquires multispectral imagery in either multispectral (4-m [13.2-ft]) or panchromatic (1-m [3.3-ft]) mode. In 2008, RapidEye was 

launched with five satellites as part of a public-private partnership with numerous 

European partners. This satellite constellation provides almost daily coverage of the 

Earth at 6.5-m (21.5-ft) resolution and was the first commercial satellite program 

to include the red-edge band, which is sensitive to changes in chlorophyll content, 

and therefore useful for vegetation mapping. WorldView-2 was launched in 2009 

with an eight-band multispectral sensor (including a red-edge band) operating at 

0.5 m (1.7 ft) in panchromatic and 1.8 m (5.9 ft) in the multispectral bands. These 

sensors provide detailed imagery with a timely repeat schedule and have been used 

to map forest habitat globally, although only IKONOS has been used in the context 

of California spotted owl mapping (Moghaddas et al. 2010). 
164 

GENERAL TECHNICAL REPORT PSW-GTR-254

Current and Emerging Technology 

LiDAR 

LiDAR provides highly detailed, extensive, and accurate vegetation structure data, 

which has long been identified as a key element of organisms’ habitats (Lefsky et 

al. 2002, Popescu and Wynne 2004, Vierling et al. 2008). LiDAR data are collected 

from a laser-emitter scanner linked to an accurate positioning system. The roundtrip time between pulse origination and return from target is measured, allowing the 

instrument to calculate the distance to a target object. LiDAR data can be broadly 

categorized into three classes depending on the type of sensor and deployment: (1) 

ground-based LiDAR, which samples the scattering returned by the entire laser 

pulse over a wide range of zenith angles and azimuth angles as it passes through the 

canopy from a stationary ground-based scanner (Henning and Radtke 2006, Strahler 

et al. 2008, Zhao et al. 2011); (2) small-footprint discrete return data in which the 

spatial coordinates of typically four discrete returns per laser pulse are recorded 

(Lefsky et al. 2002); and (3) large-footprint waveform data in which the pulse-return 

intensity over time is digitized (Lefsky 2010, Merrick et al. 2013, Vierling et al. 

2008). Aircraft-based systems use onboard global positioning system (GPS) and 

inertial measurement units to establish position, whereas ground-based LiDAR 

uses GPS alone. The resolution and quality of the data depend on both the scanner 

and the pulse density (Merrick et al. 2013). The resulting data are either a detailed 

three-dimensional point cloud (e.g., ground and airborne LiDAR) or a collection of 

intensity returns (waveform); each of these can be manipulated in numerous ways 

to derive point-based and raster-based LiDAR metrics that capture aspects of the 

forest structure such as individual trees (Jakubowski et al. 2013b, Li et al. 2012) or 

other derived metrics. Most of the current literature describing LiDAR and wildlife 

habitat focuses on aircraft-based discrete return small-footprint LiDAR. 

LiDAR metrics— 

Numerous LiDAR metrics derived from the LiDAR point cloud have proved to be 

useful in wildlife habitat studies. Merrick et al. (2013) outlines primary metrics 

(those that can be derived directly from the LiDAR point cloud) and secondary metrics (those that are modeled based on LiDAR and field data) that have been used in 

wildlife studies. Primary metrics include canopy metrics (e.g., canopy surface model, canopy cover/closure, canopy/vegetation height model, canopy/vegetation profiles, canopy base height, canopy volume); vertical profile metrics (e.g., coefficient 

of variation of hits, foliage height diversity, standard deviation of vegetation height, 

mean absolute deviation height, vertical distribution of hits); topographic products 
165 

The California Spotted Owl: Current State of Knowledge

(e.g., Digital Terrain Model, Digital Elevation Model and LiDAR return intensity). 

Secondary metrics include aboveground biomass, basal area, canopy complexity/diversity, tree diameter at breast height (d.b.h.), leaf area index (l.a.i.), timber/vegetation volume, and vertical distribution ratio. These metrics have been used to predict 

vegetation structure (e.g., biomass) and to scale-up field measurements to broader 

scales (Gonzalez et al. 2010; Hyde et al. 2005; Kane et al. 2011, 2015, 2013; Vierling 

et al. 2008, Wulder et al. 2008) to predict species performance based on structural 

associations (Lesak et al. 2011), to aid in vegetation classification and mapping 

(Swatantran et al. 2011), and in species distribution models to predict species presence or diversity. 

Very high resolution imagery and microsatellites— 

The 21st century can be characterized, in remote sensing terms, by the increased 

interest by private industry in the Earth observation domain (Melesse et al. 2007). 

There are several private companies providing high spatial resolution imagery at 

cost (e.g., IKONOS, QuickBird, Rapideye, and GeoEye). Additionally, there are 

numerous companies pioneering the deployment of so-called microsatellites, which 

are small and operate in low Earth orbit (Kramer and Cracknell 2008). Many of 

these have spatial resolutions of less than 1 m and operate in the multispectral and 

panchromatic mode. With multiple satellites operating in a constellation, image 

acquisition rates are expected to increase to more than one per day for some areas 

of the Earth. Finally, Google EarthTM (http://earth.google.com) has transformed the 

ways in which scientists and researchers can access and use high spatial resolution 

imagery, including assessing wildlife habitat (e.g., Hughes et al. 2011). 

Characterizing Habitat Across Scales 

Eighteen peer-reviewed journal articles from 1992 through 2013 revealed use of 

mapping technology to investigate California spotted owl habitat across scales 

(table 6-2). The organization of this review follows the habitat scales discussed in 

chapter 3 (i.e., nest, nest stand, core area, foraging habitat, and home range), but 

it was unclear from reading some papers what was the scale of investigation, so I 

categorized them loosely. There are tradeoffs among desired resolution, scale of 

imagery, and needed data given the application (e.g., moderate- to course-scale 

imagery such as Landsat is not appropriate for fine-scale mapping of habitat). Most 

papers used mapping technology to characterize forest structure around owl sites. 

The characterization of forest structure often involves the use of a fixed-radius 

buffer centered on nest sites or primary roost areas. The radius length dictates the 
166 

GENERAL TECHNICAL REPORT PSW-GTR-254

 

 

 

 

 

 

 

 

 

 

 

Table 6-2—Literature describing the mapping of California spotted owl habitat across scales 

Core Foraging Home 

Reference Map product/type Nest Stand area habitat range Study area 

Bias and Gutiérrez 1992 Landsat-5 TM ✓ Eldorado and Tahoe NF 

(Thematic mapper) 

Call et al. 1992 Field surveys; timber ✓ ✓ Tahoe NF 

strata maps 

Moen and Gutiérrez 1997 Landsat-5 TM ✓ ✓ ✓ Central Sierra Nevada 

Lahaye et al. 2001 Landsat ✓ ✓ San Bernardino 

Mountains 

Temple and Gutiérrez 2002 Landsat: USFS EVEG ✓ Eldorado and Tahoe NF 

Bond et al. 2002 CalFire Fire perimeter- ✓ Arizona, California, 

maps New Mexico 

Bond et al. 2004 Landsat: USFS EVEG ✓ ✓ Eldorado NF 

Blakesley et al. 2005 Color aerial ✓ ✓ Lassen NF 

photography 

Hines et al. 2005 Landsat: USFS EVEG ✓ Southern California 

Hyde et al. 2005 LiDAR: Waveform ✓ Sierra NF 

Seamans and Gutiérrez 2007 Digital orthophoto ✓ ✓ Central Sierra Nevada 

quadrangles 

Irwin et al. 2007 Timber strata maps ✓ Northern California 

Bond et al. 2009 Landsat: USFS EVEG; ✓ ✓ ✓ Sequoia NF 

Relative differenced 

normalized burn ratio 

(dRNBR) 

Phillips et al. 2010 Digital orthophoto ✓ ✓ Eldorado and Tahoe NF 

quadrangles and color 

aerial photographs 

Moghaddas et al. 2010 IKONOS ✓ ✓ ✓ Plumas-Lassen NF 

García-Feced et al. 2011 Discrete return LiDAR ✓ ✓ Eldorado NF 

Roberts et al. 2011 RdNBR ✓ ✓ Yosemite NP 

Williams et al. 2011 NAIP ✓ Eldorado and Tahoe NF 

Lee et al. 2013 Color and CIR aerial ✓ San Bernardino 

photography; NAIP Mountains 

USFS EVEG = U.S. Forest Service existing vegetation; NAIP = National Agriculture Imagery Program; LiDAR = light detecton and range, national 

forest; np = national park, CIR = color infrared. 
167 

The California Spotted Owl: Current State of Knowledge

scale of focus; and literature reports examples of radii <100 m (330 ft) (e.g., Call 

et al. 1992, Hyde et al. 2005) to >1 km (3,300 ft) (e.g., Dugger et al. 2011, Seamans 

and Gutiérrez 2007) covering circular areas from 1 ha (nest tree and stand scale) to 

greater than 1000 ha (2,500 ac; home range scale). The circular area described is 

then characterized using mapped data: either created new from field surveys, black 

and white or color air photos, or other remotely sensed imagery such as Landsat, or 

through the use of existing mapped products such as timber survey maps, Landsatderived vegetation maps, or fire-severity maps. These results are often compared 

with an area of similar size that does not contain nest trees (e.g., a randomly 

selected stand). Other methods include the characterization of forests within some 

other noncircular area (e.g., minimum convex polygons describing nest and roost 

sites as in Moen and Gutiérrez (1997)] and Irwin et al. (2007). Existing mapped 

products have also been used to aid in sampling design, as in Bond et al. (2004) 

who used the USFS EVEG habitat map to identify the four strata in which to locate 

their random plots. 

Mapping Nests and Nest Trees 

Spotted owls nest in forests with dense canopy cover and large (>76 cm [30.5 in] 

d.b.h.) trees. They will use forests with medium-sized trees if they have dense 

canopy cover and residual trees (Bias and Gutiérrez 1992, Moen and Gutiérrez 

1997). The ability to map individual trees and critical structural elements from 

remote sensing has been enhanced recently through the use of LiDAR (GarcíaFeced et al. 2011, Hyde et al. 2005). Although canopy cover estimates from optical 

remote sensing are reliable, the mapping of individual and residual trees is difficult 

with coarse-scale optical imagery such as Landsat, particularly in dense canopy. 

García-Feced et al. (2011) evaluated the ability of LiDAR data to map these critical habitat elements in the Tahoe National Forest. They surveyed for spotted owls 

within this area during 2007 through 2009 and located four nest trees. They then 

used the LiDAR data to estimate the number, density, and pattern of residual trees 

(90 cm [36 in] d.b.h.) and to estimate canopy cover within 200 m of each of the nest 

trees (a circular area of 12.6 ha [31.5 ac]). They found that nest trees were surrounded by large numbers of residual trees and high canopy cover, and the LiDARbased estimates agreed closely with residual tree counts and canopy cover estimates 

based on field data collected within 100 m (3 ha [7.5 ac]) of these nest trees. 

Mapping Nest Stand Characteristics 

California spotted owls nest and roost in complex, multlayered, late-successional 

forests with high canopy closure and cover, and numerous large trees (chapter 3). 

Using the classical buffer approach, Blakesley et al. (2005) mapped the forest stands 
168 

GENERAL TECHNICAL REPORT PSW-GTR-254

surrounding 67 spotted owl sites using color aerial photographs, digital orthophoto 

quadrangles (from 1993 and 1998), and timber sale information within circular plots 

(with radii up to 2.4 km) in northeastern California. They examined the relationships between habitat composition in the area surrounding nest trees and variation 

in nest success over time (1990 through 2000) and site occupancy, apparent survival 

probability, and reproductive output over time (1993 through 1998). They found that 

large trees with high canopy cover were important for site occupancy at the stand 

scale (e.g., 203 ha) within the nest area, and the amount of nonforested areas and 

forest cover types not used for nesting or foraging negatively influenced occupancy. 

Additionally, the presence of large remnant trees within the nest stand facilitated 

nest success. They conducted their analysis at two spatial scales: nest area (203 ha 

[507 ac]) and core area (814 ha [325 ac]). 

The first study to evaluate the use of LiDAR for mapping California spotted 

owl habitat was Hyde et al. (2005). They used large footprint, waveform LiDAR 

data acquired for the Sierra National Forest in October 1999 (leaf-on) to map 

forest structure: canopy height, canopy cover, and aboveground biomass. They 

used a LiDAR called Laser Vegetation Imaging Sensor, which is a full waveformdigitizing system that records the vertical distribution of target surfaces with 30-cm 

(12-in) vertical resolution. This was a large footprint system with a 12.5-m (31.5-ft) 

radius footprint on the ground. They compared LiDAR footprint returns to field 

data gathered in circular plots with an inner plot of 0.07-ha (1.18-ac; 15-m or 50-ft 

radius) and an outer plot of 1-ha (2.5-ac; 56.4-m or 186.1-ft radius). Results were 

encouraging: field and LiDAR canopy structure measures showed good agreement across a range of elevation and slope. They suggested that the correlation 

between the field plots and LiDAR data was amenable to scaling, and thus LiDAR 

was useful to characterize montane forest canopy structure over the wide range 

of environmental conditions that occur over the Sierra National Forest and might 

be useful to use for habitat mapping over large areas. The location of nest trees in 

relation to forest edges was examined by Phillips et al. (2010), who used a vegetation map of the Eldorado and Tahoe National Forests that had been created using 

aerial photography and digital orthophoto quadrangles from 1998 and 2000. Their 

geographic information system (GIS) database included a vegetation map with eight 

cover types, elevation data, nest tree locations, and one random location within 

each nest stand. Distances to forest edge from each nest and random location were 

compared, and they found no evidence in their study area that California spotted 

owls used nest sites closer to forest edges than one would expect by chance, and this 

was consistent over a wide range of elevations. They also suggested that the owls in 

the study area nested farther from high-contrast edges than expected by chance. 
169 

The California Spotted Owl: Current State of Knowledge

Mapping Core Use Area Characteristics 

The primary areas used by spotted owls for nesting and foraging (core use areas) 

contain the contiguous forest an owl or owl pair uses consistently, including the nest 

and roosting area (Blakesley et al. 2005, Williams et al. 2011). It is pointed out in 

chapter 3 that because these forests contain nest sites, the characteristics between 

territory and nest stand often overlap. When mapping large areas such as owl core 

use areas (e.g., territories) on the order of 150 to 400 ha (500 to 1,000 ac), moderateresolution imagery such as Landsat (resolution 30 m [99 ft]) has had a dominant yet 

contested role. 

Hunter et al. (1995) used Landsat imagery and landscape metrics to understand 

spotted owl core use areas. While they focused on the northern spotted owl (Strix 

occidentals caurina), I have discussed the paper here because of the precedent it set. 

They used a single date Landsat-5 TM image and classified the core use area of the 

northern spotted owl in Humboldt County, California, into broad vegetation lifeform classes. They then compared the landscape characteristics (land cover, fragmentation, and heterogeneity) within circular areas of 800-m (2,640-ft) radius (200 

ha [520 ac]) around each spotted owl nest, roost, and random sites between 1988 

and 1992. Nest and roost sites were characterized by lower amounts of nonvegetation and herbaceous land cover, and by greater amounts of mature and old-growth 

coniferous forest, which was less fragmented than random sites. They noted that 

the spectral similarities in the Landsat images between structurally similar seral 

stages made some age classification difficult. For example, differences between 

mature and old-growth forests were difficult to map using these data. Moen and 

Gutiérrez (1997) also used classified Landsat-5 TM imagery to examine the landscape characteristics within a 457-ha [1,142-ac] area surrounding 25 owl centers. 

They mapped minimum convex polygons that included both roosts and nests. The 

Landsat-5 image was classified by dominant species, size class, and canopy closure. 

This paper highlighted early on one of the main challenges for wildlife researchers 

using Landsat imagery and products—the typically poor ability of the Landsat pixel 

to capture the large tree (> 60 cm [24 in] d.b.h.) component of forests that appears to 

be critical to the spotted owl in particular. 

Numerous researchers have focused on the impact of fire on spotted owl core 

area habitat. In a geographically broad study, Bond et al. (2002) examined the 

response of all three spotted owl subspecies to wildfire in Arizona, California, and 

New Mexico. They examined the response of owls after large (>540-ha [1,350-ac]) 

wildfires occurred within their territories. Large-fire locations were derived from 

the Fire Resources and Assessment Program fire perimeter database, which is a 
170 

GENERAL TECHNICAL REPORT PSW-GTR-254

statewide geodatabase with wildfire history, prescribed burns, and other fuel modification projects current through 2013, and from the USFS. These digital fire data 

sets were critical for the study, and they called for more large-scale experiments to 

understand the effects of prescribed burning on spotted owls. 

Seamans and Gutiérrez (2007) modeled the probability of territory colonization, territory extinction, and breeding dispersal in relation to the amount of mature 

conifer forest in the central Sierra Nevada. They used an existing map of forest 

cover developed from aerial photographs, digital-orthophoto-quarter quadrangles, 

and extensive ground sampling of the forest to classify tree size class and canopy 

closure (Chatfield 2005) and to estimate the amount of each forest class within a 

400-ha (1,000-ac) circle (radius = 1128 m [0.7-mi] or half the mean nearest neighbor 

distance of occupied territories in their study area averaged over the years 1990 to 

2002). They found that the amount of mature conifer forest (i.e., dominated by trees 

≥30.4 cm (12 in) d.b.h. with canopy cover ≥70 percent) was correlated with spotted 

owl occupancy. Territories with more mature conifer forest had a higher probability 

of being colonized and a lower probability of becoming unoccupied. They also 

reported that alteration of mature conifer forest appeared to decrease the probability 

of colonization. 

Roberts et al. (2011) examined the effects of fire severity on spotted owl site 

occupancy in late-successional montane forest in Yosemite National Park using a 

relatively new burn-severity metric called the relative differenced normalized burn 

ratio (RdNBR) (Miller and Thode 2007). Using images of an area before and after a 

fire remotely sensed by Landsat bands 4 and 7, they calculated the RdNBR to create 

a relative measure of vegetation change, which is then classified into four levels of 

fire severity: 

• Unburned or unchanged 

• Low severity 

• Moderate severity 

• High severity 

A polygon map of fire severity for fires in Yosemite was used to compare owl 

site occupancy, and the authors reported that density estimates of California spotted 

owl pairs were similar in burned and unburned forests. They suggested that low- to 

moderate-severity fires might maintain habitat characteristics essential for spotted 

owls, and further that managed fires that emulate the historical fire regime of these 

forests may help maintain spotted owl habitat and protect this species from the 

effects of future catastrophic fires. 
171 

The California Spotted Owl: Current State of Knowledge

Lee et al. (2013) also examined the impact of fire and disturbance on spotted 

owl occupancy. They mapped the 203-ha (500-ac) forested area (radius approximately 800 m [0.5 mi]) surrounding a single owl nest tree location within each owl 

territory before and after fires in the San Bernardino and San Jacinto Mountains of 

southern California to investigate the influence of fire and salvage logging on spotted owls. Spotted owl sites affected by fire were those where the perimeter of the 

203-ha (500-ac) core area overlapped the perimeter of one of the fires that occurred 

in the area from 2003 to 2007. The prefire map was created using 1-m resolution 

CIR aerial photographs and stereo pairs of color aerial photographs. Imagery from 

NAIP taken for the San Bernardino National Forest in October 2009 was used to 

remap vegetation in core areas that burned between October 2003 and October 

2007. They also used Google Earth imagery to estimate the amount of the 203-ha 

(500-ac) area affected by extensive postfire tree removal. They found that sites 

where high-severity fire affected >50 ha (125 ac) of forested habitat could still support spotted owls and recommended that all burned sites should be monitored for 

occupancy before management actions such as salvage logging were undertaken. 

Other researchers have modeled fire behavior to predict future impacts of fires 

on spotted owl habitat. Moghaddas et al. (2010) used two common fire modeling 

software programs FlamMap and FARSITE that were parameterized with vegetation maps derived from IKONOS imagery, ground-based plot data, and integrated 

data from ARCFUELS and the Forest Vegetation Simulator. They modeled conditional burn probability under 97th percentile weather conditions across Meadow 

Valley in the Plumas National Forest to investigate the impact of forest fuel treatments. The study area contained California spotted owl habitat areas, protected 

activity centers, and home range core areas. Fourteen percent of the study area was 

spotted owl core area. The modeled results indicated that the average conditional 

burn probability was reduced between pre- and posttreatment landscapes, and the 

stands designated for management of spotted owls as well as other resources were 

assumed to benefit from the landscape fuel treatments. 

Mapping Characteristics of Foraging Habitat 

Spotted owls forage in forests characterized by a mosaic of vegetation types and 

seral stages interspersed within mature forest as well as in contiguous stands of 

mature and old-growth forest (chapter 3). Landsat imagery was used by Lahaye 

et al. (2001) to classify vegetation into four categories: owl nesting and roosting 

habitat, owl foraging habitat, nonforested vegetation, and other non-owl habitats. 

They used this classification to estimate the proportion of the study area supporting 
172 

GENERAL TECHNICAL REPORT PSW-GTR-254

owl nesting and foraging habitat in a study investigating timing and patterns of owl 

dispersal in the San Bernardino Mountains in southern California. This is a highly 

fragmented region with only 2 percent of the landscape covered by vegetation types 

that support spotted owls. They showed that the majority of owl dispersers settled 

in territories that were occupied by either pairs or single owls the previous year, 

some settled in vacant territories next to occupied sites, and a few settled at sites of 

unknown occupancy. No owls settled at unoccupied sites that were not adjacent to 

occupied sites. 

Detailed forest habitat maps have been commonly made by private landowners 

and can be used in spotted owl research. For example, Irwin et al. (2007) used owl 

telemetry and existing vegetation maps provided by a private forestry company 

to evaluate owl foraging habitat. Sierra Pacific Industries inventoried their forests 

from August 1997 to March 1999 on an 80- by 200-m (264- by 660-ft) grid. They 

used this map to compare habitat values at owl and random locations within 95 

percent minimum convex polygon home ranges. Results indicated that stands more 

likely to be chosen for foraging included those with intermediate values of the 

combined basal areas of three conifer species Douglas-fir (Pseudotsuga menziesii), 

white fir (Abies concolor), and red fir (A. magnifica) and greater basal area of 

large-diameter hardwoods. The relative probability of selection for foraging habitat 

decreased with increasing basal area of ponderosa pine (Pinus ponderosa Lawson 

& C. Lawson). Topographic position, habitat heterogeneity, tree species composition, and forest density also influenced foraging site selection. 

In 2002, the McNally Fire burned 610 km2 of land in the southern Sierra 

Nevada, including forests containing four California spotted owl territories. Four 

years later, Bond et al. (2009) examined effects of fire on these seven radiomarked 

owls from these territories by quantifying, as a function of fire severity, owl use 

of forests for nesting, roosting, and foraging. They used the Landsat-based EVEG 

vegetation map to establish habitat within foraging ranges of spotted owl and 

Landsat-based RdNBR to quantify fire severity. They reported that within 1 km of 

the center of their foraging areas, spotted owls selected all severities of burned forest and avoided unburned forest. Beyond 1.5 km of a center of foraging area, there 

were no discernable differences in use patterns among burn severities, and owls 

foraged at low rates in burned and unburned areas. Owls foraged in high-severity 

burned forest with greater basal area of snags and higher shrub and herbaceous 

cover more than in all other burn categories. 
173 

The California Spotted Owl: Current State of Knowledge

Mapping Home Ranges 

Owl home ranges encompass the area used by an owl to meet its requirements for 

survival and reproduction (chapter 3) and are large (e.g., 600 to 2200 ha [1,500 to 

5,500 ac]). Mapping owl home ranges often requires moderate-scale resolution 

imagery. The first use of Landsat for California spotted owl habitat research was 

described in Bias and Gutiérrez (1992), who used Landsat imagery to investigate 

spotted owl home range characteristics across ownership. They used Landsat-5 

TM images from 1986 and 1987 to measure the interspersion, or rate of change, 

of different habitat types along 50 randomly located transect lines throughout owl 

territories. Their study area crossed the boundaries of the Eldorado and Tahoe 

National Forests, and had a mixed ownership: 60 percent was public land and 40 

percent was private land. Their analysis was largely pre-digital: they superimposed the Landsat-5 images onto base maps using a stereo zoom transfer scope 

and interpreted vegetation changes from the Landsat images based on recognition 

and identification of image characteristics (i.e., tone, texture, color). They defined 

habitat interspersion as the number of habitat changes along a segment divided by 

the scale-equivalent length of that segment. This metric (habitat change per kilometer) was then compared across public land, private land, and nest sites. Ownership 

pattern influenced roosting and nesting behavior: the majority of observed roosts 

and all owl nests were on public lands. 

Tempel and Gutiérrez (2002) investigated whether the environment within 

an owl territory might affect stress hormone levels. They collected fecal samples 

from spotted owls in Eldorado and Tahoe National Forests to determine if certain 

environmental factors were correlated with elevated fecal corticosterone levels. The 

environmental variables they examined were largely derived from the USFS EVEG 

Landsat product, and included the amount of core and edge habitat, number of 

habitat patches, and the total length of roads within an owl territory. While a linkage between fecal corticosterone and environment was not found, they suggested 

protocols for sampling corticosterone in birds. Bond et al. 2009 used both the USFS 

vegetation EVEG map product and the RdNBR product to understand how spotted 

owls were using habitat after a fire. They found that spotted owls at two areas on 

the Sequoia National Forest foraged in a range of burn severities, illustrating that a 

mosaic of burn severities in California spotted owl territories apparently allows owl 

use 4 years after a fire. 

The accuracy of the Landsat-derived vegetation maps were explicitly tested 

by Hines et al. (2005) who performed a sensitivity analysis of the EVEG product 

developed for the USFS in southern California to estimate how mapping errors in 
174 

GENERAL TECHNICAL REPORT PSW-GTR-254

vegetation type, forest canopy cover, and tree crown size might affect the delineation of suitable habitat for the California spotted owl. In this cautionary note on the 

use of existing coarse-scale land cover products, the authors reported an increase in 

the estimated area of suitable habitat types for the spotted owl solely resulting from 

map uncertainty. 

High spatial resolution imagery has also been used to map forest structure 

and owl habitat in greater detail than possible by Landsat. Williams et al. (2011) 

used the USFS, NAIP imagery from 2005 to estimate tree size, canopy cover, and 

hardwood or conifer forest in the Eldorado and Tahoe National Forests study area. 

They digitized the boundaries of vegetation patches and then classified the patches 

into eight vegetation classes based on tree size and canopy cover consistent with the 

California Wildlife Habitat Relationships system (Mayer and Laudenslayer 1988). 

The vegetation of every owl home range in the Eldorado and Tahoe National Forests 

study area as well as 2,161 random locations throughout the study area was mapped 

and compared. They found that landscape heterogeneity (number of patches) was an 

important additional positive factor in owl home-range size, as well as owl foraging 

site selection. 

Accuracy Assessment 

Understanding the accuracy of a remotely sensed product is critical for determining 

its usefulness. I reviewed all papers assessed in this chapter for a description of 

accuracy, and the way in which accuracy might play a role in the use of the product. 

Under half (eight) of them explicitly discussed accuracy of products used. Currently, 

best practices for assessing and reporting accuracy of classified remotely sensed 

maps include the development of an “error matrix” in which reference values are 

checked against classified values across the types of land cover values (Congalton 

and Green 1999, Foody 2002). Reference data ideally should come from field data 

gathered contemporaneously with imagery. Because this is often difficult, many 

researchers use as reference data imagery at higher resolutions than the source 

imagery. Metrics derived from an error matrix include overall accuracy (percentage), and errors of omission (or Producer’s accuracy) and errors of commission (or 

User’s accuracy) for each land cover class mapped. These are important measures 

to evaluate prior to use of land cover maps as the most important classes for owl 

biology might be the classes that are difficult to accurately map. An additional 

metric—the kappa statistic—is often reported and gives the likelihood that a classification is better than random. When a remote sensing product is presented as a 
175 

The California Spotted Owl: Current State of Knowledge

physical measure, such as canopy cover, its accuracy is reported using a correlation 

coefficient (r2) or root-mean-square error (RMSE), which is based on regression 

between field-derived reference data and remotely sensed values. 

Papers of several researchers I reviewed used the error matrix approach to 

evaluate mapped products (Chatfield 2005, Hunter et al. 1995, Phillips et al. 2010, 

Ripple et al. 1997, Seamans and Gutiérrez 2005, Williams et al 2011) reporting 

overall accuracies of mapped product from aerial photography interpretation generally above 80 percent and overall accuracies of Landsat classification at 76 percent 

(Hunter et al. 1995). Moen and Gutiérrez (1997) reported an accuracy of 76 percent 

for the Landsat habitat map, but noted that the product lacked the “residual tree” 

component that appears critical for owls for their use of medium-sized tree forests. 

Bond et al. (2009) used the error matrix approach to evaluate a burn-severity map, 

and found it was 93 percent correct (with 80 field validation sites). 

The implication of the accuracy of the Landsat-derived vegetation maps was 

explicitly examined by Hines et al. (2005), who performed a sensitivity analysis 

of the EVEG product developed for the USFS in southern California to estimate 

how mapping errors in vegetation type, forest canopy cover, and tree crown size 

might affect the delineation of suitable habitat for the California spotted owl. They 

reported the overall accuracy for USFS Landsat-derived vegetation map was 73 percent, but individual class accuracy ranged from 25 to 100 percent. They used these 

error values in a simulation experiment to evaluate the role of mapped error in over 

or underpredicting owl habitat. In this cautionary note on the use of existing coarsescale land cover products, the authors reported an increase in the estimated area of 

suitable habitat types for the spotted owl solely resulting from map uncertainty. 

Accuracy assessment of LiDAR mapped products is more complicated than 

for optical imagery. Hyde et al. (2005) evaluated LiDAR-derived canopy height 

measures using regression between field and LiDAR canopy height measures 

and reported high r2

 and low RMSE. The positional accuracy of LiDAR-derived 

locations of individual trees requires taking a sample of tree locations in the field 

using high-quality GPS, and reporting the RMSE in x and y directions between 

reference and LiDAR. This is often not done owing to the difficulties in gathering 

sufficient samples in the field. García-Feced et al. (2011) compared in general terms 

the number and pattern of residual trees and canopy cover in the area surrounding 

four nest trees between LiDAR and field-derived values and show concordance of 

LiDAR with field sampling. 
176 

GENERAL TECHNICAL REPORT PSW-GTR-254

Chapter Summary 

Mapping technology has been critical to understanding the ways owls use their 

forest habitat and to help manage forests for their sustainability. Many studies have 

relied on moderate-resolution Landsat imagery to map large areas of forest, but this 

is not without challenges. Of primary importance is the assessment of accuracy 

in mapped products. Despite the need to understand product quality, the accuracy 

of mapped products is not routinely evaluated. Fewer than half of the articles 

I reviewed included a description of any accuracy assessment. Recommended 

accuracy assessment approaches are not universally adopted in the remote sensing 

community (Foody 2002). Remotely sensed or GIS-derived products are often used 

as predictor variables in regression models without consideration of uncertainty. 

This is problematic as traditional regression-based statistical models assume that 

the covariates are measured without error when this is never the case. Additionally, 

although the overall accuracies of mapped products reviewed here were generally 

high (greater than 75 percent), individual class accuracies vary considerably, and 

can be quite low. Also of importance is the difficulty of optical remote sensing to 

capture much of the structural elements so critical to owls (e.g., high concentrations 

of large trees, multilayered canopy). 

We can expect that new developments in high-resolution, multitemporal imagery, and particularly in active remote sensing methods such as LiDAR, will play 

increasing roles in wildlife research and management as their costs decrease. These 

tools provide more detail about the horizontal and vertical structure of forests, and 

when linked to accurate and often dynamic measures of animal location, a richer 

understanding of the use of the forest by these species can be developed. Yet despite 

great improvements in mapping provided by LiDAR and other high-resolution 

sensors, there are considerable outstanding needs for mapping of wildlife habitat. 

First, there is a need to better map important wildlife habitat elements within forests 

such as snags and large broken-top trees, which may be important to many wildlife 

species, including the spotted owl (Gutiérrez et al. 1992). Currently, remote sensors 

map these structural elements indirectly based on the vertical heterogeneity of the 

forest canopy (e.g., Martinuzzi et al. 2009), but they remain difficult to estimate 

accurately, particularly in dense forests (Blanchard et al. 2011). Second, research is 

ongoing to develop better metrics of vertical canopy structure for assessing habitat. 

Analysis of the discrete return point cloud can produce hundreds of structural and 

physics-based metrics (e.g., coefficient of variation of hits, or vertical distribution 

of hits), but many of these cannot be field verified, and they lack any management 

meaning. Simpler metrics that can be linked to management goals and ascertained 

in the field are needed. Synergies between ground-based LiDAR and airborne 
177 

The California Spotted Owl: Current State of Knowledge

LiDAR data might help to improve the characterization of vertical structure (e.g., 

Henning and Radtke 2006, Iavarone 2005). Third, species classification needs to 

be improved, particularly in mixed forests. The integration of LiDAR with other 

optical imagery (at fine and coarse resolutions) are proving very useful in mapping 

forests with increased species discrimination, as well as providing information on 

stress and biomass (Asner and Mascaro 2014, Gonzalez et al. 2010, Ke et al. 2010, 

Swatantran et al. 2011). Finally, optical and LiDAR fusion might also help to scale 

important forest structural measurements such as heterogeneity over spatial scales 

that are commensurate with owl home ranges (e.g., Chopping et al. 2012). These 

developments will likely augment the ways in which we map wildlife habitat in the 

near future. 

Literature Cited 

Asner, G.P.; Mascaro, J. 2014. Mapping tropical forest carbon: calibrating plot 

estimates to a simple LiDAR metric. Remote Sensing of Environment. 140: 

614–624. 

Bias, M.A.; Gutiérrez, R. 1992. Habitat associations of California spotted owls 

in the central Sierra Nevada. The Journal of Wildlife Management: 584–595. 

Blakesley, J.A.; Franklin, A.B.; Gutiérrez, R. 1992. Spotted owl roost and nest 

site selection in northwestern California. The Journal of Wildlife Management: 

388–392. 

Blakesley, J.A.; Noon, B.R.; Anderson, D.R. 2005. Site occupancy, apparent 

survival, and reproduction of California spotted owls in relation to forest stand 

characteristics. Journal of Wildlife Management. 69(4): 1554–1564. 

Blanchard, S.; Jakubowski, M.; Kelly, M. 2011. Object-based image analysis of 

downed logs in a disturbed forest landscape using lidar. Remote Sensing. 3(11): 

2420–2439. 

Bond, M.L.; Gutierrez, R.; Franklin, A.B.; LaHaye, W.S.; May, C.A.; 

Seamans, M.E. 2002. Short-term effects of wildfires on spotted owl survival, 

site fidelity, mate fidelity, and reproductive success. Wildlife Society Bulletin: 

1022–1028. 

Bond, M.L.; Lee, D.E.; Siegel, R.B.; Ward, J.P. 2009. Habitat use and selection 

by California spotted owls in a postfire landscape. The Journal of Wildlife 

Management. 73(7): 1116–1124. 
178 

GENERAL TECHNICAL REPORT PSW-GTR-254

Bond, M.L.; Seamans, M.E.; Gutiérrez, R.J. 2004. Modeling nesting habitat 

selection of California spotted owls (Strix occidentalis occidentalis) in the central 

Sierra Nevada using standard forest inventory metrics. Forest Science. 50(6): 

773–781. 

Call, D.R.; Gutiérrez, R.; Verner, J. 1992. Foraging habitat and home-range 

characteristics of California spotted owls in the Sierra Nevada. Condor: 

880–888. 

Chatfield, A.H. 2005. Habitat selection by a California spotted owl population: 

a landscape scale analysis using resource selection functions. St. Paul, MN: 

University of Minnesota. 59 p. M.S. thesis. 

Chopping, M.; North, M.; Chen, J.; Schaaf, C.B.; Blair, J.; Martonchik, J.V.; 

Bull, M.A. 2012. Forest canopy cover and height from MISR in topographically 

complex Southwestern US landscapes assessed with high quality reference data. 

IEEE Journal of Selected Topics in Applied Earth Observations and Remote 

Sensing. 5(1): 44–58. 

Cleve, C.; Kelly, M.; Kearns, F.; Moritz, M. 2008. Classification of urban 

environments for fire management support: a comparison of pixel- and objectbased classifications using high-resolution aerial photography. Computers, 

Environment and Urban Systems. 32: 317–326. 

Cohen, W.B.; Goward, S.N. 2004. Landsat’s role in ecological applications 

of remote sensing. BioScience. 54(6): 535–545. 

Cohen, W.B.; Kushla, J.D.; Ripple, W.J.; Garman, S.L. 1996. An introduction 

to digital methods in remote sensing of forested ecosystems: focus on the Pacific 

Northwest, USA. Environmental Management. 20(3): 421–435. 

Congalton, R.C.; Green, K. 1999. Assessing the accuracy of remotely sensed 

data: principles and practices. Boca Raton, FL: CRC Press, Inc. 137 p. 

Dugger, K.M.; Anthony, R.G.; Andrews, L.S. 2011. Transient dynamics 

of invasive competition: barred owls, spotted owls, habitat, and the demons 

of competition present. Ecological Applications. 21(7): 2459–2468. 

Foody, G.M. 2002. Status of land cover classification accuracy assessment.

Remote Sensing of Environment. 80(1): 185–201. 

Franklin, J.; Woodcock, C.E.; Warbington, R. 2000. Multi-attribute vegetation 

maps of forest service lands in California supporting resource management 

decisions. Photogrammetric Engineering & Remote Sensing. 66(10): 1209–1217. 
179 

The California Spotted Owl: Current State of Knowledge

García-Feced, C.; Temple, D.; Kelly, M. 2011. Characterizing California spotted 

owl nest sites and their associated forest stands using Lidar data. Journal of 

Forestry. 108(8): 436–443. 

Gonzalez, P.; Asner, G.P.; Battles, J.J.; Lefsky, M.A.; Waring, K.M.; 

Palace, M. 2010. Forest carbon densities and uncertainties from Lidar, 

QuickBird, and field measurements in California. Remote Sensing of 

Environment. 114: 1561–1575. 

Gottschalk, T.; Huettmann, F.; Ehlers, M. 2005. [Review article]: Thirty years 

of analysing and modelling avian habitat relationships using satellite imagery 

data: [a review]. International Journal of Remote Sensing. 26(12): 2631–2656. 

Gutiérrez, R.J.; Verner, J.; McKelvey, K.S.; Noon, B.R.; Steger, G.N.; Call, 

D.R.; LaHaye, W.S.; Bingham, B.B.; Senser, J.S. 1992. Habitat relations of the 

California spotted owl. In: Verner, J.; McKelvey, K.S.; Noon, B.R.; Gutierrrez, 

R.J.; Gould, G.I., Jr.; Beck, T.W., tech. coords. The California spotted owl: 

a technical assessment of its current status. Gen. Tech. Rep. PSW-GTR-133. 

Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest 

Research Station: 79–98. Chapter 5. 

Henning, J.G.; Radtke, P.J. 2006. Ground-based laser imaging for assessing 

three-dimensional forest canopy structure. Photogrammetric Engineering & 

Remote Sensing. 72(12): 1349–1358. 

Hines, E.M.; Franklin, J.; Stephenson, J.R. 2005. Estimating the effects of map 

error on habitat delineation for the California spotted owl in southern California. 

Transactions in GIS. 9(4): 541–559. 

Hughes, B.J.; Martin, G.R.; Reynolds, S.J. 2011. The use of Google EarthTM 

satellite imagery to detect the nests of masked boobies Sula dactylatra. Wildlife 

Biology. 17(2): 210–216. 

Hunter, J.E.; Gutiérrez, R.; Franklin, A.B. 1995. Habitat configuration around 

spotted owl sites in northwestern California. Condor. 97(3): 684–693. 

Hyde, P.; Dubayah, R.; Peterson, B.; Blair, J.B.; Hofton, M.; Hunsaker, C.; 

Knox, R.; Walker, W. 2005. Mapping forest structure for wildlife habitat 

analysis using waveform lidar: validation of montane ecosystems. Remote 

Sensing of Environment. 96: 427–437. 

Iavarone, A. 2005. Case study: merging airborne and ground-based Lidar data. 

Photogrammetric and Engineering and Remote Sensing. 71(9): 1001–1003. 
180 

GENERAL TECHNICAL REPORT PSW-GTR-254

Irwin, L.L.; Clark, L.A.; Rock, D.C.; Rock, S.L. 2007. Modeling foraging 

habitat of California spotted owls. The Journal of Wildlife Management. 71(4): 

1183–1191. 

Jakubowski, M.K.; Guo, Q.; Collins, B.; Stephens, S.; Kelly, M. 2013a. 

Predicting surface fuel models and fuel metrics using lidar and imagery in dense, 

mountainous forest. Photogrametric Engineering & Remote Sensing. 79(1): 

37–49. 

Jakubowski, M.K.; Li, W.; Guo, Q.; Kelly, M. 2013b. Delineating individual 

trees from lidar data: a comparison of vector- and raster-based segmentation 

approaches. Remote Sensing. 5: 4163–4186. 

Jensen, J.R. 2000. Remote sensing of the environment: an Earth resource 

perspective. 2nd ed. Upper Saddle River, NJ: Prentice Hall. 608 p. 

Kane, V.R.; Gersonde, R.F.; Lutz, J.A.; McGaughey, R.J.; Bakker, J.D.; 

Franklin, J.F. 2011. Patch dynamics and the development of structural and 

spatial heterogeneity in Pacific Northwest forests. Canadian Journal of Forest 

Research. 41(12): 2276–2291. 

Kane, V.R.; Lutz, J.A.; Cansler, C.A.; Povak, N.A.; Churchill, D.J.; Smith, 

D.F.; Kane, J.T.; North, M.P. 2015. Water balance and topography predict fire 

and forest structure patterns. Forest Ecology and Management. 338: 1–13. 

Kane, V.R.; Lutz, J.A.; Roberts, S.L.; Smith, D.F.; McGaughey, R.J.; Povak, 

N.A.; Brooks, M.L. 2013. Landscape-scale effects of fire severity on mixedconifer and red fir forest structure in Yosemite National Park. Forest Ecology

and Management. 287: 17–31. 

Ke, Y.; Quackenbush, L.J.; Im, J. 2010. Synergistic use of QuickBird 

multispectral imagery and LIDAR data for object-based forest species 

classification. Remote Sensing of Environment. 114(6): 1141–1154.

Kramer, H.J.; Cracknell, A.P. 2008. An overview of small satellites in remote 

sensing. International Journal of Remote Sensing. 29(15): 4285–4337. 

Lahaye, W.S.; Gutiérrez, R.; Dunk, J.R. 2001. Natal dispersal of the spotted owl 

in southern California: dispersal profile of an insular population. Condor. 103(4): 

691–700. 

Lauer, D.T.; Morain, S.A.; Salomonson, V.V. 1997. The Landsat program: its 

origins, evolution, and impacts. Photogrammetric Engineering & Remote 

Sensing. 63(7): 831–838. 
181 

The California Spotted Owl: Current State of Knowledge

Lee, D.E.; Bond, M.L.; Borchert, M.I.; Tanner, R. 2013. Influence of fire and 

salvage logging on site occupancy of spotted owls in the San Bernardino and 

San Jacinto Mountains of Southern California. Journal of Wildlife Management. 

77(7): 1327–1341. 

Lefsky, M.A. 2010. A global forest canopy height map from the Moderate 

Resolution Imaging Spectroradiometer and the Geoscience Laser Altimeter 

System. Geophysical Research Letters. 37: L15401. 

Lefsky, M.A.; Cohen, W.B.; Parker, G.G.; Harding, D.J. 2002. Lidar remote 

sensing for ecosystem studies. BioScience. 52(1): 19–30. 

Lesak, A.A.; Radeloff, V.C.; Hawbaker, T.J.; Pidgeon, A.M.; Gobakken, T.; 

Contrucci, K. 2011. Modeling forest songbird species richness using LiDARderived measures of forest structure. Remote Sensing of Environment. 115: 

2823–2835. 

Levien, L.M.; Fischer, C.; Roffers, P.; Maurizi, B.; Suero, J. 2002. Monitoring 

land cover changes in California: northeastern California Project Area. 

Sacramento, CA: U.S. Department of Agriculture, Forest Service. 165 p. 

Li, W.; Guo, Q.; Jakubowski, M.; Kelly, M. 2012. A new method for segmenting 

individual trees from the lidar point cloud. Photgrammetric Engineering and 

Remote Sensing. 78(1): 75–84. 

Martinuzzi, S.; Vierling, L.A.; Gould, W.A.; Falkowski, M.J.; Evans, J.S.; 

Hudak, A.T.; Vierling, K.T. 2009. Mapping snags and understory shrubs for 

a LiDAR-based assessment of wildlife habitat suitability. Remote Sensing of 

Environment. 113(12): 2533–2546. 

Mayer, K.E.; Laudenslayer, W.F. 1988. A guide to wildlife habitats of California. 

Sacramento, CA: California Department of Forestry and Fire Protection. 166 p. 

McDermid, G.J.; Franklin, S.E.; LeDrew, E.F. 2005. Remote sensing for largearea habitat mapping. Progress in Physical Geography. 29(4): 449–474. 

Melesse, A.M.; Weng, Q.; Thenkabail, P.S.; Senay, G.B. 2007. Remote sensing 

sensors and applications in environmental resources mapping and modelling. 

Sensors. 7(12): 3209–3241. 
182 

GENERAL TECHNICAL REPORT PSW-GTR-254

Merrick, M.J.; Koprowski, J.L.; Wilcox, C. 2013. Into the third dimension: 

benefits of incorporating LiDAR data in wildlife habitat models. In: Gottfried, 

G.J.; Ffolliott, P.F.; Gebow, B.S.; Eskew, L.G.; Collins, L.C., comps. Merging 

science and management in a rapidly changing world: biodiversity and 

management of the Madrean Archipelago III and 7th confeence on research and 

resource management in the southwest deserts. Proceedings, RMRS-P-67. Fort 

Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain 

Research Station: 389–395. 

Miller, J.D.; Thode, A.E. 2007. Quantifying burn severity in a heterogeneous 

landscape with a relative version of the delta Normalized Burn Ratio (dNBR). 

Remote Sensing of Environment. 109(1): 66–80. 

Moen, C.A.; Gutiérrez, R.J. 1997. California spotted owl habitat selection in the 

central Sierra Nevada. Journal of Wildlife Management. 61: 1281–1287. 

Moghaddas, J.J.; Collins, B.M.; Menning, K.; Moghaddas, E.E.; Stephens, 

S.L. 2010. Fuel treatment effects on modeled landscape-level fire behavior in the 

northern Sierra Nevada. Canadian Journal of Forest Research. 40(9): 1751–1765. 

Morgan, J.L.; Gergel, S.E.; Coops, N.C. 2010. Aerial photography: a rapidly 

evolving tool for ecological management. BioScience. 60(1): 47–59. 

Phillips, C.E.; Tempel, D.J.; Gutiérrez, R. 2010. Do California spotted owls 

select nest trees close to forest edges? Journal of Raptor Research. 44(4): 311–314. 

Popescu, S.C.; Wynne, R.H. 2004. Seeing the trees in the forest: using LiDAR 

and multispectral data fusion with local filtering and variable window size for 

estimating tree height. Photogrammetric Engineering & Remote Sensing. 70(5): 

589–604. 

Ripple, W.J.; Lattin, P.D.; Hershey, K.; Wagner, F.F.; Meslow, E.C. 1997. 

Landscape composition and pattern around northern spotted owl nest sites in 

southwest Oregon. Journal of Wildlife Management: 151–158. 

Roberts, S.L.; van Wagtendonk, J.W.; Miles, A.K.; Kelt, D.A. 2011. Effects 

of fire on spotted owl site occupancy in a late-successional forest. Biological 

Conservation. 144(1): 610–619. 

Rogan, J.; Miller, J.; Stow, D.; Franklin, J.; Levien, L.; Fischer, C. 2003. 

Land-cover change monitoring with classification trees using Landsat TM

and ancillary data. Photogrammetric Engineering & Remote Sensing. 69(7): 

793–804. 
183 

The California Spotted Owl: Current State of Knowledge

Seamans, M.E.; Gutiérrez, R. 2007. Habitat selection in a changing environment: 

the relationship between habitat alteration and spotted owl territory occupancy 

and breeding dispersal. Condor. 109(3): 566–576. 

Strahler, A.H.; Jupp, D.L.; Woodcock, C.E.; Schaaf, C.B.; Yao, T.; Zhao, F.; 

Yang, X.; Lovell, J.; Culvenor, D.; Newnham, G. 2008. Retrieval of forest 

structural parameters using a ground-based lidar instrument (Echidna®). 

Canadian Journal of Remote Sensing. 34(sup2): S426–S440. 

Swatantran, A.; Dubayah, R.; Roberts, D.; Hofton, M.; Blair, J.B. 2011. 

Mapping biomass and stress in the Sierra Nevada using lidar and hyperspectral 

data fusion. Remote Sensing of Environment. 115(11): 2917–2930. 

Vierling, K.T.; Vierling, L.A.; Gould, W.A.; Martinuzzi, S.; Clawges, R.M. 

2008. Lidar: shedding new light on habitat characterization and modeling. 

Frontiers for Ecology and the Environment. 6(2): 90–98. 

Williams, P.J.; Gutiérrez, R.; Whitmore, S.A. 2011. Home range and habitat 

selection of spotted owls in the central Sierra Nevada. Journal of Wildlife 

Management. 75(2): 333–343. 

Wulder, M. 1998. Optical remote-sensing techniques for the assessment of forest 

inventory and biophysical parameters. Progress in Physical Geography. 22(4): 

449–476. 

Wulder, M.A.; Bater, C.W.; Coops, N.C.; Hilker, T.; White, J.C. 2008. The 

role of LiDAR in sustainable forest management. The Forestry Chronicle. 84(6): 

807–826. 

Wulder, M.A.; Masek, J.G.; Cohen, W.B.; Loveland, T.R.; Woodcock, C.E. 

2012. Opening the archive: how free data has enabled the science and monitoring 

promise of Landsat. Remote Sensing of Environment. 122: 2–10. 

Zhao, F.; Yang, X.; Schull, M.A.; Román-Colón, M.O.; Yao, T.; Wang, Z.; 

Zhang, Q.; Jupp, D.L.; Lovell, J.L.; Culvenor, D.S. 2011. Measuring effective 

leaf area index, foliage profile, and stand height in New England forest stands 

using a full-waveform ground-based lidar. Remote Sensing of Environment. 

115(11): 2954–2964. 
184 

GENERAL TECHNICAL REPORT PSW-GTR-254
185 

The California Spotted Owl: Current State of Knowledge

Chapter 7: Threats to the Viability of California 

Spotted Owls 

John J. Keane1 

Introduction 

The California spotted owl (Strix occidentalis occidentalis) is a species of conservation concern owing to threats to its habitat and populations. Verner et al. (1992) first 

assessed the status of the California spotted owl “The California Spotted Owl: A 

technical Assessment of it’s current status” (CASPO) and identified four factors as 

either threats or potential threats to the viability of California spotted owl populations: (1) timber harvest and forest management, (2) wildfire, (3) development of 

gaps in owl distribution across the Sierra Nevada, and (4) human population growth 

and development. Since the publication of CASPO, other factors have emerged 

as threats to California spotted owl population viability: (1) the invasion of the 

barred owl (Strix varia) into the Sierra Nevada, (2) climate change that could affect 

owls and their habitat, (3) the invasion of West Nile virus in the owl’s range, (4) 

the potential impact to owls from secondary ingestion of rodenticides used to kill 

rodents that eat marijuana, Cannabis sp., and (5) reduction in genetic diversity. In 

this chapter, I review threats identified in CASPO and emerging threats to California spotted owls in the Sierra Nevada that have arisen since CASPO. I have relied 

on key findings from peer-reviewed literature of forest ecology and management 

and California spotted owl ecology. 

Evaluation of Threats Identified in CASPO

Forest Management 

Logging and fire suppression were identified in CASPO as primary threats to California spotted owls and their habitat in the Sierra Nevada (McKelvey and Johnston 

1992, McKelvey and Weatherspoon 1992, Weatherspoon et al. 1992, chapter 5). Key 

uncertainties were (1) whether critical habitat elements (old, large-diameter trees and 

associated large downed logs) would be maintained and perpetuated under current 

and proposed even-aged silvicultural prescriptions; and (2) whether dense, high 

canopy cover stands important to owls could be maintained given increasing risk of 

high-severity fire owing to historical fire suppression (chapter 5). In general, both 

public and private lands were managed similarly prior to CASPO (McKelvey and 

Johnston 1992). McKelvey and Weatherspoon (1992) recommended development, 

1

 John J. Keane is a research wildlife ecologist, U.S. Department of Agriculture, Forest 

Service, Pacific Southwest Research Station, 1731 Research Park Dr., Davis, CA 95618. 
186 

GENERAL TECHNICAL REPORT PSW-GTR-254

adaptive monitoring, and experimental testing of forest management approaches 

that would move forest structure and composition toward a heterogeneous condition 

that likely persisted under the area’s natural fire regime and to evaluate the effects 

of these approaches on California spotted owls and their habitat. 

Following adoption of CASPO guidelines, forest management on national 

forests diverged from private land management. Overall, 83.4 percent of the timber 

volume harvested from 1994 through 2013 came from private lands (chapter 5). 

During this time, group selection, shelterwood removal, and clearcutting were 

dominant on private land. In contrast, commercial thinning, salvage logging 

following wildfires, and hazard tree removal were dominant on national forest 

lands. About 73 to 80 percent of important California spotted owl habitat types 

occur on national forest lands in the Sierra Nevada (chapter 5). Differences in forest 

management among national forests, national parks, and private lands, along with 

variation in wildfire, have produced variable and complex landscapes across much 

of the Sierra Nevada. The scope and scale of cumulative effects is illustrated using 

case study demonstration areas. Figures 7-1 to 7-4 illustrate the complex landscape 

patterns generated by fire and forest management treatments within and surrounding four long-term demographic studies (Lassen, Eldorado, and Sierra National 

Forests and Sequoia and Kings Canyon National Parks) and within an area of mixed 

private-public ownership in the central Sierra Nevada. 

Effects of forest management on California spotted owls— 

Despite extensive research on spotted owls, the effect of forest management on owls 

is not well understood (USFWS 2011). Empirical field studies have been observational and correlative. Further, the complex mix of treatment types and wildfire 

across space and time impedes research efforts to isolate effects of specific treatment types because few owls receive the same type of treatment (figs. 7-1 to 7-4). 

Although experimental studies designed to understand the effects of logging have 

long been advocated (e.g., Gutiérrez 1985, McKelvey and Weatherspoon 1992, 

Noon and Franklin 2002, Verner et al. 1992), such studies have not been conducted, 

in part because they are technically, logistically, politically, and financially challenging. Such studies require organizational leadership, capacity, and institutional 

will to integrate multiple management objectives in the development and sustained 

testing of alternative land management strategies over large enough spatial and temporal scales to generate meaningful results (Gutiérrez et al. 2015). Although observational and correlative studies are of significant value, especially when replicated, 

they cannot produce strong inference (Romesburg 1981). 
187 

The California Spotted Owl: Current State of Knowledge Figure 7-1—Distribution of forest management treatments on national forest and private industrial forest lands on the Plumas and Lassen National Forests in the region surrounding the long-term Lassen Demographic Study area during 1990–2014. CDFW = California Department of Fish and Wildlife, NFS = National Forest System. See text for further details. 
188 

GENERAL TECHNICAL REPORT PSW-GTR-254

Figure 7-2—Distribution of forest management treatments on national forest and private industrial forest lands on the Eldorado National Forest in the region surrounding the long-term Eldorado Demographic Study area during 1990–2014. CDFW = California Department of Fish and Wildlife, NFS = National Forest System.See text for further details. 
189 

The California Spotted Owl: Current State of Knowledge Figure 7-3—Distribution of forest management treatments on national norest and private industrial forest lands on the Sierra and Sequoia National Forests and Sequoia and Kings Canyon National Parks in the region surrounding the long-term Sierra and Sequoia-Kings Canyon Demographic Study during 1990–2014. CDFW = California Department of Fish and Wildlife, NFS = National Forest System. 
190 

GENERAL TECHNICAL REPORT PSW-GTR-254

Figure 7-4—Distribution of forest management treatments on national forest and private industrial forest lands in a region of mixed public and private ownership on the Stanislaus and Eldorado National Forests during 1990–2014. CDFW = California Department of Fish and Wildlife, NFS = National Forest System. 
191 

The California Spotted Owl: Current State of Knowledge

Only three studies have explicitly addressed the effects of habitat change on 

California spotted owls at territory (Seamans and Gutiérrez 2007a, Tempel et al. 

2014) and landscape spatial scales (Stephens et al. 2014). Seamans and Gutiérrez 

(2007a) concluded that California spotted owl territories with greater amounts of 

mature conifer forest defined as >70 percent canopy cover dominated by medium 

and large trees [30.4 to 60.9 cm (11.9 to 23.6 in) diameter at breast height (d.b.h.), 

and >60.9 cm >11.9 in d.b.h., respectively), had higher probabilities of being colonized and lower probability of being unoccupied relative to territories with lower 

amounts of mature conifer forest. Territories in which >20 ha (>49.4 ac) of mature 

forest was altered experienced a 2.5 percent decline in territory occupancy probability. Breeding dispersal probability (the probability of territorial owls dispersing 

from an established site) did not change when >20 ha (>49.4 ac) of habitat was 

altered in territories with >150 ha (>370.7 ac) of mature forest within a 400-ha 

(988.4-ac) circle centered on the site at the start of the study. However, an increase 

in breeding dispersal probability was observed at territories that started with 

<150 ha (<370.7 ac) of mature forest and experienced >2 ha (>49.4 ac) of habitat 

alteration. Thirty-eight of 66 territories in this study experienced habitat alteration, including fire at two territories and timber harvest at the other 36 territories. 

Timber harvest included clearcutting, thinning, and other prescriptions, but inferences were not made relative to a specific silviculture prescription. 

Unlike earlier studies, Tempel et al. (2014) treated habitat change as dynamic 

over time and related annual patterns of change to owl survival, reproduction, 

population growth rate, and occupancy. Tempel et al. (2014) concluded that the 

amount of mature conifer forest >70 percent canopy cover; medium tree density 

(30.4 to 60.9 cm [11.9 to 23.6 in] d.b.h.) and large tree density (>60.9 cm [>23.6 in] 

d.b.h.) was the most important predictor associated with variation in demographic 

rates. This variable explained a large proportion of the variation in population 

growth rate and equilibrium occupancy, and was positively correlated with survival, equilibrium occupancy, and population growth, and negatively correlated 

with territory extinction probability. Further, medium-intensity treatments (such as 

thinning) were negatively correlated with reproduction and appeared to be related 

to reduced survival and territory occupancy when logging occurred in mature 

conifer forest that moved a class to a lower canopy cover state (e.g., canopy cover 

state changed from >70 percent cover to >40 to 70 percent). Of note, the probability 

of a territory going extinct was lower when the amount of mature conifer forest and 

high-intensity treatments (e.g., group selection, clearcut) increased and owl survival 
192 

GENERAL TECHNICAL REPORT PSW-GTR-254

and population growth were positively related to the amount of habitat edge. Tempel 

et al. (2014) hypothesized that the juxtaposition of mature conifer forest and edge 

habitat with shrub/saplings may be important for increasing owl prey populations. 

Only a single study has investigated the effects of landscape forest management on the owl (Stephens et al. 2014). They monitored owl territories annually 

after forest treatments within the 23 823-ha (58,867-ac) Meadow Valley Project 

Area (MVPA). Approximately 4161 ha (10,282 ac) of treatments were conducted 

during 2002–2008 (1784 ha [4,408 ac] of Defensible Fuels Profile Zone (DFPZ) 

treatments, 272 ha (672 ac) of group selections, 1440 ha (3,558 ac) of thinning, and 

665 ha [1,643 ac] of prescribed fire). Seven to nine spotted owl sites were occupied 

in the MVPA before and during implementation of treatments during 2002–2007. 

However, the number of occupied sites declined to six from 2008 through 2010. In 

the third and fourth years of posttreatment, the number of occupied owl sites had 

declined to four (a 43 percent reduction in occupied owl sites in the MVPA). Thus, 

the landscape management strategy had negative short-term effects on spotted owls 

in the first 4 years after project completion; because there was a decline in occupancy of territories, owls responded to treatments by using larger areas. Further, 

there appeared to be a 2- to 3-year lag in spotted owl response time to the treatments. Although owls have been declining across the demographic study area over 

the past 25 years (Conner et al. 2013), the greatest magnitude of decline has been 

observed in the MVPA treatment landscape, suggesting a negative effect of the 

landscape treatment strategy (Stephens et al. 2014). Although, this study represents 

a quasi-experiment (observing behavior of owls after a treatment), the study is the 

first to monitor California spotted owl responses to a landscape-scale fuels treatment and logging strategy. It appears this landscape-scale management negatively 

affects spotted owls, which highlights the lack of robust adaptive management 

monitoring to assess the effects of fuels reduction and timber harvest on spotted 

owls. 

Key findings from recent research on California spotted owl habitat

associations— 

There have been many studies of spotted owls since CASPO (chapters 2, 3, 4). 

These studies either confirm what was previously known, add detail (increased precision to estimates or nuances to early findings), or provide new insight (e.g., there 

is now strong evidence that California spotted owl populations are declining on 

areas of mixed U.S. Forest Service (USFS)–private land in the Sierra Nevada). The 

general patterns are that spotted owls are K-selected species having high survival 

and low annual reproductive output, that they select mature and old forest having 

high canopy cover (>60 to 70 percent) disproportionate to its availability, and that 
193 

The California Spotted Owl: Current State of Knowledge

their occupancy is related to both the amount of this high canopy forest in their territories and the amount of forest that is lost to treatments. Moreover, the configuration of landscape types, amount, and distribution is apparently related to owl fitness 

(Dugger et al. 2005, Franklin et al. 2000, Tempel et al. 2014). 

Current management— 

Verner et al. (1992) identified habitat loss from forest management practices (logging and fire suppression), as a primary threat to California spotted owls. The 

CASPO strategy (1992) caused USFS forest management to diverge from private 

lands. The different forest management approaches by private and public land managers, along with wildfire and other disturbances, has resulted in spatially complex vegetation landscapes (see figs. 7-1 to 7-4 as examples of that complexity; see 

chapter 5 for details on available information on national forest and private lands 

treatment summaries). 

Private industrial forests in the Sierra Nevada are managed using predominantly even-age silvicultural prescriptions (seed tree, shelterwood, and clearcut 

(chapter 5), although some private owners use uneven-age management. Because 

the owl is not federal or state listed, it does not receive special regulation on private 

land. Typically, a no-harvest buffer of 6 to 12 ha (15 to 30 ac) is established around 

active California spotted owl nest/activity centers (USFWS 2006). Previously 

known owl territories that are not currently occupied during project planning may 

receive no protection. McKelvey and Weatherspoon (1992) identified even-age management as a threat to owl habitat because critical habitat elements (old, large-diameter trees and associated large downed logs) and older forest stands would either 

decline or be lost eventually under this general system. There is no research on 

the specific effect of even-age management on owls and their habitat in the Sierra 

Nevada, but the northern spotted owl (S. o. caurina) was listed partially because of 

this type of silviculture. Recently, Sierra Pacific Industries (SPI) initiated research 

to assess the effects of even-age management on California spotted owls in the 

Sierra Nevada. Results from 2012 through 2014 indicate that owls are present across 

five study areas consisting of mixed SPI–Forest Service–other private owner lands, 

although further work is needed to assess habitat quality (chapter 4, Roberts et al.2

). 

Alternatively, uneven-age forest management (e.g., hazard tree removal, selection 

harvest, thinning) remains a threat because of uncertainty regarding its effects on 

2

 Roberts, K.; Hall, W.E.; Shufelberger, A.J.; Reno, M.A.; Schroeder, M.M. 2015. The 

occurrence and occupancy status of the California spotted owl on Sierra Pacific Industries’ 

lands in the Sierra Nevada of California. 11 p. Unpublished document. On file with: Sierra 

Pacific Industries, 3950 Carson Rd., Camino, CA 96049. 
194 

GENERAL TECHNICAL REPORT PSW-GTR-254

owls and their habitat (e.g., loss of residual trees, reduction of canopy cover, simplification of forest structure). 

After implementation of the CASPO guidelines (Verner et al. 1992) in the 

Sierra Nevada, national forests experienced a decline in the area logged annually with the majority of logging being commercial thinning and thinning from 

below to reduce fire risk (chapter 5). McKelvey and Weatherspoon (1992) and 

Weatherspoon and Skinner (1996) proposed that tree thinnings should incorporate 

heterogeneity into prescriptions; commercial thinning as implemented has tended 

to produce homogeneous conditions within treatment units. As typically implemented, thinning has emphasized reduction in surface and ladder fuels, maintaining trees >76 cm (>30 in) d.b.h., and posttreatment canopy cover >40 percent. 

Usually the remaining overstory trees are regularly spaced with little forest floor 

and understory diversity and low horizontal and vertical heterogeneity in stand 

structure (Knapp et al. 2012). Recent evidence suggests that these types of thinning 

prescriptions may have negative effects on California spotted owls (Stephens et al. 

2014, Tempel et al. 2014). In recent years, emphasis has refocused on silvicultural 

prescriptions that attempt to restore finer scale vertical and horizontal heterogeneity 

that would mimic predicted historical vegetation patterns (Knapp et al. 2012). 

Fire suppression also has significantly affected forest structure with changes 

in vegetation patterns at the landscape scale, as well as increases in stand density 

and shade-tolerant species, reductions in forest understory vegetation diversity, and 

reductions of vertical and horizontal heterogeneity at the stand scale (e.g., Dolanc et 

al. 2014, Knapp et al. 2013; chapter 5). At the landscape-scale, fire suppression has 

contributed to increased homogeneity in vegetation with increases in the distribution, amount and continuity of younger to mid-aged stands across the landscape, 

which under a more active natural fire regime would have likely been characterized 

by a finer scale, heterogeneous vegetation landscape. Fire suppression also has 

contributed to increased fuel loads and ladder fuels, which has increased risk of 

stand-replacing fire effects (see chapter 5 for further details). 

Forest management remains a threat to California spotted owl habitat and 

populations. Significant uncertainty persists about the effects of both public and 

private land management on California spotted owls and their habitat, and whether 

current vegetation trajectories on forest lands in the Sierra Nevada will support 

viable populations of owls because long-term monitoring of several owl populations 

across the Sierra Nevada document that owls are declining except on one study area 

on a national park (see chapter 4). The only consistent difference among these owl 

populations is forest management. Logging in national parks has been limited to 
195 

The California Spotted Owl: Current State of Knowledge

very specific purposes such as roadside hazard tree removal or fuels hazard reduction around infrastructure, whereas logging has been more prevalent on private and 

national forests. Additionally, national parks make greater use of prescribed fire 

and managed wildfire. Other differences between national forest and national park 

study areas are discussed in Franklin et al. (2004) and Blakesley et al. (2010). The 

greatest population declines are occurring on the Lassen and Eldorado National 

Forests study areas (Conner et al. 2013, Tempel and Gutiérrez 2013). Although 

causative linkages have not been established, the higher rates of decline on these 

two study areas are coincident with the greater amount and extent of national 

forest and private lands treatments (see chapter 5 for details on types of treatments 

used on national forest and private lands since CASPO) within the study areas and 

surrounding landscapes relative to the Sierra National Forest study site (figs. 7-1 to 

7-3). Recent research has indicated that dispersal dynamics and recruitment dynamics across larger landscapes and regions outside of study areas may have significant 

effects on owl population dynamics within fixed study areas (Schumaker et al. 

2014, Tempel et al. 2014, Yakusic et al. 2014; chapter 4). Although there still 

remains uncertainty regarding the effects of USFS and private land management on 

California spotted owls and their habitat, the declining owl populations on the three 

national forest study areas coupled with two studies that show declines related to 

forest management indicate that forest management remains a threat to California 

spotted owls and their habitat throughout the Sierra Nevada. 

Research on owl habitat associations at the territory-scale clearly demonstrate 

the importance of dense-canopy stands composed of medium-large trees for owl 

reproduction, survival, occupancy, and population trends. On the other hand, 

research documents that when foraging, owls will expand their habitat use to 

patches of younger forest having shrubs and along habitat edges between mature 

forest and other vegetation types (Franklin et al. 2000; Irwin et al. 2007, 2013; 

Williams et al. 2011). Studies relating owl demographic parameters to habitat 

patterns indicate the importance of territory-scale habitat configurations consisting 

of core amounts of complex-structured mature forest with intermediate amounts of 

habitat edges between forest and other vegetation types that produce heterogeneity 

and foraging habitat. However, neither the optimal mix of patches nor the optimal 

spatial configuration of vegetation is known. This pattern has also been reported for 

owls that occupy areas that experience mixed-severity fires including low amounts 

of stand-replacing fires (e.g., Bond et al. 2009). Thus, California spotted owls may 

respond favorably to forest management designed to produce fine-scale heterogeneity that benefits prey, such as woodrats, Neotoma sp. and Peromyscus sp. However, 
196 

GENERAL TECHNICAL REPORT PSW-GTR-254

there is significant uncertainty about the amounts of edge and fine-scale heterogeneity that might be beneficial to owls. Little information is available to evaluate 

how edges created by different mechanisms (e.g., fire versus mechanical treatment) 

affect the value of habitat over both short and long timeframes. Nevertheless, 

although incomplete, available information is adequate to formulate hypotheses 

regarding amounts and patterns of habitat at territory and within-territory scales 

that could have been tested through adaptive management. This is a suggestion 

articulated both in CASPO and the Sierra Framework documents, but was not done. 

Management in the Sierra Nevada is challenging because of vegetation and 

topographic variability owing to elevation and latitudinal gradients. This variation 

is further influenced by multiple ownerships, each of which is managing the land 

differently. Consequently, landscapes are diverse and subject to a mix of cumulative effects. Despite this reality, most studies center on either the territory-scale 

and within-territory-scale habitat associations. Less research has been conducted 

on landscape scales (Zabel et al. 2003). The spotted owl is a territorial species 

whose spatial organization appears to be structured according to an ideal despotic 

distribution (Franklin et al. 2000, Zimmerman et al. 2003). Understanding of the 

relationship between variation in landscape condition and population density and 

occupancy of owl territories is an important existing information gap to understand 

the status of owls in the Sierra Nevada, and to predict how density may be affected 

by changes in habitat proposed under alternative forest management scenarios. 

Wildfire

At the time of the CASPO, little information existed about the response of spotted 

owls to wildfire (Verner et al. 1992). Wildfire was recognized as a potential threat 

to owl habitat because of increasing fuels loads resulting from fire suppression policies and the vulnerability of owl habitat to high-severity wildfire (McKelvey and 

Weatherspoon 1992, Weatherspoon et al. 1992). 

Wildfire distribution and severity patterns in the Sierra Nevada: 1993–2013— 

Since CASPO, research has documented an increase in the amount of highseverity wildfire in the Sierra Nevada (Miller and Stafford 2012, Miller et al. 2009). 

Increases have occurred in both the amounts of high-severity fire and also the 

percentage of each fire burning at high severity for low- and mid-elevation conifer 

forest types. Loss of owl habitat to high-severity wildfire is an increasing threat 

to California spotted owls and their habitat, particularly in the context of climate 

change, high tree densities, high levels of tree mortality, and high forest fuels loads 

(Westerling et al. 2006; chapter 5). 
197 

The California Spotted Owl: Current State of Knowledge

Information on wildfire extent and severity patterns is available through the 

USFS Pacific Southwest Region Fire History database (Miller et al. 2009). About 

445 154 ha (1.1 million ac) of conifer, hardwood, and mixed-conifer-hardwood 

vegetation types within the range of the California spotted owl in the Sierra Nevada 

experienced wildfire between 1993 and 2013 (table 7-1; figs. 7-5 to 7-8). About 35 

612 ha (88,000 ac) of owl protected activity centers (PACs), representing about 15 

percent of the total PACs acres, burned during 1993–2013. The PACs are a 121-ha 

(300-ac) management unit established to protect core nest/roost areas of owl territories (chapters 2 and 3). Recent research has documented the value of PACs as a 

management strategy (Berigan et al. 2012, Ganey et al. 2014). However, the effect of 

high-severity wildfire on PACs is of concern. Comparison of overall burn severity 

patterns in vegetation types that comprise PACs (conifer, hardwood, and mixedconifer hardwood) across the Sierra Nevada to burn severity patterns in PACs indicates that the percentage of high-severity fire in PACs (28 percent) is similar to the 

percentage of high-severity fire across all burned acres (hectares) (26 percent). The 

percentage of moderate-severity fire is slightly higher in PACs (27 percent) versus 

overall (20 percent), while amounts of low-severity fire (PACs 36 percent, overall 

40 percent), and unburned acres within fire perimeters (PACs 11 percent, overall 

12 percent) are similar (table 7-1) (Keane, unpubl. data). These results indicate that 

PACs burned with similar proportions of high-severity fire compared to overall 

landscape fire severity patterns during 1993–2013. Similar to patterns throughout 

the Sierra Nevada (Miller et al. 2012), the number of PAC acres (hectares) experiencing fire, and high-severity fire has increased in recent years (fig. 7-9). 

Table 7-1—Distribution of wildfire acres by burn severity class in protected 

activity centers (PACs) and across the range of the California spotted owl 

in the Sierra Nevada, 1993–2013a 

Burn severity class for acres within wildfire perimeters

Total Burned 

acres acres High Moderate Low Unburned 

Percent 

Rangewide 7,466,532 1,092,814 26 27 36 11 

PACs 557,165 88,021 28 20 40 12 a

 Percentages by burn severity class only include acres for conifer, hardwood, and mixed-coniferhardwood vegetation type life forms that experienced wildfire. See text for further details. 

Sources: Vegetation type life forms from California Fire and Resource Assessment Program 2006 30-m 

raster; fire severity from U.S. Forest Service (USFS) Pacific Southwest Region (R5) vegetation burn 

severity data; owl PACS from USFS R5 and Sierra Nevada National Forest management units; owl range 

from California Department of Fish and Wildlife. 
198 

GENERAL TECHNICAL REPORT PSW-GTR-254

Figure 7-5—Distribution of wildfire hectares (ha) by burn severity class and California spotted owl protected activity centers 

(PACs) on the Plumas, Tahoe and Lassen National Forests in the northern Sierra Nevada and southern Cascades, 1993–2013. 
199 

The California Spotted Owl: Current State of Knowledge

Figure 7-6—Distribution of wildfire acres (ac) by burn severity class and California spotted owl protected activity centers (PACs) on 

the Tahoe and Eldorado National Forests in the Sierra Nevada, 1993–2013. Includes the 2014 King Fire for comparison. 
Vegetation burn severity 

1111 High 

Moderate 

Low 

1111 Unburned or very low 

0 

.. Owl PAC (former and current) 

20 40 

Kilometers 

Fire History 

1993-2013 

Stanislaus and Sierra 

National Forests 

Rim 2013 (105 000 ha) 

[Ackerson 1996 and Rogge 

1996 are contained within] 

Inyo 

National 

Forest 

GENERAL TECHNICAL REPORT PSW-GTR-254

Figure 7-7—Distribution of wildfire hectares (ha) by burn severity class and California spotted owl protected activity centers 

(PACs) on the Stanislaus and Sierra National Forests in the Sierra Nevada, 1993–2013. 

200 
201 

The California Spotted Owl: Current State of Knowledge

Figure 7-8—Distribution of wildfire hectares (ha) by burn severity class and California spotted owl protected activity 

centers (PACs) on the Sequoia National Forest in the Sierra Nevada, 1993–2013. 
202 

GENERAL TECHNICAL REPORT PSW-GTR-254

Figure 7-9—Distribution of wildfire hectares (ha) by burn severity class in protected activity center (PAC) and across the range of the 

California spotted owl by year, 1993–2013. Percentages by burn severity class for conifer, hardwood, and mixed-conifer-hardwood 

vegetation type life forms only include hactares (ha) within fire perimeters. See text for further details. 

California spotted owl–wildfire associations— 

Recent research indicates that California spotted owls persist at territories that 

experience low-moderate severity and mixed-severity (i.e., low-moderate fires 

with inclusions of high-severity) wildfire (see chapter 3) (Lee et al. 2012, Lee et 

al. 2013, Lee and Bond 2015, Roberts et al. 2011). Occupancy of sites by owls after 

fire appears to be a function of the amount of suitable habitat remaining postfire, 

the amount of suitable habitat burned at high severity, and whether postfire salvage 

logging is conducted. Available evidence indicates that postfire salvage logging 

may negatively affect postfire habitat suitability and confounds our understanding 

of owl response to fire (Lee et al. 2013). However, little is known about how salvage 

of commercially valuable trees affects owls. Further, no information is available to 

assess the response of owls to a range of postfire restoration management approaches that might emphasize primary objectives of ecological restoration rather than a 

sole focus on maximizing commercial value. Experiments are required to compare 

owl response at territories with and without salvage or postfire restoration management to disentangle the effects of treatments from the effects of high-severity 
203 

The California Spotted Owl: Current State of Knowledge

fire, particularly at owls sites where >50 to 100 percent of suitable habitat burns at 

high severity (Lee et al. 2012, Lee 2013). Clark et al. (2013) concluded that northern spotted owl site occupancy declined in the short term (3 to 5 years) following 

fire, with postfire occupancy jointly influenced by prefire habitat conditions owing 

to management, fire severity patterns, and postfire salvage logging. Information 

on California spotted owl foraging in postfire landscapes is limited to one study 

conducted at four owl territories that experienced limited amounts of high-severity 

fire (mean = 9 percent, range 4 to 12 percent of owl home ranges) (Bond et al. 2009, 

2013). Further research is needed on owl foraging habitat use across a broader gradient of territories to assess California spotted owl foraging habitat use patterns in 

postfire landscapes that experience greater total amounts, and increased patch sizes, 

of high-severity fire. While owls use the edges of high-severity fire patches, it is uncertain if they will use the interior of large patches of high-severity fire, such as the 

large patches observed in the 2013 Rim and 2014 King Fires. 

Current status on the threat of wildfire— 

While recent studies indicate that California spotted owls continue to occupy sites 

that experience low-moderate severity and mixed-severity wildfire, the threshold 

of the proportion of high-severity fire that owls can tolerate within their territory is 

unknown. No information exists on long-term survival, reproduction, and fitness 

of owls within burned territories. Further, no information is available to assess owl 

foraging behavior and habitat use patterns at territories that experience 50 to 100 

percent high-severity fire. There is no information available to evaluate how landscape-scale population density is affected by large fires. These information gaps 

are important given increases in the amounts and patch sizes of large-scale, standreplacing fires in the Sierra Nevada (Miller et al. 2009, 2012; chapter 5). 

California spotted owls may exhibit both short- and long-term responses to fire. 

Owls may persist over the short term even when habitat quality is reduced because 

of site fidelity. No information is available about short- versus long-term occupancy 

dynamics and demographic relationships to fire and habitat quality. While recent 

research suggests owls persist in territories after low-moderate and some mixedseverity fire, current and projected future increases in the amount and patch sizes 

of high-severity fire is an increasing threat to owl viability. 
204 

GENERAL TECHNICAL REPORT PSW-GTR-254

Integration of Forest Management and Wildfire

A key recommendation from CASPO was the need to develop, test, and monitor 

forest management strategies that reduce fuels accumulation and increase stand 

and landscape-scale heterogeneity to provide habitat for California spotted owls 

(McKelvey and Weatherspoon 1992). Limited progress has been made toward 

evaluating these activities of forest management (see Stephens et al. 2014, Tempel 

et al. 2014 for examples). Simulation studies have suggested that fuels reduction and 

forest restoration treatments may be compatible with reducing fire risk and providing owl habitat (Ager et al. 2012, Gaines et al. 2010, Lee and Irwin 2005, Roloff et 

al. 2012). However, no empirical studies have been conducted to test and validate 

modelling predictions. Recent work by Tempel et al. (2015) suggests that fuels 

treatments may provide long-term benefits to California spotted owls if sites experience fire under extreme conditions, but in the absence of fire, fuels treatments can 

have long-term negative effects on owls. Recent increasing trends in high-severity 

fire amounts and patch sizes (Miller 2009, Miller and Stafford 2012) coupled with 

projected future increases in high-severity fire under future climate scenarios 

(Liu et al. 2013, Westerling et al. 2006) emphasize the risk posed by high-severity 

fire to owl viability. Comprehensive, spatially explicit population models are 

not available to estimate how many owls and in what distributional pattern are 

needed to provide a high probability of sustaining a viable population and how 

owl population size and territory quality are predicted to change under alternative 

fuels reduction and forest restoration scenarios. Of particular note, large trees are 

well-documented to be key habitat elements for owl nesting and roosting; however, 

large trees are declining across the Sierra Nevada, driven by multiple factors acting 

separately or synergistically including logging, hazard tree removal, drought, insect 

mortality, fire suppression (increased stress owing to competition with other trees), 

wildfire, and climate change (Dolanc et al. 2014, Knapp et al. 2013, Lutz et al. 

2009). The fundamental need to develop and test integrated strategies to reduce fire 

risk, restore forests, and provide habitat for a viable owl population identified by 

CASPO remains unresolved. 

Areas of Concern: Gaps in the Distribution of California Spotted 

Owls in the Sierra Nevada 

Beck and Gould (1992) reported that there appeared to be no gaps in the distribution of owls in the Sierra Nevada. However, they identified eight land areas of 

concern (AOCs) within the Sierra Nevada where potential gaps in the distribution 

could develop because of the following conditions: (1) naturally fragmented distribution of habitat and owls, (2) populations become isolated, (3) habitat becomes 
205 

The California Spotted Owl: Current State of Knowledge

highly fragmented, and (4) areas where crude density of owls becomes low (table 

7-2, fig. 7-10). No research is available to assess change in owl numbers or distribution across each of the AOCs. However, AOCs 2 (Northern Plumas County) and 

4 (Northern Eldorado County) could be assessed for the long-term demographic 

monitoring study areas on the Lassen and Eldorado National Forests where owl 

populations have been declining (Conner et al. 2013, Tempel and Gutierrez 2013, 

Tempel et al. 2014) (chapter 4). Extensive forest management treatments have been 

implemented within AOCs 1 (Lassen County) and 3 (Northeastern Tahoe National 

Forest), while AOCs 5 (Northwestern Stanislaus National Forest) and 8 (Northeastern Kern County) have experienced extensive wildfire from 1990 through 2013. 

AOC 7 (Northwestern Sierra National Forest) also has experienced lower levels of 

disturbance (app. 7-1). 

Table 7-2—Descriptions and reasons for areas of concern identified in the assessment of 

the California spotted owl report 

Area 

number Name Reason for concern 

1 Lassen County (FS, NPS, IP) Habitat in this area is discontinuous, naturally 

fragmented, and poor in quality owing to 

drier conditions and lava-based soils. 

2 Northern Plumas County (FS, IP, pvt.) A gap in known distribution, mainly on 

private lands, extends east-west in a band 

almost fully across the width of the owl’s 

range. 

3 Northeastern Tahoe NF (FS, IP, pvt.) An area of checkerboard lands; much dominated by granite outcrops and red fir forests; 

both features guarantee low owl densities. 

4 Northern Eldorado NF (FS, IP, pvt.) Checkerboarded lands and large, private 

inholdings; owl densities unknown on some 

private lands and very low on others. 

5 Northwestern Stanislaus NF (FS, IP, pvt.) Has large private inholdings; owl densities 

unknown on most private lands. 

6 Southern Stanislaus NF (FS) Burned in recent years; the little remaining 

habitat is highly fragmented. 

7 Northwestern Sierra NF (FS) Habitat naturally fragmented, owing partly to 

low elevations and dry conditions; fragmentation accentuated by logging. 

8 Northeastern Kern County (FS) Only small, semi-isolated groups of owls in 

the few areas at elevations where habitat persists at the south end of the Sierra 

Nevada. 

Ownership codes: FS = USDA Forest Service; NF = national forest, NPS = National Park Service; IP = industrial private 

lands; pvt. = multiple, small, private ownerships. 

Source: Verner et al. 1992; fig. 14, p. 47 and discussion p. 45. 
206 

GENERAL TECHNICAL REPORT PSW-GTR-254

Figure 7-10—Areas of concern identified in the California spotted owl assessment area (CASPO) 

report (1992) where land ownership, topographic features, habitat fragmentation or amounts that may 

lead to future gaps in the distribution of California spotted owl populations or habitat in the Sierra 

Nevada. FS = Forest Service, NPS = National Park Service, IP = private industrial lands, CDFW = 

California Department of Fish and Wildlife. 
207 

The California Spotted Owl: Current State of Knowledge

Available evidence indicates that the threat of gaps in distribution has likely 

increased since CASPO. Documented owl population declines in AOCs 2 and 4, 

along with uncertainty about the status of owls within AOCs 1, 3, 5, and 8 where 

extensive forest management treatments have occurred, contribute to the increased 

threat. Development of gaps in owl distribution in the Sierra Nevada could have 

negative demographic effects because dispersal among geographic areas likely 

would be reduced. Spotted owls in the Sierra Nevada have low genetic diversity 

(chapter 4), and future fragmentation and isolation of owl populations within the 

Sierra could lead to further reductions in genetic diversity. 

Human Development 

McKelvey and Weatherspoon (1992) identified human population growth as a threat 

to owls and their habitat within the low to mid elevations of the Sierra Nevada. No 

information is available to evaluate effects of human population and residential 

development growth on owls and their habitat. Low- and mid-elevation zones of the 

west slope of the Sierra Nevada continue to experience growing human populations, 

expansion of communities, and increased dispersed, low-density housing (FRAP 

2010). These human-induced changes result in habitat loss, habitat degradation, 

disturbance, and increased fuels treatments and forest thinning in wildland-urbaninterface (WUI) zones to protect communities. About 50 percent of known owl 

sites occur within WUIs. Despite extensive forest management conducted within 

WUIs, no monitoring studies have been conducted to evaluate effects. These sites 

provide an opportunity to examine, retrospectively, the effects of fuels treatments 

and forest thinning on owls and their habitat. 

Evaluation of Emerging Threats 

Barred Owls 

Barred owl range expansion has posed a significant threat to the viability of the 

northern spotted owl (Gutiérrez et al. 2007, Weins et al. 2014). Along with past 

and current habitat management, barred owls are considered a primary threat to 

northern spotted owl persistence (USFWS 2011). Barred owls have invaded western 

North America over the past century (Livezey 2009). Barred owls were first documented in British Columbia in 1943, and have dispersed southward through Washington, Oregon, and California (USFWS 2011). They are now sympatric across the 

entire range of the northern spotted owl (Gutiérrez et al. 2007). Barred owls are 

currently expanding their range into the Sierra Nevada and are an increasing threat 

to California spotted owls (Dark et al. 1998, Keane 2014). 
208 

GENERAL TECHNICAL REPORT PSW-GTR-254

Ecology and interactions with spotted owls— 

Gutiérrez et al. (2007) predicted that two similar-sized, congeneric owls in newly 

established areas of sympatry would likely compete and that stable coexistence was 

unlikely. Recent work indicates this is occurring through competition for food and 

habitat as well as interference competition with barred owls being the dominant 

species (Dugger et al. 2011; Wiens et al. 2014; Yackulic et al. 2012, 2014). For example, northern spotted owl detection rates and site occupancy probabilities are lower 

in the presence of barred owls (Bailey et al. 2009; Crozier et al. 2006; Dugger et al. 

2011; Kroll et al. 2010; Olson et al. 2005; Yackulic et al. 2012, 2014), with increased 

extinction probabilities and decreased colonization probabilities when barred owls 

are present (Dugger et al. 2011, Olson et al. 2005, Yackulic et al. 2014). 

Dugger et al. (2011) reported that site occupancy dynamics of northern spotted 

owls were correlated through an additive interaction of habitat and barred owls. 

Extinction probabilities increased as the amount of old-forest habitat decreased 

around core areas, and these probabilities increased by a factor of two to three times 

when barred owls were detected. Colonization probabilities ranged from 0.33 to 

0.73 and decreased with increasing fragmentation of older forest around core areas, 

and were much lower (0.03 to 0.20) when barred owls were detected. Occupancy 

probabilities increased when the proportion of old forest increased, and decreased 

with increasing fragmentation, and occupancy probabilities decreased dramatically 

when barred owls were present regardless of habitat condition (Dugger et al. 2011). 

Dugger et al. (2011) also noted that barred owls were increasing on their study 

area and had not reached an equilibrium population size and that the relationship 

between habitat and barred owls may change as barred owls continue to increase. 

Yackulic et al. (2012) modeled hypothesized relationships between barred 

owls on spotted owls. Theoretically, these relationships were influenced by local 

and regional population sizes of each species that affects the numbers of recruits 

available for colonization (Yackulic et al. 2012), and dynamic patterns of competition that shift over time in response to the populations sizes of both species and 

amounts of important habitat types (Yackulic et al. 2014). Yackulic et al. (2012) 

also predicted that both the regional occupancy status of barred owls (i.e., regional 

population size available to produce recruits) and habitat were important factors 

affecting barred owl site occupancy dynamics. In contrast to previous speculation 

that habitat constraints would limit expansion of barred owls, Yackulic et al. (2012) 

concluded that habitat segregation would not likely limit either habitat use by barred 

owls or its numerical increase. 

Yackulic et al. (2014) extended their previous work by examining the joint 

occupancy dynamics of barred and spotted owls over a 22-year period, as well 
209 

The California Spotted Owl: Current State of Knowledge

as how intraspecific and interspecific occupancy dynamics were related to local 

competition, habitat, and local and regional population sizes. Dynamic changes 

in the availability of recruits to colonize sites for each species and their overlap in 

preferred habitat appeared to be key factors in determining the role of competition. 

Yackulic et al. (2014) found that including competition between the two species 

at the site scale resulted in increased extinction probabilities for spotted owls and 

reduced equilibrium occupancies, or population sizes, but was unlikely to lead to 

full competitive exclusion under the hypothesized scenarios they examined. 

Competition between barred and spotted owls is likely because of broad overlap 

in their habitat use and diets (Hamer et al. 2001, 2007; Wiens et al. 2014) as well 

as the aggressive behavior of barred owls. Both species show similar preference 

for old-forest habitat with large trees and high canopy closures (Hamer et al. 2007, 

Singleton et al. 2010, Wiens et al. 2014), but barred owls use a broader suite of 

vegetation types (Hamer et al. 2007, Wiens et al. 2014). Spotted owls tend to use 

areas with steeper slopes relative to barred owls (Hamer et al. 2007, Singleton et al. 

2010, Wiens et al. 2014). Using radio-marked birds, Wiens et al. (2014) estimated 

that mean overlap in proportional use of habitat types was 81 percent (range = 30 to 

99 percent) and that both species used old-conifer forest (>120 years old) in greater 

proportion to its availability. In addition, both species used riparian-hardwood 

types along streams for foraging. Spotted owls concentrated foraging and roosting 

in forest patches with large trees (>19 in [>50 cm] d.b.h.) on steep slopes in ravines, 

whereas barred owls showed strongest associations with patches of large hardwood 

and conifer trees on relatively flatter slopes. 

Wiens et al. (2014) further investigated spatial patterns of resource use between 

barred and spotted owls and found that home ranges overlapped between adjacent 

home ranges but that there was minimal overlap of core-use areas, suggesting that 

interference competition has resulted in interspecific territoriality. Spotted owl 

home ranges increased in size as the probability of barred owl presence increased, 

suggesting that spotted owls expanded their home ranges presumably to avoid 

barred owls. Further, relative probability of habitat use by spotted owls declined 

as a function of increased proximity to barred owl core areas. Wiens et al (2014) 

concluded that the patterns of spatial segregation and habitat use of these sympatric 

owls provided strong evidence of interference competition. Aggressive interactions 

between barred owls and spotted owls provided further support for interference 

competition and indicated that barred owls are the behaviorally dominant species 

(Van Lanen et al. 2011) 

There is significant diet overlap between species, yet barred owls prey on more 

species (Hamer et al. 2001, Livezey and Bednarz 2007, Wiens et al. 2014). Both 
210 

GENERAL TECHNICAL REPORT PSW-GTR-254

species prey primarily on small mammals, including flying squirrels, tree voles, 

woodrats, pocket gophers, mice, and lagomorphs, but barred owls also prey on a 

wider variety of terrestrial and aquatic prey, and diurnally active prey such as tree 

squirrels, birds, and reptiles (Hamer et al. 2001, Wiens et al. 2014). Diet overlap 

also appears to vary regionally and seasonally possibly because of spatial and 

temporal variation in prey availability and abundance (Graham 2012, Hamer et al. 

2001, Wiens et al. 2014). Wiens et al. (2014) concluded that similarity in habitat use 

patterns and dietary overlap provided evidence for exploitative competition between 

the species, and that the magnitude of this competition may vary over space and 

time in response to variation in prey availability. 

Barred owl home ranges are two to four times smaller than those of sympatric 

spotted owls (Hamer et al. 2007, Singleton et al. 2010, Wiens et al. 2014). Differences 

in home range sizes are likely a function of differences in diet; presumably the broader 

diet allows barred owls to meet their energetic demands with less foraging area. Thus, 

barred owls have the potential to reach population densities two to four times greater 

than spotted owls. Wiens et al. (2014) provided the first evidence of demographic 

performance of the species. Over the course of their study, barred owls had higher 

survival estimates than spotted owls (0.92 vs. 0.81), and barred owl pairs produced an 

average of 4.4 times more young than spotted owl pairs over the 3-year study period. 

Spotted owl pairs nesting within 0.9 mi (1.5 km) of a nest used by barred owls failed 

to successfully produce, and the number of young produced increased linearly with 

increasing distance from a barred owl core area (Wiens et al. 2014). 

Barred owl removal experiments— 

Barred owl removal experiments have been started to test the effects of barred owls 

on northern spotted owls and to assess whether removal may be a feasible management strategy (Diller 2013; Diller et al. 2012; USFWS 2008, 2011). Preliminary results suggested that barred owl presence causes declines in spotted owl occupancy 

and reductions in spotted owl calling behavior (Crozier et al. 2006, Diller et al. 

2012). Diller et al. (2012) removed barred owls from nine historical northern spotted owl sites located on private timberland in northern California. All sites were 

reoccupied by spotted owls within 1 year. One site was occupied by a female not 

detected for 7 years, while overall, four sites were occupied by the original resident 

spotted owls and five sites were occupied by new, unknown spotted owls. Barred 

owls again displaced spotted owls at three sites in 1 to 4 years after initial removal. 

Diller et al. (2012) hypothesized that preliminary results suggested that barred owl 

removal may have broader positive neighborhood effects on spotted owls by increasing density of owls, which serves as a cue to settlement by dispersing owls (see 

Seamans and Gutierrez 2006). 
211 

The California Spotted Owl: Current State of Knowledge

Barred owl status and distribution within the range of the California 

spotted owl— 

Through 2013, 51 barred and 27 “sparred” (hybrids between the two species) 

owls, and 1 unknown (fig. 7-11) (Keane, unpublished data) have been detected 

in the Sierra Nevada. None have been found in either southern or central coastal 

California. All sightings are incidental because no formal surveys for barred owls 

have been conducted. 

The first record of barred owl detected in the Sierra Nevada was in Lassen 

County in 1989 (Keane, unpublished data). Only four owls (three barred owls, 

one sparred owl) were found between 1989 and 2001 and were limited to Sierra, 

Plumas, and Lassen Counties in the northern Sierra Nevada and southern Cascade 

Range (Dark et al. 1998, Keane unpublished data). There was an extensive survey 

effort by the USFS to inventory spotted owls from 1987 through 1992, which established a baseline for barred owls. Detections of barred and sparred owl increased 

between 2002 and 2013, largely because of increased spotted owl survey effort on 

spotted owl demographic study areas in the northern Sierra and southern Cascade 

Range. The first detections in the central and southern Sierra Nevada were in 2004 

(Seamans et al. 2004, Steger et al. 2006). Six barred owls were detected in the 

southern Sierra Nevada during 2011–2012. The number of barred and sparred owls 

on the four long-term demographic study areas has remained low, although they 

may be increasing gradually in the northern Sierra Nevada, with eight barred and 

two sparred owls present on the Lassen National Forest demography study area in 

2013. This is the pattern observed in the range of the northern spotted owl—a slow 

increase followed by a rapid one. 

The invasion of the barred owl into the Sierra Nevada poses a significant threat 

to California spotted owls. Based on the limited observations discussed above, it is 

possible that they will ultimately colonize the entire Sierra Nevada. Without control 

efforts, barred owls can potentially become a primary threat to the California spotted owl in the Sierra Nevada. 

Climate Change 

Climate change is projected to have significant effects on Sierra Nevada forests 

(GEOS Institute 20133 Lenihan et al. 2008; chapter 5). Long-term climate change 

may have both direct and indirect effects on the owl. Increases in temperature and 

3 GEOS Institute. 2013. Future climate, wildfire, hydrology, and vegetation projections for 

the Sierra Nevada, California: a climate change synthesis in support of the vulnerability 

assessment/adaptation strategy process. Unpublished report. On file with: Geos Institute 84 

Fourth Street, Ashland, OR 97520. 
212 

GENERAL TECHNICAL REPORT PSW-GTR-254

Figure 7-11—Barred owl and sparred owl records within the range of the California spotted owl in 

the Sierra Nevada, 1989–2013. 
213 

The California Spotted Owl: Current State of Knowledge

changes in precipitation patterns may have direct effects on spotted owl physiology, 

survival, reproduction, recruitment, and population growth. Climate change may 

also precipitate indirect effects such as (1) geographical shifts in habitat distribution, abundance, and quality; (2) increase of high-severity wildfire; (3) increase 

in mature/large tree mortality caused by insects and disease; (4) changes in prey 

distribution, abundance, and population dynamics; (5) changes in interspecific 

interactions with competitors and predators; and (6) changes in disease dynamics 

associated with changing temperature and precipitation patterns. 

Weathers et al. (2001) determined the thermal profile, upper and lower critical 

temperatures, and basal and field metabolic rates of California spotted owls. The 

thermal neutral zone ranged from 18.2 to 35.2 °C. Above the upper critical temperature, owls experienced heat stress at rates greater than predicted for birds of similar 

size. Many studies have documented the negative effects of wet, cold weather 

during the winter and early-breeding season on spotted owl reproduction (Dugger 

et al. 2005, Franklin et al. 2000, Olson et al. 2004), survival (Franklin et al. 2000, 

Glenn et al. 2011, Olson et al. 2004), recruitment (Franklin et al. 2000), and population growth (Glenn et al. 2010). Wet, cold winter weather may increase energetic 

demands on owls by raising thermoregulation energy costs or reducing availability 

of prey and hunting success during inclement weather, which may negatively affect 

survival and reproduction. Wet, cold weather during the early breeding season may 

affect spotted owls by reducing egg viability owing to chilling, cause direct mortality of nestlings, or lower prey abundance or availability (Rockweit et al. 2012). 

Inclement winter weather may also affect recruitment through overwinter mortality 

of dispersing juvenile spotted owls (Franklin et al. 2000; Glenn et al. 2010, 2011). 

Increases in late summer precipitation have been linked to increased survival, 

recruitment, and reproduction (Glenn et al. 2010, 2011; Olson et al. 2004; Seamans 

et al. 2002). Late-season precipitation may either reduce negative effects of summer 

drought, support greater plant production and primary productivity such as seeds 

and fungi that are important food for small mammal prey, or support increases in 

prey species abundance and availability. Drought and hot temperatures during the 

previous summer have been linked to lower survival and recruitment of spotted 

owls (Franklin et al. 2000, Glenn et al. 2011). 

Across their range, spotted owls exhibit population-specific demographic 

responses to local weather and regional climates (Franklin et al. 2000; Glenn et al. 

2010, 2011; Peery et al. 2012). These results indicate that population-specific variation may lead to population-specific responses to future climate scenarios, which 

may range from neutral to significantly negative effects and increased vulnerability 

(Glenn 2011, Glenn et al. 2010, Peery et al. 2012). 
214 

GENERAL TECHNICAL REPORT PSW-GTR-254

Glenn et al. (2010, 2011) investigated relationships among survival, recruitment, and population growth rate of six northern spotted owl populations in Oregon 

and Washington relative to local weather and regional climate. Local weather 

and regional climate variables explained 3 to 85 percent of the annual variation 

in growth rate in these populations, with the relative importance of weather and 

climate factors varying among the six populations. Peery et al. (2012) similarly 

found evidence for population-specific and regional variation in the relationship 

between spotted owl survival and reproduction with climate and projected response 

to future climate scenarios. Mexican spotted owl populations in New Mexico and 

Arizona were negatively associated with hot, dry conditions, and populations were 

projected to decline rapidly under future climate scenarios. In contrast, a population 

of California spotted owls in the mountains of southern California was negatively 

associated with cold, wet springs, with the population projected to exhibit low 

response to projected future climate conditions. In general, projected population 

growth rates were more affected by changes in temperature than precipitation, and 

by stronger climate effects on reproduction than survival (Peery et al. 2012). 

Seamans and Gutiérrez (2007b) reported that temperature and precipitation during incubation most affected reproductive output, and conditions in winter associated with the Southern Oscillation Index (SOI) most affected adult survival on the 

Eldorado National Forest. Weather variables explained a greater proportion of the 

variation in reproductive output than they did survival. Further, these two weather 

variables were also included in the best models predicting annual population growth 

rate (Seamans and Gutiérrez 2007b). Subsequently, MacKenzie et al. (2012) found 

that SOI or other weather variables explained little variation in annual reproduction 

for this same population of owls over a longer time series. Unlike results for California spotted owls in southern California reported in Peery et al. (2011), subsequent 

analyses testing for effects of weather variables on demographic parameters showed 

no clear temporal associations for owls on the Eldorado National Forest in the Sierra 

Nevada. Other than the assessment conducted for the population of California 

spotted owls in the mountains of southern California (Peery et al. 2012), no studies have conducted similar analyses relating spotted owl demographic parameters 

(survival, reproduction, recruitment, and population growth) to climate variables 

and subsequently projected population growth under future climate scenarios for 

any California spotted owl populations in the Sierra Nevada. 

In addition to direct effects on spotted owl vital rates, climate-induced changes 

in temperature, precipitation, and water moisture may lead to shifts in the distribution of California spotted owls. Siegel et al. (2014) assessed the potential vulnerability 

of California spotted owls in the Sierra Nevada to future climate scenarios using 
215 

The California Spotted Owl: Current State of Knowledge

NatureServe’s Climate Change Vulnerability Index (CCVI) and predicted California 

spotted owls to be presumed stable over the next 50 years under the climate scenarios 

they investigated. Carroll (2010) recommended that ecological niche models based 

on temperature and precipitation envelopes have value for projecting potential effects 

of climate change on spotted owl distribution, although these types of coarse models 

have limitations because they do not incorporate additional important factors. A 

more rigorous assessment of climate change on spotted owls requires development of 

dynamic models that relate owl vital rates or occupancy to vegetation dynamics and 

effects of competitor and key prey species, in addition to climate variables. 

Responses of California spotted owls to climate change are likely to be 

governed by complex interactions of factors that directly affect owls and their 

habitat, as well as indirect factors that can affect habitat (e.g., insect pests, disease, 

increased fire risk, vegetation type conversions, and distributional shifts) and ecological relationships (e.g., disease, competitors, predators, prey). While ecological 

niche models suggest that projected changes in temperature and precipitation may 

have minimal effects on California spotted owl distribution in the Sierra Nevada, 

results from demographic assessments and projections suggest that future climate 

change may have population-specific effects that likely will vary over geographical, 

elevational, and ecological gradients. Further, climate change projections of future 

vegetation distribution in the Sierra Nevada suggest that much of the low- and midelevation forests that currently comprise owl habitat are vulnerable to conversion 

of forests to woodlands, shrublands, and grasslands, especially with increased fire 

probabilities (chapter 5). 

Climate change has emerged as a threat to California spotted owls in the Sierra 

Nevada given uncertainty regarding direct and indirect effects on owls and the 

potential for significant effects on the distribution and amounts of owl habitat. 

This threat may be partially mitigated over ecological time scales if mixed-conifer 

forests advance upslope, thereby providing habitat for owls where none now exists 

(e.g., Peery et al. 2012). However, it should be recognized that individual plant 

species exhibit species-specific responses to changes in temperature and precipitation, with vegetation communities reorganizing as a result of individual species 

responses (Briles et al. 2011; Davis 1981, 1986). Climate change may result in novel 

future vegetation communities that differ in species composition and richness 

relative to contemporary communities. Further, large trees that function as nest 

trees for owls and help moderate within-stand temperatures require many decades 

to centuries to attain large diameters and complex structures. Thus, it may require 

long time periods to develop the large tree vertical structure used by owls in areas 

where such structure does not now exist. 
216 

GENERAL TECHNICAL REPORT PSW-GTR-254

Disease, Parasites, and Contaminants 

Little information exists on disease prevalence in spotted owl populations, and no 

information exists about the effects of disease on individual fitness or population 

viability. West Nile virus (WNV), a primarily mosquito-borne flavivirus that was 

first detected in eastern North America in 1999 and then throughout California 

by 2004, has been a concern (Reisen et al. 2004). West Nile virus has been demonstrated to be highly lethal to owls (Gancz et al. 2004, Marra et al. 2004). The 

primary route of infection is through the bite of an infected mosquito, with secondary routes of infection possible through consumption of infected prey and possibly 

feces (Kipp et al. 2006, Komar et al. 2003). 

There has been no evidence to indicate that WNV has affected California 

spotted owl populations. Hull et al. (2010) screened samples for WNV antibodies 

from 209 California spotted owls collected from the southern (Sierra National 

Forest, Sequoia and Kings Canyon National Parks) or northern (Plumas and Lassen 

National Forests) Sierra Nevada during 2004–2008. Positive test results for antibodies would indicate exposure and survival (Hull et al. 2010). Results were negative 

for all 209 California spotted owls. Hull et al. (2010) hypothesized that populations 

either may have had little to no exposure to WNV, or infected birds had high mortality and were not available to be sampled, or no birds attained detectable immune 

response by antibody titers. However, because spotted owls have high annual survival rates, it is possible that WNV has not yet made a large impact on these birds 

(Blakesley et al. 2010, Conner et al. 2013). Because there is no general surveillance 

program through the Sierra Nevada, it has been unclear if owls have been locally 

affected by WNV or if climate change will change the disease dynamics. 

Several species of ectoparasites (Hunter et al. 1994, Young et al. 1993), endoparasites (Gutiérrez 1989; Hoberg et al. 1989, 1993), and blood parasites (Gutiérrez 

1989, Ishak et al. 2008) have been identified in spotted owls. Gutiérrez (1989) 

reported 100 percent blood parasite infection rates across all three spotted owl subspecies, suggesting long-term adaptation to high parasitism rates. Ishak et al. (2008) 

reported a prevalence of 79 percent for blood parasites of California spotted owls 

in the northern Sierra Nevada, with 79 percent of individuals positive for at least 

one infection, while 44 percent of individuals tested positive for multiple infections 

(Ishak et al. 2008). Ishak et al. (2008) reported that infection rates were higher in 

California spotted owls (79 percent) than in northern spotted owls (52 percent) and 

west coast barred owls (15 percent). Ishak et al. (2008) documented the first case 

of a Plasmodium sp. infection in a northern spotted owl and noted that barred owls 

may pose the risk of introducing novel infections into spotted owl populations. High 

rates of infection in California spotted owls compared to barred owls may position 
217 

The California Spotted Owl: Current State of Knowledge

them at a competitive disadvantage compared to barred owls (Ishak et al. 2008), 

or the opposite could be true. The potential effects of parasites on spotted owl 

behavior, survival, or reproductive success has not been studied. However, disease 

and parasites can interact with other stressors to affect the condition of individuals, 

resulting in lower survival or other impacts. 

Environmental contaminants have not been identified as potential ecological 

stressors on California spotted owls. However, recent reports of high exposure rates 

of fisher (Pekania pennanti) to rodenticides, likely associated with illegal marijuana 

cultivation, across the southern Sierra Nevada (Gabriel et al. 2012) may have implications for spotted owls and other forest carnivores, as they feed extensively on 

rodents. Ongoing research has reported 62 percent exposure of barred owls (44/71 

owls) to rodenticides on the Hupa Reservation in northern California.4 

Available evidence suggests that disease and parasites do not pose a significant threat at the current time, although WNV remains a possible future threat. 

Rodenticides pose a significant emerging threat to California spotted owls, though 

no information is available at the time to evaluate the magnitude and demographic 

consequences of this threat. High exposure rates recently recorded in barred owls 

in an area where they are sympatric with spotted owls indicates that spotted owls 

likely have experienced high exposure rates given broad dietary overlap between 

the species. 

Human Recreation and Disturbance 

Disturbance resulting from human recreation and management activities can potentially affect California spotted owls. Impacts from recreation can range from the 

presence of hikers near owl nests and roosts to loud noises made by chainsaws or 

motorized vehicles. Additionally, disturbances can be acute (short term) or chronic 

(long term) depending on the type of impact. Measures of behavioral response or 

fecal corticosterone hormone levels (hormones that reflect stress) have been used to 

assess spotted owl response to disturbance. 

Mexican spotted owls exhibited low behavioral responses of any type to hikers 

who were ≥55 m (>180 ft) distance, and juveniles and adults were unlikely to flush 

from hikers at distances >12 or >24 m (>39 or 78 ft), respectively (Swarthout and 

Steidl 2001). Additionally, owls did not change their behavior when hikers were 

near nests, although cumulative effects of high levels of recreational hiking near 

4 Higley, M. 2016. Personal communication. Wildlife biologist, Hoopa Valley Tribal 

Forestry, 40 Orchard St., Hoopa, CA 95546. 
218 

GENERAL TECHNICAL REPORT PSW-GTR-254

nests may be detrimental (Swarthout and Steidl 2003). No differences in reproductive success were observed between Mexican spotted owl nests exposed to helicopter and chainsaw noise; however, owls exhibited behavioral responses to both 

stimuli but with greater behavioral response to chainsaw noise than helicopter noise 

(Delaney et al. 1999). Results from this study supported management guidelines 

of a 400-m (0.25-mi) disturbance buffer around active Mexican spotted owl nests. 

Wasser et al. (1997) reported higher corticosterone levels in male northern spotted 

owls within 0.41 km (0.25 mi) of roads in Washington, suggesting that higher stress 

levels were correlated with proximity to roads. In contrast, Tempel and Gutiérrez 

(2003, 2004) found little evidence for disturbance effects from chainsaws and roads, 

as measured by fecal corticosterone hormone levels for California spotted owls in 

the central Sierra Nevada. Recently, Hayward et al. (2011) reported a more complex 

association between road noise and northern spotted owl response on the Mendocino National Forest in California. They found no association between baseline 

hormone levels and distance to roads. Rather, owls had higher corticosterone levels 

when exposed to continuous traffic exposure, and they found that owl response 

may vary with age of owls and physiological body condition. Of note, they reported 

lower reproductive success for owls near roads with continuous loud noise versus 

owls near quiet roads. 

The effect of disturbance will likely remain high across the Sierra Nevada, 

but probably localized in space and time. Current limited operating period (LOPs) 

management standards and guidelines used on national forest lands that limit noise 

within 400 m of nest/roost areas during the nesting period appear effective for 

mitigating acute, direct noise and activity disturbance on owls at the project level. 

Genetics 

Current information supports the subspecies classification of California spotted 

owls. Further, genetic differences between California spotted owl populations in the 

Sierra Nevada and southern California owls suggests that these populations could 

be considered as distinct management units. Within the Sierra Nevada, genetic 

variation is low, raising concern that adaptation to future environmental change 

may be constrained (chapter 4). Further reduction in genetic diversity of owls in 

the Sierra Nevada is likely to be an increasing threat if current population declines 

continue and gaps in owl distribution develop. However, the types of genetic assays 

completed so far are not reflective of adaptive genetic traits, so additional genetic 

work needs to be done. 
219 

The California Spotted Owl: Current State of Knowledge

Chapter Summary 

California spotted owls are faced with significant threats. Overall, the population 

of this subspecies appears to be declining, although population trajectories differ between national forest and national park lands (see chapter 4). The CASPO 

identified timber harvest and even-aged forest management, fire suppression and 

increased wildfire, potential development of gaps in distribution, and human development as threats to these owls (Verner et al. 1992). These threats have remained or 

increased since publication of the CASPO report. Since CASPO, range expansion of 

barred owls, climate change, contaminants, and low genetic diversity have arisen as 

additional significant threats. 

Forest management remains a primary factor for California spotted owl habitat 

and populations on national forest and private industrial forest lands. Timber harvest on national forest lands has declined over the past few decades and most timber 

volume taken from the Sierra Nevada is harvested from private land. McKelvey and 

Weatherspoon (1992) identified both even-aged forest management and fire suppression as threats to California spotted owls and their habitat. They recommended 

development and experimental evaluation of forest management strategies to reduce 

fuel accumulation and the presence of ladder fuels and their associated risk of 

high-severity fire; increase vegetation heterogeneity at stand and landscape scales; 

and produce habitat to maintain populations of California spotted owls. Little 

progress has been made toward testing the effects of forest management strategies 

and silvicultural prescriptions that reduce wildfire risk on California spotted owls 

and their habitat, even though many treatments have occurred since CASPO (but 

see Stephens et al. 2014 and Tempel et al. 2014 for exceptions). Forest management 

practices on both national forest and private lands have likely exacerbated the 

concerns expressed in CASPO (Seamans and Gutiérrez 2007a, Stephens et al. 2014, 

Tempel et al. 2014, Verner et al. 1992). Dominant management activities on national 

forests have been mechanical thinning and fire suppression, and there is growing 

recognition that standard prescriptions for thinning to reduce fuels promotes stand 

homogeneity, as does fire suppression. In addition, even-aged forest management 

on private lands has likely reduced the amount of older, large-diameter tree, closedcanopy forest habitat. Further, widespread declines in large trees, a key owl nesting 

and roosting habitat element, have been reported from across the Sierra Nevada. 

Emerging strategies that protect existing, and increase future recruitment of, large 

trees integrated with prescriptions that create tree clumps and canopy gaps hold 

promise for providing favorable habitat conditions for owls while reducing the risk 

of habitat loss to fire or climate change-driven drought and insect tree mortality. 
220 

GENERAL TECHNICAL REPORT PSW-GTR-254

Much has been learned about California spotted owl response to fire, although 

significant scientific uncertainty and concern remains regarding effects of largescale, stand-replacing fire effects on owls and their habitat. Recent increases in the 

amounts and patch sizes of high-severity fire, such as observed on the 2013 Rim and 

2014 King Fires, along with projected future increases in fire activity associated with 

climate change, indicate the increased risk associated with high-severity fire. 

Declining owl populations, uncertainty about effects of Forest Service and 

private land forest management, and increasing risk of high-severity fire contribute to increased risk of gaps developing within the distribution of the owl in the 

Sierra Nevada. Owl populations are documented to have declined in two areas of 

concern identified in CASPO. Continued loss and degradation of habitat because of 

residential development on private lands, primarily at low and mid elevations, is an 

increasing threat given continuing human population growth across the west slope 

of the Sierra Nevada. 

Range expansion of the barred owl into the Sierra Nevada poses a significant 

new threat to California spotted owls. Unlike the situation with northern spotted 

owls, it is unlikely to have contributed to documented declines of California spotted owls because their density is low and they are largely restricted to the northern 

Sierra Nevada. However, recent increases in their number and dispersal into the 

central and southern Sierra Nevada portend an expansion throughout the Sierra 

Nevada. If such an expansion follows the pattern within the northern spotted owl 

range, California spotted owls will likely face extirpation. Research has shown that 

barred owl removal is technically and economically feasible (Diller 2013). 

Direct effects of climate change on California spotted owls are difficult to 

project and may differ along elevational and latitudinal gradients across the Sierra 

Nevada. Of particular concern are related impacts of climate change such as 

drought and its indirect impacts on owl habitat characteristics and important habitat 

elements such as large trees, as well as the potential for vegetation type conversions 

from conifer forest types to hardwood, shrub, and grass vegetation types within 

the low- and mid-elevation zones of the Sierra Nevada. Recent reports of wildlife 

contamination from rodenticides associated with illegal marijuana cultivation in 

the Sierra Nevada poses an increasing threat to California spotted owls and their 

prey. To date, no available evidence has demonstrated negative effects of West Nile 

virus on California owls, though this remains a potential threat given high mortality 

from this disease that has been observed in many captive owl species. Disturbance 

from human management and recreational activities does not appear to be a significant threat to California spotted owls as existing standards and guidelines (e.g., 

LOPs) appear to be sufficient for mitigating direct, short-duration effects of forest 
221 

The California Spotted Owl: Current State of Knowledge

management activities (e.g., timber harvest, prescribed fire, etc.), while recreational 

effects appear to be localized with potential impacts to a few owl sites. 

Evaluating the current status of threats to California spotted owls is hampered 

by lack of reliable information on the current status, and recent trends, of California 

spotted owl habitat across the Sierra Nevada. Given the preeminent importance of 

understanding the current status and past trends in owl populations and habitat, 

lack of such habitat information could be considered a threat to successful owl 

management and conservation, as well as for comprehensive forest management for 

wildlife. Further detailed discussion of owl habitat mapping issues is presented in 

chapter 6. 

Based on the best scientific information available, there are significant threats 

to California spotted owls that have either increased in magnitude or arisen since 

CASPO (Verner et al. 1992). The most significant primary threats are (1) continued 

effects of forest management on both public and private land; (2) increasing trends 

in large-scale, stand-replacing fire; (3) invasion of barred owls; (4) potential climate 

change direct effects on owl populations or climate-driven vegetation type conversions and increased fire activity; and (5) increasing human population growth and 

development. Two additional issues that can potentially become significant threats 

are (1) illegal rodenticide use and (2) West Nile virus. These threats can potentially, 

functioning singly or in concert, contribute to development of gaps in the distribution of owls, which can have negative demographic consequences for owls. For 

example, climate change, fire, and forest management activities may interact to 

limit the amounts and distribution of habitat available to owls, which can be further 

affected by increases in the barred owl population. This overall threat assessment 

coupled with documented ongoing declines in owl populations clearly indicates the 

need for careful management, monitoring, and research to address key uncertainties 

for these threats. 

Significant challenges exist for addressing the multiple threats to owls and for 

developing forest management strategies that integrate owl conservation needs 

within the broader context of forest ecosystem management and restoration in the 

face of increasing fire risk and climate change. Over 20 years have passed and only 

limited progress has been made toward resolving the questions, threats, and challenges posed in the CASPO report. Progress will involve development and testing 

forest management strategies, with success predicated upon increased organizational capacity and effective collaboration between management and research. 
222 

GENERAL TECHNICAL REPORT PSW-GTR-254

Literature Cited 

Ager, A.A.; Vaillant, N.M.; Finney, M.A.; Preisler, H.K. 2012. Analyzing 

wildfire exposure and source-sink relationships on a fire prone forest landscape. 

Forest Ecology and Management. 267: 271–283. 

Bailey, L.L.; Reid, J.A.; Forsman, E.D.; Nichols, J.D. 2009. Modeling 

co-occurence of northern spotted owls and barred owls: accounting for detection probability differences. Biological Conservation. 142: 2983–2989. 

Beck, T.W.; Gould, G.I., Jr. 1992. Background and the current management 

situation for the California spotted owl. In: Verner, J.; McKelvey, J.K.; Noon, 

B.R.; Gutiérrez, R.J.; Gould, G.I., Jr.; Beck, T.W., tech. coords. The California 

spotted owl: a technical assessment of its current status. Gen. Tech. Rep. PSWGTR-133. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific 

Southwest Research Station: 37–53. 

Berigan, W.J.; Gutiérrez, R.J.; Tempel, D.J. 2012. Evaluating the efficacy of 

protected habitat areas for the California spotted owl using long-term monitoring 

data. Journal of Forestry. 110(6): 299–303. 

Blakesley, J.A.; Seamans, M.E.; Conner, M.M.; Franklin, A.B.; White, G.C.; 

Gutiérrez, R.J.; Hines, J.E.; Nichols, J.D.; Shaw, D.W.H.; Munton, T.E.; 

Keane, J.J.; Steger, G.N.; Noon, B.R.; McDonald, T.L.; Britting, S. 2010. 

Population dynamics of spotted owls in the Sierra Nevada, California. Wildlife 

Monographs. 174. 

Bond, M.L.; Lee, D.E.; Siegel, R.B.; Tingley, M.W. 2013. Diet and home-range 

size of California spotted owls in burned forest. Western Birds. 44: 114–126. 

Bond, M.L.; Lee, D.E.; Siegel, R.B.; Ward, J.P.J. 2009. Habitat use and 

selection by California spotted owls in a postfire landscape. Journal of Wildlife 

Management. 73(7): 1116–1124. 

Briles, C.E.; Whitlock, C.; Skinner, C.N.; Mohr, J. 2011. Holocene forest 

development and maintenance on different substrates in the Klamath Mountains, 

northern California, USA. Ecology. 92(3): 590–601. 

Carroll, C. 2010. Role of climatic niche models in focal-species-based conservation 

planning: assessing potential effects of climate change on northern spotted owl 

in the Pacific Northwest, USA. Biological Conservation. 143(6): 1432–1437.

Clark, D.A.; Anthony, R.G.; Andrews, L.S. 2013. Relationship between wildfire, 

salvage logging, and occupancy of nesting territories by northern spotted owls. 

Journal of Wildlife Management. 77(4): 672–688. 
223 

The California Spotted Owl: Current State of Knowledge

Conner, M.M.; Keane, J.J.; Gallagher, C.V.; Jehle, G.; Munton, T.E.; Shaklee, 

P.A.; Gerrard, R.A. 2013. Overall realized population change for long-term 

monitoring: California spotted owl case study. Journal of Wildlife Management. 

77(7): 1449–1458. 

Crozier, M.L.; Seamans, M.E.; Gutiérrez, R.J.; Loschl, P.J.; Horn, R.B.; 

Sovern, S.G.; Forsman, E.D. 2006. Does the presence of barred owls suppress 

the calling behavior of spotted owls? The Condor. 108(4): 760–769. 

Dark, S.J.; Gutiérrez, R.J.; Gould, G.I., Jr. 1998. The barred owl (Strix varia) 

invasion in California. The Auk. 115(1): 50–56. 

Davis, M.B. 1981. Quaternary history and the stability of forest communities: 

forest succession. New York: Springer: 132–153. 

Davis, M.B. 1986. Climatic instability, time lags, and community disequilibrium. 

In: Diamond, J.M.; Case, T.J., eds. Community ecology. New York: Harper & 

Row: 269–284. 

Delaney, D.K.; Grubb, T.G.; Beier, P.; Pater, L.L.; Reiser, M.H. 1999. Effects 

of helicopter noise on Mexican spotted owls. Journal of Wildlife Management. 

63(1): 60–75. 

Diller, L.; Hamm, K.; Lamphear, D.; McDonald, T. 2012. Two decades of 

research and amonitoring of northern spotted owls on private timberlands in the 

redwood region: what do we know and what challenges remain? In: Standiford, 

R.B.; Weller, T.J.; Piirto, D.D.; Stuart, J.D., eds. Proceedings of coast redwood 

forests in a changing California: a symposium for scientists and managers. Gen. 

Tech. Rep. PSW-GTR-238. Albany, CA: U.S. Department of Agriculture, Forest 

Service, Pacific Southwest Research Station: 399–407.

Diller, L.V. 2013. To shoot or not to shoot: the ethical dilemma of killing one raptor 

to save another. The Wildlife Professional. Winter: 54–57. 

Dolanc, C.R.; Safford, H.D.; Thorne, J.H.; Dobrowski, S.Z. 2014. Changing 

forest structure across the landscape of the Sierra Nevada, California, since the 

1930s. Ecosphere. 5: 1–26. 

Dugger, K.M.; Anthony, R.G.; Andrews, L.S. 2011. Transient dynamics of 

invasive competition: barred owls, spotted owls, habitat, and the demons of 

competition present. Ecological Applications. 21(7): 2459–2468. 

Dugger, K.M.; Wagner, F.; Anthony, R.G.; Olson, G.S. 2005. The relationship 

between habitat characteristics and demographic performance of northern 

spotted owls in southern Oregon. Condor. 107: 863–878. 
224 

GENERAL TECHNICAL REPORT PSW-GTR-254

Franklin, A.B.; Anderson, D.R.; Gutiérrez, R.J.; Burnham, K.P. 2000. Climate, 

habitat quality, and fitness in northern spotted owl populations in northwestern 

California. Ecological Monographs. 70(4): 539–590. 

Franklin, A.B.; Gutiérrez, R.J.; Nichols, J.D.; Seamans, M.E.; White, G.C.; 

Zimmerman, G.S.; Hines, J.E.; Munton, T.E.; LaHaye, W.S.; Blakesley, 

J.A.; Steger, G.N.; Noon, B.R.; Shaw, D.W.H.; Keane, J.J.; McDonald, T.L.; 

Britting, S. 2004. Population dynamics of the California spotted owl (Strix 

occidentalis occidentalis): a meta-analysis. Ornithological Monographs. 54: 56. 

Fire and Resource Assessment Program [FRAP]. 2010. California forest and 

rangelands assessment. California Department of Forestry and Fire Protection. 

http://frap.cdf.ca.gov/assessment/2010/assessment2010.php. (January 2015). 

Gabriel, M.W.; Woods, L.W.; Poppenga, R.; Sweitzer, R.A.; Thompson, C.; 

Matthews, S.M.; Higley, J.M.; Keller, S.M.; Purcell, K.; Barrett, R.H.; 

Wengert, G.M.; Sacks, B.N.; Clifford, D.L. 2012. Anticoagulant rodenticides 

on our public and community lands: spatial distribution of exposure and 

poisoning of a rare forest carnivore. PLoS ONE. 7(7): e40163. 

Gaines, W.L.; Harrod, R.J.; Dickinson, J.; Lyons, A.L.; Halupka, K. 2010. 

Integration of northern spotted owl habitat and fuels treatments in the eastern 

Cascades, Washington. Forest Ecology and Management. 260: 2045–2052. 

Gancz, A.Y.; Barker, I.K.; Lindsay, R.; Dibernardo, A.; McKeever, K.; Hunter, 

B. 2004. West Nile virus outbreak in north American owls, Ontario, 2002. 

Emerging Infectious Diseases. 10(12): 2135–2142. 

Ganey, J.L.; Ward, J.P., Jr.; Jenness, J.S.; Block, W.M.; Hedwall, S.; Jonnes, 

R.S.; Apprill, D.L.; Rawlinson, T.A.; Kyle, S.C.; Spangle, S.L. 2014. Use 

of protected activity centers by Mexican spotted owls in the Scaramento 

Mountains, New Mexico. Journal of Raptor Research. 48(3): 201–218. 

Glenn, E.M.; Anthony, R.G.; Forsman, E.D. 2010. Population trends in northern 

spotted owls: associations with climate in the Pacific Northwest. Biological 

Conservation. 143(11): 2543–2552. 

Glenn, E.M.; Anthony, R.G.; Forsman, E.D.; Olson, G.S. 2011. Local weather, 

regional climate, and annual survival of the northern spotted owl. Condor. 113(1): 

159–176. 

Graham, S.A. 2012. Diet compostition, niche and geographic characteristics, and 

prey size preference of barred owls (Strix varia) in the Pacific Northwest. Boise, 

ID: Boise State University. 169 p. 
225 

The California Spotted Owl: Current State of Knowledge

Gutiérrez, R.J. 1985. Information and research needs for spotted owl management. 

In: Gutiérrez, R.J.; Carey, A.B., tech. eds. Ecology and management of the 

spotted owl in the Pacific Northwest. Gen. Tech. Rep. PNW-GTR-185. Portland, 

OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research 

Station: 115–118. 

Gutiérrez, R.J. 1989. Hematozoa from the spotted owl. Journal of Wildlife 

Diseases. 25(4): 614–618. 

Gutiérrez, R.J.; Cheng, A.S.; Becker, D.R.; Cashen, S.; Ganz, D.; Gunn, J.; 

Liquori, M.; Merrill, A.; Saah, D.S.; Price, W. 2015. Legislated collaboration 

in a conservation conflict: a case study of the Quincy Library Group, California. 

In: Redpath, S.; Gutiérrez, R.J.; Wood, K.; Young, J.C., eds. Conflicts in 

conservation: navigating towards solutions. Cambridge, United Kingdom: 

Cambridge Unversity Press: 271–283. 

Gutiérrez, R.J.; Cody, M.; Courtney, S.; Franklin, A.B. 2007. The invasion 

of barred owls and its potential effect on the spotted owl: a conservation 

conundrum. Biological Invasions. 9: 181–196. 

Hamer, T.E.; Forsman, E.D.; Glenn, E.M. 2007. Home range attributes and 

habitat selection of barred owls and spotted owls in an area of sympatry. Condor. 

109(4): 750–768. 

Hamer, T.E.; Hays, D.L.; Senger, C.M.; Forsman, E.D. 2001. Diets of northern 

barred owls and northern spotted owls in an area of sympatry. Journal of Raptor 

Research. 35(3): 221–227. 

Hayward, L.S.; Bowles, A.E.; Ha, J.C.; Wasser, S.K. 2011. Impacts of acute 

and long-term vehicle exposure on physiology and reproductive success of the 

northern spotted owl. Ecosphere. 2(6): art65. 

Hoberg, E.; Cawthorn, R.J.; Hedstrom, O. 1993. Enteric coccidia (Apicomplexa) 

in the small intestine of the northern spotted owl (Strix occidentalis caurina). 

Journal of Wildlife Diseases. 29(3): 495–497. 

Hoberg, E.; Miller, G.S.; Wallner-Pendleton, E.; Hedstrom, O. 1989. Helminth 

parasites of northern spotted owls (Strix occidentalis caurina) from Oregon. 

Journal of Wildlife Diseases. 25(2): 246–251. 

Hull, J.M.; Keane, J.J.; Tell, L.; Ernest, H.B. 2010. West Nile Virus antibody 

surveillance in three Sierra Nevada raptors of conservation concern. Condor. 

112(1): 168–172. 
226 

GENERAL TECHNICAL REPORT PSW-GTR-254

Hunter, J.E.; Gutiérrez, R.; Franklin, A.B.; Olson, D. 1994. Ectoparasites of the 

spotted owl. Journal of Raptor Research. 28(4): 232–235. 

Irwin, L.L.; Clark, L.A.; Rock, D.C.; Rock, S.L. 2007. Modeling foraging 

habitat of California spotted owls. Journal of Wildlife Management. 71(4): 

1183–1191. 

Irwin, L.L.; Rock, D.F.; Rock, S.C. 2013. Do northern spotted owls use harvested 

areas? Forest Ecology and Management. 310(0): 1029–1035. 

Ishak, H.D.; Dumbacher, J.P.; Anderson, N.L.; Keane, J.J.; Valkiūnas, G.; 

Haig, S.M.; Tell, L.A.; Sehgal, R.N.M. 2008. Blood parasites in owls with 

conservation implications for the spotted owl (Strix occidentalis). PLoS ONE. 

3(5): e2304. 

Keane, J.J. 2014. California spotted owl. In: Long, J.W.; Quinn-Davidson, L.; 

Skinner, C.N., eds. Science synthesis to promote resilience of social-ecological 

systems in the Sierra Nevada and southern Cascades. Gen. Tech. Rep. PSWGTR-247. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific 

Southwest Research Station: 437–467. 

Kipp, A.M.; Lehman, J.A.; Bowen, R.A.; Fox, P.E.; Stephens, M.R.; Klenk, 

K.; Komar, N.; Bunning, M.L. 2006. West Nile virus quantification in feces of 

experimentally infected American and fish crows. American Journal of Tropical 

Medicine and Hygiene. 75(4): 688–690. 

Knapp, E.; North, M.; Benech, M.; Estes, B. 2012. The variable-density thinning 

study at Stanislaus-Tuolumne Experimental Forest. Managing Sierra Nevada 

Forests. Gen. Tech. Rep. PSW-GTR-237. Albany, CA: U.S. Department of 

Agriculture, Forest Service, Pacific Southwest Research Station: 127–140. 

Knapp, E.; Skinner, C.K.; North, M.P.; Estes, B.L. 2013. Long-term overstory 

and understory change following logging and fire exclusion in a Sierra Nevada 

mixed-conifer forest. Forest Ecology and Management. 310: 903–914. 

Komar, N. 2003. West Nile Virus: epidemiology and ecology in North America. 

Advances in Virus Research. 61: 185–234. 

Kroll, A.J.; Fleming, T.L.; Irwin, L.L. 2010. Site occupancy dynamics of 

northern spotted owls in the eastern Cascades, Washington, USA. Journal of 

Wildlife Management. 74(6): 1264–1274. 

Lee, D.C.; Irwin, L.L. 2005. Assessing risk to spotted owls from forest thinning 

in fire-adapted forests of the western United States. Forest Ecology and 

Management: 191–200. 
227 

The California Spotted Owl: Current State of Knowledge

Lee, D.E.; Bond, M.L. 2015. Occupancy of California spotted owl sites following 

a large fire in the Sierra Nevada, California. Condor. 117(2): 228–236.

Lee, D.E.; Bond, M.L.; Bochert, M.I.; Tanner, R. 2013. Influence of fire and 

salvage logging on site occupancy of spotted owls in the San Bernardino and 

San Jacinto mountains of southern California. Journal of Wildlife Management. 

77: 1327–1341. 

Lee, D.E.; Bond, M.L.; Siegel, R.B. 2012. Dynamics of breeding-season site 

occupancy of the California spotted owl in burned forests. Condor. 114(4): 

792–802. 

Lenihan, J.M.; Bachelet, D.; Neilson, R.P.; Drapek, R. 2008. Response of 

vegetation distribution, ecosystem productivity, and fire to climate change 

scenarios for California. Climatic Change. 87(1): 215–230. 

Liu, Y.; Goodrick, S.L.; Stanturf, J.A. 2013. Future U.S. wildfire potential trends 

projected using a dynamically downscaled climate change scenario. Forest 

Ecology and Management. 294: 120–135. 

Livezey, K.B.; Bednarz, J.C. 2007. Barred owl habitat and prey: a review and 

synthesis of the literature. Journal of Raptor Research. 41(3): 177–201. 

Livezey, K.B. 2009. Range expansion of barred owls, part I: chronology and 

distribution. American Midland Naturalist. 161: 49–56. 

Lutz, J.A.; van Wagtendonk, J.W.; Franklin, J.F. 2009. Twentieth-century 

decline of large-diameter trees in Yosemite National Park. Forest Ecology and 

Management. 257: 2296–2307. 

MacKenzie, D.I.; Seamans, M.E.; Gutiérrez, R.J.; Nichols, J.D. 2012. 

Investigating the population dynamics of California spotted owls without 

marked individuals. Journal of Ornithology. 152(2): 597–604. 

Marra, P.P.; Griffing, S.; Caffrey, C.; Kilpatrick, A.M.; McLean, R.; Brand, 

C.; Saito, E.; Dupuis, A.P.; Kramer, L.; Novak, R. 2004. West Nile Virus and 

wildlife. BioScience. 54(5): 393–402. 

McKelvey, K.S.; Johnston, J.D. 1992. Historical perspectives on forests of 

the Sierra Nevada and the Transverse Ranges of Southern California: forest 

conditions at the turn of the century. Gen. Tech. Rep. PSW-GTR-133. Albany, 

CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research 

Station: 225–246. 
228 

GENERAL TECHNICAL REPORT PSW-GTR-254

McKelvey, K.S.; Weatherspoon, C.P. 1992. Projected trends in owl habitat. Gen. 

Tech. Rep. PSW-GTR-133. Albany, CA: U.S. Department of Agriculture, Forest 

Service, Pacific Southwest Research Station: 261–273. 

Miller, J.D.; Safford, H. 2012. Trends in wildfire severity: 1984 to 2010 in the 

Sierra Nevada, Modoc Plateau, and southern Cascades, California, USA. Fire 

Ecology. 8(3): 41–57. 

Miller, J.D.; Safford, H.D.; Crimmins, M.; Thode, A.E. 2009. Quantitative 

evidence for increasing fire severity in the Sierra Nevada and southern Cascade 

mountains, California and Nevada, USA. Ecosystems. 12: 16–32. 

Noon, B.R.; Franklin, A.B. 2002. Scientific research and the spotted owl (Strix 

occidentalis): opportunities for major contributions to avian population ecology. 

The Auk. 119(2): 311–320. 

Olson, G.S.; Anthony, R.G.; Forsman, E.D.; Ackers, S.H.; Loschl, P.J.; Reid, 

J.A.; Dugger, K.M.; Glenn, E.M. 2005. Modeling of site occupancy dynamics 

for nothern spotted owls with emphasis on the effects of barred owls. Journal of 

Wildlife Management. 69(3): 918–932. 

Olson, G.S.; Glenn, E.M.; Anthony, R.G.; Forsman, E.D.; Reid, J.A.; Loschl, 

P.J.; Ripple, W.J.; Flaspohler 2004. Modeling demographic performance of 

northern spotted owls relative to forest habitat in Oregon. Journal of Wildlife 

Management. 68(4): 1039–1053. 

Peery, M.Z.; Gutiérrez, R.J.; Kirby, R.; LeDee, O.E.; LaHaye, W. 2012. 

Climate change and spotted owls: potentially contrasting responses in the 

Southwestern United States. Global Change Biology. 18(3): 865–880. 

Reisen, W.; Lothrop, H.; Chiles, R.; Madon, M.; Cossen, C.; Woods, L.; 

Husted, S.; Kramer, V.; Edman, J. 2004. West Nile Virus in California. 

Emerging Infectious Diseases. 10: 1369–1378. 

Roberts, S.L.; van Wagtendonk, J.W.; Miles, A.K.; Kelt, D.A. 2011. Effects 

of fire on spotted owl site occupancy in a late-succesional forest. Biological 

Conservation. 144: 610–619. 

Rockweit, J.T.; Franklin, A.B.; Bakken, G.S.; Gutiérrez, R.J. 2012. Potential 

influences of climate and nest structure on spotted owl reproductive success: a 

biophysical approach. PLoS ONE 7(7): e41498.doi:1371/journal.pone.0041498. 

Roloff, G.J.; Mealey, S.P.; Bailey, J.D. 2012. Comparative hazard assessment for 

protected species in a fire-prone landscape. Forest Ecology and Management. 

277: 1–10. 
229 

The California Spotted Owl: Current State of Knowledge

Romesburg, H.C. 1981. Wildlife science: gaining reliable knowledge. Journal 

of Wildlife Management. 45: 293–313. 

Schumaker, N.H.; Brookes, A.; Dunk, J.R.; Woodbridge, B.; Heinrichs, 

J.A.; Lawler, J.J.; Caroll, C.; LaPlante, D. 2014. Mapping sources, sinks, 

and connectivity using a simulation model of northern spotted owls. Landscape 

Ecology. 29: 579–592. 

Seamans, M.E.; Corcoran, J.; Rex, A. 2004. Southernmost record of a spotted 

owl x barred owl hybrid in the Sierra Nevada. Western Birds. 35: 173–174. 

Seamans, M.E.; Gutiérrez, R.J.; May, C.A. 2002. Mexican spotted owl 

population dynamics: influence of climatic variation on survival and 

reproduction. Auk. 119(2): 321–334. 

Seamans, M.E.; Gutiérrez, R.J. 2006. Spatial dispersion of spotted owl sites and 

the role of conspecific attraction on settlement patterns. Ecology, Ethology, and 

Evolution. 18: 99–111. 

Seamans, M.E.; Gutiérrez, R.J. 2007a. Habitat selection in a changing environment: the relationship between habitat alteration and spotted owl territory 

occupancy and breeding dispersal. Condor. 109: 566–576. 

Seamans, M.E.; Gutiérrez, R.J. 2007b. Sources of variability in spotted owl 

population growth rate: testing predictions using long-term mark–recapture data. 

Oecologia. 152(1): 57–70. 

Siegel, R.B.; Pyle, P.; Thorne, J.H.; Holguin, A.J.; Howell, C.A.; Stock, S.; 

Tingley, M. 2014. Vulnerability of birds to climate change in California’s Sierra 

Nevada. Avian Conservation and Ecology. 9(1): 7 http://www.ace-eco.org/vol9/ 

iss1/art7/. (January 2015). 

Singleton, P.H.; Lehmkuhl, J.F.; Gaines, W.L.; Graham, S.A. 2010. Barred owl 

space use and habitat selection in the Eastern Cascades, Washington. Journal of 

Wildlife Management. 74(2): 285–294. 

Stephens, S.L.; Bigelow, S.W.; Burnett, R.D.; Collins, B.M.; Gallagher, 

C.V.; Keane, J.; Kelt, D.A.; North, M.P.; Roberts, L.J.; Stine, P.A. 2014. 

California spotted owl, songbird, and small mammal responses to landscape fuel 

treatments. BioScience. doi: 10.1093/biosci/biu137. 

Swarthout, E.C.; Steidl, R.J. 2001. Flush responses of Mexican spotted owls to 

recreationists. The Journal of Wildlife Management. 65(2): 312–317. 
230 

GENERAL TECHNICAL REPORT PSW-GTR-254

Swarthout, E.C.; Steidl, R.J. 2003. Experimental effects of hiking on breeding 

Mexican spotted owls. Conservation Biology. 17(1): 307–315. 

Tempel, D.J.; Gutiérrez, R.J. 2003. Fecal corticosterone levels in California 

spotted owls exposed to low-intensity chainsaw sound. Wildlife Society Bulletin. 

31(3): 698–702. 

Tempel, D.J.; Gutiérrez, R.J. 2004. Factors related to fecal corticosterone 

levels in California spotted owls: implications for assessing chronic stress. 

Conservation Biology. 18(2): 538–547. 

Tempel, D.J.; Gutiérrez, R.J. 2013. The relationship between occupancy and 

abundance for a territorial species, the California spotted owl. Conservation 

Biology. 27(5): 1087–1095. 

Tempel, D.J.; Gutiérrez, R.J.; Battles, J.J.; Fry, D.L.; Su, Y.; Guo, Q.; Reetz, 

M.J.; Whitmore, S.A.; Jones, G.M.; Collins, B.M.; Stephens, S.L.; Kelly, 

M; Berigan, W.J.; Peery, M.Z. 2015. Evaluating short- and long-term impacts 

of fuels treatments and simulated wildfire on an old-forest species. Ecosphere. 

6(12): Art 261. 

Tempel, D.; Gutiérrez, R.J.; Whitmore, S.; Reetz, M.; Stoelting, R.; Berigan, 

W.; Seamans, M.E.; Peery, M.Z. 2014. Effects of forest management on 

California spotted owls: implications for reducing wildfire risk in fire-prone 

forests. Ecological Applications. 24(8): 2089–2106. 

U.S. Department of the Interior, Fish and Wildlife Service [USFWS]. 2006. 50 

CFR Part 17 Endangered and threatened wildlife and plants; 12-month finding 

for a petition to list the California spotted owl (Strix occidentalis occidentalis) 

as threatened or endangered. Federal Register. 71(100): 29886–29908. 

U.S. Department of the Interior, Fish and Wildlife Service [USFWS]. 2008. 

Final recovery plan for the northern spotted owl (Strix occidentalis caurina).

 Portland, OR. 142 p. https://www.fws.gov/ecos/ajax/docs/recovery_plan/NSO_ 

Final_Rec_Plan051408.pdf. (22 November 2016). 

U.S. Department of the Interior, Fish and Wildlife Service [USFWS]. 2011. 

Revised recovery plan for the northern spotted owl (Strix occidentalis caurina). 

Portland, OR. 258 p. 

Van Lanen, N.J.; Franklin, A.B.; Huyvaert, K.P.; Reiser, R.F., II; Carlson, 

P.C. 2011. Who hits and hoots at whom? Potential for interference competition 

between barred and northern spotted owls. Biological Conservation. 144: 

2194–2201. 
231 

The California Spotted Owl: Current State of Knowledge

Verner, J.; McKelvey, K.S.; Noon, B.R.; Gutiérrez, R.J.; Gould, G.I.; Beck, 

T.W. 1992. The California spotted owl: a technical assessment of its current 

status. Gen. Tech. Rep. PSW-GTR-133. Albany, CA: U.S. Department of 

Agriculture, Forest Service, Pacific Southwest Research Station. 285 p.

Wasser, S.K.; Bevis, K.; King, G.; Hanson, E. 1997. Noninvasive physiological 

measures of disturbance in the northern spotted owl. Conservation Biology. 

11(4): 1019–1022. 

Weathers, W.W.; Hodum, P.J.; Blakesley, J.A. 2001. Thermal ecology and 

ecological energetics of California spotted owls. Condor. 103: 678–690. 

Weatherspoon, C.P.; Husari, S.J.; Van Wagtendonk, J. 1992. Fire and fuels 

management in relation to owl habitat in forests of the Sierra Nevada and 

southern California. Gen. Tech. Rep. PSW-GTR-133., Albany, CA: U.S. 

Department of Agriculture, Forest Service, Pacific Southwest Research Station: 

247–260. 

Weatherspoon, C.P.; Skinner, C.N. 1996. Landscape-level strategies for forest 

fuel management. In: SNEP Science Team and Special Consultants, eds. Sierra 

Nevada ecossytem project: final report to Congress. Vol. II: assessments and 

scientific basis for management options. Report No. 37. Davis, CA: Centers for 

Water and Wildlands Resources, University of California–Davis: 1471–1492. 

Westerling, A.L.; Hidalgo, H.G.; Cayan, D.R.; Swetnam, T.W. 2006. Warming 

and earlier spring increase western U.S. forest wildlfire activity. Science. 313: 

940–943. 

Wiens, J.D.; Anthony, R.G.; Forsman, E.D. 2014. Competitive interactions and 

resource partitioning between northern spotted owls and barred owls in western 

Oregon. Wildlife Monographs. 185(1): 1–50. 

Williams, P.J.; Gutiérrez, R.J.; Whitmore, S. 2011. Home range and habitat 

selection of spotted owls in the central Sierra Nevada. Journal of Wildlife 

Management. 75(2): 333–343. 

Yackulic, C.B.; Reid, J.; Davis, R.; Hines, J.E.; Nichols, J.D.; Forsman, E. 

2012. Neighborhood and habitat effects on vital rates: expansion of the barred 

owl in the Oregon Coast Ranges. Ecology. 93(8): 1953–1966. 

Yackulic, C.B.; Reid, J.; Nichols, J.D.; Hines, J.E.; Davis, R.; Forsman, E. 

2014. The roles of competition and habitat in the dynamics of populations and 

species distributions. Ecology. 95(2): 265–279. 
232 

GENERAL TECHNICAL REPORT PSW-GTR-254

Young, K.E.; Franklin, A.B.; Ward, J.P. 1993. Infestation of northern spotted 

owls by hippoboscid (Diptera) flies in northwestern California. Journal of 

Wildlife Diseases. 29(2): 278–283. 

Zabel, C.J.; Dunk, J.R.; Stauffer, H.B.; Roberts, L.M.; Mulder, B.S.; Wright, 

A. 2003. Northern spotted owl habitat models for research and management 

application in California, USA. Ecological Applications. 13(4): 1027–1040. 

Zimmerman, G.S.; LaHaye, W.S.; Gutiérrez, R.J. 2003. Empirical support for a 

despotic distribution in a California spotted owl population. Behavioral Ecology. 

14(3): 433–437. 
233 

The California Spotted Owl: Current State of Knowledge

Appendix 7-1—Distribution of Forest Management 

Treatments and Wildfire During 1990–2014 Within the 

Areas of Concern Identified in the 1992 CASPO Report

The following maps show the distribution of Forest Management Treatments and 

Wildfire During 1990–2014 Within the areas of concern Identified in the 1992 

“The California Spotted Owl: A Technical Assessment of Its Current Status” 

CASPO report. 
234 

GENERAL TECHNICAL REPORT PSW-GTR-254

Figure A-1—Distribution of forest management treatments and wildfire on national forest and private industrial forest lands in the California spotted owl assessment (1992) areas of concern 1 (Lassen County) and 2 (Northern Plumas County) on the Plumas and Lassen National Forests, 1990–2014. Sources: National Forest System (NFS) treatments extracted from U.S. Forest Service (USFS) Forest Activities Tracking System courtesy of Joe Sherlock (Pacific Southwest Region [R5] silviculturist), private industrial treatments courtesy of California Department of Forestry Forest Practice Geographic Information System (Suzanne Lang), fire perimeters from USFS R5 vegetation burn severity data, National Agricultural Imagery Program photography from U.S. Department of Agriculture Farm Service Agency Aerial Photography Field Office, owl range from California Department of Forestry, and Wildlife (CDFW), owl areas of concern from general technical report PSW-GTR-133 (1992). 
235 

The California Spotted Owl: Current State of Knowledge Figure A-2—Distribution of forest management treatments and wildfire on national forest and private industrial forest lands in the California spotted owl assessment (1992) area of concern 3 (Northeastern Tahoe National Forest) on the Tahoe National Forest, 1990–2014. Sources: National Forest System (NFS) treatments extracted from U.S. Forest Service (USFS) Forest Activities Tracking System courtesy of Joe Sherlock (Pacific Southwest Region [R5] silviculturist), private industrial treatments courtesy of California Department of Forestry Forest Practice Geographic Information System (Suzanne Lang), fire perimeters from USFS R5 vegetation burn severity data, National Agricultural Imagery Program photography from U.S. Department of Agriculture Farm Service Agency Aerial Photography Field Office, owl range from California Department of Forestry and Wildlife (CDFW), owl areas of concern from 

general technical report PSW-GTR-133 (1992). 
236 

GENERAL TECHNICAL REPORT PSW-GTR-254

Figure A-3—Distribution of forest management treatments and wildfire on national forest and private industrial forest lands in the California spotted owlassessment (1992) area of concern 4 (Northern Eldorado National Forest) on the Eldorado National Forest, 1990–2014. Sources: National Forest System (NFS)treatments extracted from U.S. Forest Service (USFS) Forest Activities Tracking System courtesy of Joe Sherlock (Pacific Southwest Region [R5] silviculturist), private industrial treatments courtesy of California Department of Forestry Forest Practice Geographic Information System (Suzanne Lang), fire perimeters from USFS R5 vegetation burn severity data, National Agricultural Imagery Program photography from U.S. Department of Agriculture Farm ServiceAgency Aerial Photography Field Office, owl range from California Department of Forestry and Wildlife (CDFW), owl areas of concern from general technicalreport PSW-GTR-133 (1992). 
237 

The California Spotted Owl: Current State of Knowledge

Figure A-4—Distribution of forest management treatments and wildfire on national forest and private industrial 

forest lands in the California spotted owl assessment (1992) areas of concern 6 (Southern Stanislaus National 

Forest) and 7 (Northwestern Sierra National Forest) on the Stanislaus and Sierra National Forests, 1990–2014. 

Sources: National Forest System (NFS) treatments extracted from U.S. Forest Service (USFS) Forest Activities 

Tracking System courtesy of Joe Sherlock (Pacific Southwest Region [R5] silviculturist), private industrial treatments courtesy of California Department of Forestry Forest Practice Geographic Information System (Suzanne 

Lang), fire perimeters from USFS R5 vegetation burn severity data, National Agricultural Imagery Program 

photography from U.S. Department of Agriculture Farm Service Agency Aerial Photography Field Office, owl 

range from California Department of Forestry and Wildlife (CDFW), owl areas of concern from general technical 

report PSW-GTR-133 (1992). 
238 

GENERAL TECHNICAL REPORT PSW-GTR-254

Figure A-5—Distribution of forest management treatments and wildfire on national forest and private industrial forest lands in the California Spotted Owl Assessment (1992) area of concern 8 (Northeastern Kern County) on the Sequoia National Forest, 1990–2014. Sources: National Forest System (NFS) treatments extracted from U.S. Forest Service (USFS) Forest Activities Tracking System courtesy of Joe Sherlock (Pacific Southwest Region [R5] silviculturist), private industrial treatments courtesy of California Department of Forestry Forest Practice Geographic Information System (Suzanne Lang), fire perimeters from USFS R5 vegetation burn severity data, National Agricultural Imagery Program photography from U.S. Department of Agriculture Farm Service Agency Aerial Photography Field Office, owl range from California Department of Forestry and Wildlife (CDFW), owl areas of concern from general technical report PSW-GTR-133 (1992). 
239 

The California Spotted Owl: Current State of Knowledge

 

Chapter 8: The Spotted Owl in Southern and Central 

Coastal California 

R.J. Gutiérrez, Douglas J. Tempel, and M. Zachariah Peery1 

Introduction 

Spotted owl populations found in southern and central coastal California have 

received much less attention than those inhabiting the Sierra Nevada because of 

economic (effect of habitat conservation measures on timber harvest) and social 

issues (community stability and desire for naturally functioning ecosystems). Yet 

there has been continued concern over the status of owl populations in this region 

since the first technical assessment of the California spotted owl “The California 

Spotted Owl: A Technical Assessment of Its Current Status” (CASPO) in 1992 

(Eliason and Loe 2011,2

 LaHaye and Gutiérrez 2005, Verner et al. 1992c). In this 

chapter, we first summarize the areas of concern for southern California and central 

coastal California (hereafter we refer to this region as “southern California”) 

portrayed in CASPO (Verner et al. 1992b). We then summarize new information 

gained since CASPO and revisit the status of threats to the owls. Finally, we provide 

some observations on the status of owls in southern California and potential management implications derived from new information. 

Since the CASPO report, most new information on spotted owls stems from 

work on the San Bernardino population, which is the largest owl population in southern California (see below). This information has been reported in scientific journals 

and symposia or as part of targeted monitoring in a few mountain ranges. Whereas 

lack of funding within the U.S. Forest Service (USFS) has limited the acquisition 

of new information, the USFS has developed a California spotted owl strategy for 

southern California (see footnote 2; Loe and Beier 20043). The original strategy was 

motivated by the extensive fires in southern California during 2003. This regionspecific strategy was developed as a response to CASPO (Verner et al. 1992b). 

1 R.J. Gutiérrez is a professor and Gordon Gullion Endowed Chair Emeritus, University 

of Minnesota, 2003 Upper Buford Circle, St. Paul, MN 55108; Douglas J. Tempel is a 

postdoctoral research associate, Department of Forest and Wildlife Ecology, University 

of Wisconsin, 1630 Linden Dr., Madison, WI 53706; M. Zachariah Peery is an associate 

professor, Department of Forest and Wildlife Ecology, University of Wisconsin, 1630 

Linden Dr., Madison, WI 53706. 

2 Eliason, E.; Loe, S. 2011. Management indicator species account for California spotted 

owl in the southern California province. 61 p. Unpublished report. On file with: USDA 

Forest Service, San Bernardino National Forest, 602 S Tippecanoe Ave., San Bernardino, 

CA 92408. 

3 Loe, S.; Beier, J.L. 2004. Conservation strategy for the California spotted owl (Strix 

occidentalis occidentalis) on the national forests of southern California. Unpublished 

report. On file with: USDA Forest Service, San Bernardino National Forest, 602 S 

Tippecanoe Ave., San Bernardino, CA 92408. 
240 

GENERAL TECHNICAL REPORT PSW-GTR-254

CASPO Assessment of Areas of Concern 

In CASPO, four general areas of concern were identified for the California spotted owl (Strix occidentalis occidentalis) in southern and central coastal California 

(Verner et al. 1992b: 7): 

1. The first was the potential loss of connectivity among mountain ranges in 

the region and between this region and the Sierra Nevada. 

2. The second was the potential fragmentation of habitat within these insular areas that define the distribution of the owls in southern California (see 

below). 

3. The third was the loss of habitat owing to water usage that leads to decline 

of riparian forest, high-severity fires that result in loss of habitat, and recreational use that results in either loss of habitat or disturbance to owls. 

4. The fourth concern was the lack of land use policies on private lands, adjacent to public lands, which could be used to mitigate the potential effects of 

development.

 The CASPO also noted that if the owl metapopulation did not function sufficiently to facilitate demographic rescue, then populations would have to function 

independently (depend on their own population dynamic processes), which meant 

that these populations would have to depend solely on the amount and quality of 

habitat available to them to remain viable. 

Distribution and Metapopulation 

The spotted owl in southern California is distributed from Monterey County south 

to Mount Palomar near the Mexican border (fig. 8-1), and is found as far south 

as the Sierra San Pedro Martir in Baja California Norte (Gutiérrez et al. 1995). 

Both the Sierra San Pedro Martir and Sierra Juarez are southern extensions of the 

Peninsular Ranges that contain most of the populations in southern California. 

The subspecies of owl found on these two Mexican ranges is unknown but by its 

geographic location is thought to be the California spotted owl. Owls also occur in 

the Tehachapi Mountains that potentially link this population, by closest proximity, 

to the Sierra Nevada (Verner et al. 1992b). Notable is the apparent absence of owls 

from the Santa Cruz Mountains, which apparently have suitable forest types for 

spotted owls. Based on geographic proximity, the Carmel Valley should not have 

presented a substantial barrier to dispersal for birds inhabiting the south side of the 
241 

The California Spotted Owl: Current State of Knowledge

Figure 8-1—Approximate territory locations and distribution of the California spotted owl in southern California, 2014. The 

Sierra Nevada is also depicted on the map to show the proximity of the Sierra Nevada population with the southern California owl 

metapopulation. 

Carmel Valley in the Santa Lucia Range. At the time of CASPO, there had been 

no systematic surveys for spotted owls in the Santa Cruz Mountains (Verner et al. 

1992b). This was still the situation in 2016. 

Within this large geographic distribution, the spotted owl in southern California is unique among west coast spotted owl populations because it occurs as a 

presumed metapopulation (LaHaye et al. 1994). Metapopulations are defined by 

distinct populations of individuals that function independently yet their dynamics 

are interrelated because of dispersal among populations (Hanski and Gilpin 1991). 

For the California spotted owl, the key issue is that the distance between populations is farther than owls typically disperse (Gutiérrez and Harrison 1996). The 

populations are generally distinct and isolated from each other because they exist 
242 

GENERAL TECHNICAL REPORT PSW-GTR-254

within the high-elevation forests that are found above the unsuitable shrub and 

semiarid vegetation zones that serve as barriers to movement among populations. 

In metapopulations, all populations have an equal likelihood of extinction, which 

predicts that persistence of the regional spotted owl population is dependent on 

there being enough populations. However, this is not the case in southern California 

so, theoretically, some populations will have to serve as source populations to “rescue” populations that go extinct (Gutiérrez and Harrison 1996, LaHaye et al. 1994, 

Noon and McKelvey 1996, Noon et al. 1992). Hence, the spatial structure of populations and habitats within and among populations is critical to the functioning of this 

metapopulation (Gutiérrez and Harrison 1996, Noon and McKelvey 1996). Thus far, 

there is scant evidence that dispersal among populations is a central property of the 

metapopulation dynamics of owls in southern California because there have been 

no records of movement even between populations in relatively close proximity 

(i.e., adjacent mountain ranges) (LaHaye et al. 2001, 2004). The lack of documented 

owl movement among populations for this region is in stark contrast to that of 

Mexican spotted owls (S. o. lucida) in the American Southwest, where movement 

among mountain ranges (i.e., populations) is common and the population is generally considered to have a metapopulation structure (Gutiérrez et al. 1996, May et 

al. 1996). In addition, two habitat conditions affect spotted owls generally (habitat 

fragmentation and habitat heterogeneity); these conditions increase the complexity 

and also the risk of extinction for owls in the southern California metapopulation 

(LaHaye et al. 1994). In this regard, most of the detailed ecological studies of 

southern California have occurred in the San Bernardino Mountains, which harbors 

the largest population of owls in southern California, and consequently these results 

likely provide the most optimistic view 

of owls in southern California. 

LaHaye et al. (1994) and Noon et al. (1992) modeled the dynamics of this metapopulation, while Beck and Gould (1992) provided verbal and visual descriptions of 

areas of potential concern for the southern California metapopulation. These studies 

clearly indicate that owl populations and habitat within populations are discontinuous. Noon et al.’s (1992: 189) simulation of the southern California metapopulation 

employed (and acknowledged) optimistic assumptions for owl survival rates in 

suitable habitat (i.e., they set survival rates high enough so that annual population 

growth rate [λ] = 1) and allowed for the possibility that λ increased by 2 percent per 

year. The reason for these assumptions was to examine how the habitat distributed 

over this large area might affect the metapopulation dynamics. At the time of their 

analysis (Noon et al 1992), there was only one owl demographic study in southern 

California, and its relatively short duration did not allow a meaningful estimation of 
243 

The California Spotted Owl: Current State of Knowledge

the effect of environmental conditions on that population. They noted that it is critical to examine the effects of both demographic and stochastic (random variation 

owing to such things as drought and fires) variation on owl population dynamics. 

Some of the assumptions posed for metapopulation theory have been that dispersal 

capability of owls to move among independent populations (i.e., mountain ranges) 

must be sufficient, that the distances between mountain ranges affect system 

dynamics, that the risks to owls when moving between or among mountain ranges 

is not excessive, and that small populations with high turnover have to be augmented by immigration to persist. From these basic assumptions, they concluded 

that the San Bernardino population was critical to the persistence of the entire 

metapopulation because the many small populations benefited in the simulation by 

having a large source population (i.e., the San Bernardino/San Gabriel Mountains). 

Noon et al. (1992) also evaluated the effect of potential habitat configurations on 

owl demography and key properties of the metapopulation (e.g., how the spacing of 

habitat islands affected dispersal). Evidently, simulated populations were strongly 

affected by dispersal risks both within and among ranges, sizes of individual 

populations, and the distances among populations. 

Noon et al. (1992) felt it was premature to assess extinction risk for the southern California owl metapopulation because there was insufficient data on several 

important variables (e.g., stochastic environmental variation, correlation in environmental conditions among populations), but LaHaye et al. (1994) had access to 

additional data and conducted such an analysis. They used a spatially structured 

metapopulation model that considered the number, size, and spatial location of 

each habitat patch and allowed for interaction among these patches (stochastic 

growth and dispersal among populations) and for correlation among environmental 

variation within the region (i.e., the degree to which environmental conditions 

were similar among areas supporting populations). Because they did not have 

information on all the populations, they relied on demographic information from 

the San Bernardino (i.e., the largest population in the metapopulation) and the San 

Jacinto Mountains (i.e., one of the smallest populations). They found that dispersal 

between these close populations was very low (no dispersal of color-marked owls 

was observed in 6 years of study), so they modeled a variety of dispersal rates and 

also modeled dispersal as a function of distance (i.e., dispersal rates declined with 

increasing distance between populations). Because they suspected that rainfall 

(a form of environmental variation) was correlated with spotted owl population 

dynamics (now demonstrated to be true; see LaHaye et al. [2004] and below), they 

modeled a range of environmental correlation even though rainfall was strongly 

correlated among the four mountain ranges examined. The correlations of rainfall 
244 

GENERAL TECHNICAL REPORT PSW-GTR-254

amounts among the San Bernardino, Santa Ynez, and Santa Ana Mountains and 

Mount Palomar ranged from 0.81 to 0.89. Their simulations suggested the metapopulation would likely either go extinct within the next 30 to 40 years or, under 

alternative hypotheses of deterministic decline and environmental fluctuations, 

would undergo a substantial decline but not go extinct, respectively. The effects of 

high environmental correlations and the vital rates were strong in influencing simulation results. They discussed a variety of alternative explanations for their results, 

most of which were not optimistic about the state of the metapopulation. 

The insular nature of these populations also presented a unique opportunity to 

study particular facets of the owl’s ecology (e.g., dispersal) that were more difficult 

to study in larger contiguous populations as shown by LaHaye et al. (1994). At the 

time of CASPO, only one long-term and several short-term studies were available 

for the technical assessment team (Verner et al. 1992c). One of these studies in the 

San Bernardino Mountains continued until 2000, while the others (San Jacinto 

Mountains and Mount Palomar) ended either before or shortly after CASPO was 

completed. Intensive study has been replaced by irregular monitoring sponsored 

by individual natinal forests (see footnote 2). Because of the very low numbers of 

birds in some populations and the apparent low dispersal, some of these populations 

appear to be in precarious conservation status, which makes this paucity of information an even greater concern (see footnote 2). 

General Ecology 

The ecology of spotted owls has been well described (e.g., chapter 2; Gutiérrez et 

al. 1995, Verner et al. 1992a), and the general ecology of spotted owls in this region 

does not appear to differ substantially from that of California spotted owls elsewhere (note: there is almost no information on spotted owls in Baja California Norte 

[Gutiérrez et al. 1995]). However, the details of environment, particularly climate, 

vegetation, and insularity, may affect the dynamics of the owl in southern California differently than they do in the Sierra Nevada (Gutiérrez and Pritchard 1990; 

Gutiérrez et al. 2011; LaHaye et al. 1992, 2001, 2004). The differences between 

owls in this region and the Sierra Nevada also have to be viewed not only within 

the context of the spatial fragmentation of populations (disjunct mountain ranges 

leading to metapopulation structure [i.e., insularity]), but also with respect to the 

spatial fragmentation of individuals (discontinuities of habitat owing to topography, 

elevation, soils, aspect, wildfire, and human impacts) and the natural heterogeneous 

makeup of cover types within owl territories. Very little is known about home range 

sizes of spotted owls in southern California but limited information suggests that 

home range size is variable (Zimmerman et al. 2001). Habitat fragmentation occurs 
245 

The California Spotted Owl: Current State of Knowledge

when habitat is discontinuous and that discontinuity affects population processes as 

a binary outcome (habitat or no habitat) (Franklin and Gutiérrez 2002). In contrast, 

habitat heterogeneity is the diversity of vegetation and successional stages within 

an area of interest (e.g., an owl territory), such that it reflects a multistate outcome 

(Franklin and Gutiérrez 2002). 

Habitat 

There are four major cover types used by spotted owls in southern California: 

riparian/hardwood forests and woodlands, live oak (Quercus chrysolepis Liebm.)/ 

bigcone Douglas-fir (Pseudotsuga macrocarpa (Vasey) Mayr) forest, mixed-conifer 

forest, and redwood (Sequoia sempervirens Lamb. ex D. Don Endl.)/California 

laurel (Umbellularia californica Hook. & Arn.) Nutt.) forest (Gutiérrez et al. 1992). 

Unlike the Sierra Nevada, most owls occur in cover types other than mixed-conifer 

forest (Gutiérrez et al. 1992) because mixed-conifer forest is only found at the highest elevations in most of these isolated mountain ranges. 

Smith et al. (2002) found owls distributed over a large altitudinal gradient (800 

to 2600 m [2,625 to 8,530 ft]) in the San Bernardino Mountains, which was the 

limit of available habitats in this mountain range. Of the major cover types used 

by owls in this range, canyon live oak/bigcone Douglas-fir cover type had both the 

most territories and the highest density of territories (56 and 0.39/km2

 [0.15 mi2], 

respectively; see also density comparisons with other areas in California below). 

The density of the 40 owl territories found in mixed-conifer/hardwood forest was 

0.29 territories/km2 (0.11 territories/mi2), and the density of the 48 territories found 

in mixed-conifer forest was 0.16 territories/km2 (0.06 territories/mi2); Smith et al. 

(2002) partitioned the mixed-conifer type of Gutiérrez et al. (1992) into two categories based on the proportion of hardwoods found in the understory and subcanopy 

layer of the forest. LaHaye et al. (1997) speculated that the high density of owls 

in canyon live oak/bigcone Douglas-fir forests may be related to high densities of 

prey in the chaparral that typically surrounds this cover type because more young 

fledged in this forest type than other types in the San Bernardino Mountains. They 

reported that owl territories were clumped in space rather than being randomly 

distributed, which resulted in the mean nearest neighbor distance (1497 m [4,911 

ft]) being significantly less than the distance between an equal number of random 

points (1787 m [5,862 ft]) (Smith et al. 1999). 

Smith et al. (2002) also assessed vegetation patterns at three arbitrary scales 

and one biologically based scale (3, 20, 79, and 177 ha [7, 49, 195, and 437 ac]) 

within owl territories and compared these patterns to those found at same-sized 

plots at randomly chosen sites. These analysis areas were circular plots with radii 
246 

GENERAL TECHNICAL REPORT PSW-GTR-254

of 100, 250, 500, and 750 m (328, 820, 1,640, and 2,460 ft), respectively. The 3-ha 

area represented the immediate area surrounding a nest or primary roost site, the 

20-ha (49-ac) area was used to assess both natural and human-induced fragmentation, the 79-ha (195-ac) area represented a larger area around the nest but probably 

much less than a core area, and the 177-ha (437-ac) area represented half the nearest 

neighbor distance, which approximated the size of a territory (see chapters 2 and 3). 

Collectively, they classified 17 cover types that they collapsed to four cover types 

for ease of analysis and to focus on forested vegetation (Smith et al. 2002: 140). At 

all analysis scales, spotted owl sites contained more closed-canopy forest and less 

nonforest, open forest, and chaparral cover types than random areas. Moreover, 

these closed-canopy areas were in fewer but larger patches. Their analysis showed 

that as the amount of closed-canopy forest increased so did the probability that a 

site contained owls. 

Although riparian/hardwood forests are used by owls in southern California, 

the owls in the San Bernardino Mountains that had riparian habitat in their home 

ranges had only minor portions of their home ranges in this cover type (Gutiérrez 

and Tempel, pers. obs.). These streamside forests and woodlands are also important 

owl habitats in other mountain ranges in southern California (Verner et al. 1992b). 

Many studies of habitat structure have shown that spotted owls are habitat 

specialists (i.e., they use some cover types in greater proportion than their availability in the landscape), and this is also true for owls inhabiting the San Bernardino 

Mountains (Gutiérrez et al. 1995, LaHaye et al. 1997, Verner et al. 1992b). LaHaye 

et al. (1997) showed that owls in the San Bernardino Mountains used areas that had 

greater canopy cover and more complex vegetation structure than what was available to them (i.e., randomly selected areas; table 8-1) (LaHaye et al. 1997). Owls 

also selected nest sites that had greater canopy cover, larger trees, and greater basal 

areas of hardwoods and conifers than what was available to them. 

Population Dynamics 

There have been many analyses of owl population dynamics in southern California 

(Franklin et al. 2004; Gutiérrez and Pritchard 1990; Gutiérrez et al. 2011; LaHaye et 

al. 1992, 1994, 2001, 2004; Noon et al. 1992; Peery et al. 2012). Of these, five were 

comprehensive studies that provided estimates of finite rate of population change; all 

of these involved the same San Bernardino long-term demography study (Franklin et 

al. 2004; LaHaye et al. 1992, 1994, 2004; Noon et al. 1992). The others were focused 

more specifically on elements of population dynamics or climate change (Gutiérrez 

and Pritchard 1990, Gutiérrez et al. 2011, LaHaye et al. 2001, Peery et al. 2012). 
247 

The California Spotted Owl: Current State of Knowledge

Table 8-1—Structure characteristics of spotted owl habitat and random areas in the San 

Bernardino Mountains 

Nest points Random points 

Habitat variable (N = 103) (N = 296) 

Meana Percentage 

CVb 

Mean Percentage 

CV 

Percentage canopy closure 79.3 22.3 52.4 49.9 

Percentage slope 54.2 49.8 32.1 68.7 

Broken-top tree basal areac 2.9 174.3 0.5 322.9 

Snag basal area 4.8 116.7 1.8 217.8 

Hardwood basal area (30.1 to 45 cm 3.2 216.7 0.9 332.8 

diameter at breast height [d.b.h.]) 

Hardwood basal area (>45 cm d.b.h) 4.9 144.7 0.8 380.4 

Total conifer basal area 37.1 59.5 20.1 85.8 

Conifer basal area (50.1 to 75 cm d.b.h.) 9.6 100.3 4.9 130.1 

Conifer basal area (>75 cm d.b.h.) 19.1 77.4 6.7 124.2 

a Includes zero values for all variables. 

b

 Percentage coefficient of variation. 

c Square meters per hectare. 

Source: Reproduced with permission from the Wilson Journal of Ornithology [formerly the Wilson Bulletin]. 

Density 

Crude densities (the density irrespective of cover types present on the landscape) 

of owls in southern California are lower than densities in other areas of California 

(table 8-2), which reflects the spatial fragmentation of suitable habitat across the 

landscape. However, ecological density (the density of owls within all suitable cover 

types on the landscape) is similar to one population of northern spotted owls in 

northwest California before its decline (Franklin et al. 1990). This suggests that the 

habitat in southern California has a similar capacity for supporting spotted owls 

as the more mesic forests in northwestern California, the latter of which have been 

highly fragmented by logging during the last half of the 20th century. We note that 

almost all populations of spotted owls are declining throughout its range so current 

densities will be lower (e.g., see table 8-2 for northwestern California example). 

Reproduction 

Franklin et al. (2004) estimated fecundity (number of female young produced per 

territorial female) for the San Bernardino Mountain population and found uncertainty among the models which represented hypothesized relationships. Their top 

model specified an even-odd pattern for reproduction, but this model was only 

slightly better than the “null” (intercept-only) model. Moreover, the parameter coef-
248 

GENERAL TECHNICAL REPORT PSW-GTR-254

Table 8-2—Crude (density over an entire study area) and ecological (density within cover types 

that are preferentially selected relative to available cover types) densities of spotted owls in 

California 

Crude density 

Time period 

1980–early Time period 

1990s most recent Location Source 

Owls/km2 

0.015 No recent estimate San Bernardino Mountains LaHaye et al. 2004, Smith et al. 2002 

0.059a 0.051b Lassen National Forest Keane 2016c 

0.18a 0.16b Eldorado National Forest Keane2016c 

0.151a 0.151b Southern Sierra Nevada Keane 2016c 

0.184a 0.184b Sequoia and Kings Canyon Franklin et al. 2004,. Keane 2016c 

1.21 No recent estimate San Jacinto Mountains Noon et al. 1992 

0.64 No recent estimate Mount Palomar Gutiérrez and Pritchard 1990 

0.235 0.123 Northwest California Franklin et al. 1990, Franklin 2016d 

Ecological density 

0.58 No recent estimate San Bernardino Smith et al. 2002 

0.544 No recent estimate Northwestern California Franklin et al. 1990 

a

 Year of lowest density within span of years (1990–2000) studied by Franklin et al. 2004; density calculated from raw data because 

density was not estimated by Franklin et al 2004. 

b

 Year of lowest density within span of years (1990–2005) studied by Blakesley et al. 2010; density calculated from raw data because 

density was not estimated by Blakesley et al. 

c Keane, J. K. 2016. Personal communication. Research wildlife ecologist, U.S. Department of Agriculture, Forest Service, Pacific 

Southwest Research Station, 1731 Research Park Dr., Davis, CA 95618. 

d Franklin, A. F. 2016. Personal communication. Supervisory research biologist and project leader, Wildlife Pathogens and Food 

Security & Safety Project, USDA/APHIS/WS National Wildlife Research Center, 4101 Laporte Avenue, Fort Collins, CO 80521-2154. 

ficient for the even-odd relationship was not significantly different than zero. The 

estimate of fecundity derived using the top model was 0.362 female young produced per female, which was similar to the Lassen (0.336), slightly lower than the 

Eldorado (0.409), and slightly higher than the Sierra and Sequoia and Kings Canyon 

(0.284 and 0.289, respectively) long-term demography studies (Franklin et al. 2004). 

LaHaye et al. (2004) used several more years of data than did Franklin et 

al. (2004) and derived different analyses from those of Franklin et al. (2004) in 

two significant ways. First, they created models that hypothesized relationships 

between weather and owl reproduction and other vital rates (see below). Second, 

they estimated rates of population change using a different approach than Franklin 

et al. (2004; see also chapter 4). Many studies of spotted owls have used weather 

variables to examine patterns in owl vital rates (e.g., Franklin et al. 2000, Seamans 
249 

The California Spotted Owl: Current State of Knowledge

et al. 2002, Seamans 2007). Weather has been shown to affect animals directly (e.g., 

by affecting energy needs) and indirectly (e.g., by affecting plants, which produce 

food for the prey upon which the owl depends). Thus, analyses that assess weather 

effects have provided insight into environmental processes and conditions that have 

the potential to affect owls. 

In the San Bernardino Mountains, LaHaye et al. (2004) showed that fecundity 

varied annually from 0.00 to 0.47 for subadult owls to 0.18 to 0.51 for adult owls. 

The top model suggested that the data were best explained by the additive effects of 

age and weather covariates. Owls experienced reduced fecundity during wet springs 

but increased fecundity when the previous weather year was wetter. Thus, owls 

reproduced best during a dry spring that followed a wet winter. Interestingly, this 

weather relationship (model) explained 100 percent of the temporal process variation in the data where 62 percent of the variation in the data was process variation 

and 38 percent was sampling variation. Process variation is the variation in the 

data that is attributable to the underlying processes that affect fecundity, whereas 

sampling variation is the variation attributable to sampling error. 

Owls use old forest for nesting sites (Gutiérrez 1985). However, as has been 

shown for other California spotted owl populations (Gutiérrez et al. 1992), owls 

in the San Bernardino Mountains will use other nest structures besides old trees 

(LaHaye et al. 1997). Spotted owls used nine tree species for nesting, and the majority of nest types were platform nests (59 percent). Cavity (24 percent) and broken 

top (17 percent) nests were used less frequently (LaHaye et al. 1997). However, they 

found no difference in nesting success among owls using different nest structures 

(LaHaye et al. 1997). Moreover, they found no difference between successful and 

unsuccessful nests with respect to habitat at the spatial scale of either the nest or 

nest stand (LaHaye et al. 1997). However, owls nesting in canyon live oak/bigcone 

Douglas-fir forests had higher reproduction than those nesting in other forest types, 

and the authors speculated that this may have been due to higher prey densities in 

chaparral surrounding this forest type. 

Survival 

We restrict our comments to the most recent population analyses for the San 

Bernardino Mountains because earlier studies used smaller datasets from this 

population (Franklin et al. 2004, LaHaye et al. 2004). Apparently, survival probabilities of adult spotted owls were estimated to be 0.815 (Franklin et al. 2004) 

or 0.796 (LaHaye et al. 2004), which were similar to estimates for most Sierra 

Nevada populations of owls. In addition, LaHaye et al. (2004) also estimated 

separate survival probabilities for first- and second-year subadults as 0.692 and 
250 

GENERAL TECHNICAL REPORT PSW-GTR-254

0.88, respectively. The slight difference between these estimates is related to the 

use of several additional years of data by LaHaye et al. (2004). LaHaye et al. (2004) 

also found that estimates of survival were most correlated with the age of owls and 

precipitation in the preceding winter, but there was uncertainty among weather 

variables in competing models. However, second-year subadult survival was higher 

than adult survival (0.88 and 0.80, respectively), which was different than other 

spotted owls populations. They also estimated juvenile survival to be 0.37. Analyses 

of the population have yielded the only unbiased estimates of juvenile spotted owls 

based on mark-recapture data (Zimmerman et al. 2003). LaHaye et al. (2004) also 

reported finding no temporal process variation in nonjuvenile survival, suggesting 

it was nearly constant over time. 

Dispersal 

The demographically closed nature of the San Bernardino study area has provided 

insight on dispersal for both juvenile and adult owls (Gutiérrez et al. 2011, LaHaye 

et al. 2001). Spotted owls exhibit obligate juvenile dispersal (i.e., they always 

disperse from the natal areas in their year of hatching) (Gutiérrez et al. 1995). In 

the San Bernardino population, of 478 juveniles banded between 1987 and 1998, 67 

males and 62 females successfully dispersed (i.e., they were relocated as a territorial 

bird elsewhere on the study area) from their natal areas. Males dispersed slightly 

less distance than females, but the difference was not statistically significant (mean 

for males = 10.1 km [6.3 mi], SD = + 7.6 km [4.7 mi]; mean for females = 11.7 km 

[7.3 mi], SD = + 8.1 km [5.0 mi]). In general, female birds disperse farther than 

male birds, but there are many exceptions to this generality (Greenwood 1980). By 

age 4, almost all birds had settled on territories, but about 40 percent of them took 

2 to 4 years to settle, which indicated they were floaters during that time. Floater 

owls, as described in chapter 2, will usually not exhibit territorial behavior (i.e., 

hoot in response to vocal lures or other owls hooting). The rather short dispersal 

distances reported by these authors were likely a reflection of a “reflective boundary” of unsuitable habitat at the edge of the study area (LaHaye et al. 2001). No 

spotted owls have been found successfully dispersing between or among the San 

Bernardino, San Gabriel, and San Jacinto Mountains, which are adjacent ranges, 

despite surveys and banding of owls within all three ranges (LaHaye et al. 2001, 

2004). This suggests that interpopulation dispersal, the key to maintenance of a 

metapopulation structure, is rare, which seems to support the more pessimistic 

projections of LaHaye et al.’s (1994) metapopulation modeling. 

LaHaye et al. (2001) evaluated the dispersal distances between and among 

juveniles fledged in pairs and triplets, respectively. They found no correlation in 
251 

The California Spotted Owl: Current State of Knowledge

the distances that related birds dispersed. In addition, they also found evidence for 

conspecific attraction because most owls settled at or near sites that were occupied 

the prior year (LaHaye et al. 2001). 

Peery and Gutiérrez (2013) used the same dataset as LaHaye et al. (2001) to 

assess whether juvenile survival was influenced by parental reproductive output 

under the hypothesis that the offspring of parents producing large broods would 

have relatively low survival probabilities, as expected under classic life-history 

theory (i.e., there is a cost to the owls because of the effort required to reproduce). 

They found that individuals that fledged in pairs had a greater probability of 

surviving their first year than individuals that fledged as singletons or in triplets. 

Moreover, improved survival for individuals that fledged in pairs carried over 

to subadult and adult stages. These authors also showed that indices of territory 

quality based purely on reproductive output were strongly correlated with indices 

based on offspring fitness that accounted for heterogeneity in survival rates. Thus, 

if reproductive output of owls within territories is known, the information could be 

used in conjunction with occupancy and survival information to rank territories for 

conservation planning. 

Almost a third (29 percent) of all territorial females and nearly a fifth (19 percent) of males on the San Bernardino study area dispersed at least once during that 

12-year study (Gutiérrez et al. 2011). Spotted owls may disperse following either the 

breaking of a pair bond or when a mate dies. Gutiérrez et al. (2011) found that birds 

that had higher reproductive output than the population average were less likely to 

disperse, which suggests that birds based their choices on the perceived quality of 

either particular territories or particular mates. The former hypothesis was supported by a post-hoc analysis that birds occupying territories of higher quality (i.e., 

territories whose occupants over time had higher than average reproductive output) 

were less likely to disperse. Of course, the territory and the individuals occupying 

a territory were confounded in their analyses, which was reflected in the relatively 

low variation explained by their models. Birds that dispersed following the death 

of their mate tended to improve their reproductive output, but it was not clear that 

birds that divorced improved their reproductive output. However, this latter result 

was likely related to paucity of data. 

Occupancy 

The most complete data on territory occupancy in southern California exists for 

the San Bernardino and San Jacinto Mountains within the San Bernardino National 

Forest (SBNF). In addition to surveys conducted under the demographic study from 

1987 through 1998 on the SBNF, extensive monitoring of known owl territories was 
252 

GENERAL TECHNICAL REPORT PSW-GTR-254

resumed from 2003 through 2011 within the two mountain ranges. This monitoring revealed (see footnote 2) a significant decline (about 50 percent) in territory 

occupancy from 1989 through 2010 on the SBNF. Although Eliason and Loe only 

reported naïve estimates of territory occupancy (i.e., conducted no statistical modeling to account for imperfect detection), their naïve occupancy estimates likely were 

unbiased because a large number of surveys (up to six) were typically conducted 

at each territory during a given year. Surveys were also conducted within other 

national forests in southern California from 2003 through 2011, but we can make no 

inferences about trends in occupancy within other mountain ranges because only a 

small number of locations were sporadically surveyed. As discussed in chapter 4, 

Lee et al. (2013) found no statistically significant effects of fire or salvage logging 

on spotted owl territory occupancy in the San Bernardino Mountains from 2003 

to 2011. However, they recognized that fire and salvage logging may have had 

negative effects on occupancy that were biologically meaningful. For example, 

territories that experienced fire had a 0.062 less probability of being occupied by an 

owl pair the following year than unburned sites; postfire salvage logging reduced 

this probability by an additional 0.046. In particular, local extinction markedly 

increased when >50 ha (124 ac) burned at high severity within a 203-ha (502-ac) 

region around territory centers. 

Population Trends 

The most comprehensive studies of the San Bernardino owl population by Franklin et al. (2004) and LaHaye et al. (2004) differed in their approaches and intent. 

Franklin et al. (2004) conducted a meta-analysis of all extant California spotted 

owl demographic studies so they were intent on keeping methodologies and data 

structures similar. In the former case, they used Pradel’s temporal symmetry model 

to estimate population rate of change because the Sierra Nevada study areas were 

demographically open and thus had biased data with respect to juvenile survival. 

However, as also noted above, the San Bernardino population was closed so they 

used a Leslie projection matrix to estimate population rate of change because 

estimates of juvenile survival were not biased by undetected emigration (LaHaye et 

al. 2004, Zimmerman et al. 2007). The Pradel model answers the question: “Are the 

owls on the study area being replaced?” The Leslie projection matrix answers the 

question “Are the owls on the study area replacing themselves?” Thus, both estimators are valid; they simply confer different inferences. 

Franklin et al. (2004) found a linear decline in population over the time considered (λ = 0.98), but the confidence interval overlapped 1.0 so there was uncertainty 
253 

The California Spotted Owl: Current State of Knowledge

about an actual decline. In contrast, LaHaye et al. (2004) estimated λ = 0.91 and 

the confidence limit did not overlap 1.0, which indicated that the population in the 

San Bernardino Mountains declined approximately 9 percent/year over the period 

of study (1987–1998). LaHaye et al. (2004) also analyzed their data using the same 

time period as Franklin et al. (2004) and estimated that λ = 0.92, which was still a 

significant decline but much lower than Franklin et al.’s (2004) estimate. This difference relates to the closed San Bernardino population, which allowed LaHaye et 

al. (2004) to use a Leslie projection matrix to estimate lambda. Finally, Franklin et 

al. (2004) developed a metric called “realized population change” that depicted the 

change in population size over time relative to the initial population size. Consistent 

with their estimate of lambda, realized change for the San Bernardino population 

was not significantly different than 1. This metric was developed because it is 

difficult to detect trends in populations when they are small (Franklin et al. 2004, 

Tempel and Gutiérrez 2013). Thus, estimates of the decline of owls in the San 

Bernardino Mountains were supported by an estimator that was able to take advantage of the internal dynamic processes (stage-specific survival and reproduction) 

exhibited by this owl population. 

Threats 

Here we return to the factors noted by CASPO as threats to the long-term viability of 

the southern California owl metapopulation (Verner et al. 1992b: 7). In addition, we 

address the concerns raised by LaHaye and Gutiérrez (2005) and provide new potential concerns that have surfaced since CASPO (see footnote 2) (Peery et al. 2012). 

Natural Connectivity Among Populations 

Successful dispersal among populations is the only way that this metapopulation 

can continue to function naturally (LaHaye et al. 2004, Verner et al. 1992b). Connectivity among populations is influenced by barriers and by dispersal habitat. In 

1992, the threat of barriers was urban and suburban development, while the threat 

to habitat was the elimination of riparian areas that might serve as corridors. The 

current situation is worse because development continues unabated within both the 

Los Angeles Basin and the surrounding deserts. Moreover, many wind turbines 

have been erected in several areas that could serve as potential dispersal corridors 

between mountain ranges and between the southern California region and the 

Sierra Nevada. Wind turbines pose a potential threat of unknown magnitude to 

owls. There are no intact riparian forests that could act as corridors to assist owls 

dispersing among mountain ranges. At the time of CASPO, reservoirs were not 

specifically considered a barrier to dispersal, but at least one owl drowned in its 
254 

GENERAL TECHNICAL REPORT PSW-GTR-254

apparent attempt to cross one in the area between the San Bernardino and San 

Gabriel Mountains.4 Thus, we add two types of infrastructure development as 

potential threats to dispersal—wind farms and large reservoirs. Finally, the link 

between the Sierra Nevada population and southern California through the Transverse Ranges has also not improved and likely has deteriorated owing to continued 

human development. 

Integrity of Habitat Supporting Each Population 

With dispersal reduced among populations, rescue effects will not be a factor in 

the functioning of the metapopulation. Rather, each population will persist or go 

extinct, in part, as a function of its own habitat conditions. Habitat loss could result 

from fires and salvage logging (see above). There are as yet, no restrictions on 

logging on private forest land within the range of the owl other than those imposed 

by the California Forest Practices Act. Habitat is also being lost or fragmented as a 

result of primary and secondary home building (LaHaye and Gutiérrez 2005). However, there is no longer any commercial timber harvest on national forests within 

the owl’s range in southern California (see footnote 2). Yet we still do not know if 

key habitat elements are declining (e.g., large residual trees). 

Water Diversion and Stream Channelization 

LaHaye and Gutiérrez (2005) provided no evidence for current loss of riparian 

habitat owing to the water diversion threat noted by Verner et al. (1992b). Yet 

this threat remains as well as the threat of channelization to control waterflow 

(i.e., flood protection). Some owls, particularly those at low elevations, have parts 

of their territories within riparian habitats and these activities either degrade or 

eliminate these riparian areas. Riparian areas have high species diversity so they 

likely represent suitable owl foraging sites if they contain tree cover. The U.S. 

Forest Service has made some progress by requiring that water be hauled into some 

vacation homes and camps instead of being diverted from streams (see footnote 2). 

This should reduce some negative impact to riparian areas, but the effect of these 

new regulations has not been quantified (see footnote 2). 

Wildfire

Wildfire has long been a concern for its potential impact on owls and their habitat, 

but its overall effect on owl populations is not clear (see chapters 3 and 4 as well as 

4 LaHaye, W.S. 1996. Personal communication. Wildlife biologist, 10156 Pine Place, 

Morongo Valley, CA 92256. 
255 

The California Spotted Owl: Current State of Knowledge

above discussion). Given the loss of habitat owing to other factors (e.g., urbanization 

and drought, see below), fires are likely a contributing factor in this loss. 

Human Recreation 

Southern California has a very large and dense human population and the surrounding mountain ranges are used heavily for recreation. LaHaye and Gutiérrez (2005) 

and others (see footnote 2) voiced concern that recreational activities could negatively affect owls indirectly through disturbance and degradation or loss of habitat 

to accommodate this recreational activity. 

Drought 

LaHaye et al. (2004) showed that precipitation was correlated with reproductive patterns. Thus, the general drought pattern that has been affecting southern California 

for the past two decades will probably have some negative impact on owl demography, primarily by reducing reproductive output. The recent and future droughts will 

only exacerbate this concern. 

Air Pollution 

Air pollution is a well-known phenomenon in southern California. It can potentially 

affect vegetation dynamics, which in term could affect the habitat of owls and their 

prey (LaHaye and Gutiérrez 2005). Although air pollution is an issue that is being 

addressed at many levels through policies and law, it still is affecting some of these 

owl habitat islands. It also poses a direct threat to owls because birds do not possess 

a DNA repair mechanism for lung tissue (Rombout et al. 1991). 

Mining 

Several owl territories in the San Bernardino Mountains are possibly being affected 

by carbonate mining operations (see footnote 2). The two impacts stemming from 

these mining operations are side-casting of rock from roads and tailings and water 

diversion that affects riparian habitat. 

Marijuana Cultivation 

We are unaware of the extent of marijuana (Cannabis sp.) cultivation in southern 

California, but it is prevalent throughout the rest of rural and mountainous California. Recent evidence indicates widespread use of rodenticides to control rodents 

that eat these plants has led to secondary poisoning of Pacific fishers (Pekania 

pennanti) in the southern Sierra Nevada (Gabriel et al. 2012). These rodenticides 
256 

GENERAL TECHNICAL REPORT PSW-GTR-254

are now being found in a high percentage of barred owls (Strix varia) in northwestern California.5

 If barred owls are being poisoned, then spotted owls are probably 

also being affected because these species often use the same habitats where they 

co-occur (Gutiérrez et al. 2007). Thus, we feel it prudent to list this activity as a 

potential threat to spotted owls in southern California. 

Cumulative Effect of Small-Scale Management Actions 

Many small-scale activities are conducted by land managers within the range of the 

owl in southern California, which by themselves may not significantly affect owls 

but could do so collectively. Some examples of these are hazard tree removals for 

roads, powerlines, building camps, building vacation homes, diverting water for 

special uses, and developing ski areas (see footnote 2). 

Invasive Species and Disease 

The barred owl potentially was first observed in southern California in January 

2016 in Los Angeles County but has not yet been verified.6 In addition, West Nile 

virus occurs in southern California, but there is no evidence it is affecting owls 

(chapter 7). However, invasive plants may be a threat to owl habitat (see footnote 

2). Plant species such as cheatgrass (Bromus tectorum L.), Chinese tree of heaven 

(Ailanthus altissima (Mill.) Swingle), and tamarisk (Tamarix ramosissima Ledeb.) 

can potentially affect owl habitat either through competition and displacement or 

providing fuel for fires. Sudden oak death syndrome has also affected owl habitat in 

some parts of the Los Padres National Forest (see footnote 2). 

Climate Change 

Intuitively, California spotted owls in southern California would seem to be vulnerable to the warmer and drier conditions expected under climate change scenarios 

given the xeric nature of this region (relative to other areas occupied by this subspecies). Peery et al. (2012) assessed the potential impacts of climate change on California spotted owls in the San Bernardino Mountains by first correlating annual 

demographic rates (survival and reproduction) to weather conditions, and then using 

demographic-weather relationships to project the population forward in time under 

5 Higley, J.M. 2015. Personal communication. Wildlife biologist, Hoopa Valley Tribe, 

80 Willow Ln, Hoopa, CA 95546. 

6 Garrett, K.L. 2016. Personal communication. Ornithology collections manager, Natural 

History Museum of Los Angeles County, 900 Exposition Blvd., Los Angeles, CA 90007. 
257 

The California Spotted Owl: Current State of Knowledge

 

alternative climate change scenarios. According to their model, viability at the end 

of the 21st century was relatively insensitive to climate change under all scenarios 

considered, whereas the viability of two Mexican spotted owl populations was 

projected to decline substantially as a function of climate change. Viability in the 

San Bernardino Mountains did not decline largely because reproduction is lower 

in cold, wet nesting seasons, and such conditions are expected to attenuate under 

climate change. At least two important caveats need to be mentioned regarding the 

findings of this study. First, neither changes in wildfires, which may increase in frequency and severity, nor other agents of disturbance (e.g., insects and diseases) were 

modeled. Second, expected changes in temperature under climate change exceeded 

the variability that occurred during the period used to develop demography-weather 

relationships. Thus, the authors assumed that relationships between weather and 

owl demography will hold under a novel climate space, an assumption that may not 

be valid. 

Although milder nesting conditions may improve reproductive success (Peery et 

al. 2012), the owl’s bioclimatic niche will almost certainly move to higher elevations 

in southern California. It is unclear whether suitable owl habitat will be able to track 

elevational changes in the owl’s bioclimatic niche within the timeframe needed to 

avoid mismatches between needed forest conditions and suitable climate. Moreover, 

the spotted owl’s bioclimatic niche will likely occur over more narrow elevational 

gradients in southern California as the climate warms, which could cause a contraction in the distribution (and reduction in abundance) of owls in the region. Finally, low 

intermountain dispersal rates in southern California suggest that spotted owls may not 

be able to track latitudinal shifts in their bioclimatic niche (LaHaye et al. 2001). 

Chapter Summary 

The status of the spotted owl in southern California is, if not dire, significantly more 

deteriorated than when it was evaluated as part of CASPO (Verner et al. 1992c). Most 

information stems from the largest population of owls in southern California, which 

should have the highest potential for self-sustaining viability. If this population is 

undergoing substantial decline (50 percent; see footnote 2) (LaHaye et al. 2004), we 

can assume other populations in southern California are declining as well. The large 

number of threats, concomitant with no apparent remedies to them, suggests that 

every effort be made to maintain the integrity of existing suitable forests. Minnich 

(1980) indicates that canyon live oak/bigcone Douglas-fir forests may have declined 

in the past century as a result of fire. Canyon live oak/bigcone Douglas-fir forests 

are often surrounded by highly flammable chaparral and scrub cover types and 
258 

GENERAL TECHNICAL REPORT PSW-GTR-254

 

therefore could be a priority for fire treatments. However, the tenuous nature of the 

metapopulation makes active management to reduce fire risk arguably a more risky 

activity than in other regions. Regardless, maintaining all habitat elements known 

to be used by owls, especially large trees (both conifers and hardwoods), diverse 

forest structure, snags, and high canopy cover in mature forests, appears to be a 

key factor in conserving owls. Areas at higher elevations are also likely to be of 

greater importance in the future given predictions of climate change and potential 

use of refugia at higher elevations (Jones et al. 2016, Peery et al. 2012). As noted 

by CASPO (Verner et al. 1992c), efforts to improve connectivity among mountain 

ranges and facilitate northerly movements to areas that may be resilient to climate 

change are important. A spatial population modeling exercise that incorporates 

climate change and evaluates functional connectivity could greatly facilitate such 

planning. Finally, assuming the San Bernardino population could first be stabilized 

and then increased, it may well be time to consider reintroducing owls from this 

population to other areas where populations have become extinct to provide artificial “rescue effects” in this metapopulation. However, if extinction of populations is 

from loss or fragmentation of habitat, translocations would not be beneficial. 

Literature Cited 

Beck, T.W.; Gould, G.I., Jr. 1992. Background and the current management 

situation for the California spotted owl. In: Verner, J.; McKelvey, K.S.; Noon, 

B.R.; Gutiérrez, R.J.; Gould, G.I., Jr. ; Beck, T.W., eds. The California spotted 

owl: a technical assessment of its current status. Gen. Tech. Rep. PSW-GTR-133. 

Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest 

Research Station: 37–53. 

Blakesley, J.A.; Seamans, M.E.; Conner, M.M.; Franklin, A.B.; White, G.C.; 

Gutiérrez, R.J.; Hines, J.E.; Nichols, J.D.; Munton, T.E.; Shaw, D.W.H.; 

Keane, J.J.; Steger, G.N.; McDonald, T.L. 2010. Population dynamics of 

spotted owls in the Sierra Nevada, California. Wildlife Monographs. 174: 1–36. 

Franklin, A.B.; Anderson, D.R.; Gutiérrez, R.J.; Burnham, K.P. 2000. 

Climate, habitat quality, and fitness in northern spotted owl populations

in northwestern California. Ecological Monographs. 70: 539–590. 

Franklin, A.B.; Gutiérrez, R.J. 2002. Spotted owls, forest fragmentation, and 

forest heterogeneity. Studies in Avian Biology. 25: 203–220. 
259 

The California Spotted Owl: Current State of Knowledge

Franklin, A.B.; Gutiérrez, R.J.; Nichols, J.D.; Seamans, M.E.; White, G.C.; 

Zimmerman, G.S.; Hines, J.E.; Munton, T.E.; LaHaye, W.S.; Blakesley, 

J.A.; Steger, G.N.; Noon, B.R.; Shaw, D.W.H.; Keane, J.J.; McDonald, T.R.; 

Britting, S. 2004. Population dynamics of the California spotted owl (Strix 

occidentalis occidentalis): a meta-analysis. Ornithological Monographs. 54: 

1–54. 

Franklin, A.B.; Ward, J.P.; Gutiérrez, R.J.; Gould, G.I., Jr. 1990. Density of 

northern spotted owls in northern California. Journal of Wildlife Management. 

54: 1–10. 

Gabriel, M.W.; Woods, L.W.; Poppenga, R.; Sweitzer, R.A.; Thompson, C.; 

Matthews, S.M.; Higley, J.M.; Keller, S.M.; Purcell, K.; Barrett, R.H.; 

Wengert, G.M.; Sacks, B.N.; Clifford, D.L. 2012. Anticoagulant rodenticides 

on our public and community lands: spatial distribution of exposure and 

poisoning of a rare forest carnivore. PLoS ONE. 7 e40163. doi:10.1371/journal. 

pone.0040163. 

Greenwood, P.J. 1980. Mating systems, philopatry and dispersal in birds and 

mammals. Animal Behaviour. 28: 1140–1162. 

Gutiérrez, R.J. 1985. An overview of recent research on the spotted owl. In: 

Gutiérrez, R.J.; Carey, A.B., tech. eds. Ecology and management of the spotted 

owl in the Pacific Northwest. Gen. Tech. Rep. PNW-GTR-185. Portland, OR: 

U.S. Department of Agriculture, Forest Service, Pacific Northwest Research 

Station: 39–49. 

Gutiérrez, R.J.; Cody, M.; Courtney, S.; Franklin, A.B. 2007. The invasion 

of barred owls and its potential effect on the spotted owl: a conservation 

conundrum. Biological Invasions. 9: 181–196. 

Gutiérrez, R.J.; Franklin, A.B.; LaHaye, W.S. 1995. Spotted owl (Strix 

occidentalis). In: Poole, A.; Gill, F., eds. The birds of North America No. 179: 

life histories for the twenty-first century. Washington, DC: The Philadelphia 

Academy of Sciences and The American Ornithologists’ Union. 28 p. 

Gutiérrez, R.J.; Harrison, S. 1996. Applying metapopulation theory to 

spotted owl management: a history and critique. In: McCullough, D., ed. 

Metapopulations and wildlife conservation. Covelo, CA: Island Press: 167–185. 

Gutiérrez, R.J.; LaHaye, W.S.; Zimmerman, G.S. 2011. Breeding dispersal in 

an isolated population of spotted owls (Strix occidentalis): evidence for improved 

reproductive output. Ibis. 153: 592–600. 
260 

GENERAL TECHNICAL REPORT PSW-GTR-254

Gutiérrez, R.J.; Pritchard, J. 1990. Distribution, density, and age structure of 

spotted owls on two southern California habitat islands. Condor. 92: 491–495. 

Gutiérrez, R.J.; Seamans, M.E.; Peery, M.Z. 1996. Intermountain movement 

by Mexican spotted owls (Strix occidentalis lucida). Great Basin Naturalist. 

56: 87–89. 

Gutiérrez, R.J.; Verner, J.; McKelvey, K.S.; Noon, B.R.; Steger, G.S.; Call, 

D.R.; LaHaye, W.S.; Bingham, B.B.; Senser, J.S. 1992. Habitat relations of the 

California spotted owl. In: Verner, J.; McKelvey, K.S.; Noon, B.R.; Gutiérrez, 

R.J.; Gould, G.I., Jr.; Beck, T.W., eds. The California spotted owl: a technical 

assessment of its current status. Gen. Tech. Rep. PSW-GTR-133. Albany, CA: 

U.S. Department of Agriculture, Forest Service, Pacific Southwest Research 

Station: 79–147. 

Hanski, I.; Gilpin, M. 1991. Metapopulation dynamics: brief history and 

conceptual domain. Biological Journal of the Linnean Society. 42: 3–16. 

Jones, G.; Gutiérrez, R.J.; Tempel, D.J.; Zuckerberg, B.; Peery, M.Z. 2016. 

Using dynamic occupancy models to inform climate change adaptation strategies 

for California spotted owls. Journal of Applied Ecology. 53. 895–905. 

LaHaye, W.S.; Gutiérrez, R.J. 2005. The spotted owl in southern California: 

ecology and special concerns for the maintenance of a forest-dwelling species in 

a human-dominated desert landscape. In: Kus, B.E.; Beyers, J.L., tech. coords. 

Planning for biodiversity: bringing research and management together. Gen. 

Tech. Rep. PSW-GTR-195. Albany, CA: U.S. Department of Agriculture, Forest 

Service, Pacific Southwest Research Station: 199–209. 

Lahaye, W.S.; Gutiérrez, R.J.; Akcakaya, H.R. 1994. Spotted owl 

metapopulation dynamics in southern California. Journal of Animal Ecology. 

63: 775–785. 

LaHaye, W.S.; Gutiérrez, R.J.; Call, D.R. 1992. Demography of an insular 

population of spotted owls (Strix occidentalis occidentalis). In: McCullough, 

D.R.; Barrett, R.H., eds. Wildlife 2001: populations. New York: Elsevier Press: 

803–814. 

LaHaye, W.S.; Gutiérrez, R.J.; Call, D.R. 1997. Nest-site selection and 

reproductive success of California spotted owls. Wilson Bulletin. 109: 42–51. 

LaHaye, W.S.; Gutiérrez, R.J.; Dunk, J.R. 2001. Natal dispersal of the spotted 

owl in southern California: dispersal profile of an insular population. Condor. 

103: 691–700. 
261 

The California Spotted Owl: Current State of Knowledge

 

LaHaye, W.S.; Zimmerman, G.S.; Gutiérrez, R.J. 2004. Temporal variation 

in the vital rates of an insular population of spotted owls (Strix occidentalis 

occidentalis): contrasting effects of weather. Auk. 121: 1056–1069. 

May, C.A.; Peery, M.Z.; Gutiérrez R.J.; Seamans, M.E.; Olson, D.R. 1996. 

Feasibility of a random quadrat study design to estimate changes in density 

of Mexican spotted owls. Res. Pap. RMRS-RP-322. Fort Collins, CO: U.S. 

Department of Agriculture, Forest Service, Rocky Mountain Research 

Station. 12 p. 

Minnich, R.A. 1980. Wildfire and geographic relationships between canyon live 

oak, Coulter pine, and big-cone Douglas-fir forests. In: Plumb, T.R., tech. coord. 

Ecology, management, and utilization of California oaks. Gen. Tech. Rep. PSWGTR-44. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific 

Southwest Research Station: 55–61. 

Noon, B.R.; McKelvey, K.S. 1992. Stability properties of the spotted owl 

metapopulation in southern California. In: Verner, J.; McKelvey, K.S.; Noon, 

B.R.; Gutiérrez, R.J.; Gould, G.I., Jr.; Beck, T.W., eds. The California spotted 

owl: a technical assessment of its current status. Gen. Tech. Rep. PSW-GTR-133. 

Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest 

Research Station: 187–206. 

Noon, B.R.; McKelvey, K.S. 1996. A common framework for conservation 

planning: linking individual and metapopulation models. In: McCullough, D., ed. 

Metapopulations and wildlife conservation. Covelo, CA: Island Press: 139–165. 

Peery, M.Z.; Gutiérrez, R.J. 2013. Life-history tradeoffs in spotted owls (Strix 

occidentalis): implications for assessment of territory quality. Auk. 130: 132–140. 

Peery, M.Z.; Gutiérrez, R.J.; Kirby, R.; LeDee, O.E.; LaHaye, W. 2012. 

Climate change and spotted owls: potentially contrasting responses in the 

Southwestern United States. Global Change Biology. 18: 865–880. 

Rombout, P.J.A.; Donans, J.A.M.A.; vanBree, L.; Marra, M. 1991. Structural 

and biochemical effects in lungs of Japanese quail following a 1-week exposure 

to ozone. Environmental Research. 54: 39–51. 

Smith, R.B.; LaHaye, W.S.; Gutiérrez, R.J.; Zimmerman, G.S. 2002. Spatial 

habitat characteristics of an insular spotted owl (Strix occidentalis) population 

in southern California. In: Newton, I.; Kavanagh, R.; Olson, J.; Taylor, I., eds. 

Ecology and conservation of owls. Collingwood, Victoria, Australia: CSIRO 

Publishing: 137–147. 
262 

GENERAL TECHNICAL REPORT PSW-GTR-254

Smith, R.B.; Peery, M.Z.; Gutiérrez, R.J.; LaHaye, W.S. 1999. The relationship 

between spotted owl diet and reproductive success in the San Bernardino 

Mountains, California. Wilson Bulletin. 111: 22–29. 

Tempel, D.J.; Gutiérrez, R.J. 2013. Relation between occupancy and abundance 

for a territorial species, the California spotted owl. Conservation Biology. 27: 

1087–1095. 

Verner, J.; Gutiérrez, R.J.; Gould, G.I., Jr. 1992a. The California spotted owl: 

general biology and ecological relations. In: Verner, J.; McKelvey, K.S.; Noon, 

B.R.; Gutiérrez, R.J.; Gould, G.I., Jr.,; Beck, T.W., eds. The California spotted 

owl: a technical assessment of its current status. Gen. Tech. Rep. PSW-GTR-133. 

Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest 

Research Station: 55–77. 

Verner, J.; McKelvey, K.S.; Noon, B.R.; Gutiérrez, R.J.; Gould, G.I., Jr.; Beck, 

T.W. 1992b. Assessment of the current status of the California spotted owl, 

with recommendations for management. In: Verner, J.; McKelvey, K.S.; Noon, 

B.R.; Gutiérrez, R.J.; Gould, G.I., Jr.; Beck, T.W., eds. The California spotted 

owl: a technical assessment of its current status. Gen. Tech. Rep. PSW-GTR-133. 

Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest 

Research Station: 3–26. 

Verner, J.; McKelvey, K.S.; Noon, B.R.; Gutiérrez, R.J.; Gould, G.I., Jr.; 

Beck, T.W. 1992c. The California spotted owl: a technical assessment of its 

current status. Gen. Tech. Rep. PSW-GTR-133. Albany, CA: U.S. Department 

of Agriculture, Forest Service, Pacific Southwest Research Station. 285 p.

Zimmerman, G.S.; Gutiérrez, R.J.; LaHaye, W.S. 2007. Finite study areas and 

vital rates: sampling effects on estimates of spotted owl survival and population 

trends. Journal of Applied Ecology. 44: 963–971. 

Zimmerman, G.S.; LaHaye, W.S.; Gutiérrez, R.J. 2001. Breeding-season home 

ranges of spotted owls in the San Bernardino Mountains, California. Western 

Birds. 32: 83–87. 

Zimmerman, G.S.; LaHaye, W.S.; Gutiérrez, R.J. 2003. Empirical support for a 

despotic distribution in a California spotted owl population. Behavioral Ecology. 

14: 433–437. 
263 

The California Spotted Owl: Current State of Knowledge

Chapter 9: Synthesis and Interpretation of 

California Spotted Owl Research Within the 

Context of Public Forest Management 

M. Zachariah Peery, R.J. Gutiérrez, Patricia N. Manley, Peter A. Stine, 

and Malcolm P. North1 

Introduction 

In this chapter, we synthesize the information presented in the preceding chapters 

of this assessment of the California spotted owl (Strix occidentalis occidentalis) and 

provide a scientific appraisal of its implications for forest management and owl conservation. We focus on the key scientific findings since the 1992 California spotted 

owl technical assessment (CASPO) (Verner et. al. 1992) and discuss priorities for 

future research that could enhance the successful conservation of California spotted 

owls and their habitat. Throughout this chapter, we acknowledge when uncertainty 

limits well-founded conclusions and articulate differences in interpretation of the 

scientific literature, where such differences exist. The development of a spotted 

owl conservation strategy will require additional, careful analysis and deliberation 

to arrive at specific and scientifically defensible management guidelines (sensu 

CASPO; Verner et al. 1992). 

Implications of Recent Research for Spotted Owl 

Conservation 

The greatest challenge for managers charged with maintaining a viable population of spotted owls on National Forest System (NFS) lands in the Sierra Nevada 

may be to embed effective, long-term owl conservation practices within an overall 

management strategy aimed at restoring resilient forest structure, composition, 

and function. We discuss how and when spotted owl conservation and forest 

ecosystem restoration are compatible based on our current understanding of the 

1 M. Zachariah Peery is an associate professor, Department of Forest and Wildlife Ecology, University of Wisconsin, 1630 Linden Dr., Madison, WI 53706; R.J. Gutiérrez is a 

professor and Gordon Gullion Endowed Chair Emeritus, University of Minnesota, 2003 

Upper Buford Circle, St. Paul, MN 55108; Patricia N. Manley is a research program manager for the Conservation of Biodiversity Program, U.S. Department of Agriculture, Forest 

Service, Pacific Southwest Research Station, 2480 Carson Rd., Placerville, CA 95667; Peter 

A. Stine is a biogeographer and retired director of Partnerships and Collaboration, U.S. 

Department of Agriculture, Forest Service, Pacific Southwest Research Station, John Muir 

Institute for the Environment, 1 Shields Ave., University of California–Davis, CA 95616; 

Malcolm P. North is a research forest ecologist, U.S. Department of Agriculture, Forest 

Service, Pacific Southwest Research Station, 1731 Research Park Dr., Davis, CA 95618. 
264 

GENERAL TECHNICAL REPORT PSW-GTR-254

scientific literature. Conversely, we identify circumstances in which reducing fuels 

and restoring desired forest structure and composition may pose significant risks 

to spotted owls so that managers and policymakers can make informed decisions 

about relevant tradeoffs. 

A number of species of conservation concern exists within the Sierra Nevada 

in addition to California spotted owls, including American marten (Martes americana), northern goshawk (Accipiter gentilis), black-backed woodpecker (Picoides 

arcticus), great gray owl (S. nebulosa), and particularly Pacific fisher (Pekania 

pennanti). The cumulative effects of meeting the current habitat conservation needs 

of each of these species may increase the challenge of achieving ecosystem management and ecological restoration objectives. However, a comprehensive old-forest 

management strategy that promotes large trees and canopy complexity within a 

landscape-scale mosaic of forest conditions could benefit many species of conservation concern, not just the California spotted owl. 

Conservation of Spotted Owls in the Context of Ecosystem 

Restoration 

Meeting the dual objectives of conserving spotted owls and promoting resilience of 

Sierra Nevada forests will require restoring some semblance of historical wildfire 

regimes without endangering already declining spotted owl populations. Conserving spotted owl populations and restoring ecosystem resilience are complementary 

objectives when management activities reduce the loss of old forest and owl habitat 

to drought and large high-severity fires. To do so will require reducing small-tree 

densities and promoting “natural” fire regimes in Sierra Nevada forests while maintaining a sufficient amount and distribution of suitable habitat to support viable owl 

populations (a key uncertainty is the amount and distribution of habitat that is sufficient). Thus, a reasonable guiding philosophy is to manage Sierra Nevada forests 

in ways that combine the objectives of spotted owl conservation, fuels management, 

and drought resilience while also recognizing that forests are dynamic ecosystems 

that will support a range of vegetation types and structures that vary over space 

and time. In practice, however, implementing effective fire management and 

ecosystem restoration programs that do not also pose risks to spotted owls will be 

challenging. In some cases, conserving habitat elements known to be important 

to spotted owls may lead to dense stands with high fuel loadings that are at risk 

from high-severity fire and other stressors such as drought, insects, pathogens, and 

air pollution (chapters 5 and 7). Conversely, fuel reduction and forest restoration 

strategies that reduce canopy cover, the complexity of forest structure, or large-tree 
265 

The California Spotted Owl: Current State of Knowledge

density can potentially affect spotted owl populations negatively in both the short 

and long term (chapters 2, 3, 4, and 7). Determining the appropriate pace, scale, and 

intensity of treatments as well as the type of treatment is complicated by scientific 

uncertainty of the potential impacts of a suite of threats including some types of 

mechanical treatments on spotted owls (chapters 3 and 7; see also the “Research 

Implications” section below). 

Two different paradigms emerged as part of this assessment regarding tradeoffs between the potential short-term negative impacts and possible long-term 

benefits of fuel and restoration treatments on spotted owls. One paradigm holds 

that treatments within spotted owl habitat pose risks to spotted owls because owls 

have declined significantly on some NFS lands in the Sierra Nevada and southern 

California over the past two decades (chapters 4 and 8). Although the cause of 

these recent declines is uncertain, the large reduction in abundance observed on the 

Eldorado National Forest study area cannot be attributed to barred owl (Strix varia) 

or fire, as estimated declines occurred before the King Fire, and very few barred 

owls have been detected on the Eldorado. Certainly non-habitat-related factors (e.g., 

climate) could have contributed to recent declines (Jones et al. 2016a), but there is 

concern that habitat features known to be important to spotted owls (e.g., forests 

with vertical structure and complex canopies) have declined during demographic 

studies as a result of forest management activities on both public and private lands, 

recognizing that these potential effects have been difficult to detect as part of 

demographic studies (chapters 4 and 7). There is also concern that the removal of 

large trees as hazards (e.g., road- and trail-side tree removal) and salvage logging 

affect owl habitat suitability and could be affecting spotted owl populations in the 

Sierra Nevada. Finally, and perhaps most importantly, pre-CASPO changes to owl 

habitat from historical even-age timber harvesting and the selective removal of 

large and “defect” trees may be contributing to recent population declines (via longterm “legacy” effects) as well as longer term (unmeasured) declines. 

The conclusion from this interpretation of the published literature is that current spotted owl populations may be small relative to historical levels and limited 

by the spatial extent of old forest and forests containing legacy elements in the 

Sierra Nevada (chapter 4). There is recognition that high-severity fire and other 

ecosystem stressors pose threats to California spotted owls (Jones et al. 2016b), but 

there is also concern that the expansion of treatments that simplify forest structure 

and decrease forest tree canopy cover in owl habitat could exacerbate population 

declines and increase the probability of extirpation of owls from the region. Moreover, whether fuel treatments will protect spotted owl habitat from high-severity 
266 

GENERAL TECHNICAL REPORT PSW-GTR-254

fire sufficiently to compensate for potential short-term impacts to populations 

is unknown. This paradigm suggests that conserving and promoting a sufficient 

amount of forest dominated by large trees, complex forest structure, and closed 

canopies at sites known to be used by spotted owls—particularly in owl nest stands, 

activity centers, and territories—is likely to enhance owl habitat and populations. 

Nevertheless, fuels and restoration treatments are considered to be an important 

component of an overall strategy intended to restore resilience to Sierra Nevada forests at larger spatial scales (chapter 5). Thus, under this paradigm, treatments would 

occur primarily in areas of the landscape dominated by younger forests with high 

small-tree density and be designed to enhance foraging habitat and foster growth 

rates of larger, retained trees to enhance resilience to fire when possible. Finally, 

it is a well-established principle of wildlife management (“Declining Population 

Paradigm”) that halting and reversing substantial recent population declines of a 

species of concern, like the spotted owl, is an essential component of a conservation 

program (Caughley 1994). Doing so will be challenging, likely requiring restoration 

of habitat conditions as well as the implementation of studies carefully designed to 

identify the cause of recent population declines more precisely (and thus facilitate 

effective and specific management actions; see below). 

The alternative paradigm that emerged from the assessment holds that increases 

in the spatial extent of high-severity fire and other disturbances to forests (e.g., 

prolonged drought, insects, and disease), resulting from over a century of fire 

suppression and now climate change, pose the primary proximate threat to spotted 

owl population persistence, owl habitat, and forest ecosystems in the Sierra Nevada. 

Current fuels and other restoration treatments are intended to retain and promote 

large-tree development, but their pace and scale is small (on national forests in 

the Sierra Nevada <12 141 ha/year (<30,000 ac/year) of mechanical thinning and 

<3642 ha/year (<9,000 ac/year) of prescribed fire occur, compared to 196 677 ha/ 

year (486,000 ac/year) that historically burned [North et al. 2012]) such that any 

detrimental effects to owls are likely to be local and short lived. In contrast, recent 

and projected wildfire and drought-related mortality, exacerbated by the fire deficit, 

affect a significantly greater area, and often kill the largest trees while also drastically reducing canopy cover. Actions consistent with this paradigm could include, 

for example, fuel- and density-reduction treatments strategically placed with less 

consideration of owl habitat impacts and an emphasis placed on reducing the spatial 

extent of high-severity fire effects and drought-induced mortality. 

This paradigm suggests that working with dynamic forest processes would be 

more effective for reducing risks to owls and the forests upon which they depend 
267 

The California Spotted Owl: Current State of Knowledge

than a strategy focused on conserving and enhancing existing owl habitat. 

Underlying this interpretation of risk is uncertainty about historical (i.e., pre-EuroAmerican) spotted owl populations when forest conditions were significantly 

different than they are today. Recent analyses of historical datasets and efforts to 

reconstruct stand structure prior to the fire-suppression era (chapter 5) suggest that 

average canopy cover was lower in landscapes with an active fire regime. Modern 

forests, by contrast, have far fewer large trees, and defect structures have declined 

while high canopy cover conditions have become more widespread. It is unclear how 

these changes have influenced owl population declines that have been documented 

in three of the four Sierra Nevada demographic studies. Furthermore, it is uncertain 

whether and to what degree observed population declines may be a result of either 

recent management practices that created a more homogeneous forest, a legacy of 

more extensive and intensive management practices in previous decades (e.g., the 

logging of large trees and snag removal), or a change in a population that is currently 

higher than was supported under historical forest conditions. In light of these uncertainties, it is possible that restoring forest conditions where high canopy cover conditions are aligned with productive sites will support an owl population equilibrated to 

forest conditions and resilient to dynamic processes even as the climate changes. 

The underlying uncertainties associated with past, present, and future conditions will present challenges to spotted owl conservation and forest management. 

However, there was consensus about new findings presented in this assessment that 

are relevant to both owl conservation and forest restoration in the Sierra Nevada. 

These include: 

• Spotted owls have declined in abundance on some NFS lands in the Sierra 

Nevada over the past two decades (chapter 4). 

• The density of large and defect trees has declined in Sierra Nevada forest as 

a result of historical (pre-CASPO) timber harvesting (chapter 5), and these 

habitat elements may well be contributing to recent spotted owl population 

declines. 

• A century of fire exclusion has led to an increase in the size of highseverity fires owing to the accumulation of surface and ladder fuels 

(chapter 5). Habitat loss resulting from large high-severity fires and other 

stressors poses an increasing risk both to spotted owls and forest ecosystems, and these risks will likely increase with climate change over coming 

decades (Jones et al. 2016b). 

• Restoring low- to moderate-severity fire regimes to the mixed-conifer zone 

could help achieve both spotted owl conservation and forest restoration 

goals. 
268 

GENERAL TECHNICAL REPORT PSW-GTR-254

• Habitat conditions in some spotted owl territories may not be viable in the 

long term because they are located in areas that have high burn probabilities or low drought tolerance. Conservation and restoration of owl habitat 

(large trees, moderate stem density, and canopy cover) in areas that could 

support these conditions (i.e., higher actual evapotranspiration and lower 

climatic water deficit rates) may align the distribution of owl habitat with 

forest restoration goals. 

These key findings and points of consensus suggest that the viability of spotted owls in the Sierra Nevada depends on carefully balancing fuel and restoration 

treatments with the maintenance and enhancement of existing owl habitat. However, as discussed above, developing a specific set of management guidelines as 

part of owl conservation efforts that will simultaneously achieve these objectives 

will be complicated by scientific uncertainty about the potential risks and rewards 

posed by fuel and restoration treatments. Thus, conservation planning efforts would 

benefit from a quantitative risk assessment, which would require close coordination 

among wildlife ecologists, forest and fire ecologists, and remote-sensing scientists, 

as well as the development of an integrated model that links fire behavior, forest 

conditions, and spotted owl habitat/demography at the appropriate spatial and 

temporal scales. Moreover, the success of future conservation planning efforts also 

would benefit from the development of a robust feedback loop that generates and 

incorporates new information and learning through implementation, for example 

by monitoring the impacts of management actions on both forest structure and owl 

population response. Possible approaches for evaluating the short- and long-term 

impacts of fire and restoration treatments, as well as possible components of an 

adaptive management strategy, are described in more detail below. 

Desired Conditions for Areas of Ecological Importance 

Research summarized in chapters 2 through 8 indicates that different ecological 

(e.g., habitat) features are important to spotted owls at each of several spatial scales, 

and that considering these scale-specific requirements will facilitate the development of forest conditions that minimize risk to owls and promote resilient forest 

ecosystems. The scales of greatest importance are the owl’s activity center, territory, and home range, embedded within the broader forested landscape. In general, 

owl territory occupancy and demographic rates are likely to fare better with a 

gradient of less intensive to more intensive forest management activities within owl 

habitat as a function of distance from activity centers. Maintaining key habitat elements within activity centers and territories will likely promote population growth 
269 

The California Spotted Owl: Current State of Knowledge

in the short term, whereas reducing risk from high-severity fire and other threats 

within owl home ranges and the broader landscape could promote population viability in the long term. Given that owls are central-place foragers and exhibit strong 

nesting site fidelity (see chapter 2), and new nesting habitat (primarily old forest) 

develops over long time scales, maintaining existing nesting habitat (particularly at 

sites that have a history of use) is likely to promote viable populations while forest 

and restoration treatments designed to reduce risks from high-severity fire and 

other environmental stressors are implemented at larger spatial scales. The retention of large trees (particularly old trees with structural defects) and the accelerated 

development of additional large trees is likely to be beneficial at all scales, given 

that such trees have declined on the landscape and are important for both forest 

resilience and spotted owls. Desired conditions for each scale of ecological importance, as well as the implications of recent research for achieving these conditions 

via forest management, are described as follows: 

• Activity-center scale: Maintaining high-quality nesting and roosting habitat (i.e., old forest) at known spotted owl activity centers (defined as the 

areas of long-term nesting and roosting use within an owl territory) will 

likely enhance occupancy and demographic performance. Forest structural 

characteristics known to be important at this scale are more likely to be 

maintained or even enhanced through low-intensity vegetation treatments 

when forest management is implemented with the intent of reducing the 

risk of high-severity fire and drought-induced large tree mortality. 

• Territory scale (outside of activity centers): Within territories, spotted 

owl occupancy and fitness appear to be positively related to the acreage of 

high-quality habitat (i.e., forests dominated by large trees and particularly 

high canopy cover), and a landscape populated by territories containing 

a sufficient amount of these habitat conditions will likely promote viable 

spotted owl populations. However, given climate change predictions and the 

likely increase in large high-severity fires and drought-induced tree mortality, reducing these risks to forests within territories will likely benefit spotted owl populations. 

• Home range scale (outside of territories): Spotted owl home ranges 

characterized by heterogeneous forests containing a mosaic of vegetation 

conditions, including patches of old forest and a mix of stand ages will 

likely confer sufficient high-quality nesting, roosting, and foraging habitat. At this scale, there is an opportunity to place greater emphasis on fuels 

management and forest restoration, particularly approaches that enhance 
270 

GENERAL TECHNICAL REPORT PSW-GTR-254

forest resilience, landscape heterogeneity, and spotted owl foraging habitat. 

Maintaining and increasing the prevalence of large trees could be particularly effective for restoring forest resilience and improving owl foraging 

habitat at this scale. 

• Landscape scale (matrix between home ranges): A landscape of heterogeneous forests containing a mosaic of vegetation conditions including 

patches of old forest will likely provide opportunities for recruitment of 

new spotted owl territories in the context of dynamic forest conditions and 

confer broad-scale ecosystem resilience. Thus, fuels and restoration treatments—in conjunction with prescribed and managed fire—that promote 

landscape heterogeneity in forest conditions and reduce risks for highseverity fire and other stressors will likely benefit spotted owls and forest 

resilience in the longer term. 

The Science Behind Scale-Specific Desired Conditions: 

Implications for Forest Management 

Here, we review the scientific basis for the above-described desired conditions and 

discuss how the knowledge accumulated about California spotted owls and forest 

ecosystems in the Sierra Nevada since CASPO could inform forest management at 

each of these scales in more detail. 

Activity-center scale— 

California spotted owl activity centers are typically characterized by old-forest 

conditions (i.e., large trees, complex structure; chapter 3) and maintaining such 

conditions within activity centers is likely important for promoting owl reproduction and population viability. Protected activity centers (PACs) were designed as 

part of the CASPO strategy to protect 120 ha (300 ac) of the “best available” nesting 

and roosting habitat (i.e., activity centers) within known spotted owl territories and 

appear to have been a useful management construct based on research demonstrating long-term use of these areas by owls (Berigan et al. 2012; chapter 2). Moreover, 

Blakesley et al. (2005) demonstrated that reproductive success in California spotted 

owls was correlated with vegetation types characterized by high-canopy and medium/large trees at the approximate scale of a PAC (see chapter 4 for more details). 

These observations, coupled with observed significant declines in owl populations 

on NFS lands (chapter 4), suggest that fuels and restoration treatments that substantially affect these habitat attributes within spotted owl activity centers could affect 

spotted owl populations adversely. Limiting treatments to approaches designed to 
271 

The California Spotted Owl: Current State of Knowledge

avoid impacts to existing spotted owl habitat such as prescribed burning and hand 

removal of small trees are more likely to provide a balance between habitat conservation and fuels management than prescriptions that appreciably affect forest overstory structure at this scale. 

Territory scale— 

The quality of nesting, roosting, and foraging habitat at the territory scale is inevitably a complex function of many variables such as the density of large trees (including snags), vertical forest structure, canopy complexity, and large woody debris, as 

well as forest heterogeneity (chapter 3). Consideration of the range of forest conditions the literature suggests is important to owls at this scale will facilitate management of spotted owl habitat in a manner that promotes viable populations (chapter 

5). Many of these characteristics, however, have proven difficult to map with sufficient accuracy and resolution at the territory scale, particularly with a large enough 

sample size of territories to understand how they affect owl occupancy and fitness, 

and thus population viability (chapter 6). In contrast, canopy cover has been relatively tractable to map at this scale and has proven to be a reasonably strong predictor of spotted owl territory occupancy and fitness (chapter 4). Recent research on 

the Eldorado study area indicates that occupancy and territory fitness (i.e., λ or annual population growth rates at the territory scale) (Franklin et al. 2000) are higher 

in territories (400-ha [988-ac] circles centered on activity centers) that contain relatively large extents of closed-canopy forest (≥70 percent cover) (Tempel et al. 2014), 

and that owls occupying high-elevation territories appear to be more sensitive to 

reductions in this forest type (Jones et al. 2016a). To date, these studies provide the 

most rigorous quantitative assessment of the association (particularly the directionality and the shape of the relationship) between the acreage of high-canopy forest 

and spotted owl demographics and provide the best available information for management of owl habitat at the territory scale. Although from a management perspective it would be valuable to understand what acreage of high-canopy cover forest 

is expected to lead to stable population growth at the territory scale, the absolute 

estimate of territory fitness (see fig. 4 in Tempel et al. 2014) is biased low owing to 

territory switching by owls and because recruitment into territories was not considered in the analysis by Tempel et al. (2014). As a result, decisions about how much 

high-canopy cover forest to retain within territories, based on this analysis, will be, 

to a certain extent, subjective (but see below). Tempel et al. (2014) suggested that 

100 to 150 ha (247 to 370 ac) of high-canopy cover forest might provide a reasonable balance between perceived tradeoffs associated with maintaining owl habitat 
272 

GENERAL TECHNICAL REPORT PSW-GTR-254

quality and promoting forest resilience. However, the mean area of high-canopycover forest in the declining Eldorado “population” was about 137 ha (338 ac), and 

thus a population of territories each containing only 100 ha (247 ac) of high-canopyforest is expected to decline even more rapidly given the association between highcanopy-cover forest and spotted owl population growth rates. By extension, a landscape populated by territories containing only 50 ha (123 ac) of forests >70 percent 

canopy cover—the mathematical inflection point in the nonlinear relationship between territory fitness and the area of high-canopy-cover forest—will likely lead to 

even more rapid population declines, at least in the short term. Importantly, ongoing research based on all four demographic study areas will provide more precise 

estimates of how much high-canopy-cover forest (as well other canopy cover types) 

is required to yield stable population growth rates at the territory scale. 

Although consistent empirical evidence supports the short-term conservation value of retaining high-canopy-cover forest (i.e., most owls are found in these 

forests), the amount of canopy cover necessary to allow owl persistence remains 

uncertain. The topography and past, active fire regimes of the Sierra Nevada created a diverse array of patches at different scales (i.e., patchy tree patterns within 

patches; successional patchworks within local landscapes; and life form, cover type, 

and structural patchworks within ecoregions). Consequently, many existing stands 

having high canopy cover are likely the result of past logging and fire suppression 

rather than locations that would normally support high canopy cover with a historical fire regime (chapter 5). For mixed-conifer forests, historical estimates of canopy 

cover generally ranged from 15 to 30 percent for forests with active fire regimes 

(Collins et al. 2011, 2015; Lydersen et al. 2013; Stephens et al. 2015), 19 to 49 percent for modern old growth with restored fire regimes (Lydersen and North 2012), 

and 60 to 65 percent for modern forests having fire-suppressed regimes (Lydersen 

et al. 2013), with lower values for pine-dominated stands (Stephens and Fry 2005, 

Taylor 2010). Particular locations, such as a group of tall trees or a grove of large 

trees supporting a dense cluster of codominant trees within the grove matrix, may 

have high canopy closure, but a stand-level average canopy cover of 70 percent 

appears to have been rare in historical forests (Collins et al. 2015, Stephens et al. 

2015). Thus, although 70-percent canopy cover may be a desired condition for owls, 

in some locations it will be difficult to achieve and harder to maintain because 

such stand conditions usually have high surface and ladder fuel loading, high stem 

density, and associated water stress that increases large tree mortality. 

Our assessment of the scientific literature indicates that reconciling perceived 

differences in desired conditions for spotted owls and Sierra Nevada forests at the 
273 

The California Spotted Owl: Current State of Knowledge

territory scale—particularly with respect to canopy cover—is one of the more significant challenges facing management intended to maintain viable owl populations 

and promote resilient forests. However, at least three general principles for simultaneously achieving both objectives are emerging from ongoing scientific research. 

First, aligning high-canopy-cover stands with locations capable of supporting these 

forest types (e.g., north-facing slopes and drainage bottoms) will likely increase 

the resilience of both forest ecosystems and owl habitat to predicted increases in 

fire and drought severity under climate change. While desired conditions for the 

distribution of high-canopy-cover areas within spotted territories are uncertain, 

ongoing research using light detection and ranging (LiDAR) and studies of actual 

evapotranspiration and climatic water deficit are investigating these issues from 

a forest-restoration perspective. Second, if other attributes of spotted owl habitat, 

such as the prevalence of large defect trees, can be enhanced via forest management, it may be possible to maintain viable owl populations with less high-canopy 

forest present within territories than has been estimated in recent studies (Tempel et 

al. 2014). Third, emerging research suggests that once a certain amount of highcanopy forest is present within territories (presumably to meet nesting and roosting 

requirements), gains in medium (40 to 70 percent) canopy forest provide similar 

benefits to expected territory occupancy rates as gains in high canopy cover forests 

(Tempel et al., in press). While additional research is needed to develop desired 

conditions likely to confer both resilient forests and high-quality spotted owl habitat 

at the territory scale, these findings collectively suggest that achieving seemingly 

incompatible objectives may be possible. 

When developing management guidelines based on studies of spotted owl 

demography and canopy cover conditions at the territory (and other) ecological 

scales, it is important to recognize that canopy cover is often estimated using different techniques in research and management applications. For example, canopy 

cover was estimated using aerial photointerpretation in Tempel et al. (2014) and 

Landsat imagery in Tempel et al. (2016), both mapping products being informed 

by ground-based vegetation measurement taken with spherical densitometers. By 

contrast, many forest management applications use Forest Vegetation Simulatorderived canopy cover values (Dixon 2002), which generally yield lower estimates 

(particularly when canopy cover is high) and do not necessarily indicate that desired 

conditions are not being met (Fiala et al. 2006). Determining whether desired 

conditions are being met requires developing appropriate calibration equations or 

“cross-walking” exercises. 
274 

GENERAL TECHNICAL REPORT PSW-GTR-254

Home range scale— 

Heterogeneous forests containing a mosaic of vegetation conditions within spotted 

owl home ranges (but outside of the territory scale) could both enhance the resilience of Sierra Nevada forests and benefit owl populations (chapters 3 and 5). In 

particular, forest restoration practices that promote the maintenance/recruitment 

of old-forest conditions within a heterogeneous matrix of other forest and vegetation types would likely increase resilience to changing climate and other stressors. Moreover, the promotion of wildfire regimes characterized by a range of burn 

severities and that produce a mosaic of vegetation conditions, including unburned 

refugia, can potentially enhance spotted owl foraging habitat and prey resources 

(chapter 3). Mast-producing trees, particularly oaks, within mixed-conifer forests 

may also benefit owl prey species, particularly woodrats (Neotoma spp.) (chapter 3). 

However, diet studies of California spotted owls suggest that management intended 

to benefit prey populations would ideally be tailored to specific elevations (chapter 

2). Shrubs and early seral stage forest may benefit the primary prey species at lower 

elevations (woodrats), whereas at higher elevations, older forests will more likely 

benefit the primary prey species (flying squirrels). Promoting woodrat habitat at the 

expense of suitable spotted owl nesting habitat (e.g., converting older forest to brush 

or early seral stands) could adversely affect owls. Moreover, as is current practice, 

we suggest that the scale at which spotted owl home range areas are managed vary 

along a latitudinal gradient because home ranges are smaller in the southern Sierra 

Nevada than in the northern Sierra Nevada. 

Identifying spotted owl home ranges (as well as sites within individual home 

ranges) that would most benefit from fuel and restoration treatments would help 

balance the potential short-term impacts and long-term benefits of such treatments. 

Specifically, treatments in home ranges containing relatively poor spotted owl 

habitat and dominated by dense stands of shade-tolerant trees species on south 

slopes or ridgetops (i.e., fire prone) could be particularly beneficial (chapters 3 and 

5). Conversely, treatments in home ranges containing habitat conditions known 

to promote reproduction, survival, and territory occupancy, and where forests did 

not depart appreciably from the “natural range” of variability are likely to be more 

detrimental to owl populations. An important issue to resolve when developing an 

owl habitat management strategy is that California spotted owl fitness appears to 

benefit from prevalent closed-canopy forests, conditions that can also increase the 

risk of high-severity fire and susceptibility to climate change and related stressors. 

A key issue is that home ranges that contribute disproportionately to population 

growth and stability may not be readily identified. It might be possible to mitigate 
275 

The California Spotted Owl: Current State of Knowledge

this uncertainty, to some extent, by emphasizing the retention and promotion of 

other habitat features known to be important to spotted owls in all home ranges, 

such as large and old trees (especially ones with defects that are used as nest sites), 

large oaks, large coarse woody debris, and other features in treated areas. Emerging remote sensing technologies will likely improve the ability to characterize 

spotted owls habitat features at home range (and landscape) scales and thus provide 

opportunities to develop habitat targets based on more than simply canopy cover 

(chapter 6 and see below). 

Landscape scale— 

Similar to the home-rangescale, landscape-scale heterogeneity in forest conditions, 

shaped by a range of burn severities, could increase both the resilience of Sierra 

Nevada forests and spotted owl population viability. Thus, restoration activities 

intended to promote historical fire regimes and forest heterogeneity at broad spatial 

scales would be consistent with owl conservation objectives, particularly when 

implemented in areas where fire risk is high and the density of owl sites is low. 

Wildland fire (managed wildfire and prescribed fire) is an option, in conjunction 

with mechanical treatments to reduce fuel loads, for reducing risk of high-severity 

fire in the broader landscape. In addition to restoring historical fire regimes, landscape-scale fuel and restoration treatments could be designed to reduce risks to owl 

territories, particularly in highly productive ones. Nevertheless, some “protected” 

habitat will inevitably be lost to high-severity fire and the recruitment of nesting 

habitat outside of home ranges (e.g., via the protection and enhancement of large 

trees) through strategic management approaches will likely be needed to maintain a 

well-distributed spotted owl population. Such an approach explicitly recognizes and 

embraces the inherent spatial and temporal variability of forest conditions that is 

characteristic of a dynamic forest landscape. A habitat monitoring program, similar 

to the one developed for Pacific fisher, and discussed in more detail below, could be 

an effective means to account for the losses and gains in suitable habitat and to increase confidence that sufficient habitat is being maintained or restored at landscape 

scales to promote viable owl populations. 

Landscape-scale conservation of spotted owl habitat is complicated by the fact 

that climate change will continue to alter the structure and composition of forests, 

as well as other environmental factors (e.g., microclimates and prey communities) 

that may affect California spotted owls (chapters 5 and 7). Projections of future 

climate suggest that pine-oak, mixed-conifer, and red fir forests, which comprise 

the majority of spotted owl habitat, are expected to shift upward in elevation but 
276 

GENERAL TECHNICAL REPORT PSW-GTR-254

remain in locations where soils and topographic features maintain relatively suitable 

growing conditions and microclimates (chapter 5). Thus, the conservation, restoration, and promotion of old-forest conditions at appropriate sites on north-facing 

slopes and in steep drainages within the current range are likely to benefit spotted 

owls in both the short and long term. In addition, spotted owl habitat may develop 

at sites where it is currently not present (e.g., by promoting old-forest characteristics 

at higher elevations in montane conifer and red fir forests), and these sites could be 

particularly important in the future if owl populations track suitable climate conditions and that of the forests they tend to occupy. The establishment and growth 

of large-diameter trees at sites where they presently do not occur will take many 

decades and perhaps centuries depending on current forest conditions. Long-term 

planning would be required to identify such sites and implement management 

activities that will promote habitat features required by owls in a timely manner. 

Postfire Management

Recent research indicates that California spotted owls persist in territories that 

experience low- to moderate- severity and mixed-severity (i.e., low to moderate 

fire regimes with inclusions of high-severity fire patches) wildfire and that small 

patches of high-severity fire may enhance foraging conditions for spotted owls 

(chapters 3 and 7). Salvage harvesting within such landscapes, particularly highintensity salvage (removal of most snags), could negatively affect spotted owl habitat via the removal of snags and ultimately the reduction of coarse woody debris on 

the forest floor (chapter 5 and 7). However, site occupancy after fire is more likely 

to be affected when large areas of forest are burned at high severity (chapters 3 and 

7). While salvage harvesting may have few short-term (<10 year) ecological benefits 

in such landscapes, over the long term (>30 years), judicious salvage practices (e.g., 

leaving some snags) followed by reforestation based on goals for desired restoration 

of historical conditions may help promote a return to mature conifer forest more 

quickly than an intensively burned area that is not harvested or replanted. Replanting trees in intensively burned landscapes, particularly large burns far from seed 

sources, will likely increase the rate at which spotted owl habitat is regenerated 

(chapter 5). However, forests resulting from reforestation efforts will be a function 

of planting design (e.g., density of tree planting) and subsequent management; 

large areas planted using conventional plantation prescriptions (i.e., high densities 

of equally spaced seedlings and homogeneous thinning) (chapter 5) could lead to 

delayed or poor habitat conditions for spotted owls, high risk to wildfire, and a 

reduction in landscape-scale diversity in vegetation conditions. In sum, spatial scale 
277 

The California Spotted Owl: Current State of Knowledge

and planting practices are both important considerations when balancing tradeoffs 

among salvage harvesting, restoration, and spotted owl conservation. Salvage 

within low-moderate and mixed-severity burned areas can potentially affect spotted owls negatively, whereas the inability to replant large patches of high-severity 

burned forest could result in the long-term loss of owl habitat (chapter 5). 

Barred Owl Range Expansion, Monitoring, and Control 

Barred owls have recently expanded their range into the northern Sierra Nevada, 

and a small but increasing number of barred owls are being detected in the central 

and southern Sierra Nevada (chapter 7). Given the apparently profound impacts 

that they have had on northern spotted owls (S. o. caurina), control measures would 

likely be most effective while barred owls still occur at low densities in the Sierra 

Nevada (Dugger et al. 2016). As is the case generally for invasive species, the 

momentum of range expansion and abundance is expected to increase exponentially 

once barred owls have reached a critical, as yet unknown, density. The control of 

barred owls throughout forest areas in the Sierra Nevada will be difficult, if not 

impossible, when such a critical density is reached. If barred owls reach a critical 

density, as they have in the range of the northern spotted owl, we can expect a rapid 

increase in their numbers, interspecific competition, predation, and other impacts to 

spotted owls (Gutiérrez et al. 2007). 

The primary control strategy for barred owls being tested in the range of the 

northern spotted owl is lethal removal. Barred owls are easy and cost-effective to 

remove from known locations (Diller et al. 2014). In the southern Cascades and 

Sierra Nevada, known barred owls could likely be removed in one or two field 

seasons by small crews of technicians using current techniques (Diller et al. 2014). 

An effective regional strategy could include a comprehensive survey and removal 

protocol that targets some specified degree of coverage and detection probability, 

and then the removal of barred owls wherever they occur in the Sierra Nevada. The 

success of such a program could be gauged based on how effectively it maintained 

barred owl numbers near their current low levels and prevented the rapid and 

sustained increases that were observed within the range of the northern spotted owl. 

The development of barred owl monitoring and control measures could be an 

important topic for the conservation of the California spotted owl. Development and 

implementation of these measures would likely benefit from an integrated effort 

by several natural resource agencies, including the USFS, U.S. Fish and Wildlife 

Service, National Park Service, and California Department of Fish and Wildlife, as 

well as the cooperation of private landowners with property in the forest matrix. 
278 

GENERAL TECHNICAL REPORT PSW-GTR-254

Establishing Benchmarks for Conservation Success 

Conservation of California spotted owls in the Sierra Nevada will require maintaining a well-distributed population of owls of sufficient abundance that the population 

will be resilient to the effects of climate change and other environmental stressors. 

A set of “conservation benchmarks” would be valuable to indicate the status of 

California spotted owl populations; such benchmarks could be used to evaluate 

monitoring results and gauge whether management activities have accomplished 

their intended objective of conserving spotted owls, or whether additional conservation measures need to be implemented, within an adaptive management framework. 

Establishing population and habitat benchmarks could reduce conflicts in interpretation of monitoring results and improve management responsiveness, at least to 

the extent that stakeholders agree on a set of predefined conservation benchmarks. 

Potential demographic metrics of spotted owl population status upon which 

conservation benchmarks could be based include abundance, population trends, 

and geographic distribution. Demographic metrics of population status could be 

feasible and cost effective to monitor and are generally indicative of the viability of 

California spotted owls in the Sierra Nevada. Habitat-based metrics, such as those 

currently under development as part of the Pacific Fisher Conservation Strategy, 

also have merit (see above) particularly at regional scales that are outside of existing spotted owl demographic study areas. However, such metrics cannot replace 

population-based metrics because spotted owls may decline for many reasons 

(e.g., barred owls, disease, etc.). In contrast, changes in habitat metrics only reflect 

changes in distribution and abundance of habitat. 

Quantifiable benchmarks based on demographic metrics could include, for 

example: (1) the number (n) of spotted owls that represent different levels of probability of long-term persistence in the Sierra Nevada; (2) population trends that represent stable, increasing, or declining populations with given degrees of confidence; 

and (3) the number of owl territories that must be occupied in each geographic zone 

(perhaps as defined by clusters of ranger districts) to achieve different levels of 

probability of long-term persistence. Establishing scientifically defensible benchmarks for demographic metrics is challenging, and the minimum viable population 

size (MVP) concept, for example, has been criticized on the grounds that no single 

“magic” population size guarantees population persistence and that modeling 

frameworks for estimating MVPs are not exact (Flather et al. 2011). Nevertheless, 

as a practical matter, quantifiable conservation benchmarks are important to trigger 

management decisions—whether they indicate problems and trigger conservation 
279 

The California Spotted Owl: Current State of Knowledge

“triage” efforts, or they indicate positive conditions and validate that management 

activities have had either a positive or neutral effect. Exploring extinction risk 

across a range of possible benchmarks and identifying thresholds below which 

extinction risk is increasingly likely can circumvent some of the perils associated 

with estimates of “absolute” conservation benchmarks. The California spotted owl 

is rare among species of conservation concern because there is a significant body of 

demographic data available to inform the development of conservation benchmarks 

for a population viability analysis perhaps guided by expert opinion. In addition, 

genetic analyses described below could provide historical context for conservation 

benchmarks developed as part of such a modeling exercise. 

Progress on CASPO Uncertainties and Remaining 

Knowledge Gaps 

Inventory 

The CASPO technical assessment identified deficiencies in inventory (i.e., surveys 

to assess spotted owl occurrence and abundance) as one of the major uncertainties 

about California spotted owls (Verner and Taylor 1992). Specifically, they mentioned 

three parts of the owl’s range where basic information on owl locations and habitat 

use is needed: (1) the mountains of southern California; (2) the foothill woodlands 

of the western Sierra Nevada; and (3) the Coast Ranges, particularly the region 

from the Santa Cruz area northward to San Mateo County. Extensive surveys in 

southern California were conducted on the San Bernardino demographic study area 

(San Bernardino and San Jacinto Mountains) from 1987 through 2000 and as part 

of a regional U.S. Forest Service (USFS) monitoring program from 2003 through 

2012 (see chapter 8). The information gained from these surveys suggests that owl 

populations in southern California have declined in the past 20 years and that little, 

if any, connectivity exists between owl populations in the various mountain ranges 

in southern California. However, based on an inspection of the California spotted 

owl databases from the California Department of Fish and Wildlife (CDFW) and the 

Pacific Southwest Region of the USFS (see figs. 4-1 and 4-2), it appears that very 

few spotted owl surveys have been conducted in other mountain ranges of southern California, the foothills of the western Sierra Nevada, and the Coast Ranges 

(particularly north of Monterey Bay) since CASPO. Some surveys may have been 

conducted on private timberlands within these geographic areas, but not incorporated into the CDFW database. Nonetheless, considerable uncertainty remains about 

the owl’s distribution and winter ranges in these areas, and the significance of these 

local and regional owl populations to the species’ rangewide persistence. 
280 

GENERAL TECHNICAL REPORT PSW-GTR-254

Monitoring 

The CASPO recommendations included the formal adoption of demographic 

approaches for monitoring California spotted owls in the Sierra Nevada by continuing and expanding four demographic studies recently established at that time. 

The rationale for a demographic, as opposed to an occupancy-based, strategy 

stemmed from concern over potentially low statistical power to detect declines in 

territory occupancy using methods developed for northern spotted owls. Moreover, 

demographic studies provide more insight into the mechanisms behind changes in 

populations, and they are likely better able to contend with lag effects associated 

with the owl’s long lifespan. These ongoing demographic studies have now yielded 

reasonably precise estimates of population trends and indicate that two of three 

study populations occurring primarily on national forests are declining in abundance, and the third is likely declining (chapter 4). 

From a statistical perspective, conclusions about population trends are limited 

to the areas encompassed by the four demographic study areas because the areas 

were not randomly selected. However, because study areas are large, span the 

length of the Sierra Nevada, have long-term (20+ years) data, and have similar 

study designs, they likely reflect the trends in California spotted owl populations 

throughout NFS lands on the west side of the Sierra Nevada. To achieve a regionalscale inference based on sampling of owls, monitoring would have to occur at a 

much larger scale using a lower intensity approach that did not sacrifice the statistical rigor of the demography study areas. Subsequent to CASPO, formal approaches 

based on presence/absence data (i.e., occupancy analysis) were developed to provide 

a statistical framework for estimating changes in occupancy over space and time 

while accounting for imperfect detection probabilities. For example, a recent analysis of territory occupancy data for the Eldorado Study Area found that annual rates 

of change in occupancy were similar to rates of change in abundance estimated 

with demographic methods (reverse-time mark-recapture models). Although this 

congruence is encouraging, developing an occupancy-based monitoring program 

outside of the demographic studies (i.e., a regionwide scale of monitoring) would 

involve beginning a new monitoring program with no prior information. Moreover, 

given the species’ long lifespan, detecting trends in spotted owl populations can 

take many years, even decades (as was the case with the existing demography studies). Nonetheless, a regional-scale, occupancy-based monitoring program would be 

highly complementary to the information provided by the demographic monitoring 

and would facilitate the assessment of barred owl impacts and serve as a valuable 

component of a barred owl management strategy (see below). 
281 

The California Spotted Owl: Current State of Knowledge

Research 

One major knowledge gap identified in CASPO involved “habitat capability” as 

little to no information existed on relationships between fitness (survival and reproduction) and habitat features. As described in chapter 4, significant progress has 

been made in this regard as several studies have investigated correlations between 

habitat features and owl demographic rates such as reproduction, survival, and 

occupancy across a range of spatial scales. A general picture has emerged where 

individual fitness and territory occupancy appear to be linked to the availability 

of closed-canopy forest with complex vertical structure. Other vegetation types 

(e.g., montane chaparral) distributed within a mosaic of forest types may constitute 

important foraging habitat, particularly when juxtaposed with closed-canopy 

forests, and may confer fitness benefits to spotted owls; however, such linkages 

have not been demonstrated conclusively for California spotted owls. 

Priority Research Needs 

The synthesis of information in each chapter, as well as progress made on knowledge gaps identified as part of the CASPO process, indicate that a paradox exists 

with respect to our understanding of the California spotted owl’s ecology and 

life-history needs. On the one hand, many aspects of its ecology have been studied 

intensively, and consequently, we have a reasonable understanding of population 

trends within specific demography study areas as well as habitat associations at a 

variety of spatial scales. On the other hand, many important uncertainties remain; 

two of the most important being (1) the environmental and anthropogenic causes 

of observed population declines, and (2) the short- and long-term effects of forest 

fuels and restoration management and wildfire regimes on spotted owls. Reducing 

these two uncertainties, as well as others described below, would be facilitated by 

creative research approaches that integrate emerging advances in animal tracking 

technology, remote sensing-based habitat mapping, and population genomic and 

other molecular approaches. Below, we highlight priority research needs for the 

coming years and briefly point to possible new research approaches that could be 

employed to reduce outstanding uncertainties. 

Identifying Environmental Causes of Population Declines 

Spotted owls are declining in abundance on some NFS lands in the Sierra Nevada 

(chapter 4). The identification of environmental factor(s) responsible for population declines is essential for halting and reversing wildlife population declines. 

Understandably, much research on California spotted owls has focused on habitat 
282 

GENERAL TECHNICAL REPORT PSW-GTR-254

associations and, more recently, the influence of habitat quality and wildfires on 

populations in an effort to inform managers of forest lands (see chapters 2, 3, and 

4). However, studies explicitly attempting to diagnose causes of the decline and 

discriminate among candidate environmental factors are lacking given the inherent 

challenges in conducting such work, and mechanism(s) behind observed declines 

thus remain uncertain. Tempel et al. (2014) suggested that reductions in the amount 

of closed-canopy forests may have contributed to observed declines in spotted owl 

abundance in the central Sierra Nevada, but the study area-level decline in owl 

habitat (about 8 percent) was considerably less than the decline in owl numbers 

(about 50 percent) over a two-decade period. The population decline may have 

exceeded the recent reduction in habitat because of lag effects related to historical 

habitat change, nonlinear effects of habitat loss, and changes in other, unmeasured 

habitat elements. However, factors unrelated to habitat could also be contributing to 

the decline (see chapter 7), and carefully designed studies are needed to understand 

the relative importance of potential threat factors. Moreover, multiple environmental factors could be responsible for observed population declines, and they could be 

acting synergistically (interacting) such that cumulative effects exceed the impacts 

of stressors when considered individually and even additively. Clearer understanding of these processes would facilitate the effective allocation of conservation 

resources and improve the likelihood of halting and reversing declines. 

Several conceptual frameworks have been developed to diagnose causes of 

decline in species of conservation concern (e.g., Peery et al. 2004). These frameworks can include comparing populations experiencing different levels of potential 

threat factors, comparing population trends before and after a threat factor emerges, 

and direct experiments designed to manipulate threat factors and measure an 

appropriate response. Carefully designed experimental studies constitute the most 

rigorous way to test hypotheses about causes of declines (e.g., effects of forest 

treatments) in spotted owls (chapter 7), but are challenging to implement because of 

the owl’s long lifespan and large spatial requirements, and previous attempts have 

met with limited success. Controlling the many factors influencing owl population 

performance, while testing specific hypotheses, is immensely difficult on landscapes with significant environmental variability and diverse historical conditions. 

Moreover, the owl is currently declining, and additional losses of habitat under the 

auspices of “experimentation” need to be carefully vetted with consideration for 

potential detrimental effects on the species. Successful research would also require 

experimental control over large geographic areas, something that has proven to be 

very difficult to execute. 
283 

The California Spotted Owl: Current State of Knowledge

Comparing populations experiencing different levels of threats (e.g., amounts of 

habitat loss, disease incidence, prevalence of barred owls) is a potentially powerful 

approach. For instance, comparisons of population growth rates among the four 

Sierra Nevada demographic study areas (owls on the three NFS study areas are 

declining, whereas owls on the National Park Service [NPS] study area appear 

stable; chapter 4) suggest that past and current management differences between 

NFS lands and NPS lands could be responsible for spotted owl declines in the 

former. However, ecological and other anthropogenic differences between NFS and 

NPS lands could be responsible for differences in population trends between land 

ownerships. For example, the NFS study areas contain a large amount of private 

land managed for commercial timber production, whereas the NPS study area does 

not. Inference about causes of declines can be strengthened by considering individual owls or territories as the sampling unit and relating spatiotemporal variation 

in demographic parameters (e.g., survival, reproduction, and occupancy) to variables that reflect potential threat factors. In fact, population-level research has thus 

far shown that the individual/territory is the key sampling level for attempting to 

partition the complex and interacting mechanisms behind the observed population 

declines in the long-term and spatially extensive monitoring demographic study 

areas. However, a key gap here involves a dearth of individual- and territory-level 

information (other than relatively coarse-scale habitat metrics) that can be used to 

explain variation in the demographic rates. Greater emphasis on collecting information about prey availability, competitor and predator abundance (particularly barred 

owls—at this time they are only a potentially important consideration within the 

Lassen Study Area), and diseases within the demographic study areas could help 

elucidate the causes of declines. Some of this information can readily be collected 

as part of captures (e.g., health parameters) or surveys (e.g., presence/absence of 

predators or competitors) that are already part of the monitoring activities conducted for each demographic study. Although correlative associations (i.e., observational studies) between owl demographic rates and environmental variables do not 

necessarily identify causative mechanisms behind population declines, concordance 

in results across study areas greatly strengthens inferences about causes. Moreover, 

once reasonably well-supported correlations between environmental factors and 

demographic rates have been identified, simple population models can be used 

to estimate the sensitivity of population growth to each variable. Environmental 

variables with a strong influence on population growth (i.e., high sensitivity) and that 

are amenable to manipulation would then be likely candidates for management intervention. Although some information (notably prey availability) may be impractical 
284 

GENERAL TECHNICAL REPORT PSW-GTR-254

to collect at a sufficiently large number of territories, there are smaller scale opportunities and indirect methods for studying prey as a limiting factor (see below). The 

research approach described here would benefit from maintaining the three demographic study areas on NFS lands and restoring the NPS study area (Sequoia and 

Kings Canyon), as doing so would facilitate both broad- and fine-scale assessment 

of relationships between owl population and changes in environmental conditions. 

Effects of Fuel Treatments and Wildfire on Population Viability

Greater understanding of the effects of fuel and restoration treatments and wildfire 

on California spotted owls is needed to inform forest management that is intended 

to recover owl populations and restore ecosystem resilience in Sierra Nevada forests 

(chapters 3 and 7). Fuel and restoration treatments may confer long-term benefits 

to spotted owls by reducing the risk of large high-severity fires (chapter 5), but it 

is unknown under what circumstances potential long-term benefits outweigh more 

certain short-term impacts to owls if treatments substantially change forest structure. In short, for fuel and restoration treatments to confer a net benefit to spotted 

owls, the following conditions all must be met: 

• Large high-severity fires have a negative effect on owl habitat and 

populations. 

• Fuel and restoration treatments effectively reduce the frequency and size of 

high-severity fires and water-stress mortality that are detrimental to spotted 

owl habitat. 

• Treatment changes to forest structure do not lead to the extirpation of spotted owls or to such low abundances that owls cannot recover to realize the 

benefits of restored fire regimes. 

High-severity fires— 

Until recently, the effects of high-severity fire on spotted owls has remained uncertain, in part, because studies had been predicated on a limited number of territories 

experiencing extensive high-severity fire, potential fire effects have been confounded by the effects of salvage logging, and the lack of marked individuals makes it 

difficult to interpret the occupancy status of territories (chapters 3 and 7). However, 

the King Fire of 2014 provided an unprecedented opportunity to understand the 

impacts of a large high-severity fire on California spotted owls given its impact on 

the Eldorado demography study area in the form of a natural “before, after, control, 

impact” experiment. Spotted owl occupancy declined markedly at severely burned 

sites 1-year postfire, and the large patch of severely burned forest was strongly 
285 

The California Spotted Owl: Current State of Knowledge

avoided for foraging by global positioning system (GPS)-tagged individuals whose 

territories were at the perimeter of the fire (Jones et al. 2016b). Because fire impacts 

were not confounded by salvage harvesting, this study thus provided compelling 

evidence that high-severity fire can negatively affect spotted owls when burn patch 

size is large. In contrast, occupancy did not appear to decline in the first breeding 

season following the Rim Fire (Bond and Lee 2015); however, caution is advisable 

when interpreting results after only 1 year given the high site fidelity of spotted 

owls and without knowledge of the distribution, size, and severity of burned forest patches. Caution is also appropriate because owls were not individually marked 

and movements among territories by unmarked individuals can give the appearance of occupancy in vacant territories. Additional opportunities will likely emerge 

for studying the effects of severe fire on spotted owls by taking advantage of these 

opportunities through monitoring responses after fires and using radiotelemetry or 

GPS tagging when feasible to augment our understanding of owl responses to fire. 

Fuel treatments— 

In terms of fuel treatment effectiveness, model-based and empirical evidence 

indicate that fuel treatments can reduce fire severity and spread, and that combining fuel treatments (e.g., Strategically Placed Landscape Treatments [SPLATs] or 

topography-based variable-density reduction) with prescribed and managed fire can 

effectively reduce the extent of high-intensity fires in the Sierra Nevada under most 

conditions (chapter 5). In some recent wildfires (e.g., Rim, King, Valley, Butte), fire 

behavior has exceeded current model predictions, producing large patches of highseverity effects, including areas with recent (<15 years) fuels treatments. It is not 

always clear what is driving this extreme behavior, but some observations and early 

analyses (Lydersen et al. 2013) suggest two likely factors: (1) heavy fuel loads sufficient for the wildfire to generate its own extreme weather, and (2) increasingly efficient early suppression of past fires, which has effectively selected for fewer, highseverity large wildfires that “escape” containment during more extreme weather 

than occurred historically (North et al. 2015). Fuels treatments, such as some 

combination of mechanical thinning, prescribed burning and managed wildfire,

will likely only be effective in the absence of extreme fire behavior. 

Treatment changes to forest structure do not lead to the extirpation of 

spotted owls— 

Little direct information exists about the short-term influence of fuel treatments 

on owls. Tempel et al. (2014) found a small negative effect on reproductive output 

related to “medium-intensity” timber harvests that included, but was not limited to, 

SPLATs and SPLAT-like thin-from-below prescriptions with relatively even spacing 
286 

GENERAL TECHNICAL REPORT PSW-GTR-254

among the remaining trees and found no effect on territory occupancy or individual 

survival. Stephens et al. (2014) reported a decline in the number of territorial owl 

sites 3 to 4 years following implementation of a landscape fuels management strategy primarily consisting of defensible fuels profile zones and forest thinning treatments. The potential effects of actual SPLATs or “newer” fuels reduction and restoration treatments that vary forest conditions with topography (North et al. 2009) 

have not been assessed. Further, studies of the potential long-term benefits of restoring historical fire regimes (reducing the extent of high-severity fire) to spotted owls 

have remained in their infancy. Recently, the Sierra Nevada Adaptive Management 

Project (SNAMP) investigated this question at the “fireshed” scale (tens of square 

kilometers) within an interdisciplinary framework. Results from this project suggest that SPLATs can reduce the risks of high-severity fire to spotted owls on the 

fireshed scale over a 30-year timeframe (Tempel et al. 2015). In the absence of fire, 

SPLATs had a persistent negative effect on spotted owl fitness over the modeling 

period (Tempel et al. 2015). Thus, additional research will be needed that incorporates absolute risks of fire into model-based evaluations of fuel treatments effects 

in order to examine relative risks in greater detail. Moreover, similar studies conducted at landscape scales will be needed to fully assess the effects of implementation of fuel treatments on the spotted owl population throughout the Sierra Nevada. 

Ultimately, rigorous characterization of the short- and long-term effects of fuel and 

restoration treatments on spotted owls will be possible with the development of a 

spatially explicit population model that could be used as a decision-support tool 

to help evaluate the potential effects of alternative forest management actions (see 

above). 

Historical (Pre-Euro-American) Abundance of California 

Spotted Owls 

The California spotted owl is unusual among species of concern because more 

than 20 years of detailed demographic data have been collected at multiple sites, 

whereas most threatened species do not have such a wealth of information to guide 

managers. However, our insight into factors affecting population trends is limited to 

recent decades although wildfire suppression and timber harvesting have altered the 

composition and structure of Sierra Nevada forests since the late 19th century (chapters 4 and 5). As discussed above, unavoidable uncertainty exists about population 

trends over the past 150 years and the full suite of factors that influenced populations. Several hypotheses have been suggested about this early history of owls, but 

these cannot be tested with information from demographic studies, and existing 

genetic information does not provide definitive information regarding historical 
287 

The California Spotted Owl: Current State of Knowledge

population size. However, emerging genetic-based methods provide exciting opportunities to evaluate demographic history in California spotted owls (chapter 4). 

Populations that have dramatically declined (i.e., effective population size) tend to 

lose genetic diversity (e.g., heterozygosity and allelic diversity) because of genetic 

drift. When historical specimens from museums are available, direct comparisons 

of genetic diversity (and thus effective population size) can be made between 

historical and modern sampling times, but few such samples are available for 

California spotted owls. However, the recent advent of next-generation-sequencing 

(NGS) has revolutionized researchers’ ability to characterize demographic history 

using genetic samples collected at a single point in time (chapter 4). Indeed, NGS 

approaches now allow for the screening of tens of thousands of loci (as opposed to 

tens of loci) and thus the evaluation of competing models of demographic history 

(e.g., population expansion or contraction over relatively short periods of time) and 

estimation of important population parameters (e.g., pre-Euro-American population 

size and subsequent trends in population size). As such, “genomic” methods could 

provide considerable potential for understanding the combined effects of forest 

management activities on spotted owls in the 20th century. Answering the question 

of whether spotted owls are more abundant currently or historically would provide 

insight into how resilient owl populations might be to future population perturbations as a function of climate, fire, management, or combinations thereof. 

Enhancing Foraging and Prey Habitat 

Evidence is mounting that California spotted owls forage in heterogeneous forests 

containing a mosaic of vegetation types and seral stages (including patches of old 

forest) because complex landscapes support a diversity and abundance of small 

mammal prey (chapter 3). Indeed, the juxtaposition of mature conifer forests 

containing a hardwood component with other vegetation types may enhance 

foraging opportunities and confer fitness benefits to owls (chapter 3). However, 

greater understanding of the vegetation conditions that shape the abundance and 

distribution of important prey species in the Sierra Nevada would facilitate the 

development of effective stand- and landscape-scale forest management strategies 

to enhance spotted owl foraging habitat. Pairing such forest management activities 

with carefully designed field studies (ideally within an experimental framework) 

could provide a rigorous means for evaluating the potential benefits of efforts to 

enhance spotted owl prey and foraging habitat. Assessments of forest management 

intended to improve foraging habitat quality would also benefit from the integration of pellet-based diet analyses, prey and vegetation sampling, as well as the use 
288 

GENERAL TECHNICAL REPORT PSW-GTR-254

of fine-scale tracking of radio- or GPS-marked owls. Ultralight GPS loggers are 

now sufficiently small (5 to 10 g [0.2 to 0.4 oz]) to be deployed safely on owls, can 

generate precise locations at rapid intervals, and would provide an effective tool for 

determining the use of different vegetation types for foraging. Such studies could 

be enhanced by the use of isotopic tracers applied to prey occurring in the target 

patches, which would allow for the estimation of the contribution of prey captured 

in different habitats to the total biomass of prey consumed by marked owls. 

Impacts of Climate Change to Spotted Owls and Their Habitat 

Climate change will inevitably alter the structure and composition of Sierra Nevada 

forests and thereby the suitability and configuration of spotted owl habitat (chapter 

5). Thus, it will be important to identify sites that are most likely to continue to support suitable habitat, as well as sites that are likely to transition into suitable habitat 

as the climate changes, and to actively manage and conserve priority sites (e.g., by 

reducing fire risk and promoting old-forest conditions). Existing habitat areas that 

are likely to retain suitability into the future are generally predicted to occur in 

cooler, north-facing areas with high topographic relief, and existing models of forest vulnerability provide a tool for predicting the specific locations of likely refugia 

on the landscape (chapter 5). Although plants and animals in montane systems are 

generally expected to make upslope shifts in distributions as the climate warms, 

novel vegetation communities are likely to emerge, and changes in the distribution 

of owl habitat and prey will likely be influenced by soil conditions and topographic 

features (chapter 5). A recent climate change-modeling study based on 20 years of 

spotted owl occupancy data from the Eldorado demography study area suggests that 

maintaining viable spotted owl populations is most likely to be achieved by managing for owl habitat at high-elevation territories (Jones et al. 2016a). This analysis 

constitutes a novel and useful first attempt to project the effects of climate change 

on spotted owl distribution in the Sierra Nevada; however, it does not explicitly consider projected changes in forests or fire regimes in this region. Future studies could 

further our understanding of potential climate change effects by linking expected 

changes in owl distribution to shifts in vegetation communities and change in fire 

dynamics—an effort that would benefit from integrative efforts involving wildlife, 

forest, and fire ecologists. 

Habitat Evaluation Tools 

Accurate and high-resolution mapping of the California spotted owl habitat elements across landscapes will enhance the effectiveness of many aspects of owl 

habitat conservation, including monitoring the effect of management actions on 
289 

The California Spotted Owl: Current State of Knowledge

habitat quality. As described in chapter 6, most previous map-based studies of 

California spotted owl habitat associations have made use of moderate-resolution 

Landsat imagery that cannot discern some key structural elements (e.g., high concentrations of residual trees or multilayered canopy) that are important to spotted 

owls. Emerging remote sensing methods such as LiDAR will continue to improve 

our ability to characterize habitat quality for spotted owl habitat at landscape scales 

and hopefully provide the spatiotemporal vegetation data needed to understand the 

effects of forest management and natural disturbance on owls. Despite improvements 

in habitat mapping provided by LiDAR and other high-resolution sensors, there are 

many outstanding needs for mapping of wildlife habitat, including the mapping of 

snags, large trees, and large broken-top trees; development of improved metrics to 

quantify vertical canopy structure; and development of tree species distributions 

in mixed-conifer forests. The integration of various LiDAR techniques with other 

optical imagery may overcome some of these hurdles and improve the mapping 

of wildlife habitat in the near future, particularly to the extent that simple, fieldvalidated remote sensing metrics can be linked to management goals (chapter 6). 

Literature Cited 

Caughley, G. 1994. Directions in conservation biology. Journal of Animal Ecology. 

62: 215–244. 

Collins, B.M.; Everett, R.G.; Stephens, S.L. 2011. Impacts of fire exclusion and 

recent managed fire on forest structure in old growth Sierra Nevada mixedconifer forests. Ecosphere. 2: 51. 

Collins, B.M.; Lydersen, J.M.; Everett, R.G.; Fry, D.L.; Stephens, S.L. 2015. 

Novel characterization of landscape-level variability in historical vegetation 

structure. Ecological Applications. 25: 1167–1174. 

Diller, L.V.; Dumbacher, J.P.; Bosch, R.P.; Bown, R.R.; Gutiérrez, R.J. 2014. 

Removing barred owls from local areas: techniques and feasibility. Wildlife 

Society Bulletin. 38: 211–216. 

Dixon, G.E. 2002. Essential Forest Vegetation Simulator: a user’s guide to the 

Forest Vegetation Simulator. Revised edition. Internal report. Fort Collins, CO: 

U.S. Department of Agriculture, Forest Service. 
290 

GENERAL TECHNICAL REPORT PSW-GTR-254

Dugger, K.M.; Forsman, E.D.; Franklin, A.B.; Davis, R.J.; White, G.C.; 

Schwarz, C.J.; Burnham, K.P.; Nichols, J.D.; Hines, J.E.; Yackulic, C.B.; 

Doherty, P.F.; Bailey, L.; Clark, D.A.; Ackers, S.H.; Andrews, L.S.; 

Augustine, B.; Biswell, B.L.; Blakesley, J.; Carlson, P.C.; Clement, M.J.; 

Diller, L.V.; Glenn, E.M.; Green, A.; Gremel, S.A.; Herter, D.R.; Higley, J. 

M.; Hobson, J.; Horn, R.B.; Huyvaert, K.P.; McCafferty, C.; McDonald, T.; 

McDonnell, K.; Olson, G.S.; Reid, J.A.; Rockweit, J.; Ruiz, V.; Saenz, J.; 

Sovern, S.G. 2016. The effects of habitat, climate, and barred owls on long-term 

demography of northern spotted owls. Condor. 118: 57–116. 

Fiala, A.C.S.; Garmman, S.L.; Gray, A.N. 2006. Comparison of five canopy 

cover estimation techniques in the western Oregon Cascades. Forest Ecology and 

Management. 232: 188–197. 

Flather, C.H.; Hayward, G.D.; Beissinger, S.R.; Stephens, P.A. 2011. Minimum 

viable populations: Is there a ‘magic number’ for conservation practitioners? 

Trends in Ecology and Evolution. 26: 307–316. 

Gutiérrez, R.J.; Cody, M.; Courtney, S.; Franklin, A.B. 2007. The invasion 

of barred owls and its potential effect on the spotted owl: a conservation 

conundrum. Biological Invasions. 9: 181–196. 

Jones, G.M.; Gutiérrez, R.J.; Tempel, D.J.; Whitmore, S.A.; Berigan, W.J.; 

Peery, M.Z. 2016a. Megafires: an emerging threat to old-forest species. Frontiers 

in Ecology and the Environment. 14: 300–306. 

Jones, G.; Gutierrez, R.J.; Tempel, D.; Zuckeberg, B.; Peery, M.Z. 2016b. 

Using dynamic occupancy models to inform climate change adaptation strategies 

for California spotted owls. Journal of Applied Ecology. 53: 895–905. 

Lydersen, J.; North, M. 2012. Topographic variation in active-fire forest structure 

under current climate conditions. Ecosystems. 15: 1134–1146. 

Lydersen, J.M.; North, M.P.; Knapp, E.E.; Collins, B.M. 2013. Quantifying 

spatial patterns of tree groups and gaps in mixed-conifer forests: reference 

conditions and long-term changes following fire suppression and logging. Forest 

Ecology and Management. 304: 370–382. 

North, M.; Stephens, S.; Collins, B.; Agee, J.; Aplet, G.; Franklin, J.; 

Fulé, P. 2015. Reform forest fire management: agency incentives undermine 

policy effectiveness. Science. 349: 1280–1281. 
291 

The California Spotted Owl: Current State of Knowledge

North, M.; Stine, P.; O’Hara, K.; Zielinski, W.; Stephens, S. 2009. An 

ecosystem management strategy for Sierran mixed-conifer forests. Gen. Tech. 

Rep. PSW-GTR-220. Albany, CA: U.S. Department of Agriculture, Forest 

Service, Pacific Southwest Research Station. 49 p.

Peery, M.Z.; Beissinger, S.R.; Newman, S.H.; Burkett, E.B.; Williams, T.D. 

2004. Applying the declining population paradigm: diagnosing causes of low 

reproductive success in marbled murrelets. Conservation Biology. 18: 1088–1097. 

Stephens, S.L.; Fry, D.L. 2005. Spatial distribution of regeneration patches in an 

old-growth Pinus jeffreyi-mixed conifer forest in northwestern Mexico. Journal 

of Vegetation Science. 16: 693–702. 

Stephens, S.L.; Lydersen, J.M.; Collins, B.M.; Fry, D.L.; Meyer, M.D. 2015. 

Historical and current landscape-scale ponderosa pine and mixed-conifer forest 

structure in the Southern Sierra Nevada. Ecosphere. 6(5): art 79. 

Taylor, A. 2010. Fire disturbance and forest structure in an old-growth Pinus 

ponderosa forest, southern Cascades, USA. Journal of Vegetation Science. 21: 

561–572. 

Tempel, D.; Gutierrez, R.J.; Whitmore, S.; Reetz, M.; Berigan,W.; Stoelting, 

R.; Seamans, M.E.; Peery, M.S. 2014. Effects of forest management on 

California spotted owls: implications for reducing wildfire risk in fire-prone 

forests. Ecological Applications. 24: 2089–2106. 

Tempel, D.J.; Keane, J.J.; Gutiérrez, R.J.; Wolfe, J.D.; Jones, G.M.; Ramirez, 

C.M.; Koltunov, A.; Berigan, W.J.; Gallagher, C.V.; Munton, T.E.; Shaklee, 

P.A.; Whitmore, S.A.; Peery, M.Z. 2016. Meta-analysis of California spotted 

owl (Strix occidentalis occidentalis) territory occupancy in the Sierra Nevada: 

habitat associations and their implications for forest management. The Condor: 

Ornithological Applications. 118(4) 747–765. 

Verner, J.; Taylor, R.J. 1992. Future directions for the California spotted owl 

effort. In: Verner, J.; McKelvey, K.S.; Noon, B.R.; Gutiérrez, R.J.; Gould, G.I., 

Jr.; Beck, T.W., eds. The California spotted owl: a technical assessment of its 

current status. Gen. Tech. Rep. PSW-GTR-133. Albany, CA: U.S. Department 

of Agriculture, Forest Service Pacific Southwest Research Station: 27–33.
292 

GENERAL TECHNICAL REPORT PSW-GTR-254

Glossary 

Clearcut—An even-age regeneration or harvest method that removes all trees in 

the stand producing a fully exposed microclimate for the development of a new age 

class in one entry. 

Commercial thin—A harvest intermediate between planting and final harvest of a 

stand with the objective of reducing stand density to improve growth, enhance forest health, and other resources objectives. Treatment can recover potential mortality 

while producing merchantable material. Thinning can include application of the 

following techniques to achieve objectives: chemical (killing of unwanted trees by 

herbicide application); crown (removal of trees from dominant and co-dominant 

strata); free (no consideration to crown position); low (removal of trees from lower 

crown classes); mechanical or row (removal of trees either in row, strips by using a 

fixed spacing interval); selection (removal of the crown class to favor those in the 

lower crown classes.) 

Conversion (forest)—Alteration of timber-growing lands to a use or uses other 

than the growing of timber. 

Core area—A management term used to refer to the area within an owl’s home 

range that receives concentrated or disproportionally high use by an owl. 

Cover type—A category within a classification scheme defined by the user that 

distinguishes among different landscape conditions (e.g., vegetation types, water, 

talus slopes). 

Damaged timberland—An area of timberland where wildfire, insects, disease, 

wind, flood, or other disturbance has occurred that has resulted in a reduced stocking of trees below “minimum stocking requirements.” 

Fuel break—An area where the cutting or removal of trees eliminates the vertical 

continuity of vegetative materials and the horizontal continuity of tree crowns for 

the purpose of reducing flammable materials and maintaining an open space to 

reduce fire spread, duration, and intensity. 

Foraging habitat—The cover types an owl uses to forage for food and that are 

found within each owl’s territory and home range. 
293 

The California Spotted Owl: Current State of Knowledge

Group selection—An uneven-age regeneration method in which trees are cut 

in small groups to establish a matrix of new age classes. The width of groups is 

commonly less than about twice the height of the mature trees. Individual trees in 

the matrix may or may not be harvested to provide improved growing conditions 

for remaining trees. Multiple entries of this activity ultimately results in an unevenaged stand of three or more age classes. 

Habitat quality—The capacity of the habitat to provide resources and conditions 

suitable for individual and population persistence. 

Home range—The area used by an individual to meet its requirements for survival 

and reproduction. 

Landscape matrix—The mosaic of cover types in a landscape outside of specified 

areas recognized as supporting spotted owls (i.e., designated territories and home 

ranges), which may or may not include areas of suitable habitat. 

Nest stand—The forest stand containing the location of a nest (note: nests can 

occur on the edge of forest stands). 

PAC—protected activity center. A management area designation on National Forest 

System lands. A (123-ha) 300-ac area of the best available habitat around a known 

nest site or center of activity within which specified habitat protective measures 

are applied. 

Overstory removal cut—The cutting of trees within the upper canopy layer with 

the purpose of “releasing” (i.e., removing competition from dominate trees) understory trees. The primary source of regeneration is advance reproduction. A minor 

(less than about 10 percent of full stocking), live component of the upper canopy 

may be retained for reasons other than regeneration. 

Rehabilitation (forest)—Activities undertaken for the purposes of restoring and 

enhancing the productivity of commercial timberlands which do not meet minimum 

stocking standards. Activities may include harvest or clearing of the site followed 

by site preparation and restocking. 

Salvage cut—An intermediate harvest that removes trees that are dead or dying 

because of injurious agents other than competition for the purpose recovering 

economic value, as well as to improve forest health. 
294 

GENERAL TECHNICAL REPORT PSW-GTR-254

Sanitation cut—An intermediate harvest removing trees to improve stand health 

by stopping or reducing the actual or anticipated spread of insects and disease, but 

not necessarily to recover economic value. 

Seed-tree cut—A type of logging where all trees are removed except those needed 

for seed production. Prepares the seed bed and creates a new age class in an 

exposed microenvironment. Additional trees may be retained to provide a minor 

(less than about 10 percent of full stocking) live component after the removal cut, 

for reasons other than regeneration. 

Shelterwood cut (includes prep and removal)—The shelterwood logging method 

results in regeneration of a stand via a series of harvests (preparatory, seed, and 

removal). The shelterwood regeneration method is normally used when some shade 

canopy is considered desirable for the establishment of regeneration. 

single-tree selection cut—An uneven-age logging method where individual trees 

of any size classes are removed more or less uniformly throughout the stand creating or maintaining a multiage structure to promote growth of remaining trees and 

to provide space for regeneration. Multiple entries of this activity ultimately results 

in an uneven-aged stand of three or more age classes. 

Territory—The area of the home range from which an owl excludes (i.e., defends 

other owls. Presumably such areas are most important for nesting, roosting, and 

foraging. The territory is commonly represented for the purposes of habitat analysis 

as a 1,000-ac circle (1.5-mi radius) around the nest site or half the average nearest 

neighbor distance of owls within a population. 
This publication is available online at www.fs.fed.us/psw/

Pacific Southwest Research Station

800 Buchanan Street

Albany, CA 94710