Prepared in cooperation with the University of Arizona, 

School of Natural Resources and the Environment

Effects of Reintroduced Beaver (Castor canadensis) on

Riparian Bird Community Structure along the Upper San Pedro 

River, Southeastern Arizona and Northern Sonora, Mexico

U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2014-1121
Cover: Photograph of beaver dam (0.8 meter height), 3.7 kilometers north of U.S. Highway 90, July 2006.

Photograph taken by Glenn E. Johnson, University of Arizona, School of Natural Resources and the 

Environment.
Effects of Reintroduced Beaver (Castor canadensis) on 

Riparian Bird Community Structure along the Upper San 

Pedro River, Southeastern Arizona and Northern Sonora, 

Mexico

By Glenn E. Johnson and Charles van Riper III

Prepared in cooperation with the University of Arizona, School of Natural Resources and the Environment

Open-File Report 2014-1121

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2014 

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Suggested citation:

Johnson, Glenn E., and van Riper III, Charles, 2014, Effects of reintroduced beaver (Castor canadensis) on riparian 

bird community structure along the upper San Pedro River, southeastern Arizona and northern Sonora, Mexico: U.S. 

Geological Survey Open-File Report 2014-1121, 98 p., http://dx.doi.org/10.3133/ofr20141121. 

ISSN 2331-1258 (online)

Any use of trade, product, or firm names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted material contained within this report.
iii

Contents

Chapter 1. Bird Abundance and Richness in a Southwestern Desert Riparian Area Following Beaver 

Reintroduction ............................................................................................................................................................. 1

Abstract ...................................................................................................................................................................... 1

Introduction ................................................................................................................................................................ 1

Description of Study Area .......................................................................................................................................... 5

Methods ..................................................................................................................................................................... 6

Sampling Design .................................................................................................................................................... 6

Bird Surveys........................................................................................................................................................... 7

Avian Species Groups............................................................................................................................................ 8

Environmental Measurements................................................................................................................................ 8

Beaver and Water Measurements.......................................................................................................................... 9

Statistical Analyses ...............................................................................................................................................10

Results......................................................................................................................................................................11

Avian Survey Effort and Detections.......................................................................................................................11

Beaver Activity.......................................................................................................................................................12

Non-Random Spatial Association between Beaver and Birds ...............................................................................13

Bird-Habitat Associations ......................................................................................................................................13

Effects of Beaver on Vegetation/Habitat................................................................................................................14

Discussion.................................................................................................................................................................15

Conclusion ................................................................................................................................................................18

Acknowledgments.....................................................................................................................................................19

References Cited ......................................................................................................................................................20

Figures......................................................................................................................................................................25

Tables .......................................................................................................................................................................28

Chapter 2. An Investigation of Important Factors Associated with Riparian Bird Community Structure 

Along the Upper San Pedro River in Southeastern Arizona and Northern Sonora, Mexico: Effects of 

Reintroduced Beaver..................................................................................................................................................57

Abstract .....................................................................................................................................................................57

Background...............................................................................................................................................................57

Methods ....................................................................................................................................................................58

Seven-Minute Point Count Sampling.....................................................................................................................58

Resampling of Historical Surveys..........................................................................................................................58

Conservation Priority Index Ranking .....................................................................................................................59

Results......................................................................................................................................................................59

Breeding Birds of the Upper San Pedro River Riparian Areas ..............................................................................59

Species of Conservation Concern.........................................................................................................................60

Incidental and Noteworthy Species, Excluded Observations.................................................................................60

Notes on Beaver Activity and Distribution .............................................................................................................61

Modeling Bird and Beaver Associations Relative to Vegetation and Surface Water .............................................62

Additional Funding Received and Updated List of Project Partners (In Bold)........................................................62

Changes to Original Proposal - Interactions with People ......................................................................................63

Conclusion ................................................................................................................................................................63

References Cited ......................................................................................................................................................64

Figures......................................................................................................................................................................66

Tables .......................................................................................................................................................................69
iv

Appendix 2-A. Table of 59 species of conservation interest detected throughout study area along the San Pedro 

River and associated riparian areas in southeastern Arizona and northern Sonora, and designation under each of 

17 lists intended to aid in the prioritization of species for conservation action. .........................................................75

Chapter 3. Southwestern Willow Flycatcher Nest Records and Potential for Future Breeding 

along the Upper San Pedro River, Arizona...............................................................................................................83

Abstract .....................................................................................................................................................................83

Introduction ...............................................................................................................................................................83

Methods ....................................................................................................................................................................84

Historical and Contemporary Breeding Records ...................................................................................................84

Upper San Pedro River 2005 Nest–Site Characteristics Compared to Regional Measurements ..........................84

Environmental Conditions Along the Upper San Pedro River................................................................................85

Results......................................................................................................................................................................85

Historical and Contemporary Breeding Records ...................................................................................................85

Upper San Pedro 2005 Nest-Site Characteristics Compared to Regional Measurements ....................................86

Environmental Conditions Along Upper San Pedro...............................................................................................87

Discussion.................................................................................................................................................................87

Conclusions and Management Implications..............................................................................................................89

Acknowledgments.....................................................................................................................................................91

References Cited ......................................................................................................................................................92

Tables .......................................................................................................................................................................95

Appendix 3-A. Discovery, Location, Subsequent Monitoring, and Fate of the 2005 Hereford Nest...........................98

Figures

Figure 1-1. Number of birds counted (Y axis) within 1 meter distance bins (X axis) along the upper San Pedro 

River, May–July 2005 and 2006. A total of 22,166 bird detections are represented.................................................... 25

Figure 1-2. Percentage of total basal area of all beaver-gnawed (but standing) cottonwood (Populus fremontii) and 

willow trees (Salix spp.), and combined basal area of cottonwood-willow stumps and live trees (unaffected by beaver 

[Castor canadensis]) across all sites within each of the low-, moderate-, and high-use beaver intensity classes, and 

sites with no beaver use .............................................................................................................................................. 25

Figure 1-3. Breeding bird species richness (mean number of 76 species detected within 50 meters of stations) at 

sites where beavers (Castor canadensis) absent (n=154) and present (n=86); includes 95-percent confidence 

intervals from t test (table 1-5)..................................................................................................................................... 26

Figure 1-4. Relative abundance of all breeding birds (average number of individuals detected per survey visit 

within 50 m) at sites where beavers (Castor canadensis) absent (n=154) and present (n=86); includes 95-percent 

confidence intervals from t test (table 1-5)................................................................................................................... 27

Figure 2-1. Scanning riparian vegetation for a bird heard during a training session lead by the investigator at Los 

Fresnos Land Trust, TNC~Mexico, in the Sonoran portion of the upper San Pedro River watershed, May 16, 2007. . 66

Figure 2-2. Beaver (Castor canadensis) sign along upper San Pedro River 7.2 kilometers south of the 

U.S.-Mexico International border................................................................................................................................. 66

Figure 2-3. Dam in riparian area north of Hereford Bridge with recent fire history. ..................................................... 67

Figure 2-4. Beaver (Castor canadensis) dam over 1 meter high, 1 kilometer south of Hunter Wash, north of 

Hereford, Arizona (8.5 x 11 inch clipboard for scale)................................................................................................... 67

Figure 2-5. Beaver (Castor canadensis) dam (same 0.8 meter height dam from figure 1, p. 1), 3.7 kilometers 

north of U.S. Highway 90, July 2006 (8.5 x 11 inch clipboard for scale)...................................................................... 68

Figure 2-6. Yellow-crowned Night-Heron (Nyctanassa violacea) observed 7 kilometers south of

international border...................................................................................................................................................... 68
v 

Tables

Table 1-1. Environmental variables used in stepwise variable selection and final habitat/covariate models............... 28

Table 1-2. Number of survey stations where beaver (Castor canadensis) dams were documented and

total number of dams documented each year from 2000 to 2006 within the San Pedro National Riparian 

Conservation Area, Arizona......................................................................................................................................... 30

Table 1-3. Beaver (Castor canadensis) activity from 2000 to 2006 at 240 stations in the San Pedro Riparian 

National Conservation Area, Arizona, used to quantify beaver presence/absence and beaver intensity..................... 30

Table 1-4. Common and scientific names of 76 breeding bird species detected within 50 meters of 240 survey 

stations on the San Pedro Riparian National Conservation Area, Arizona, 2005–2006............................................... 31

Table 1-5. Species richness and relative abundance of species groups at sites with and without beavers (Castor 

canadensis) ................................................................................................................................................................. 34

Table 1-6. Mean relative abundance for most commonly detected bird species (n=31) at sites with and without 

beaver (Castor canadensis) ........................................................................................................................................ 35

Table 1-7. Habitat and Environmental Covariate models for species and species groups from multiple linear 

regression (n=240) using stepwise variable-selection (minimum BIC to enter) selected model terms......................... 36

Table 1-8. Comparison of models incorporating beaver-related variables to habitat- and environmental 

covariate-only models for species and species groups, respectively........................................................................... 39

Table 1-9. Result of multiple linear regression (n=240) models after adding beaver-related factors to 

habitat/covariate models (table 1-7), for species and species groups ......................................................................... 43

Table 1-10. Name and location of the 240 point count stations used to survey breeding birds in 2005–2006 

on the San Pedro Riparian National Conservation Area, listed from northern-most to southernmost station. ............. 51

Table 2-1. Four components of the investigation of bird community dynamics in riparian areas in the upper 

San Pedro River watershed and details of survey technique, effort, total detections, and study design...................... 69

Table 2-2. Bird species encountered during summer surveys of riparian areas in upper San Pedro River

watershed within the United States and Mexico .......................................................................................................... 70

Table 3-1. Records of breeding Southwestern Willow Flycatchers (E.t. extimus) within the upper San Pedro River 

watershed in Arizona, including: locality, number of nests documented, date(s) of record, sources used to find or 

investigate record (specimen information given in parentheses: institution abbreviation*, catalog number, and 

collector), and further information and comments on the record.................................................................................. 95

Table 3-2. Comparison of mean and range of values from 240 sites, value at nest site, and rank and percentile 

(= percent sites with lesser values) of nest site in relation to range of values for all stations, for each of seven 

environmental variables: percent Goodding’s willow (Salix gooddingii) cover in 3–5 m and 5–7 m canopy layers, 

total number of Goodding’s willow, seep willow and all woody plant stems, mean diameter-at-breast-height of all 

woody species’ main stems, and total width of riparian vegetation.............................................................................. 97
vi

Conversion Factors and Datums

Conversion Factors

SI to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

kilometer (km) 0.5400 mile, nautical (nmi) 

meter (m) 1.094 yard (yd) 

Area

square meter (m2) 0.0002471 acre 

hectare (ha) 2.471 acre

square hectometer (hm2) 2.471 acre

square kilometer (km2) 247.1 acre

square centimeter (cm2) 0.001076 square foot (ft2)

square meter (m2) 10.76 square foot (ft2) 

hectare (ha) 0.003861 square mile (mi2) 

Datums

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88). 

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83). 

Elevation, as used in this report, refers to distance above the vertical datum. 
1 

Effects of Reintroduced Beaver (Castor canadensis) on 

Riparian Bird Community Structure along the Upper San 

Pedro River, Southeastern Arizona and Northern Sonora, 

Mexico

By Glenn E. Johnson and Charles van Riper III

Chapter 1. Bird Abundance and Richness in a Southwestern Desert Riparian Area 

Following Beaver Reintroduction

Abstract

We measured bird abundance and richness along the upper San Pedro River in 2005 and 2006 to 

investigate how beavers (Castor canadensis) may act as ecosystem engineers after reintroduction to a 

desert riparian area in the Southwestern United States. In areas where beavers colonized, we found higher 

bird abundance and richness of bird groups, such as all breeding birds, insectivorous birds, and riparian 

specialists, and higher relative abundance of many individual species—including several avian species of 

conservation concern. After accounting for environmental factors, such as presence or persistence of 

surface water, and extent of Frémont cottonwood (Populus fremontii) and Goodding’s willow (Salix 

gooddingii), the relative influence of beaver activity was not as strong as these other environmental 

factors. However, there was still evidence of an association between beaver activity and bird abundance 

and richness, as models that included beaver-related variables better explained variation in bird abundance 

and richness for 71 percent of species groups and 46 percent of individual species for which we built 

models. Although the effect sizes associated with the beaver influence on the bird community were 

smaller than similar studies conducted in other regions of North America, the biological significance of 

beaver activity in the upper San Pedro River riparian area will likely become even stronger with increasing 

time. 

Introduction

Interactions among organisms are often examined by ecologists to help understand dynamic 

processes affecting the distribution and persistence of species. For example, interactions can be trophic or 

chemical, competitive or mutualistic, or intra- or inter-specific. The concept of ecosystem engineering is 

used to investigate interactions that arise when an organism physically creates or modifies its environment 

and thereby changes the availability of abiotic resources required by other organisms, which may result in 

direct, indirect, or cascading biotic effects (Jones and others, 1994, 1997, 2010; Berke, 2010; Thomsen 

and others, 2010). The strength of ecosystem engineering and the effects it can exert on community 

dynamics can be explicitly parameterized to help generate testable hypotheses and predictions, as 

demonstrated in observational, experimental, and modeling studies investigating hundreds of species and 

ecological contexts (Wright and others, 2003; Badano and Cavieres, 2006; Wright and Jones, 2006; 

Cuddington and others, 2007, 2009; Wright, 2009; Jones and others, 2010). The ecosystem engineering 
2 

concept focuses attention on (1) the process by which organisms may change the physical structure or 

other abiotic dimensions of their environment, and then (2) the consequence(s) physical changes may have 

on biotic elements in the community (Jones and Gutiérrez, 2007; Jones and others, 2010). Although there 

is some overlap with other terms (for example, foundation species, dominants, habitat cascades, niche 

construction, facilitation, and keystone species), ecosystem engineering is not synonymous with these 

concepts, for example because it is not dependent on disproportionate effects of a species or organism’s

biomass, trophic interactions, evolutionary consequences of habitat formation, or purely beneficial 

interactions (Jones and others, 1994; Wright and Jones, 2006; Berke, 2010). When focusing on a specific 

organism, one could classify both “autogenic” ecosystem engineers that modulate abiotic resources 

through their physical structure alone (for example, coral reefs, kelp beds, large trees) as well as 

“allogenic” engineers that can modify their environment through behavior (for example, mammals 

burrowing in the earth or woodpeckers excavating cavities). However, because all organisms modify their 

environment to some degree, the question “is this species an engineer?” is not particularly informative. 

Rather, by focusing on the process when it is relevant, for example, “how and when do ecosystem 

engineering activities of an organism exert influence on other organisms?” new insights can be gained 

regarding the direct or indirect interactions that influence community dynamics and structure (Jones and 

others, 2010). 

The concept of ecosystem engineering is relevant and has proved useful in applied contexts where 

the goal is to conserve or restore populations or ecosystem function (Wright and others, 2002; Boogert, 

2006; Byers and others, 2006; Crain and Bertness, 2006; Moore, 2006; Griffiths and others, 2010; Laland 

and Boogert, 2010). Beavers (Castor canadensis) are often cited as the quintessential ecosystem 

engineers, and have consistently been promoted by resource managers in North America and Europe as an 

inexpensive tool for restoring ecological function in riparian areas (Apple, 1985; Olson and Hubert, 1994; 

Fredlake, 1997; Albert and Trimble, 2000; Baker and Hill, 2003; Rosell and others, 2005; Boyle and 

Owens, 2007; Prettyman, 2009). Throughout North America, beavers historically had a profound impact 

on stream ecology and watershed function, and after extensive extirpation by trapping in the 18th and 19th

centuries, the species returned to portions of its former range during the 20th century—both through 

natural recolonization and human reintroduction (Naiman and others, 1988; Baker and Hill, 2003; Rosell

and others, 2005). Despite beavers’ consumption of riparian vegetation and use of plant material to build 

dams, many land managers and restoration ecologists believe the benefits to the ecosystem as a whole 

outweigh any consequences of herbivory or felling of trees (Apple, 1985; Olson and Hubert, 1994; 

Fredlake, 1997; Albert and Trimble, 2000; Fouty, 2002). Ostensibly, the potential benefits of beaver on 

ecosystems that are sought by managers are the same as the ecosystem engineering effects, although this is 

not always explicitly stated as such. 

A beaver’s ecosystem engineering process begins with several abiotic effects on the environment. 

Beavers create dams that impound water, reduce water velocity, promote deposition and retention of 

sediments, organic matter, and dissolved nutrients, while reducing turbidity downstream. At larger scales, 

beaver dams can modulate high-flow events and capture precipitation that is then released slowly into the 

groundwater and stream channel, which ultimately can alter erosion patterns, stream channel morphology, 

and stream gradients (Naiman and others, 1988; Gurnell, 1998; Baker and Hill, 2003). In addition to 

maintaining hydrologic stability, or even de-entrenching some down-cut stream channels (Albert and 

Trimble, 2000), the totality of beaver influences typically make some resources more available, such as 

water, organic matter, and nutrients such as dissolved nitrogen and phosphorus (Naiman and others, 1988; 

Baker and Hill, 2003;Wright and others, 2003). Additionally, through creation of canopy gaps resulting 

from tree felling, beaver activity can alter the temperature and moisture gradients in their terrestrial 

environment, and create a mosaic of disturbed and undisturbed patches with different levels of abiotic 

conditions or resources required by other species (Wright and others, 2002, 2003). 
3 

Given such dramatic effects on the abiotic components of the environment, many studies have 

found biotic consequences associated with beaver activity; for example, raised water tables and increased 

soil-water interface can facilitate germination, recruitment, and primary production of trees and shrubs, 

thus having a positive feedback on beavers and other species dependent on riparian vegetation (Naiman 

and others, 1988). Beaver activity has been associated with higher species richness and abundance of a 

broad range of taxa including plants, odontanate insects, aquatic invertebrates, fish, amphibians, reptiles, 

and small mammals (Wright and others, 1996, 2003; Baker and Hill, 2003; Rosell and Parker, 1996; 

Rosell and others, 2005). Many anecdotal natural history accounts have noted that bird communities 

appear to thrive where beavers are found (Carr, 1940; Muller-Schwarze, 1992; Wilkinson, 2003; Nijhuis, 

2011). The majority of rigorous (for example, experimental) studies regarding the effects of beaver 

activity on birds have focused on waterfowl (Anatidiae), oftentimes demonstrating positive effects of 

beaver ponds on habitat use and vital rates (Speake, 1956; Arner, 1963; Renouf, 1972; Nummi, 1989, 

1992; Merendino and others 1995; Nummi and Poyosa, 1997; McKinstry and others, 2001; Hartke and 

Hepp, 2004; Nummi and Hahtola, 2008). Observational studies on other groups of birds (for example,

songbirds), although fewer in number, also have consistently found that beaver activity is correlated with 

higher species richness and abundance in mesic environments of Eastern North America (Reese and Hair, 

1976; Grover and Baldassarre, 1995; Edwards and Otis, 1999; Bulluck and Rowe, 2006; Aznar and 

Desrochers, 2008) and in more arid regions in Western North America (Medin and Clary, 1990; Brown 

and Parsons, 1979; Cooke and Zack, 2008). Yet in these studies, explanations for these effects typically 

are correlative, invoking increases in riparian vegetation, open water, prey (for example, insects) 

availability, and structural complexity at small and/or large scales. 

Despite the presumed effects of beaver activity on birds, specific mechanisms by which beaver 

activity affects avian communities have only been studied for waterfowl. Past studies on the association 

between beaver activity and bird communities have usually assumed that observed differences between 

areas with beaver activity and surrounding environments could be attributed to the beaver activity. Beaver 

habitat selection also may explain some of these observed patterns, yet few studies have attempted to 

account for co-varying factors that could simultaneously influence the habitat selection of both birds and 

beavers (Cooke and Zach, 2008; Chandler and others, 2009). 

Beaver were once widespread in riparian areas in the Southwestern United States, although they 

were largely extirpated in the 19th century (Baker and Hill, 2003; Webb and others, 2007). The gallery 

forests of cottonwood (Populus fremontii) and Goodding’s willow (Salix gooddingii) in the Southwest 

support breeding bird communities known for being among the highest density and diversity in North 

America (Carothers, 1974; Hunter and others, 1987). In southeastern Arizona, beavers were so common 

along the San Pedro River in the 1800s that James Ohio Pattie named it the “Beaver River” in 1825 (Webb 

and others, 2007), yet the mammal was extirpated there by the early 20th century . 

In 1984, when the Bureau of Land Management (BLM) assumed stewardship of approximately 75 

km of the upper San Pedro River watershed and created the San Pedro Riparian National Conservation 

Area (SPRNCA), conservation of birds and other wildlife became a central management objective. In 

cooperation with Arizona Game and Fish Department and others, BLM personnel reintroduced beaver in 

the SPRNCA from 1999 to 2002, resulting in at least 40 individuals in 13 different sites by 2005 (M. 

Fredlake, Bureau of Land Management, oral commun.). Along with returning a potentially important 

native species to the ecosystem, goals of this program were to increase water retention, restore the 

structural heterogeneity of vegetation, and enhance conditions for wildlife (Fredlake, 1997). 

Similar to other locations where beaver activity exists in conjunction with a management priority 

on birds (for example, Longcore and others, 2007), some have voiced concerns that beaver activity may 

jeopardize the riparian forests and the bird community of the SPRNCA (M. Fredlake, Bureau of Land 

Management, oral commun..). Tree-felling by beaver could reduce canopy cover needed by some birds, 
4 

such as the Yellow-billed Cuckoo (Coccyzus americanus), a candidate for listing under the Endangered 

Species Act that is relatively common along the San Pedro River compared to other river systems in the 

region (Krueper and Rich 2003). Conversely, reduced over-story canopy of cottonwood, an increase in 

early successional (that is, shrub-form) growth of willows, and other components of beaver activity are 

associated with habitat for Southwestern Willow Flycatcher (Empidonax traillii extimus; Finch and others, 

2000; Sogge and Marshall, 2000), which is listed as threatened in Arizona at both the State and Federal 

level. Increases in habitat for the flycatcher in this area may contribute to recovery, given the species’ 

historical and contemporary nest records in the local area combined with the presence of large populations 

to the north along the lower reach of the San Pedro River. However, if beaver heavily utilized willow or 

cottonwood and caused net losses to either of these woody species, it could affect both of these bird 

species of conservation concern, as well as many others that depend on riparian vegetation. 

Within a few years after the reintroduction of beaver, the reduction in over-story canopy and/or 

construction of dams and retention of water were expected to be the potential pathways for abiotic 

influences of beaver activity and, thus, initial components of the ecosystem engineering process. The 

primary abiotic driver for the majority of bird species on the upper San Pedro River is the depth or 

availability of water, because this directly affects the extent and vigor of riparian vegetation, which in turn 

can affect vegetative and insect food availability, nesting substrate, cowbird parasitism, and nest predation 

(Brand and others, 2010a, 2010b, 2011). Previous studies along the San Pedro have empirically 

demonstrated that groundwater depth is the key driver of vegetation dynamics, especially determining the 

extent of cottonwood and willow in reaches with relatively shallow (more available) groundwater versus 

reaches more dominated by tamarisk (Tamarix spp.) and/or mesquite (Prospis velutina) in areas with 

deeper (less available) water (Stromberg,1998; Leenhouts and others, 2005; Stromberg and Tellman, 

2010). The extent and persistence of surface water also is affected by groundwater depth, and although 

surface water does not directly drive vegetation dynamics (Stromberg, 1998), it is crucial to waterdependent birds, such as Mallards (Anas platyrhynchos) (Brand and others, 2010a). 

We investigated the potential for beaver activities to affect the riparian ecosystem and influence 

breeding bird communities along the upper San Pedro River. We hypothesized that if beavers were 

affecting the environment through the ecosystem engineering process, we would find that differences in 

the breeding bird communities along the river would be non-randomly associated with places where 

beavers had settled. However, we recognized the possibility that any non-random association could result 

either from an ecosystem engineering process and its effects on water and riparian vegetation structure 

(that is, increased shrub cover resulting from greater water availability and/or direct reduction in canopy 

over-story resulting from tree-felling) or simply the result of both beaver and certain bird species being 

drawn to the same environments. Depending on the length of time beaver had been in the system (3–6 

years, depending on the site and settlement date), bird community structure could be influenced more by 

water and vegetation dynamics than by beaver activity. 

To consider the full complement of environmental conditions present on the upper San Pedro 

River, we sampled throughout this portion of the watershed along a gradient of vegetative and hydrologic 

conditions, which included areas with and without beaver, but that were otherwise similar. We specifically 

assessed three questions: 

1. Were there differences in the number of breeding bird species and the abundance of individual 

species where beavers were present in the SPRNCA? 

2. Were there differences in the parameters in question 1 attributable to environmental factors that 

were merely correlated with beaver habitat selection? Or, alternatively,

3. Was there evidence that observed differences could be attributed to beaver activity—either from 

tree felling, dam creation, or both? 
5 

Because the area is specifically managed for birds, we wanted to assess if the distribution of birds—

especially species of concern—and beaver were spatially related in any way, and thus whether beaver 

activity might lead to management concerns or opportunities in relation to the beaver reintroduction 

program. We extensively sampled birds and environmental attributes, and then modeled the potential 

relationships between the avian community and factors in their environment including vegetation, water, 

and beaver activity. 

Description of Study Area

We studied interactions between bird communities and beaver in the lower riparian floodplain 

terrace and channel (and “channel shelf” and “floodplain bank,” especially when vegetated; Hupp and 

Osterkamp, 1996) along 68.5 km of the upper San Pedro River, within the boundaries of the SPRNCA. 

The SPRNCA extends northward from the U.S. border with Mexico approximately 60 km to St. David, 

Arizona, and is managed by BLM. Elevation ranged from 1,125 m at the northern-most portion of our 

study to 1,285 m at the southern-most point, 2.5 km north of the international border. Prior research 

conducted on the upper San Pedro River has described the interaction between groundwater, surface 

water, and vegetation community dynamics within and adjacent to the riparian areas (see Stromberg and 

Tellman, 2010; and Webb and others, 2007, for reviews), providing the basis for the following summary. 

The upper San Pedro River contains reaches with perennial, intermittent, and ephemeral surface water. 

Riparian vegetation consists primarily of Frémont cottonwood (Populus fremontii) and Goodding’s willow 

(Salix gooddingii) as the dominant over-story canopy, with seepwillow (Baccharis salicifolia) often 

present in the understory along wetter reaches. Mesquite occurs on high floodplain terraces adjacent to the 

primary riparian vegetation. Mesquite and non-native tamarisk (that is, saltcedar Tamarix chinensis and 

closely related species) are regular yet minor components of the vegetation throughout the length of the 

study area, and dominate along some of the drier, ephemeral reaches in the north of the study area where 

groundwater is deep and less accessible. Tamarisk, especially, forms fairly homogeneous stands in some 

areas in the northern section of the study area near the community of St. David, although there are 

scattered remnant or senescing cottonwood trees. Additional woody species present in the study area 

include velvet ash (Fraxinus velutina), Arizona walnut (Juglans microcarpa v. major), netleaf hackberry 

(Celtis reticulata), western soapberry (Sapindus saponaria), and desert willow (Chilopsis linearis). 

Emergent vegetation at sites with perennial, slow-moving water include bulrush (Scirpus ssp.), cattail 

(Typha domingensis), and various species of sedge (Carex ssp.). 

In the early the 19th century, the San Pedro River Basin was swampy, with beaver dams, isolated 

patches of gallery cottonwood forest, and a high water table and extensive sacaton grasslands (Sporobolus 

wrightii) within the primary floodplain (Stromberg and Tellman, 2010). Similar to much of the Southwest 

in the second half of the 19th century, channel incision and arroyo formation started to occur on the upper 

San Pedro River, draining wetlands and resulting in a lower water table. Like previous arroyo formation, 

channel widening, and fill-in cycles over the last 8,000 years, the entrenchment on the San Pedro River

and throughout the Southwest is correlated with a climate-flood cycle (Webb and others, 2007). However 

in the 19th century, anthropogenic factors (for example, livestock grazing, removal of beaver dams, wood 

cutting, agriculture, and groundwater use) may have contributed to the speed and severity of 

entrenchment. River down-cutting and channel widening continued through the 1930s, after which 

cottonwood and willow established itself more widely as the channel stabilized and secondary, lower 

floodplains formed adjacent to the main channel. Throughout the later 20th century, riparian vegetation 
6 

continued to expand in extent and height along the new, lowered floodplain of upper San Pedro River, 

especially during the 1980s and early 1990s (Webb and others, 2007; Stromberg and Tellman, 2010). The 

removal of cattle grazing in the 1990s probably contributed to increased riparian vegetation production 

and the substantial increases in bird abundance in the SPRNCA documented by Krueper and others 

(2003). 

Due largely to the current extent of cottonwood-willow cover along the upper San Pedro River, the 

area is recognized as a regionally important area for breeding and migrating birds, and has been the site of 

multiple avian studies (Skagen and others, 1998; Krueper, 1999; Krueper and others, 2003; Brand and 

others, 2008, 2010a, 2011; McFarland and others, 2012. In a review, Brand and others (2010b) found that 

breeding bird species richness estimates from the upper San Pedro River were approximately twice as high 

as reported from other Southwest rivers. The river is perhaps best known for its recreational bird-watching 

opportunities and for the near-certainty that local groundwater pumping is depleting the aquifer that 

supplies the river’s surface water and supports the adjacent cottonwood-willow riparian forests (Stromberg 

and Tiller, 1996; Stromberg, 2001; Rojo and others, 1999; Steiner and others, 2000; Davis, 2004; Steinitz 

and others, 2005; Mac Nish and others, 2010). 

Methods

To estimate abundance of breeding birds, we used variable-radius point count surveys combined 

with distance sampling (Reynolds and others, 1980; Ralph and others, 1993; Buckland and others, 2001; 

Thomas and others, 2010) from May 24 to July 24, 2005 and 2006. To quantify environmental conditions 

at point count stations, we documented the presence of surface water and its persistence through the dry 

season, canopy cover and basal area of all woody plants, and width of riparian vegetation areas along the 

river channel. To quantify the presence and intensity of use by beaver, we also used data on beaver 

activity and location collected by BLM staff from the time of initial release in late 1999 through spring 

2005, and then documented all beaver activity at and near the survey stations in 2005 and 2006. We then 

used model selection techniques to evaluate the strength of associations between bird population and 

community parameters and beaver activity environmental factors. 

Sampling Design

To achieve extensive spatial coverage across riparian vegetation areas targeted for sampling, we

randomly chose a start point and placed survey points systematically throughout the SPRNCA. Survey 

stations (n=240) were located every 250–285 m along the upper San Pedro River in riparian vegetation 

adjacent to the river on the lowest available floodplain terrace, which was typically just above (0.5–2 m) 

the river level. The initial stations were located randomly at 100, 200, or 300 m from bridges or other 

access points along the river, and then additional stations were systematically placed within available 

riparian vegetation, utilizing GarminTM 12XL and 12X GPS units to estimate distances between stations. 

The distance of a given station from the river’s edge varied slightly depending on the perpendicular width 

of the riparian vegetation corridor, but was typically placed under canopy cover within 10 m of the riverside edge of riparian vegetation. At some locations (<10 percent) with very wide riparian corridors and/or 

where floods had scoured vegetation, stations were placed 16–46 m from the edge of the river. Previous 

observations by BLM personnel indicated beaver were building dams in the San Pedro River on or near 

larger (>5 m) wash inlets, so when beaver dams or wash inlets were encountered more than 100 m from a 

previously established survey station, the station was replaced with a new station located 5–15 m up-river 

of the dam or wash inlet. 
7 

Bird Surveys

We used modified point-count survey methods (Reynolds and others, 1980; Ralph and others, 

1993) incorporating field procedures for distance sampling (Buckland and others, 2001; Thomas and 

others, 2010) to estimate bird abundance and breeding bird species richness. Seven-minute point-count 

surveys were conducted from mid-May to early August, which represents the most active period of 

breeding bird activity in this riparian area. Although surveys were conducted from May 15 to August 5, 

we only utilized surveys from May 24 to July 24; lingering winter birds and/or late-spring migrants 

complicated detection of territorial singing birds earlier in May, and rain and flood events during late July 

and early August precluded consistent and/or safe sampling in the lower floodplain thereafter. After 

excluding these surveys (before May 24 and after July 24) for which detectability of breeding birds was 

compromised, total effort at each station over the 2 years ranged from five to eight visits. Field personnel 

surveyed 8–15 stations per morning between 0500 and 0900 hours. Throughout the survey period each 

year, stations were visited at a variety of times during the day by randomizing start points of successive 

visits. For each bird detected, the observer estimated the distance (to nearest meter) visually or with 

Bushnell Yardage-Pro® laser rangefinders, and also recorded the detection type (song, call, visual, 

moving/flying within riparian vegetation, or flyover) and whether birds were detected within the first 5 

minutes or the last 2 minutes. Indications of breeding activity were documented at every opportunity 

during or between surveys, along with incidental sightings of rare species and those of conservation 

concern. 

To account for variation in detection probabilities (for example, because of bird behavior, observer 

skill, environmental factors, and because the adjacent upland environment generally had much fewer birds 

that were not as likely to be affected by beaver activity), we included only birds detected within 50 m of 

stations for analyses. We arrived at this distance cut-off point through several lines of evidence. A line 

fitted to a plot of the number of birds detected within 100 m of the stations approached a horizontal 

asymptote at approximately 50 m (fig. 1-1), because of the decreased detectability with increasing distance 

coupled with the proximity of the edge of the riparian corridor. Histograms of detection distances from 

point count stations typically demonstrate an increasing number of detections within distance categories 

farther away from the center point, because of the effect of increasing area of each successive distancebin, and the point at which detectability decreases significantly can be inferred by noting where the 

“shoulder” to the right of the peak is located (Emlen, 1971; Buckland and others, 2001). We checked this 

assumption by evaluating the relationship between detectability-adjusted estimates of density and relative 

abundance. 

We used the multiple covariate distance sampling approach within program DISTANCE (Marques 

and others, 2007; Thomas and others, 2010) to construct detection probabilities and density estimates per 

hectare for two of the more common species in the study, Yellow Warbler (Setophaga petechia) and Song 

Sparrow (Melospiza melodia). These density estimates for each station were then compared to the 

maximum number of singing males detected per visit, median number of singing males detected across all 

visits, and relative abundance (total number of birds detected at each station divided by effort at the 

station, that is, the average number of birds per visit) calculated from our raw data but truncated at 30, 50, 

60, 80, and 100 m. We used the 50-m truncation distance for all bird species as it was most closely 

correlated to the program DISTANCE-derived density estimates (see section, “Results”). Although this 

obviously left out many detections (that is, singing birds beyond 50 m), we felt this truncation distance 

was an appropriate balance of considerations related to detection probability, the extent of vegetation 

measurements (the majority of which were within 30 m), and inclusion of a representative sample of birds 

using the station-area. 
8 

We recorded all birds detected, including migrants and vagrants, yet for our analyses considered 

only species known to breed in the upper San Pedro River Basin (Krueper, 1999; Krueper and others, 

2003; Brand and others, 2008, 2010a). Birds were excluded if detected flying over the canopy at the 

station and not utilizing the surrounding riparian environment. 

Avian Species Groups

In addition to considering only breeding bird species (using our own observations as well as those 

of Krueper, 1999; and Brand and others, 2010a), we grouped some species that use similar environments 

or have similar foraging strategies. We used prior research (Hunter and others, 1987,; Krueper, 1999; 

Brand and others; 2010a) to determine: (1) which bird species are, or are not, associated with riparian 

environments in the Southwestern United States, and then (2) which of the riparian-associated species are 

considered riparian generalists or riparian obligates. We grouped species as a “riparian generalist” if they 

were known to utilize multiple riparian vegetation types (for example, mesquite and tamarisk, either more 

often or in equal proportion to cottonwood/willow use), and we grouped species as a “riparian specialist” 

if either obligated or strongly associated with cottonwood-willow forests and/or surface water on the upper 

San Pedro River according to Brand and others (2010a), Krueper (1999), or Hunter and others (1987). For 

all species detected, we also determined whether each was of conservation concern status if the species 

was indicated by Hunter and others (1987) as “declining” in Southwestern broadleaf riparian forests, or if 

it was included in the “Birds of Conservation Concern” list of the U.S. Fish and Wildlife Service (2008). 

Environmental Measurements

We quantified a range of environmental conditions at each survey station. To estimate canopy 

cover for each species of tree and shrub, we added all percent cover estimates from nine different height 

strata measured (0–1, 1–3, 3–5, 5–7, 7–10, 10–15, 15–20, 20–25, 25–30 m) for each of five dominant 

woody species, that is, cottonwood, willow, tamarisk, mesquite, and seepwillow. To estimate upper levels 

of canopy cover for cottonwood, we also included “canopy cover from 20 to 30 m” to ensure we 

accounted for this potentially important variable. To measure standing tree density, we recorded diameterat-breast-height (DBH) for all trees within 30 m. To estimate relative abundances of different-size classes 

of trees and shrubs, we considered all woody stems within a 30-m radius of the station center and used 

Biltmore sticks and/or DBH tapes to check sizes and recorded measurements to the nearest centimeter. We

also recorded when trees had signs of beaver gnawing (these are reported with other living trees when 

reporting canopy cover and for most basal area measurements) and measured the diameter of all beaver 

stumps, beaver-created snags, or felled trees, measuring as close to 1.3 m from the ground to stem as

possible, or as high as possible for stumps. To analyze tree mortality and canopy openings related to 

beaver within the 30-m vegetation plots, we combined beaver-created stumps, snags, and recently downed 

trees of willow and cottonwood into one grouping, because it was often difficult or impossible to 

distinguish between stumps of willow or cottonwood (especially after a year or two of weathering), and 

because there were few beaver-created snags or beaver-felled trees. For analyses, DBH measurements 

were transformed to individual tree basal areas (DBH2

* 0.00007854; Dunster and Dunster, 1996), then all 

basal area measurements for each plot were summed for each species within each of six DBH size classes 

(size 1 = 1–4 cm DBH, 2 = 5–11 DBH, 3 = 12–18 DBH, 4 = 19–25 DBH, 5 = 26–52 DBH, 6 = >53 cm 

DBH) for five dominant species (cottonwood, willow, mesquite, tamarisk, and Baccharis) and for stumps. 

Because of colinearity between these classes, for analyses, we condensed the live woody stems into three 

classes (<12 cm DBH, 12–25 cm DBH, and >25 cm DBH), and beaver-created stumps, snags, and 

recently felled trees were condensed into one basal area class, which was the sum of their values at each 

site. Because other woody species are relatively minor components of the canopy, we did not test for 

effects for all species. For example, cottonwood, willow, mesquite, and tamarisk made up 95.3 percent
9 

(16,907) of all stem and stump diameters recorded (17,724), with seepwillow comprising another 2.3 

percent (418 stems). Basal area calculations are presented directly as measured, then summarized (in 

square meters) for each 30-m radius plot surrounding the station, or 2,826 m2 (0.2826 ha) rather than 

extrapolated to a single, larger unit (hectare, square kilometer, etc.). An index of vegetation structure was 

created by allocating a “1” to each height strata level above 1 m (all sites had some vegetation below 1 m) 

with vegetation cover, which were then summed to get a "Vegetation Height Diversity" at each site 

(scored from 1 to 7). 

To categorize riparian vegetation and geomorphological influences at the stations outside 30 m, we

measured the over-all width of riparian vegetation perpendicular to orientation of river, distance from the 

survey center point to edges of riparian vegetation, riverbank, and other habitats farther away (for 

example, grassland, mesquite savannah, or mesquite bosque), width and depth of river, and other 

vegetative and geomorphic factors, such as vegetation community type(s) in each area and depth of 

entrenchment/terracing. During the analysis phase, we reduced these variables considerably (from >100 to 

<20 columns), for example, reducing the canopy coverage classes to either the “average percent cover” 

(all percent cover measurements within each height strata, divided by the number of strata with 

measurements), and by not including more than one of any two highly correlated variables in the final 

analysis. Table 1-1 summarizes environmental measurements used for analyses. 

Beaver and Water Measurements

Immediately after each avian survey, and while transiting along the river between survey stations, 

we recorded beaver sign (dams, gnawed trees, stumps, downed trees) and river condition (dry, recently 

dried, or water present, and if present whether shallow or deep, <0.2 and >0.2 m, respectively) at the 

stations. Water data were summarized to provide a measure of the persistence of surface water throughout 

the dry season before the late-summer monsoons. We considered three classes of surface-water conditions 

as potentially important to birds—if a station was dry from May through June (unless a rain or flood event 

occurred, that is, “ephemeral”); if a station had flowing water that persisted until at least late May (but 

later dried up at some sites) when many birds are setting up territories or have already settled into their 

breeding activities; and if a site contained perennial water that flowed throughout the summer (table 1-1).

We used indicator variables for the latter two classes (that is, =1 if condition is true at the site) and sites 

without either of these water conditions were used as the reference levels (that is, =0; table 1-1). 

Data were provided for beaver activity locations from late 1999 through early 2005 by BLM 

personnel, and we collected these data from 2005 to 2006, which we categorized by year according to an 

annual cycle that started after the majority of rains and flooding from the summer monsoon season had 

subsided (October 1) and extending for the following year. To determine beaver influence at each avian 

survey station, we projected two circles of 50- and 100-m radius around each station in ArcMap 9 

(Environmental Systems Research Institute) displaying map at a 1:3,200 scale). For each year period, we 

projected all spatial beaver information around each station location and determined proximity of dams 

and sign within each 50- or 100-m distance bin, or within 150 m downriver because downriver dam 

locations regularly had backwater up to 150 m upriver, and beavers use this water to travel and forage. If a 

dam or location coordinates appeared to span both sides of a distance-boundary (for example, from 45 to 

55 m), we included it within the closer category. 

To model beaver activity as an explanatory (independent) variable at each avian survey station, we

included three variables in our analyses—presence/absence, number of years where a dam was present 

within 50 m, and an intensity scale that ranked sites according to beaver presence and both the distance to 

dams and the number of years dams were located in the vicinity of stations (tables 1-1–1-3). Presence was 

inferred by beaver sign documented within 100 m of the station (in any year), and included dams, gnawed 

trees, stumps, bank dens, and other signs of occupation. We counted the number of years in which a dam 
10

was present within 50 or 100 m of a given station, or 150 m if the dam was downriver, but used just the 

number of years stations had a dam within 50 m as the only continuous variable related to beaver activity. 

In order to rank the intensity of beaver use at sites, we considered the results that the BLM and we 

documented (tables 1-2 and 1-3) and created the following levels of indicator variables, with the reference 

level set to “beaver absence”—low-use sites that did not include sites with any dams but did include any 

other “beaver presence” sign, such as bank dens and/or at least 10 gnawed/felled trees documented within 

100 m of the avian survey station (“beaver level 1”); moderate-use sites where no more than one dam was 

located within 50 m of a station, and/or where stations had less than three dams documented within 100 m 

of a station, in any year (“beaver level 2”); and high-use sites where dams were located within 50 m of 

stations for 2 or more years, and/or where dams were located within 100 m for 3 or more years (“beaver 

level 3”) (table 1-1). 

Statistical Analyses

To assess non-random associations between relative bird abundance or richness when comparing 

areas with and without beavers, we first categorized all 240 survey stations into two groups based on the 

documented presence of beaver (that is, dams or any other sign within 100 m). We then conducted a twotailed t test for the difference in mean relative abundance at stations with beaver activity versus those with 

no activity, both for species most commonly detected (n=31) and species groups. Because bird abundance 

is affected by a complex of environmental conditions (not just beaver activity) at each station, non-random 

associations between bird abundance and beaver presence do not suggest mechanistic or causal 

relationships. Therefore, to further address the question of how beaver may conduct ecosystem 

engineering, we first explained variation in bird abundance because of environmental factors other than 

beaver. We used multiple linear regression to describe “habitat relationships” for species with a greater 

than or equal to 50 percent difference in abundance when comparing stations with and without beaver (for 

all species where the difference was significant at the P ≤ 0.05 level and that had greater than 50 

detections), and similarly constructed “environmental correlate models” for the total number of breeding 

birds and riparian birds detected within 50 m of stations (“habitat” is species-specific so we do not use this 

term for groups of species). 

We first screened for highly correlated variables by computing pairwise correlations (Pearson’s 

correlation coefficients) and then eliminated one variable of any pair where r ≥ 0.65 (retaining the most 

biologically significant variable), yielding a set of 18 environmental covariates for potential inclusion in 

habitat/covariate models (table 1-1). To choose a candidate model of environmental covariates for each 

species or species group, we fit all possible models because we had no a priori expectations. The stepwise 

variable selection platform in JMP 9.0.2 (SAS Institute, Inc.) was used to select an environmental habitat 

model (using “all possible models” option with “minimum BIC” as a stopping rule, fitting up to eight

parameters). Due to the possibility of potentially correlated variables (biologically if not statistically), and 

to guard against over-fitting models, we used Bayesian Information Criterion (BIC; Schwarz, 1978) to 

rank support for each model: 

-2loglikelihood + k × ln(n), 

where

k is the number of parameters, and 

n is the sample size (that is, the number of data points in the study; n=240). 
11

We chose BIC over AIC (Akaike Information Criteria) for initial model selection because it 

applies a larger penalty for each additional model parameter that is dependent on the sample sizes 

(Ramsey and Schafer, 2002); therefore, models were much more parsimonious and less likely to include 

insignificant or redundant variables. Each model explained relative abundance, or richness, at all stations 

sampled. 

After building models that attempted to account for variation attributable to habitat of each species 

or for environmental correlates of the different species groups, we examined whether or not beaver-related 

variables explained additional variation over the habitat/covariate models alone, and the strength of the 

additional model parameters. The models were ranked and compared using AICc values, ΔAIC, and 

Akaike weights (wi). AICc were used for initial ranking of models. Models with beaver-related variables 

in which ΔAIC ≤ 2 of the top ranked variable are reported (that is, regression coefficients), although we

give summary model statistics (R2, AICc, delta AIC, Akaike weights) for all models ranked used Burnham 

and Anderson’s (2004) “simple rules of thumb” to assess the relative merits of models in the set, where 

models having ΔAIC ≤ 2 have substantial support (evidence), those in which 4 ≤ ΔAIC ≤ 7 have 

considerably less support, and models having ΔAIC > 10 have essentially no support. Finally, we 

considered evidence that the top-ranked models (highest-ranked habitat/environment and beaver models) 

contained the most accurate model using wi, which reflects the likelihood that a particular model was best 

among those considered (Burnham and Anderson, 2004; Chandler and others, 2009). 

We used JMP 9.0.2 (SAS Institute, Inc.) for all statistical analyses. To better meet assumptions, 

most continuous variables were log transformed ([X + 1] log 10) before regression analyses. Although this 

transformation did not always result in normal univariate distributions, residuals were checked after fitting 

final models to confirm approximate normality. For example, the distribution of variables with percent 

values were compared after arcsine and log transformations, and the log-transformed variables were 

retained after determining that the distribution of the residuals was approximately normal (Ramsey and 

Schafer, 2002. We used P ≤ 0.05 as our accepted level of significance for all t-tests and inferences 

regarding individual regression estimates, and based the latter on the variables included within the ranked 

models (that is, rather than using model averaging) because each beaver variable was entered in only one 

model. 

Results

Avian Survey Effort and Detections 

A total of 1,480 survey visits (759 in 2005; 721 in 2006) were conducted at the 240 stations; 

greater than 90 percent of stations were surveyed three times per year. Relative abundance at stations after 

truncating detection data at 50 m was most strongly correlated with the density calculated by program 

DISTANCE, for both Yellow Warbler (r=0.93, P < 0.0001) and Song Sparrow (r=0.892, P < 0.0001). 

After excluding birds detected at distances greater than 50 m from stations, flyovers, wintering birds and 

obvious migrants, juveniles, or other birds for which detectability may have been compromised (for 

example, vagrants), we analyzed 13,590 detections of 76 species known to breed in the upper San Pedro 

River Basin (table 1-4). This included 42 species that primarily are associated with riparian vegetation and 

surface water in the region, and 34 species that more typically use upland vegetation and yet also visit 

adjacent riparian areas (Hunter and others, 1987; Krueper, 1999; Brand and others, 2010a). Of the 42 

riparian-associated species, 28 were riparian specialists, and 14 were riparian generalists. Additionally, 68 

percent of species (52 of 76) were primarily insectivorous, and 25 percent were species of regional 

conservation concern (Hunter and others, 1987; U.S. Fish and Wildlife Service, 2008). Of the 19 species 

of regional conservation concern, 14 also were riparian-associated species (table 1-4), and 11 of these had 

sufficient detections to test for differences in relative abundance between sites with and without beaver. 
12

Beaver Activity

Beaver census data yielded 2,943 observations of sign (bank dens, gnawed, or felled trees) and at 

least 122 instances where dams were found on the upper San Pedro River from 2000 to -2006, allowing us 

to determine the number of years in which beaver may have influenced the environment at avian survey 

station locations. At 64 percent of stations (n=154) beavers were apparently absent as no sign was 

documented, with the remaining 36 percent of stations (n=86) divided between low-, moderate-, and highuse sites (14, 14, and 8 percent of stations, respectively). Beaver dams were located within the following 

distance categories from stations—within 50 m at 14 percent of stations (n=33), within 51–100 m at 5 

percent of stations (n=13), and 100–150 m downstream of 3 percent of stations (n=6) (see table 1-2 for 

totals by year). Including all signs documented over the 6-year period at the 36 percent of stations that had 

some level of beaver activity within 100 m, 14 percent were stations (n=34) where only presence was 

documented, whereas dams were documented at the 22 percent (n=52) of stations (that is, those 

categorized as moderate- and high-use (table 1-3). 

Virtually all woody vegetation that beaver felled, gnawed, or consumed living materials from was 

either willow or cottonwood. No tamarisk, mesquite, or other woody species was observed gnawed more 

than a few times, although limbs of these species were occasionally seen in dam construction. The stem 

density (DBH) counts within 30 m of each station revealed that extant beaver stumps represented a 

maximum of 15, 19, and 20 percent of total basal area of cottonwood/willow stems within beaver low-, 

moderate-, and high-use sites, respectively. However, the sites with the highest percentage of stump basal 

area within each level of beaver intensity all hosted a relatively low number of stems (for example, total of 

10 trees and/or stumps), meaning that a few stumps could make up a large proportion of stems and thus 

basal area. Considering the summed basal area for cottonwood and willow (because the stumps of these 

two species were not always readily distinguished) across all sites within each of the low-, moderate-, and 

high-use beaver intensity classes, extant beaver stumps represented a total of 1.9, 2.2, and 1.6 percent of 

total cottonwood/willow basal area within each class, respectively (fig. 1-2). Within the low-, moderate-, 

and high-use sites, beaver-gnawed (but standing) trees represented 5.5, 3.5, and 5.2 percent of the summed 

basal area of cottonwood trees, and 4.2, 7.9, and 3 percent of willow trees, respectively (fig. 1-2). 

Beaver dams typically were located at or near wash inlets or other areas where obvious sediment 

deposits or exposed bedrock would obstruct channel morphology to such an extent that water might be 

pooled upriver from the inlet. The water impounded by beaver dams raised the water level in the river 

from 0.1 to greater than 1 m (in correlation with the size of dams), and in some cases also affected the 

water table in the surrounding riparian floodplain (U.S. Geological Survey, unpub. groundwater data, 

1999). However, in many cases, the impounded backwater was restricted to areas where water was 

naturally pooled already or where the backwater was within an incised channel (that is, ≥ 1 m below the 

lower floodplain). Thus, the beaver-impounded water rarely breached the channel bank or created classic 

stream braiding along the lowest channel shelf and other areas of the lower floodplain. An exception to the 

above pattern was a unique area (between Hereford Road and Hunter Wash) that had reduced canopy 

cover because of two fires within the prior 10 years (one in 1999, another in 2003, M. Fredlake, Bureau of 

Land Management, oral commun.), and a generally less-incised channel. Here, braiding, multiple smaller 

channels, and a higher water table appeared to be associated with beaver presence. Perhaps because of the 

reduced woody canopy cover in this reach, beaver apparently consumed mostly cattail, bulrush, and other 

non-woody emergent riparian vegetation, in addition to willow and cottonwood where available. Dams in 

this area were largely constructed of mud, rocks, and a higher percentage of non-woody vegetation than 

seen in other reaches. 
13

Non-Random Spatial Association between Beaver and Birds 

The majority of the eight species groupings we considered had higher richness and/or abundance 

(P <0.001, from two-tailed t tests for difference in means, degrees of freedom=239) at sites where beaver 

presence had been documented as compared to where they had not (table 1-5, figs. 1-3 and 1-4). The 

greatest difference in the number of species was riparian specialists, with a 26.9 percent mean difference 

(2.3 species/survey), and the least significant difference was the number of conservation concern species 

with 7.4 percent more species (P = 0.035) found at sites where beaver have been documented (table 1- 5). 

Total breeding bird community species richness was greater at sites where beaver had been documented, 

with a mean difference of 12.8 percent (= 2.4 species). The only non-significant difference in mean 

number of species detected at beaver presence versus absence sites was for the number of riparian 

generalist species, with only a 0.2 percent difference (P=0.947, table 1-5). As well, relative abundance 

was higher at sites with beaver for the two groups for which this metric was analyzed, with 25.2 percent

more insectivorous birds (= 1.66 more birds) and total breeding bird relative abundance 27.1 percent

higher (= 2.7 more birds) on average per visit. 

Of the 76 breeding species that we considered, a total of 31 had sufficient detections to test for 

non-random spatial association (two-tailed t tests, 239 degrees of freedom) with beaver (table 1-6). Of 

these 31 species, relative abundance was significantly greater (P ≤ 0.05) for 11, including three species of 

conservation concern—Yellow-billed Cuckoo, Northern Flicker (Colaptes auratus), and Yellow Warbler. 

Two species had significantly fewer (P ≤ 0.05) individuals detected per visit at sites where beaver were 

documented, Ash-throated Flycatcher (Myiarchus cinerascens) and Northern Beardless-Tyrannulet 

(Camptostoma imberbe), the latter being a species of conservation concern. Additionally, there were seven

species that were more common at sites where beaver had been documented, although the differences 

were non-significant at the P ≤ 0.05 level; this group included Mallard, Ladder-backed Woodpecker 

(Picoides scalaris), and Gila Woodpecker (Melanerpes uropygialis) (table 1-6).

Bird-Habitat Associations 

We modeled bird-habitat associations for 13 species (table 1-7), 12 of which showed at least a 50

percent difference in relative abundance for beaver presence than for beaver absence sites and for which 

this difference was significant (P≤ 0.05) (table 1-6). Although Yellow Warbler relative abundance was 

only 32 percent greater at sites where beaver had been documented, we included this species in habitat 

models because the difference in relative abundance was highly significant (P <0.001), and because the 

Yellow Warbler was the most common bird detected, and is a species of regional conservation concern. 

Additionally, we built environmental covariate models (table 1-7) for the seven species groups that 

showed a non-random association with beaver presence (table 1-5). 

Relative abundance of bird populations and species groups was consistently explained by a small 

set of the 18 environmental variables that we considered. Surface water was an important explanatory 

factor in more than one-half of models (table 1-7). For individual species, both the presence of surface 

water in May or the persistence of perennial surface water throughout the summer were included in 46 and 

62 percent of the models, respectively, and for species groups, the presence of water in May was 

especially important (included in 86 percent of models) and perennial surface water was included in two 

models (29 percent). Perennial water was the only factor in the models that explained relative abundance 

of Brown-headed Cowbirds (Molothrus ater). Cottonwood and willow (cover and/or abundance) were 

included in 62 and 31 percent, respectively, of the individual species relative abundance models, whereas 

for species groups, cottonwood and willow were included in 43 and 86 percent of models, respectively. 

Relative abundance of 23 percent of species and no species group richness or relative abundance included 

mesquite cover or basal area, and relative abundance of two (15 percent) species and two (29 percent) 

species groups included tamarisk cover or basal area.
14

Despite the consistent importance of several factors in explaining relative abundance of birds, the 

proportion of variation explained by vegetation and water factors varied somewhat depending on the 

response variable. Percent variation in abundance that was explained by habitat models (R2

) ranged from 

as low as 7–8 percent for species with fewer detections, such as Black Phoebe (Sayornis nigricans), 

Brown-headed Cowbird, and Yellow-billed Cuckoo, to as high as 35–42 percent for more common species 

such as Song Sparrow, Yellow Warbler, Common Yellowthroat (Geothlypis trichas), and Lesser 

Goldfinch (Spinus psaltria) (table 1-8). Among seven species groups, percent of variation in richness 

explained by the environmental covariates (table 1-7) was low for total number of conservation concern 

species (R2=0.13) and highest for the number of riparian specialist species (R2=0.44) (table 1-8). Percent 

variation explained by the two species groups’ relative abundance models was relatively high (R2=0.44), 

both for all breeding species and all insectivorous birds. 

Effects of Beaver on Vegetation/Habitat 

After accounting for variation in relative abundance and species richness attributable to 

environmental factors (table 1-7), models that included beaver presence, intensity, and number of years 

with dams within 50 m ranked higher for 46 percent of species and 71 percent of species groups (tables 1-

8 and 1-9) than the habitat/covariate-only models. Conversely, models containing beaver-related factors

did not explain more variation in relative abundance better than the habitat-only models for 54 percent of 

the 13 species, or more species richness for 28 percent of the 7 species groups (tables 1-8 and 1-9). 

Models with beaver-related variables did not perform better for either of the two bird species for 

which mean relative abundance was negatively associated with beaver activity (Ash-throated Flycatcher 

and Northern Beardless-Tyrannulet, table 1-6), and although R2 values were marginally higher when 

beaver terms were included in the models, the habitat-only models were weighted with higher 

probabilities (wi=0.39 and 0.53 probability) of containing the best model, after delta AIC rankings (table 

1-8). For the 11 species that were positively associated with beaver activity from t-tests (table 1-6), the 

multiple linear regression models that included beaver factors were ranked higher than those with only 

water or vegetation factors for 55 percent of species. For the remaining 45 percent of these 11 species, 

model weights indicated a <0.005–0.34 probability that beaver factors might explain relative abundance 

(table 1-8). For the four species of conservation concern included in the model-fitting because of their 

non-random association with beaver, beaver models for only one (Yellow Warbler, habitat + number of 

years dams present within 50 m) indicated this as the highest probability model (wi=0.57), whereas for the 

other three species (Yellow-billed Cuckoo, Northern Flicker, and Northern Beardless Tyrannulet), the 

habitat-only model was the best performing and model weights indicated a 0.22 to 0.28 probability that 

beaver factors explained relative abundance (table 1-8). For the six species for which beaver-related

variables improved model performance, four of these [Common Yellowthroat, Lesser Goldfinch, Whitebreasted Nuthatch (Sitta carolinensis), and Cassin’s Kingbird (Tyrannus vociferans)] had habitat models 

further than 2.0 ΔAIC from the top-model, and two species (Yellow Warbler and Song Sparrow) still 

retained the habitat-only model within 2.0 ΔAIC. 

There was little evidence that beaver-related variables explained relative abundance for 2 of the 7 

species groups positively associated with beaver activity, including for the number of 14 riparian 

conservation concern species, or the relative abundance of insectivorous birds (table 1-8). For five of the 

seven species groups that were positively associated with beaver activity, model performance improved 

when we constructed models containing beaver-related variables over the environmental covariate-only 

model. These groups included the overall richness and relative abundance for all breeding bird species, 

and species richness of all 42 riparian-associated species, riparian specialists, and insectivorous birds 
15

table 1-8). For the five groups for which beaver-related variables improved model performance, four had 

environmental covariate-only models that were displaced by the beaver models to greater than 2.0 ΔAIC 

within the top-model, with only one group (insectivorous bird species richness) still retaining the 

covariate-only model within 2.0 ΔAIC (table 1-8). 

Multi-model inference, according to lowest AIC score and Akaike weights (wi), indicated that 

models including beaver-related variables better explained bird abundance or richness (6 of 13 species, 

and 5 of 7 species groups), although the estimates of the effects of beaver and their relative magnitude of 

influence were usually smaller relative to other covariates (table 1-9). For example, after accounting for 

habitat variables, there was some evidence (P=0.06) that Yellow Warbler abundance might decline with 

the greater number of years a site had beaver dams present within 50 m (table 1-9). However, because

both the response and explanatory variables are in the log(10) scale, the effect (a 1 percent increase in the 

number of years with a dam would lead to a 0.097 percent decrease in mean relative abundance) turns out 

to be small, even though statistically significant and contributing to model performance (table 1-9). 

Similarly, a 1 percent increase in the number of years a site had dams within 50 m was associated with 

0.07 percent increase in Song Sparrow abundance. For species groups, the models containing beaverrelated variables contained estimates that were both on the same order of magnitude as other factors and 

the beaver-related variables were significant, or at least nearly so, at the P <0.05 level. For example, a 1 

percent increase in the number of years a site had dams was associated with a 2.5 percent increase in the 

number of breeding species. 

Discussion

We found evidence that relative bird abundance and species richness along the upper San Pedro 

River were better explained by models that included beaver presence or intensity of use than models that 

incorporated only environmental habitat factors. Whereas other researchers—all from outside the 

Southwestern United States—reported greater differences in bird communities in areas with or without 

beaver influence (for example, Bulluck and Rowe, 2006; Cooke and Zack, 2008), and sometimes soon 

after a reintroduction occurred (for example, Medin and Clary, 1990) on the upper San Pedro River the 

effect sizes of beaver-related variables were relatively small (although highly significant) compared to the 

other environmental factors. Considering previous avian research in the study area (Brand and others, 

2008, 2010a, 2011; Krueper and others, 2003) and in the region (Carothers and others, 1974; Anderson 

and Ohmart, 1984), the finding that riparian vegetation and surface water strongly influence avian 

communities on the upper San Pedro is not unexpected. However, the strong association between beaver 

activity and bird abundance and richness on the upper San Pedro River indicates that beaver reintroduction 

in southwestern riparian systems may have an important additive influence on avian populations—

especially given the relatively short length of time beavers had been in system. 

In the majority of species or species groups where model performance was enhanced by fitting rich 

models containing beaver-related variables, the significance and effect size of the beaver variables was 

relatively small compared to the environmental factors related to woody vegetation and surface water. As 

well, the statistically significant relationship observed between beaver activity and bird metrics (from ttests) and the associated effect sizes were reduced after accounting for the habitat/environment covariates 

(from multiple linear regressions). However, although the beaver-related parameter estimates associated 

with bird species groups and relative abundance of individual bird species may seem biologically 

negligible, these estimates pertain to average abundance and species richness at the scale of a 50-m 

sampling stations across a 68.5-km river reach. When extrapolated across the study area, even a seemingly 

negligible difference across 50-m plots may in fact scale up to be biologically important across the entire 

study area. For example, a 73.6 percent difference in Song Sparrow abundance at sites with beaver as 

compared to sites without beaver equates to only 0.33 more birds per station-visit (table 1-6); however, 
16

this is a biologically significant number of individuals when considering all individual birds at the stations 

where both beavers and Song Sparrows were detected (78 stations=78 *0.33=25.74 more birds), or when

extrapolated to the proportion of the study area, these sampling stations represent (78/240=32.5 percent of 

the study reach, 22.26 km is 32.5 percent of 68.5 km, 22.26 km would host 445.2 plots of 50-m, so [445.2 

* 0.33]=146.9 birds per visit across the study area). For birds of conservation concern that are rare on the 

landscape (for example, Yellow-billed Cuckoo), even small differences in the total number of individual 

birds may be crucially important for population maintenance (table 1-6).

Jones and others (2010) described predictive equations related to four cause-and-effect criteria for 

interpreting ecosystem engineering. These included (1) the engineer causes structural change, (2) 

structural change leads to abiotic change, (3) structural and abiotic change leads to biotic change, and (4) 

structural, abiotic, and/or biotic change can feedback to the engineer (Jones and others, 2010). Although 

we did not formally incorporate their approach in our study, the upper San Pedro River would be wellsuited for inclusion as a replicate sampling site in a comparative study that included other sites with 

different levels of the parameters described by Jones and others (2010; that is, time since engineering 

species establishment, magnitude of potential engineer effects given abiotic conditions and other species 

present, etc.).

In our study system, the anticipated abiotic effect of beavers on the bird community was either a 

reduction in canopy cover because of tree-felling, an increase in surface-water retention, or both. We did 

not find evidence that beavers had substantially reduced canopy cover since being reintroduced, as sites 

with beaver had similar tree stem density as sites without beaver. The potential for increased surface-water 

retention because of beaver dam construction was compromised because the dams on the San Pedro River 

typically failed every summer during monsoon flooding, and because active dams raised water levels 

within a highly entrenched channel that was often a natural backwater area already (M. Fredlake, Bureau 

of Land Management, oral commun.), and thus there was little widening of the water courses or a braiding 

effect on floodplain. Thus, the initial (abiotic) pathways of an ecosystem engineering interaction were not 

pronounced or obvious within the time between when the reintroduction began and when we conducted 

our study. 

Nonetheless, USGS groundwater wells adjacent to beaver ponds showed a measurable increase in 

groundwater levels (C. van Riper, U.S. Geological Survey, oral commun.). Any increase in the soil-water 

interface during the dry season may facilitate increased riparian vegetation production, thus providing 

more vegetation and other habitat components for the breeding riparian bird community. An increase in 

water retention also likely will benefit water-obligate bird species. We included all water obligates 

detected within 50 m and the cottonwood-willow obligates into the “riparian specialist” group (because of

low numbers of water-obligate birds detected), which as an entire group was indeed strongly associated 

with beaver activity. 

Although beaver activity was associated with sites that contained a high number of species and 

relative abundance of many species and species groups, we could not rule out the possibility that beaver 

may have just selected habitat that already contained these features. The fairly short length of time 

between the reintroduction and our study, and the apparent lack of signature, pronounced ecosystemengineering effects of beavers, supports this possibility. Future studies of the ecosystem engineering 

effects of beavers in relation to birds or other taxa need to account for the possibility that beaver habitat 

selection may confound the finding of an effect of beavers in relation to birds. This is especially the case 

in the desert Southwest, where riparian vegetation provides abiotic resources (amelioration of extreme 

temperatures and aridity, nest substrate, predator-free-space, etc.) and facilitates trophic opportunities (for 

example, herbivory, insect habitat and foraging, etc.). In fact, willow (Salicaceae) has recently been cited 

for its ecosystem engineer-like ability to alter river hydrology and how this feeds back to the (willow) 
17

population as well as the surrounding riparian vegetation (Moggridge and Gurnell, 2009). No work to date 

has explicitly evaluated the relative or interactive ecosystem engineering effects of beaver and willow (or 

cottonwood) in relation to birds, but this is an important question for future research. 

Beavers are potentially beneficial for some riparian attributes and the ecosystem engineering 

concept can provide valuable insights in the study of beaver, birds, and riparian restoration. However, land 

managers, scientists, and the public should have realistic expectations informed by the site-specific 

context of a potential reintroduction, and ideally make comparisons with other, similar sites before moving 

forward with a reintroduction project. Because the relative effect of any organism on its environment will 

likely be different in a given situation, it may be important to investigate potential engineering effects 

along environmental gradients in order to predict when the strongest “engineering” effects (that is, 

feedbacks to the engineer and other species in the area) are likely to occur (Wright and others, 2006; Jones 

and Gutiérrez, 2007). The reach of the San Pedro River we surveyed contains an environmental gradient in 

terms of water and riparian vegetation cover—both important explanatory factors for beaver and birds, 

and is situated in a more arid region than previous investigations of the relationship between bird metrics 

and beaver activity. In the Southwest region, beaver could potentially provide resources (namely water) 

and/or disturbance processes (opening canopy gaps through herbivory) that might not otherwise occur, or 

at least in greater proportion than would occur without the “engineer” species. 

Although we found evidence that beaver activity was strongly associated with bird abundance and 

richness, the magnitude of the ecosystem engineering effect may have been reduced by the unique 

environmental context occurring on the San Pedro River. First, each year all beaver dams failed during 

heavy monsoon-season flooding (M. Fredlake, Bureau of Land Management, oral commun.). Second, 

because water is limited to certain reaches where groundwater from the local aquifer supplies the surface 

water in the river, and beavers were limited to these reaches, beaver are only able to settle in certain sites 

where water is already present. Third, geomorphic processes exert a dominant influence in the system, 

including large flood disturbances that create patches of early successional habitat, and also tributary 

washes dumping sediment loads within the main channel and creating a back-water behind them (an 

abiotic process mimicking the consequences of dam-building), where beavers often chose to settle. Within 

these back-water areas, channel morphology remained incised, and thus there was little opportunity for 

immediate increases in channel width, even after beaver built dams that raised the water level. Fourth, 

beavers were newly reintroduced (5–6 years before the study) to the system, and thus there was little time 

for the engineering effects to accrue, or for bird populations to respond to these changes. Lastly, the upper 

San Pedro River bird community is already highly diverse, and yet within this diverse community, there is

actually a limited pool of the bird species groups that have (in other studies) shown the most marked 

responses to beaver activity (for example, Medin and Clary, 1990; Cook and Zack, 2008). For example, 

during the breeding season, there are only one waterfowl species, three woodpeckers, one shorebird, and 

two herons regularly present on the San Pedro River (Krueper, 1999). 

Spatial or temporal scales can be important considerations in the study of ecosystem engineers, for 

example, beaver activity can influence stream morphology at the watershed scale, especially when many 

beavers are present (Wright and others, 2006). Similarly, the effect of a single beaver’s activity can extend 

for many decades beyond its lifetime; for example, impounded areas can become full of sediment and 

undergo succession from a wetland, to meadow, and finally to forest (Wright and others, 2002, 2003; 

Wright, 2009; Jones and others, 2010). For 2 years, we studied a 70-km reach of a watershed and 240 

sampling stations within that reach, but did not investigate multiple temporal or spatial scales (for 

example, station as compared to a 1, 2, or 10-km reach). To gain further insight, it would likely be 

beneficial to consider other watersheds, or finer scale reaches within a watershed (for example, 1–2 km), 

and naturally occurring beavers at the same time as reintroduced beavers, and ideally within a beforeafter/control-impact study design. A collaboration is underway with other researchers who conducted 
18

studies prior to beaver reintroduction on the San Pedro River (Krueper and others, 2003; Brand and others, 

2010a) to re-survey sites with and without beaver influence (Johnson and van Riper, Chapter 2 of this 

report). We anticipate this study and future work in the upper San Pedro River will help determine the 

extent to which the bird community changed after beaver reintroduction in addition to our current study, 

which measured the association after the reintroduction occurred. 

Conclusion

Most organisms modify their environment to various degrees and interact with other organisms in 

that environment. To the extent that an ecosystem engineering process and its consequences affect 

community dynamics, ecologists can consider whether this environmental modification constitutes an 

interaction, either through indirect, direct, or cascading effects. We found that although bird abundance 

and species richness were certainly associated with beaver activity, the ecosystem engineering-type 

interaction was not as pronounced compared to what has been found in other regions. More study is 

needed to ascertain whether this was because of the relatively short time beavers had been in the system, 

unique environmental conditions, or both. The environmental context of the San Pedro River that could 

limit the ecosystem engineering effect of beavers included disturbance effects greater than those of the 

beaver (monsoon floods) and a deeply entrenched channel that (together) may have resulted in an 

ephemeral or marginal influence of beaver dams. Other conditions important to consider at this site 

include the presence of cottonwood and willow, reaches without surface water (limiting the beaver 

settlement opportunities), and a bird community that was already known for being highly diverse and 

abundant (Brand and others, 2010b). These factors likely contributed to a situation in which the potential 

ecosystem engineering effects of beaver may not be as profound as those seen in other studies. 

Riparian bird communities, especially in the Southwestern United States, are noted for their 

generally high bird abundance and diversity as well as other important ecosystem services, and 

consequently land managers, scientists, and the public support efforts to predict how human-influenced 

impacts such as groundwater withdrawal or climate change will affect this ecosystem type (for example, 

Steiner and others, 2000; Steinitz and others, 2005; Brand and others, 2011). Considerable effort and more 

than 400 million dollars has been allocated toward riparian restoration projects in the Southwest since 

1990, and providing habitat for birds and other wildlife is one of the major intentions—as well as criterion 

for success—for these projects (Follstad-Shah and others, 2007). Restoration ecologists, land managers, 

and the public need to set realistic goals for ecosystem restoration (Hobbs, 2007), and part of this process, 

especially in the Southwest, should involve an evaluation of when and where beaver reintroduction may 

assist in well-defined restoration objectives. 

Beaver reintroduction did not appear to have detrimental effects on any species of conservation 

concern and, in fact, there was evidence that a breeding bird community is more abundant and more 

diverse where beavers were present. Future research is needed to determine when, and under what 

conditions, beaver may be helpful to the full complement of riparian restoration goals as defined by 

Fredlake (1997) and under what conditions they may be detrimental to conservation objectives (for 

example, recovery or maintenance of Yellow-billed Cuckoo populations). Our findings indicate that there 

appears to be no cause for alarm when beavers are found actively utilizing riparian vegetation (for 

example, see also Longcore and others, 2007), at least in areas where beaver are native. Our study, as well 

as most of the prior avian research in the region, also suggests that beaver reintroduction and conservation 

of riparian bird communities in the Southwestern United States can co-exist, especially if monitoring of 

cottonwood and willow occurs in conjunction with the effort. 
19

Although more time and investigation is needed to determine the full extent of beaver influence on 

the upper San Pedro River, it is clear that the spatial association of beaver activity and a highly abundant 

and diverse bird community represent an important opportunity to evaluate the ecosystem engineering 

effects of beaver in a unique environmental context. Although we did not investigate other attributes of 

ecosystem restoration that beaver may contribute to, we conclude that beaver are more likely to benefit 

rather than harm San Pedro’s bird community—as long as an adaptive management program continues 

along with monitoring in areas with beaver activity, dense willow or other riparian vegetation, and bird 

species of conservation concern. Rigorous assessment of beaver reintroduction projects need to 

incorporate before-after/control-impact studies (Johnson and van Riper, Chapter 2 of this report) and 

thorough evaluation of unique, site-level ecological context and history, so that the potential for beavers to 

act as riparian restoration agents through their ecosystem engineering behaviors and its consequences can 

be evaluated and repeated where appropriate. 

Acknowledgments

Many people made this research possible and a valuable undertaking for the authors. Mark 

Fredlake, Bill Childress, and Marcia Radke with the Bureau of Land Management’s Sierra Vista office for 

the San Pedro Riparian National Conservation Area kindly facilitated access to the study area, housing 

during the field season, and data from previous research; discussions with Mark Fredlake and his 

introduction to the study area early in the project planning phase were invaluable. Funding for the field 

research was generously provided the U.S. Fish and Wildlife Service Sonoran Joint Venture, T & E, Inc., 

and the U.S. Geological Survey. We sincerely appreciate the funding awarded through the following 

University of Arizona programs: the Silliman Memorial Research Award, the Institute for the Study of 

Planet Earth Graduate Travel Award, Graduate and Professional Student Council Travel Grant, and both 

the International Wildlife Foundation Scholarship and the Arrington Memorial Scholarship provided 

through the College of Agriculture. 

We were privileged to have some extraordinary field assistants: Brian Bielfelt, Morgan Jackson, 

and Sonja Smith were all focused, energetic, and thoroughly capable field biologists who collected 

excellent bird survey data; Jessica Gist, Josh Brown, Stacy Cossell, and Claire Zugmeyer all helped 

immensely with many different aspects of the extensive vegetation measurements and subsequent data 

entry process. We cannot thank all of them enough for their many contributions and great company. We

also thank William Mannan, Courtney Conway, and Cecil Schwalbe, who all provided critical advice and 

much-needed guidance. Arriana Brand and Dave Krueper were both generous with their time, data, and 

wisdom derived from multiple years of avian study in the upper San Pedro River Valley—the solid 

foundation provided by their work made our research possible. In addition to the above, we also thank 

Robert Steidl, William Pine, Joseph Watkins, and Aaron Flesch for advice on different sampling design, 

statistical, or modeling considerations. 
20

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25

Figures

0

100

200

300

400

500

Birds Counted

-10 0 10 20 30 40 50 60 70 80 90 100110

Distance

Figure 1-1. Number of birds counted (Y axis) within 1 meter distance bins (X axis) along the upper San Pedro River, 

May–July 2005 and 2006. A total of 22,166 bird detections are represented.

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

C-W Basal Area at Sites 

Live C/W Tree

Gnawed

Cottonwood

Gnawed Willow

Stumps

Figure 1-2. Percentage of total basal area of all beaver-gnawed (but standing) cottonwood (Populus fremontii) and 

willow trees (Salix spp.), and combined basal area of cottonwood-willow stumps and live trees (unaffected by beaver 

[Castor canadensis]) across all sites within each of the low-, moderate-, and high-use beaver intensity classes, and 

sites with no beaver use.
26

Figure 1-3. Breeding bird species richness (mean number of 76 species detected within 50 meters of stations) at 

sites where beavers (Castor canadensis) absent (n=154) and present (n=86); includes 95-percent confidence 

intervals from t test (table 1-5).

18.9

21.3

0

2

4

6

8

10

12

14

16

18

20

22

24

Absent

No. Breeding Species 

Beaver - / +

Present
27

Figure 1-4. Relative abundance of all breeding birds (average number of individuals detected per survey visit within 

50 m) at sites where beavers (Castor canadensis) absent (n=154) and present (n=86); includes 95-percent 

confidence intervals from t test (table 1-5).

10.0

12.7

0

2

4

6

8

10

12

14

Absent Present

All Bird Abundance (N/effort)

Beaver - / +
28

Tables

Table 1-1. Environmental variables used in stepwise variable selection and final habitat/covariate models. 

[Variables followed by “Lg” were transformed using Log10 (X +1)] 

Abbreviation Description

Wash <200 Indicator (=1) if a wash inlet within 200 m (brings in sediment, beavers may settle).

H2O May Indicator (=1) if there is water at the point in late May.

H2O PER Indicator (=1) if water persists throughout summer (perennial).

Rip Veg Width Lg Total width of riparian area vegetation, measured perpendicular to river (includes river channel).

BACH Av%Cov Lg Average percent cover of Baccharis (seep willow) within 3 lowest height layers.

MESQ Av%Cov Lg Average percent cover of Mesquite within 5 lowest height layers.

TAMI Av%Cov Lg Average percent cover of Tamarisk within 4 lowest height layers.

SALI Av%Cov Lg Average percent cover of Willow (Salix goodingi) within 6 lowest height layers.

POFR Av%Cov Lg Average percent cover of Cottonwood (Populus freemontii) within 9 height layers measured.

SumPOFR %Cov 20-30m Lg The summed percent cover of Cottonwood from both of the upper canopy strata (20–25 m, 25–30 m).

ALL TAMI LT Lg Summed basal area of live Tamarisk stems, in m2/2,826 m2 (tree basal area per 30-m radius plot), all 

size classes summed.

ALL MESQ Lg Summed basal area of live Mesquite stems, in m2/2,826 m2 (tree basal area per 30-m radius plot), all 

size classes summed.

SALI <12cmDBH Lg Summed basal area of small Willow stems (<12 cm DBH size class), per m2/2,826 m2 (tree basal area, 

per 30-m radius plot).

POFR 12-25cmDBH Lg Summed basal area of medium Cottonwood stems (12–25 cm DBH size class), per m2/2,826 m2 (tree 

basal area, per 30-m radius plot).

SALI 12-25cmDBH Lg Summed basal area of medium Willow stems (12–25 cm DBH size class), per m2/2,826 m2 (tree basal 

area, per 30-m radius plot).

POFR >25cmDBH Lg Summed basal area of large Willow trees (>25 cm DBH size class), per m2/2,826 m2 (tree basal area, 

per 30-m radius plot).

SALI >25cmDBH Lg Summed basal area of large Cottonwood trees (>25 cm DBH size class), per m2/2,826 m2 (tree basal 

area, per 30-m radius plot).

Veg Diversity Lg Index of vegetation structure, from 1-8, summed values from allocating a "1" to each height strata 

level with vegetation in it. 

Beaver Presence Presence inferred by dams and/or sign within 100 m of the station (in any year), and included gnawed 

trees, stumps, bank dens, and other signs of occupation (= 1, indicator variable).

Num Yr BD <51 Lg Number of years a beaver dam was within 50 m of point (continuous variable)
29

Abbreviation Description

Beaver Level 1 Low use: beaver occupancy in one or more years (2000–2006) within 100 m of the avian survey 

station, no dams documented (n=34)

Beaver Level 2 Moderate use: stations where a dam was documented within 100 m upstream or 150 m downstream 

during 1 or 2 years, and no more than 1 year with a dam within 50 m (n=33).

Beaver Level 3 High use: stations where dams were located within 50 m for2 or more years, and/or where dams were 

located within 100 m for 3 or more years (n=19). 
30

Table 1-2. Number of survey stations where beaver (Castor canadensis) dams were documented and total number of 

dams documented each year from 2000 to 2006 within the San Pedro National Riparian Conservation Area, Arizona. 

[Survey stations were established in 2005. Dams were documented using GPS and/or GIS prior to spring 2005 by Bureau of 

Land Management personnel and by bird surveyors in 2005 and 2006, and were complete censuses] 

2000 2001 2002 2003 2004 2005 2006

1All Dams on upper River 5 6 17 20 25 25 32

Stations with Dam ≤ 50 m 2 5 8 8 11 8 22

Stations with Dam 51–100 m 2 1 5 11 9 5 7 2

Stations w/ Dam 101–150 m 0 0 2 1 3 2 3

Total stations with Dams 4 6 15 20 23 15 32

1

Includes total count of dams, that is, when more than one dam was located within 100 m of a station, or when dams were more 

than 150 m downstream or 100 m upstream of any station. 2

Includes only dams 100–150 m downstream of stations. 

Table 1-3. Beaver (Castor canadensis) activity from 2000 to 2006 at 240 stations in the San Pedro Riparian National 

Conservation Area, Arizona, used to quantify beaver presence/absence and beaver intensity.

Beaver influence Number of stations

Beaver occupancy within 100 m ≥ 1year, but no dams 34

Beaver dams impounding water within 100 m:

Dam present 1 year 26

Dam present 2 years 8

Dam present 3 years 8

Dam present 4 years 5

Dam present 5 years 2

Dam present 6 years 2

Dam present 7 years 1

Total stations with dams 52

Total stations with some beaver influence 86

No beaver activity documented (within 100 m) 154

Total number of stations 240
31

Table 1-4. Common and scientific names of 76 breeding bird species detected within 50 meters of 240 survey stations on the San Pedro Riparian National 

Conservation Area, Arizona, 2005–2006. 

[Includes total detections, number and percent of stations where detected. Riparian and species groupings used for species richness, riparian and insectivorous species group 

analyses include the following grouping codes: Riparian (R) and if so whether a cottonwood, willow, and/or water specialist (CW) or riparian generalist (Gen) in our study 

system according to Kruper (1999) and/or Brand and others (2010a), if a conservation concern species as designated by the U.S. Fish and Wildlife Service (2008; “C”) 

and/or Hunter and others (1987, “c”), and if primarily insectivorous (I). Common and scientific nomenclature follow the American Ornithologists Union (2011).] 

Common name Scientific name

Total 

detections

Number of 

stations

detected Freq (%)

Riparian/

Spp. group

Yellow Warbler Setophaga petechial 1,721 224 93 R(CW),C, I

Yellow-breasted Chat Icteria virens 1,170 227 95 R(Gen),c, I

Lesser Goldfinch Spinus psaltria 991 191 80 R(CW)

Bewick's Wren Thryomanes bewickii 724 217 90 R(Gen), I

Song Sparrow Melospiza melodia 702 177 74 R(CW), I

House Finch Carpodacus mexicanus 616 192 80

White-winged Dove Zenaida asiatica 612 199 83

Brown-headed Cowbird Molothrus ater 564 192 80 R(Gen), I

Abert's Towhee Melozone aberti 562 196 82 R(Gen)

Vermilion Flycatcher Pyrocephalus rubinus 518 183 76 R(CW),c, I

Cassin's Kingbird Tyrannus vociferans 510 163 68 R(CW), I

Gila Woodpecker Melanerpes uropygialis 494 188 78 R(CW),C, I

Common Yellowthroat Geothlypis trichas 472 172 72 R(CW), I

Summer Tanager Piranga rubra 424 178 74 R(CW),c, I

Bell's Vireo Vireo bellii 357 149 62 R(Gen),C, I

Mourning Dove Zenaida macroura 242 123 51

White-breasted Nuthatch Sitta carolinensis 233 130 54 R(CW), I

Western Wood-Pewee Contopus sordidulus 227 116 48 R(CW), I

Brown-crested Flycatcher Myiarchus tyrannulus 174 99 41 R(CW),c, I

Lucy's Warbler Oreothlypis luciae 169 112 47 R(Gen),C, I

Blue Grosbeak Passerina caerulea 166 104 43 R(Gen), I

Ash-throated Flycatcher Myiarchus cinerascens 136 87 36 I

Ladder-backed Woodpecker Picoides scalaris 129 94 39 R(Gen), I

Northern Flicker Colaptes auratus 104 66 28 R(CW),c, I

Black-chinned Hummingbird Archilochus alexandri 99 76 32 R(Gen)

Phainopepla Phainopepla nitens 99 42 18 C

Mallard Anas platyrhynchos 90 45 19 R(CW)

Northern Beardless-Tyrannulet Camptostoma imberbe 89 62 26 R(Gen),C, I

Common Ground-Dove Columbina passerina 84 61 25
32

Common name Scientific name

Total 

detections

Number of 

stations detected Freq (%)Riparian/

Spp. group

Northern Mockingbird Mimus polyglottos 76 55 23I

Bullock's Oriole Icterus bullockii 74 57 24 R(CW), I

Western Kingbird Tyrannus verticalis 70 37 15I

Gambel's Quail Callipepla gambelii 66 44 18

Northern Rough-winged Swallow Stelgidopteryx serripennis 65 32 13 R(CW), I

Black PhoebeSayornis nigricans 64 44 18 R(CW), I

Cliff Swallow Petrochelidon pyrrhonota 62 146 R(CW), I

Gray Hawk Buteo nitidus 62 39 16 R(CW)

Yellow-billed Cuckoo Coccyzus americanus 61 45 19 R(CW),C, I

Bushtit Psaltriparus minimus 58 23 10I

Northern Cardinal Cardinalis cardinalis 55 42 18 R(Gen), I

Great Blue Heron Ardea herodias 50 35 15 R(CW)

Red-winged Blackbird Agelaius phoeniceus 44 156 R(CW), I

Botteri's Sparrow Peucaea botterii 26 177 C, I

Black-throated Sparrow Amphispiza bilineata 24 125I

Verdin Auriparus flaviceps 22 146I

Great Horned Owl Bubo virginianus 21 146 R(CW)

Turkey Vulture Cathartes aura 1783

Chihuahuan Raven Corvus cryptoleucus 1594

Killdeer Charadrius vociferus 1552 R(CW), I

Cooper's Hawk Accipiter cooperii 1394 R(CW),c

Common Raven Corvus corax 1273

House Sparrow Passer domesticus 1221

Tropical Kingbird Tyrannus melancholicus 1152 R(CW), I

Varied Bunting Passerina versicolor 1194 C, I

Black-headed Grosbeak Pheucticus melanocephalus 10 104 R(Gen), I

Bridled Titmouse Baeolophus wollweberi 1042 R(CW), I

Canyon Towhee Melozone fusca963C

Crissal Thrasher Toxostoma crissale963I

Red-tailed Hawk Buteo jamaicensis873

American Kestrel Falco sparverius773I

Cassin's Sparrow Peucaea cassinii773 ,C, I

Hooded Oriole Icterus cucullatus763 R(Gen),c, I

Lesser Nighthawk Chordeiles acutipennis642I

Common Nighthawk Chordeiles minor552I

Greater Roadrunner Geococcyx californianus442

Curve-billed Thrasher Toxostoma curvirostre331I

European Starling Sturnus vulgaris310I
33

Common name Scientific name

Total 

detections

Number of 

stations

detected Freq (%)

Riparian/

Spp. group

Pyrrhuloxia Cardinalis sinuatus 3 2 1

Say's Phoebe Sayornis saya 3 3 1 I

Willow Flycatcher Empidonax traillii 3 2 1 R(CW),c, I

Cactus Wren Campylorhynchus brunneicapillus 2 2 1 I

Green Heron Butorides virescens 2 2 1 R(CW)

Scaled Quail Callipepla squamata 2 2 1

Eastern Meadowlark Sturnella magna 1 1 0 I

Western Screech-Owl Megascops kennicottii 1 1 0 R(Gen)

White-tailed Kite Elanus leucurus 1 1 0
34

Table 1-5. Species richness and relative abundance of species groups at sites with and without beavers (Castor canadensis). 

(Two sided t test for difference in means, DF=239). 

Beaver absent

(n= 154)

Beaver present

(n=86)

Difference 

(%) Significance

Species roup

Freq. 

(%) Mean SE

Freq. 

(%) Mean SE t P

Number of Breeding Bird Species (of 76 spp.) 100 18.90 0.29 100 21.31 0.38 12.8 5.44 <0.001

Relative Abundance of Breeding Bird Species 100 9.99 0.23 100 12.70 0.31 27.1 7.04 <0.001

Number of Riparian Species (of 42) 100 14.76 0.25 100 17.08 0.33 15.7 5.67 <0.001

Number of Riparian Specialists Species (of 14) 100 8.56 0.21 100 10.87 0.28 26.9 6.61 <0.001

Number of Riparian Generalist Species (of 28) 100 6.19 0.13 100 6.21 0.17 0.2 0.07 0.947

Number of Riparian Conservation C. spp. (of 14) 100 6.29 0.13 100 6.76 0.17 7.4 2.12 0.035

Number of Insectivorous spp. (of 52) 100 12.42 0.22 100 14.20 0.29 14.3 4.90 <0.001

Relative Abundance of Insectivorous Group 100 6.59 0.18 100 8.25 0.24 25.2 5.60 <0.001
35

Table 1-6. Mean relative abundance for most commonly detected bird species (n=31) at sites with and without beaver (Castor canadensis) . 

[Two sided t test for difference in means, DF=239). * denotes species of conservation concern from U.S. Fish and Wildlife Service (2008) and/or Hunter and others (1987). 

Beaver absent

(n= 154)

Beaver present

(n=86)

Difference (%) Significance

Species Freq. 

(%)

Mean SE Freq. 

(%)

Mean SE t P

Yellow-billed Cuckoo* 12 0.03 0.010 30 0.08 0.013 167.4 3.14 0.002

Common Yellowthroat 63 0.28 0.029 87 0.57 0.039 105.2 5.95 <0.001

Black Phoebe 15 0.04 0.010 24 0.07 0.014 90.4 2.06 0.041

Lesser Goldfinch 73 0.63 0.059 92 1.13 0.078 79.4 5.11 <0.001

Northern Flicker* 21 0.07 0.014 38 0.12 0.018 78.0 2.27 0.024

Song Sparrow 64 0.45 0.042 91 0.78 0.056 73.6 4.75 <0.001

White-breasted Nuthatch 50 0.15 0.019 62 0.25 0.025 65.2 3.24 0.001

Mallard 14 0.06 0.017 27 0.10 0.023 59.4 1.27 0.205

Western Wood-Pewee 39 0.15 0.020 65 0.24 0.027 55.8 2.58 0.011

Brown-headed Cowbird 77 0.39 0.034 85 0.59 0.046 50.2 3.41 0.001

Cassin's Kingbird 64 0.35 0.035 76 0.52 0.047 48.6 2.91 0.004

Ladder-backed Woodpecker 34 0.09 0.012 49 0.13 0.017 45.1 1.94 0.054

Blue Grosbeak 40 0.12 0.015 49 0.17 0.020 44.2 2.01 0.045

Mourning Dove 45 0.17 0.021 62 0.24 0.028 43.4 2.11 0.036

House Finch 74 0.44 0.041 91 0.61 0.054 40.6 2.61 0.010

White-winged Dove 81 0.44 0.032 87 0.60 0.043 37.9 3.07 0.002

Yellow Warbler* 90 1.24 0.062 99 1.64 0.083 32.1 3.81 0.000

Gila Woodpecker* 75 0.37 0.032 85 0.46 0.042 25.8 1.80 0.073

Common Ground-Dove 25 0.07 0.011 26 0.08 0.015 13.9 0.49 0.626

Lucy's Warbler* 45 0.13 0.016 49 0.14 0.021 5.5 0.28 0.780

Summer Tanager* 74 0.34 0.026 74 0.35 0.034 3.2 0.25 0.799

Yellow-breasted Chat* 95 0.93 0.046 93 0.94 0.061 1.6 0.20 0.844

Abert's Towhee 81 0.45 0.029 83 0.45 0.039 -0.7 -0.06 0.951

Brown-crested Flycatcher* 39 0.14 0.018 45 0.14 0.023 -0.9 -0.04 0.966

Gray Hawk* 15 0.05 0.010 19 0.05 0.014 -4.7 -0.14 0.892

Bell's Vireo* 65 0.29 0.025 57 0.27 0.034 -6.7 -0.47 0.642

Vermilion Flycatcher 75 0.43 0.029 78 0.39 0.039 - 8.8 -0.79 0.433

Bullock's Oriole 25 0.06 0.010 21 0.06 0.013 -10.8 -0.40 0.686

Bewick's Wren 90 0.61 0.034 91 0.53 0.046 -13.4 -1.42 0.158

Ash-throated Flycatcher 44 0.14 0.015 23 0.06 0.020 -55.3 -3.03 0.003

Northern Beardless-Tyrannulet* 31 0.09 0.011 17 0.04 0.014 -60.0 -3.02 0.003
36

Table 1-7. Habitat and Environmental Covariate models for species and species groups from multiple linear 

regression (n=240) using stepwise variable-selection (minimum BIC to enter) selected model terms. 

[Significant P-values (≤0.05) indicated by *] 

Parameter Estimate SE t P

Yellow-billed Cuckoo

Intercept 0.002 0.005 0.420 0.6745

H2O PER 0.016 0.006 2.790 0.0057*

SumPOFR %Cov 20-30m Lg 0.017 0.006 2.610 0.0097*

Common Yellowthroat

Intercept -0.094 0.073 -1.280 0.2034

H2O May 0.048 0.019 2.530 0.0121*

BACH Av%Cov Lg 0.102 0.021 4.800 <.0001*

MESQ Av%Cov Log -0.061 0.017 -3.490 0.0006*

TAMI Av%Cov Lg -0.047 0.019 -2.500 0.0132*

POFR Av%Cov Lg -0.121 0.028 -4.380 <.0001*

POFR 12-25cmDBH Lg -0.163 0.060 -2.700 0.0075*

Veg Diversity Lg 0.306 0.098 3.110 0.0021*

Black Phoebe

Intercept 0.000 0.008 -0.060 0.9494

H2O PER 0.020 0.006 3.420 0.0007*

POFR Av%Cov Lg 0.040 0.012 3.190 0.0016*

POFR >25cmDBH Lg -0.047 0.016 -2.970 0.0033*

Lesser Goldfinch

Intercept 0.087 0.020 4.310 <.0001*

H2O May 0.086 0.028 3.100 0.0022*

H2O PER 0.106 0.023 4.680 <.0001*

SALI <12cmDBH Lg 1.573 0.494 3.180 0.0016*

POFR 12-25cmDBH Lg 0.437 0.085 5.110 <.0001*

SumPOFR %Cov 20-30m Lg -0.073 0.023 -3.160 0.0018*

Northern Flicker

Intercept 0.046 0.008 5.760 <.0001*

H2O PER 0.026 0.008 3.440 0.0007*

MESQ Av%Cov Lg -0.036 0.010 -3.470 0.0006*

POFR >25cmDBH Lg -0.034 0.014 -2.350 0.0197*

Song Sparrow

Intercept 0.014 0.015 0.950 0.3417

H2O May 0.148 0.019 7.610 <.0001*

H2O PER 0.047 0.017 2.840 0.0049*

SALI <12cmDBH Lg 1.555 0.374 4.160 <.0001*
37

Parameter Estimate SE t P

White-breasted Nuthatch

Intercept 0.027 0.009 3.000 0.0030*

POFR >25cmDBH Lg 0.055 0.023 2.360 0.0192*

SumPOFR %Cov 20-30m Lg 0.034 0.013 2.540 0.0117*

Western Wood-Pewee

Intercept -0.003 0.011 -0.320 0.7528

H2O May 0.054 0.013 4.320 <.0001*

POFR >25cmDBH Lg 0.069 0.021 3.330 0.0010*

Brown-headed Cowbird

Intercept 0.117 0.010 11.950 <.0001*

H2O PER 0.068 0.015 4.630 <.0001*

Cassin's Kingbird

Intercept 0.091 0.011 8.320 <.0001*

H2O PER 0.058 0.016 3.600 0.0004*

SALI <12cmDBH Lg 0.976 0.401 2.430 0.0157*

Yellow Warbler

Intercept 0.080 0.023 3.470 0.0006*

Wash <200 0.057 0.021 2.710 0.0073*

H2O May 0.148 0.020 7.290 <.0001*

POFR Av%Cov Lg 0.180 0.026 6.820 <.0001*

SALI >25cmDBH Lg 0.242 0.092 2.620 0.0094*

Ash-throated Flycatcher

Intercept 0.072 0.009 8.310 <.0001*

Wash <200 -0.026 0.010 -2.580 0.0105*

H2O May -0.039 0.009 -4.150 <.0001*

ALL TAMI LT Lg 0.419 0.174 2.410 0.0168*

Northern Beardless-Tyrannulet

Intercept 0.037 0.004 8.780 <.0001*

H2O PER -0.029 0.006 -4.740 <.0001*

ALL MESQ Lg 0.155 0.045 3.480 0.0006*

Species Richness 76 Breeding Bird Species (BBS)

Intercept 22.075 2.076 10.630 <.0001*

H2O May 2.679 0.590 4.540 <.0001*

Rip Veg Width Lg -2.977 0.972 -3.060 0.0025*

SALI Av%Cov Lg 2.279 0.834 2.730 0.0068*

Relative Abundance of 76 BBS

Intercept 3.168 1.575 2.0100 0.0455*

H2O May 2.675 0.456 5.8700 <.0001*

H2O PER 1.045 0.361 2.8900 0.0042*

SALI <12cmDBH Lg 36.390 8.085 4.5000 <.0001*

Veg Diversity Lg 5.465 1.951 2.8000 0.0055*
38

Parameter Estimate SE t P

Species Richness of 42 Riparian BBS

Intercept 11.811 0.630 18.760 <.0001*

H2O May 2.027 0.547 3.700 0.0003*

TAMI Av%Cov Lg -1.512 0.537 -2.820 0.0053*

SALI Av%Cov Lg 2.008 0.664 3.030 0.0028*

POFR Av%Cov Lg 1.876 0.559 3.360 0.0009*

Species Richness of Riparian Specialists

Intercept 0.893 1.403 0.640 0.5254

H2O May 2.771 0.403 6.870 <.0001*

H2O PER 1.115 0.315 3.540 0.0005*

Veg Diversity Lg 6.774 1.739 3.900 0.0001*

Species Richness of 14 conservation concern species

Intercept 1.629 0.976 1.670 0.0966

SALI >25cmDBH Lg 3.143 1.134 2.770 0.0060*

Veg Diversity Lg 5.346 1.134 4.720 <.0001*

Species Richness of Insectivorous Riparian Birds

Intercept 10.350 0.576 17.960 <.0001*

H2O May 1.446 0.501 2.890 0.0042*

TAMI Av%Cov Lg -1.457 0.491 -2.970 0.0033*

SALI Av%Cov Lg 1.494 0.608 2.460 0.0147*

POFR Av%Cov Lg 1.447 0.512 2.830 0.0051*

Relative Abundance of Insectivorous Riparian Birds

Intercept 1.013 1.201 0.840 0.3996

H2O May 2.065 0.339 6.090 <.0001*

BACH Av%Cov Lg 1.403 0.426 3.290 0.0012*

POFR 12-25cmDBH Lg -2.926 1.053 -2.780 0.0059*

SALI 12-25cmDBH Lg 6.220 1.960 3.170 0.0017*

Veg Diversity Lg 4.261 1.525 2.790 0.0056*
39

Table 1-8. Comparison of models incorporating beaver-related variables to habitat- and environmental covariate-only models for species and species groups, 

respectively. 

[Habitat and Environmental Covariate models for species and species groups, from multiple linear regression (n=240) using stepwise variable selection (minimum BIC to 

enter) selected model terms] 

Species Model R2 K LL AICc BIC ΔAIC Wi

Yellow-billed Cuckoo

Habitat 0.083 3 10122.93 -829.52 -815.77 0.00 0.4261

Habitat + Beaver presence 0.088 4 13541.18 -828.69 -811.55 0.83 0.2819

Habitat + No. years with dam 0.083 4 13522.09 -827.52 -810.38 1.99 0.1572

Habitat * Beaver presence 0.093 6 20414.80 -825.81 -801.93 3.71 0.0666

Habitat + Beaver intensity 0.088 6 20383.28 -824.53 -800.65 4.99 0.0352

Habitat * No. years with dam 0.086 6 20372.06 -824.08 -800.20 5.44 0.0281

Habitat * Beaver intensity 0.097 9 30825.23 -820.58 -786.73 8.94 0.0049

Common Yellowthroat

Habitat * Beaver presence 0.400 10 19799.39 -472.30 -435.17 0.00 0.4046

Habitat + Beaver presence 0.391 9 17686.49 -470.82 -436.98 1.48 0.1933

Habitat + No. years with dam 0.390 9 17668.01 -470.33 -436.48 1.97 0.1512

Habitat 0.384 8 15634.42 -470.25 -439.71 2.04 0.1456

Habitat + Beaver intensity 0.399 11 21750.17 -469.61 -429.21 2.69 0.1053

Black Phoebe

Habitat 0.074 4 13283.43 -812.92 -795.77 0.00 0.4767

Habitat + No. years with dam 0.078 5 16654.03 -811.88 -791.36 1.03 0.2842

Habitat + Beaver presence 0.076 5 16639.10 -811.16 -790.63 1.76 0.1975

Habitat + Beaver intensity 0.080 7 23405.33 -808.04 -780.82 4.88 0.0416

Lesser Goldfinch

Habitat + Beaver presence 0.353 7 7309.42 -252.35 -225.13 0.00 0.6760

Habitat + Beaver intensity 0.359 9 9400.81 -250.25 -216.41 2.10 0.2370

Habitat 0.334 6 6118.41 -247.50 -223.62 4.85 0.0598

Habitat + No. years with dam 0.336 7 7123.20 -245.92 -218.70 6.43 0.0272
40

Species Model R2 K LL AICc BIC ΔAIC Wi

Northern Flicker

Habitat 0.118 4 11451.70 -700.82 -683.67 0.00 0.5347

Habitat + Beaver presence 0.119 5 14340.68 -699.11 -678.59 1.71 0.2272

Habitat + No. years with dam 0.118 5 14334.03 -698.78 -678.26 2.04 0.1932

Habitat + Beaver intensity 0.123 7 20156.18 -695.87 -668.65 4.95 0.0450

Song Sparrow

Habitat + No. years with dam 0.424 5 7734.22 -377.04 -356.52 0.00 0.4922

Habitat 0.417 4 6148.79 -376.29 -359.15 0.75 0.3383

Habitat + Beaver presence 0.417 5 7675.94 -374.20 -353.68 2.84 0.1189

Habitat + Beaver intensity 0.423 7 10789.54 -372.50 -345.27 4.55 0.0507

White-breasted Nuthatch

Habitat + Beaver presence 0.134 4 9265.38 -567.02 -549.88 0.00 0.5456

Habitat + Beaver intensity 0.144 6 13982.59 -565.62 -541.73 1.41 0.2700

Habitat 0.116 3 6884.05 -564.11 -550.36 2.91 0.1272

Habitat + No. years with dam 0.117 4 9191.67 -562.51 -545.36 4.51 0.0572

Western Wood-Pewee

Habitat 0.172 3 6783.03 -555.83 -542.08 0.00 0.4971

Habitat + Beaver presence 0.175 4 9062.17 -554.59 -537.44 1.25 0.2666

Habitat + No. years with dam 0.172 4 9049.93 -553.84 -536.69 2.00 0.1833

Habitat + Beaver intensity 0.178 6 13630.09 -551.36 -527.48 4.48 0.0530

Brown-headed Cowbird

Habitat 0.083 2 2940.18 -362.93 -352.59 0.00 0.3884

Habitat + Beaver presence 0.088 3 4421.74 -362.34 -348.58 0.59 0.2889

Habitat + Beaver intensity 0.101 5 7413.46 -361.41 -340.88 1.52 0.1814

Habitat + No. years with dam 0.083 3 4404.28 -360.91 -347.15 2.02 0.1413

Cassin's Kingbird

Habitat + Beaver intensity 0.134 6 8457.73 -342.13 -318.25 0.00 0.6016

Habitat 0.101 3 4144.77 -339.64 -325.89 2.49 0.1736

Habitat + No. years with dam 0.108 4 5544.08 -339.29 -322.14 2.84 0.1453

Habitat + Beaver presence 0.103 4 5524.34 -338.08 -320.93 4.05 0.0795
41

Species Model R2 K LL AICc BIC ΔAIC Wi

Yellow Warbler

Habitat + No. years with dam 0.420 6 7810.14 -315.93 -292.05 0.00 0.5665

Habitat 0.411 5 6450.42 -314.46 -293.94 1.47 0.2712

Habitat + Beaver presence 0.412 6 7729.67 -312.68 -288.80 3.26 0.1113

Habitat + Beaver intensity 0.418 8 10343.60 -311.12 -280.57 4.82 0.0510

Ash-throated Flycatcher

Habitat 0.122 4 10847.07 -663.82 -646.67 0.00 0.3943

Habitat + No. years with dam 0.129 5 13611.57 -663.56 -643.04 0.25 0.3472

Habitat + Beaver presence 0.126 5 13594.09 -662.71 -642.19 1.11 0.2268

Habitat + Beaver intensity 0.127 7 19081.82 -658.78 -631.56 5.04 0.0317

Northern Beardless-Tyrannulet

Habitat 0.126 3 9572.06 -784.38 -770.63 0.00 0.5395

Habitat + Beaver presence 0.128 4 12789.83 -782.71 -765.57 1.67 0.2345

Habitat + No. years with dam 0.126 4 12784.03 -782.36 -765.21 2.02 0.1964

Habitat + Beaver intensity 0.128 6 19247.15 -778.57 -754.69 5.80 0.0296

Species Groups

Species Richness 76 Breeding Bird Species 

Habitat + Beaver intensity 0.296 7 -36014.83 1243.37 1270.59 0.00 0.7948

Habitat + Beaver presence 0.274 5 -25572.86 1246.68 1267.20 3.31 0.1520

Habitat + No. years with dam 0.265 5 -25631.20 1249.52 1270.04 6.15 0.0367

Habitat 0.254 4 -20443.72 1251.11 1268.26 7.74 0.0165

Relative Abundance of 76 Breeding Bird Species

Habitat + Beaver presence 0.457 6 -27095.11 1096.04 1119.92 0.00 0.6868

Habitat + Beaver intensity 0.460 8 -36532.86 1098.84 1129.38 2.80 0.1689

Habitat 0.443 5 -22561.15 1099.86 1120.38 3.82 0.1016

Habitat + No. years with dam 0.444 6 -27232.54 1101.59 1125.48 5.56 0.0426

Species Richness of 42 Riparian Breeding Bird Species

Habitat + Beaver intensity 0.393 8 -37986.41 1142.56 1173.10 0.00 0.5584

Habitat + Beaver presence 0.379 6 -28279.08 1143.93 1167.81 1.37 0.2818

Habitat + No. years with dam 0.373 6 -28329.78 1145.98 1169.86 3.42 0.1010

Habitat 0.365 5 -23529.50 1147.06 1167.59 4.50 0.0588
42

Species Model R2 K LL AICc BIC ΔAIC Wi

Species Richness of Riparian Specialists

Habitat + Beaver intensity 0.467 7 -30194.09 1042.42 1069.64 0.00 0.3540

Habitat + Beaver presence 0.457 5 -21384.78 1042.51 1063.03 0.09 0.3380

Habitat 0.450 4 -17052.20 1043.56 1060.71 1.14 0.1998

Habitat + No. years with dam 0.452 5 -21431.51 1044.79 1065.31 2.37 0.1082

Species Richness of 14 Conservation Concern Spp.

Habitat 0.134 3 -10851.80 889.24 903.00 0.00 0.5283

Habitat + No. years with dam 0.135 4 -14558.93 890.98 908.12 1.73 0.2223

Habitat + Beaver presence 0.134 4 -14562.27 891.18 908.33 1.94 0.2007

Habitat + Beaver intensity 0.139 6 -22100.92 894.01 917.89 4.77 0.0487

Species Richness of Insectivorous Riparian Birds

Habitat + Beaver intensity 0.319 8 -36691.52 1103.61 1134.16 0.00 0.3587

Habitat + Beaver presence 0.306 6 -27286.92 1103.79 1127.68 0.18 0.3275

Habitat 0.298 5 -22658.75 1104.61 1125.14 1.00 0.2174

Habitat + No. years with dam 0.299 6 -27347.42 1106.24 1130.12 2.63 0.0964

Relative Abundance of Insectivorous Riparian Birds

Habitat 0.433 6 -23896.12 966.63 990.51 0.00 0.3866

Habitat + Beaver presence 0.436 7 -28025.13 967.53 994.76 0.90 0.2461

Habitat + Beaver intensity 0.446 9 -36351.82 967.70 1001.55 1.07 0.2267

Habitat + No. years with dam 0.433 7 -28057.59 968.65 995.88 2.02 0.1406
43

Table 1-9. Result of multiple linear regression (n=240) models after adding beaver-related factors to habitat/covariate 

models (table 1-7), for species and species groups. 

[Significant P-values (≤ 0.05) indicated by *] 

Parameter Estimate SE t P

Yellow-billed Cuckoo

Habitat + Beaver Presence

Intercept 0.001 0.005 0.310 0.7554

H2O PER 0.012 0.007 1.860 0.0644

SumPOFR %Cov 20-30m Lg 0.016 0.007 2.430 0.0160*

Beaver Presence 0.008 0.007 1.110 0.2660

Habitat + Number of years with dam

Intercept 0.002 0.005 0.420 0.6744

H2O PER 0.016 0.006 2.570 0.0108*

SumPOFR %Cov 20-30m Lg 0.017 0.006 2.570 0.0107*

Num Yr BD <51 Lg 0.005 0.018 0.300 0.7627

Common Yellowthroat

Habitat * Beaver presence

Intercept -0.051 0.075 -0.680 0.4983

BACH Av%Cov Lg 0.100 0.021 4.720 <.0001*

MESQ Av%Cov Lg -0.056 0.018 -3.210 0.0015*

TAMI Av%Cov Lg -0.036 0.019 -1.860 0.0640

POFR Av%Cov Lg -0.122 0.028 -4.410 <.0001*

POFR 12-25cmDBH Lg -0.155 0.060 -2.570 0.0108*

Veg Diversity Lg 0.296 0.098 3.020 0.0028*

H2O May -0.012 0.035 -0.330 0.7397

Beaver Presence 0.059 0.024 2.480 0.0137*

(H2O May)*(Beaver Presence) -0.171 0.091 -1.880 0.0609

Habitat + Beaver Presence

Intercept -0.088 0.073 -1.200 0.2316

H2O May 0.043 0.019 2.260 0.0245*

BACH Av%Cov Lg 0.100 0.021 4.660 <.0001*

MESQ Av%Cov Lg -0.055 0.018 -3.120 0.0020*

TAMI Av%Cov Lg -0.040 0.019 -2.090 0.0378*

POFR Av%Cov Lg -0.117 0.028 -4.230 <.0001*

POFR 12-25cmDBH Lg -0.153 0.061 -2.530 0.0121*

Veg Diversity Lg 0.287 0.099 2.910 0.0040*

Beaver Presence 0.022 0.014 1.630 0.1045
44

Parameter Estimate SE t P

Habitat +Number of years with dam

Intercept -0.087 0.073 -1.190 0.2358

H2O May 0.048 0.019 2.520 0.0123*

BACH Av%Cov Lg 0.098 0.022 4.550 <.0001*

MESQ Av%Cov Lg -0.058 0.017 -3.310 0.0011*

TAMI Av%Cov Lg -0.043 0.019 -2.280 0.0234*

POFR Av%Cov Lg -0.118 0.028 -4.290 <.0001*

POFR 12-25cmDBH Lg -0.158 0.060 -2.620 0.0094*

Veg Diversity Lg 0.292 0.098 2.970 0.0032*

Num Yr BD <51 Lg 0.055 0.037 1.480 0.1413

Black Phoebe

Habitat + Number of years with dam

Intercept -0.001 0.008 -0.080 0.9398

H2O PER 0.018 0.006 2.870 0.0044*

POFR Av%Cov Lg 0.039 0.012 3.140 0.0019*

POFR >25cmDBH Lg -0.046 0.016 -2.910 0.0040*

Num Yr BD <51 Lg 0.019 0.019 1.030 0.3064

Habitat + Beaver presence

Intercept -0.001 0.008 -0.130 0.8967

H2O PER 0.018 0.007 2.570 0.0108*

POFR Av%Cov Lg 0.040 0.012 3.170 0.0017*

POFR >25cmDBH Lg -0.047 0.016 -2.950 0.0035*

Beaver Presence 0.004 0.007 0.580 0.5628

Lesser Goldfinch

Habitat + Presence

Intercept 0.087 0.020 4.360 <.0001*

H2O May 0.077 0.028 2.790 0.0057*

H2O PER 0.080 0.024 3.310 0.0011*

SALI <12cmDBH Lg 1.516 0.488 3.110 0.0021*

POFR 12-25cmDBH Lg 0.448 0.085 5.300 <.0001*

SumPOFR %Cov 20-30m Lg -0.079 0.023 -3.440 0.0007*

Beaver Presence 0.060 0.023 2.620 0.0093*

Habitat + Beaver intensity

Intercept 0.085 0.020 4.260 <.0001*

H2O May 0.073 0.028 2.620 0.0094*

H2O PER 0.087 0.025 3.510 0.0005*

SALI <12cmDBH Lg 1.583 0.491 3.230 0.0014*

POFR 12-25cmDBH Lg 0.459 0.085 5.410 <.0001*

SumPOFR %Cov 20-30m Lg -0.077 0.023 -3.310 0.0011*

Beaver Level 1 0.082 0.029 2.870 0.0045*

Beaver Level 2 0.030 0.031 0.950 0.3406

Beaver Level 3 0.059 0.037 1.600 0.1108
45

Parameter Estimate SE t P

Northern Flicker

Habitat + Beaver presence

Intercept 0.045 0.008 5.570 <.0001*

H2O PER 0.023 0.009 2.580 0.0104*

MESQ Av%Cov Lg -0.036 0.010 -3.440 0.0007*

POFR >25cmDBH Lg -0.033 0.014 -2.330 0.0207*

Beaver Presence 0.006 0.009 0.620 0.5359

Song Sparrow

Number of years with dam

Intercept 0.014 0.015 0.970 0.3329

H2O May 0.146 0.019 7.580 <.0001*

H2O PER 0.040 0.017 2.320 0.0209*

SALI <12cmDBH Lg 1.514 0.374 4.050 <.0001*

Num Yr BD <51 Lg 0.077 0.046 1.680 0.0949

White-breasted Nuthatch

Habitat + Beaver presence

Intercept 0.022 0.009 2.360 0.0193*

POFR >25cmDBH Lg 0.060 0.023 2.560 0.0110*

SumPOFR %Cov 20-30m Lg 0.026 0.014 1.840 0.0671

Beaver Presence 0.023 0.010 2.230 0.0268*

Habitat + Beaver intensity

Intercept 0.022 0.009 2.400 0.0172*

POFR >25cmDBH Lg 0.058 0.023 2.470 0.0142*

SumPOFR %Cov 20-30m Lg 0.026 0.014 1.900 0.0590

Beaver Level 1 0.036 0.014 2.540 0.0118*

Beaver Level 2 0.022 0.015 1.540 0.1261

Beaver Level 3 0.001 0.018 0.050 0.9634

Western Wood-Pewee

Habitat + Beaver presence

Intercept -0.004 0.011 -0.350 0.7252

H2O May 0.049 0.014 3.650 0.0003*

POFR >25cmDBH Lg 0.070 0.021 3.380 0.0008*

Beaver Presence 0.010 0.011 0.910 0.3641

Habitat + Number of years with dam

Intercept -0.003 0.011 -0.310 0.7566

H2O May 0.055 0.013 4.290 <.0001*

POFR >25cmDBH Lg 0.068 0.021 3.320 0.0011*

Num Yr BD <51 Lg -0.009 0.031 -0.300 0.7648
46

Parameter Estimate SE t P

Brown-headed Cowbird

Beaver presence

Intercept 0.115 0.010 11.410 <.0001*

H2O PER 0.056 0.017 3.220 0.0015*

Beaver Presence 0.022 0.018 1.210 0.2277

Beaver intensity

Intercept 0.113 0.010 11.280 <.0001*

H2O PER 0.061 0.018 3.440 0.0007*

Beaver Level 1 0.038 0.022 1.710 0.0880

Beaver Level 2 -0.009 0.025 -0.350 0.7293

Beaver Level 3 0.033 0.030 1.100 0.2711

Cassin's Kingbird

Beaver intensity

Intercept 0.086 0.011 7.830 <.0001*

H2O PER 0.060 0.019 3.200 0.0016*

SALI <12cmDBH Lg 0.998 0.398 2.510 0.0128*

Beaver Level 1 0.053 0.023 2.260 0.0245*

Beaver Level 2 -0.016 0.026 -0.600 0.5485

Beaver Level 3 -0.031 0.031 -1.020 0.3089

Yellow Warbler

Habitat + Number of years with dam

Intercept 0.078 0.023 3.400 0.0008*

Wash <200 0.065 0.021 3.040 0.0027*

H2O May 0.155 0.020 7.550 <.0001*

POFR Av%Cov Lg 0.178 0.026 6.800 <.0001*

SALI >25cmDBH Lg 0.272 0.093 2.910 0.0040*

Num Yr BD <51 Lg -0.098 0.052 -1.880 0.0614

Ash-throated Flycatcher

Habitat + Number of uears w/ dam

Intercept 0.072 0.009 8.260 <.0001*

Wash <200 -0.024 0.010 -2.270 0.0243*

H2O May -0.036 0.010 -3.790 0.0002*

ALL TAMI LT Lg 0.407 0.174 2.340 0.0201*

Num Yr BD <51 Lg -0.034 0.025 -1.350 0.1787

Habitat + Beaver presence

Intercept 0.072 0.009 8.300 <.0001*

Wash <200 -0.024 0.010 -2.350 0.0198*

H2O May -0.035 0.010 -3.450 0.0007*

ALL TAMI LT Lg 0.405 0.175 2.320 0.0211*

Beaver Presence -0.009 0.009 -0.990 0.3234
47

Parameter Estimate SE t P

Northern Beardless-Tyrannulet

Habitat + Beaver presence

Intercept 0.037 0.004 8.700 <.0001*

H2O PER -0.026 0.007 -3.610 0.0004*

ALL MESQ Lg 0.155 0.045 3.470 0.0006*

Beaver Presence -0.005 0.008 -0.640 0.5206

Species Groups

Species Richness 76 Breeding Bird Species (BBS)

Intensity

Intercept 22.075 2.076 10.630 <.0001*

H2O May 2.679 0.590 4.540 <.0001*

Rip Veg Width Lg -2.977 0.972 -3.060 0.0025*

SALI Av%Cov Lg 2.279 0.834 2.730 0.0068*

Beaver Level 1 1.849 0.604 3.060 0.0025

Beaver Level 2 0.012 0.064 0.020 0.9855

Beaver Level 3 2.061 0.777 2.650 0.0086

Beaver presence

Intercept 22.423 2.057 10.900 <.0001*

H2O May 2.248 0.607 3.700 0.0003*

Rip Veg Width Lg -3.114 0.963 -3.230 0.0014*

SALI Av%Cov Lg 1.996 0.832 2.400 0.0173*

Beaver Presence 1.204 0.473 2.550 0.0115*

Number of years with dam

Intercept 22.338 2.069 10.800 <.0001*

H2O May 2.538 0.592 4.290 <.0001*

Rip Veg Width Lg -3.082 0.968 -3.180 0.0017*

SALI Av%Cov Lg 2.124 0.834 2.550 0.0115*

Num Yr BD <51 Lg 2.522 1.321 1.910 0.0574

Relative Abundance of 76 BBS

Beaver presence

Intercept 3.3385 1.5608 2.14 0.0335*

H2O May 2.5347 0.455 5.57 <.0001*

H2O PER 0.6368 0.3954 1.61 0.108

SALI <12cmDBH Lg 35.849 8.0052 4.48 <.0001*

Veg Diversity Lg 5.232 1.9332 2.71 0.0073*

Beaver Presence 0.9223 0.3808 2.42 0.0162*
48

Parameter Estimate SE t P

Intensity

Intercept 3.2037 1.5676 2.04 0.0421*

H2O May 2.4921 0.4573 5.45 <.0001*

H2O PER 0.702 0.403 1.74 0.0828

SALI <12cmDBH Lg 36.726 8.0481 4.56 <.0001*

Veg Diversity Lg 5.3918 1.9411 2.78 0.0059*

Beaver Level 1 1.1451 0.4729 2.42 0.0162*

Beaver Level 2 0.4944 0.5214 0.95 0.344

Beaver Level 3 1.099 0.6189 1.78 0.0771

Species Richness of 42 Riparian BBS

Intensity

Intercept 11.472 0.629 18.250 <.0001*

H2O May 1.814 0.547 3.320 0.0011*

TAMI Av%Cov Lg -1.159 0.542 -2.140 0.0335*

SALI Av%Cov Lg 1.957 0.658 2.980 0.0032*

POFR Av%Cov Lg 2.030 0.553 3.670 0.0003*

Beaver Level 1 1.336 0.514 2.600 0.0099*

Beaver Level 2 0.076 0.522 0.150 0.8840

Beaver Level 3 1.541 0.656 2.350 0.0197*

Beaver presence

Intercept 11.600 0.631 18.390 <.0001*

H2O May 1.804 0.551 3.270 0.0012*

TAMI Av%Cov Lg -1.253 0.544 -2.300 0.0222*

SALI Av%Cov Lg 1.859 0.661 2.810 0.0053*

POFR Av%Cov Lg 1.992 0.557 3.580 0.0004*

Beaver Presence 0.890 0.391 2.280 0.0237*

Species Richness of Riparian Specialists

Intensity

Intercept 0.804 1.396 0.580 0.5653

H2O May 2.621 0.404 6.480 <.0001*

H2O PER 0.957 0.354 2.700 0.0074*

Veg Diversity Lg 6.865 1.730 3.970 <.0001*

Beaver Level 1 0.947 0.421 2.250 0.0256*

Beaver Level 2 -0.048 0.463 -0.100 0.9169

Beaver Level 3 0.864 0.552 1.570 0.1186

Beaver presence

Intercept 1.002 1.399 0.720 0.4744

H2O May 2.677 0.405 6.610 <.0001*

H2O PER 0.846 0.349 2.420 0.0162*

Veg Diversity Lg 6.622 1.733 3.820 0.0002*

Beaver Presence 0.602 0.341 1.760 0.0791
49

Parameter Estimate SE t P

Species Richness of 14 Conservation Concern Spp.

Number of years with dam

Intercept 1.694 0.984 1.720 0.0864

SALI >25cmDBH Lg 3.022 1.154 2.620 0.0094*

Veg Diversity Lg 5.254 1.146 4.580 <.0001*

Num Yr BD <51 Lg 0.372 0.630 0.590 0.5552

Beaver presence

Intercept 1.684 0.989 1.700 0.0898

SALI >25cmDBH Lg 3.048 1.163 2.620 0.0093*

Veg Diversity Lg 5.255 1.161 4.530 <.0001*

Beaver Presence 0.084 0.217 0.380 0.7010

Species Richness of Insectivorous Riparian Birds

Intensity

Intercept 10.115 0.580 17.450 <.0001*

H2O May 1.300 0.504 2.580 0.0106*

TAMI Av%Cov Lg -1.219 0.500 -2.440 0.0154*

SALI Av%Cov Lg 1.481 0.607 2.440 0.0154*

POFR Av%Cov Lg 1.541 0.510 3.020 0.0028*

Beaver Level 1 1.142 0.474 2.410 0.0166*

Beaver Level 2 -0.046 0.481 -0.100 0.9240

Beaver Level 3 0.801 0.605 1.320 0.1866

Beaver presence

Intercept 10.205 0.580 17.590 <.0001*

H2O May 1.293 0.507 2.550 0.0114*

TAMI Av%Cov Lg -1.280 0.501 -2.560 0.0112*

SALI Av%Cov Lg 1.392 0.608 2.290 0.0230*

POFR Av%Cov Lg 1.527 0.512 2.980 0.0032*

Beaver Presence 0.611 0.360 1.700 0.0907

Relative Abundance of Insectivorous Riparian Birds

Beaver presence

Intercept 1.092 1.203 0.910 0.3649

H2O May 1.969 0.350 5.630 <.0001*

BACH Av%Cov Lg 1.385 0.427 3.250 0.0013*

POFR 12-25cmDBH Lg -2.746 1.065 -2.580 0.0106*

SALI 12-25cmDBH Lg 5.756 2.004 2.870 0.0045*

Veg Diversity Lg 4.160 1.527 2.720 0.0069*

Beaver Presence 0.297 0.271 1.100 0.2736
50

Parameter Estimate SE t P

Intensity

Intercept 0.955 1.199 0.800 0.4265

H2O May 1.903 0.350 5.440 <.0001*

BACH Av%Cov Lg 1.476 0.430 3.430 0.0007*

POFR 12-25cmDBH Lg -2.683 1.062 -2.530 0.0122*

SALI 12-25cmDBH Lg 6.549 2.047 3.200 0.0016*

Veg Diversity Lg 4.266 1.522 2.800 0.0055*

Beaver Level 1 0.743 0.353 2.100 0.0365*

Beaver Level 2 -0.136 0.377 -0.360 0.7188

Beaver Level 3 0.111 0.453 0.240 0.8070
51

Table 1-10. Name and location of the 240 point count stations used to survey breeding birds in 2005–2006 on the 

San Pedro Riparian National Conservation Area, listed from northern-most to southernmost station. 

[Included are the unique station number identifier, station identifier used during field work (an abbreviation of local 

landmarks and station number within the reach), and geographic coordinates (Universal Transverse Mercator, North 

American Datum of 1983)]

Unique Station ID Reach-station ID 

(Field Name) Easting Northing

1 AF1 574901 3525266

2 AF2 575028 3524992

3 AF3 574785 3524935

4 AF3.5 574515 3524859

5 AF4 574439 3524571

6 AF5 574615 3524397

7 AF6 574797 3524203

8 AF7 574627 3524028

9 AF7.5 574712 3523791

10 AF8 574845 3523549

11 AF9 574682 3523324

12 AF10 574420 3523215

13 AF11 574252 3522987

14 AF12 574143 3522718

15 AF13 574200 3522452

16 AF14 574356 3522217

17 AF15 574230 3521984

18 AF16 574151 3521718

19 AF17 574156 3521469

20 AF18 574136 3521192

21 AF19 574122 3520942

22 SDD7 574118 3520681

23 SDD6 574155 3520425

24 SDD5 574264 3520157

25 SDD4 574452 3519970

26 SDD3 574566 3519741

27 SDD2 574725 3519550

28 SDD1 574887 3519362

29 CWW13 574910 3519121

30 CWW12 574714 3518965

32 CWW10 574401 3518602

33 CWW9 574269 3518379

34 CWW8 574078 3518169

35 CWW7 573997 3517911

36 CWW6 574057 3517653

37 CWW5 573972 3517389

38 CWW4 573648 3517281

39 CWW3.5 573500 3517056

40 CWW2 573847 3516907

41 CWW1 574124 3516815

42 CON16 574184 3516549

43 CON15 574124 3516323

44 CON14 574288 3516098

45 CON13 574529 3516035
52

Unique Station ID Reach-station ID 

(Field Name) Easting Northing

46 CON12 574803 3515915

47 CON11 575041 3515798

48 CON10 575199 3515617

49 CON9 575108 3515394

50 CON8 574891 3515248

51 CON7 575109 3515099

52 CON6 575310 3514923

53 CON5 575584 3514876

54 CON4 575620 3514579

55 CON3 575502 3514345

56 CON2 575429 3514100

57 CON1 575464 3513848

58 FBN15 575450 3513571

59 FBN14 575531 3513326

60 FBN13 575686 3513093

61 FBN12 575818 3512857

62 FBN11 576005 3512672

63 FBN10 576124 3512457

64 FBN9 576197 3512191

65 FBN8 576184 3511902

66 FBN7 576245 3511662

67 FBN6 576325 3511393

68 FBN5 576351 3511128

69 FBN4 576253 3510871

70 FBN3 576160 3510603

71 FBN2 576160 3510352

72 FBN1 576373 3510194

73 FBS1 576390 3509781

74 FBS2 576471 3509555

75 FBS3 576567 3509305

76 FBS4 576786 3509104

77 BRWE12 576833 3508725

78 BRWE11 577034 3508499

79 BRWE10 577294 3508372

80 BRWE9 577454 3508108

81 BRWE8 577532 3507875

82 BRWE7 577597 3507615

83 BRWE6 577606 3507376

84 BRWE5 577485 3507144

85 BRWE4 577498 3506872

86 BRWE3 577465 3506607

87 BRWE2 577264 3506432

89 BRS3 576723 3506516

90 BRS4 576474 3506505

91 BRS5 576226 3506427

92 BRS6 576206 3506186

93 BRS7 576384 3505993

94 BRS8 576574 3505814

95 BRS9 576760 3505649

96 BRS10 576982 3505505

97 BRS11 577058 3505279

98 BRS12 577278 3505126
53

Unique Station ID Reach-station ID 

(Field Name) Easting Northing

99 BRS2 577346 3504874

100 BRS1 577556 3504777

101 GNAR1 577486 3504504

102 GNAR2 577727 3504398

103 GNAR3 577947 3504225

104 GNAR4 578041 3503974

105 GNAR5 577879 3503743

106 GNAR6 577725 3503521

107 GNAR7 577574 3503290

108 GNAR8 577588 3503031

109 GNAR9 577504 3502787

110 GNAR10 577570 3502517

111 GNAR11 577839 3502440

112 GNAR12 578029 3502283

113 NAR1 578111 3502022

114 NAR2 578066 3501761

115 NAR3 577930 3501529

116 NAR4 577851 3501263

117 NAR5 577883 3501015

118 NAR6 578059 3500823

119 NAR7 578152 3500593

120 NAR8 578214 3500334

121 NAR9 577969 3500176

122 NAR10 577849 3499920

123 NAR11 578016 3499696

124 NAR12 578179 3499506

125 GHEW10 578467 3499057

126 GHEW9 578706 3498949

127 GHEW8 578885 3498701

128 GHEW7 579086 3498514

129 GHEW6 579234 3498276

130 GHEW5 579030 3498115

131 GHEW4 578923 3497841

132 GHEW3 579118 3497623

133 GHEW2 579444 3497557

134 GHEW1 579512 3497308

135 GRHA5 579651 3497103

136 GRHA4 579908 3497110

137 GRHA3 580144 3497196

138 GRHA2 580300 3496972

139 GRHA1 580378 3496729

140 LBN18 580866 3495255

141 LBN17 580975 3494966

142 LBN16 581095 3494719

143 LBN15 581060 3494416

144 LBN14 581005 3494154

145 LBN13 581188 3493975

146 LBN12 581400 3493808

147 LBN11 581563 3493598

148 LBN10 581615 3493296

149 LBN9 581468 3493090

150 LBN8 581375 3492868
54

Unique Station ID Reach-station ID 

(Field Name) Easting Northing

151 LBN7 581301 3492646

152 LBN6 581351 3492435

153 LBN5 581400 3492159

154 LBN4 581485 3491940

155 LBN3 581558 3491745

156 LBN2 581628 3491504

157 LBN1 581616 3491265

158 LBS1 581937 3490730

159 LBS2 582100 3490579

160 LBS3 582194 3490372

161 LBS4 582354 3490194

162 LBS5 582453 3489917

163 LBS6 582424 3489692

164 LBS7 582322 3489458

165 LBS8 582344 3489235

166 LBS9 582481 3489045

167 LBS10 582643 3488804

168 LBS11 582705 3488630

169 LBS12 582810 3488358

170 LBS13 582792 3488145

171 LBS14 582776 3487930

172 COT1 582723 3487684

173 COT2 582707 3487444

174 COT3 582659 3487192

175 COT4 582755 3486965

176 COT5 582918 3486757

177 COT6 582858 3486505

178 COT7 582756 3486323

179 COT8 582658 3486074

180 COT9 582696 3485827

181 COT10 582938 3485807

182 COT11 583040 3485564

183 COT12 582890 3485363

184 COT13 582874 3485090

185 COT14 583089 3484917

186 HNWA1 583061 3484658

187 HNWA2 583126 3484415

188 HNWA3 583342 3484319

189 HNWA4 583567 3484124

190 HNWA5 583544 3483860

191 HNWA6 583603 3483641

192 HNWA7 583723 3483474

193 HNWA8 583722 3483253

194 HNWA9 583829 3483006

195 HNWA10 584097 3482901

196 HNWA11 583987 3482632

197 HNWA12 584174 3482402

198 HBN21 584338 3482229

199 HBN20 584588 3482259

200 HBN19 584777 3482112

201 HBN18 584626 3481876

202 HBN17 584878 3481868
55

Unique Station ID Reach-station ID 

(Field Name) Easting Northing

203 HBN16 585000 3481646

204 HBN15 584961 3481376

205 HBN14 584689 3481339

206 HBN13 584594 3481102

207 HBN12 584859 3481033

208 HBN11 585050 3480836

209 HBN10 584930 3480641

210 HBN9 584711 3480497

211 HBN8 584575 3480260

212 HBN7 584590 3480040

213 HBN6 584733 3479805

214 HBN5 584928 3479601

215 HBN4 584755 3479391

216 HBN3 584701 3479123

217 HBN2 584711 3478872

218 HBN1 584768 3478665

219 HBS1 584825 3478417

220 HBS2 584843 3478161

221 HBS6 585325 3477388

222 HBS7 585332 3477124

223 HBS8 585206 3476894

224 HBS9 585109 3476659

225 HBS10 585031 3476416

226 HBS11 584991 3476175

227 HBS12 585067 3475932

228 HBS13 585112 3475645

229 HBS14 585269 3475437

230 PAL1 584489 3471806

231 PAL2 584554 3471553

232 PAL3 584330 3471407

233 PAL4 584135 3471274

234 PAL5 584006 3471046

235 PAL6 583821 3470897

236 PAL7 583844 3470661

237 PAL8 583653 3470486

238 PAL9 583441 3470367

239 PAL10 583441 3470125

240 PAL11 583278 3469912

241 PAL12 583296 3469647

242 PAL13 583083 3469509
56

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57

Chapter 2. An Investigation of Important Factors Associated with Riparian Bird 

Community Structure Along the Upper San Pedro River in Southeastern Arizona 

and Northern Sonora, Mexico: Effects of Reintroduced Beaver

Abstract

We conducted bird surveys in riparian areas along the upper San Pedro River in southeastern 

Arizona (United States) and northern Sonora (Mexico) in order to describe factors influencing bird 

community dynamics and the distribution and abundance of species of conservation concern. These 

surveys were conducted to determine the effects of the ecosystem-altering activities of a recently 

reintroduced beaver population. Beginning in the summer of 2005, we established and surveyed 245 avian 

point count stations within the San Pedro Riparian National Conservation Area in Arizona, where we also 

re-visited bird survey sites in riparian portions of two different avian studies that were originally surveyed 

prior to the beaver reintroduction. In 2006 and 2007, we established and surveyed an additional 70 point 

count survey stations in northern Sonora, starting along the main stem of the San Pedro River just south of 

the international border (33 stations on collectively owned Ejido lands) and extending up and to the 

western portion of the upper watershed at Las Nutrias Reservoir (8 stations on private land) and Los 

Fresnos Land Trust [28 stations on private land jointly owned and managed by TNC-Mexico and two 

other Mexican non-government organizations— Biodiversidad y Desarrollo Armónico (BIDA). and 

Naturalia]. Here, we report a summary of our efforts and a comparison of breeding bird species found in 

the United States and Mexico. More than one-half of 126 species that we detected are considered

“conservation priority” at some level by at least one of several initiatives to recommend management and

monitoring of bird populations. Additional activities documented herein include field-skills training of 

several Mexican biologists, both at a formal avian identification and monitoring workshop in May 2007 at 

Los Fresnos Land Trust, as well as continual individualized training of several biologists throughout the 

collaboration in 2006 and 2007. 

Background

The upper San Pedro River riparian corridor of southeastern Arizona is recognized as a regionally 

important area for breeding and migrating birds (Skagen and others, 1998; Krueper and others, 2003). It is 

a well-known area for bird-watching opportunities and as a region where there exists a controversy, 

regarding local groundwater pumping and its effect on the river’s surface water (Stromberg and Tiller, 

1996; Rojo and others, 1999; Davis, 2004; Caicedo and others, 2006). In 1999, the Bureau of Land 

Management (BLM), in cooperation with the Arizona Game and Fish Department, began to successfully 

reintroduce beaver (Castor canadensis) in to the San Pedro Riparian National Conservation Area 

(SPRNCA) in the United States, where historical accounts show it once thrived. The purpose was to 

increase total riparian area water retention and to restore habitat structural heterogeneity, thus enhancing 

conditions for birds and other wildlife (Fredlake, 1997). 

Many land managers and restoration ecologists consider the re-establishment of beaver as an 

efficient method of restoring riparian attributes and functionality (Apple, 1985; Olson and Hubert, 1994; 

Albert and Trimble, 2000). To assess this expectation, as well as describe other variables important to 

riparian bird community structure along the upper San Pedro River, we studied avian abundance along a 

gradient of riparian conditions along the upper San Pedro River in southeastern Arizona (2005–2007) and 

northern Sonora (2006–2007). Sites varied naturally in vegetation cover and species assemblage, tree 

age/size classes and basal area, surface-water depth and persistence (dry, intermittent, and perennial), and 

intensity of beaver activity. Our study is one of the first to investigate the influence of beaver on avian 

communities in a Southwest riparian ecosystem. 
58

Methods

Seven-Minute Point Count Sampling 

Avian survey stations (point counts) were established every 280 m along the San Pedro River. Our

initial survey points were placed at 100, 200, or 300 m (that is, randomly chosen among these three) from 

bridges or other road access points. We used modified variable-circular plot point counts (Reynolds and 

others, 1980; Ralph and others, 1993), incorporating distance sampling methods of Buckland and others 

(2001) to estimate bird abundance and species richness at a total of 245 stations in the United States and 

70 stations in Mexico. In 2005–2007, we conducted 7-minute point count surveys from mid-May to late 

July or early August. For every bird detected, we noted distance from observer (as measured with a 

Bushnell Yardage-Pro® laser Range Finder), detection type (song, call, visual/perched, or moving/flying, 

flying high overhead), if detected in first 5 or last 2 minutes, and any breeding information (for example, 

nest found, carrying nest material or fecal sacs or food, young fledglings being fed, etc.). Indications of 

breeding activity were documented at every opportunity during or between surveys; as well, incidental 

sightings of rare species and those of conservation concern were noted when walking between survey 

stations and while scouting new routes (for example, at reservoirs in Mexico adjacent or near riparian 

areas). Data from all birds encountered through the 7-minute point count sampling effort are summarized 

and presented in section, “Results” of this chapter. 

Immediately after each visit, we recorded beaver sign (dam, gnawed trees, stumps, downed trees) 

and condition of river (deep or shallow water, drying or recently dried, etc). Additionally, we measured 

more than 40 environmental variables at each survey site, including percent canopy cover for each tree 

and shrub species in seven different height strata (0–1, 1–3, 3–5, 5–10, 10–15 m, etc.) and diameter breast 

height of all trees within 30 m, width of riparian vegetation perpendicular to orientation of river, distance 

from the survey center point to river edges and to riparian vegetation edges, width and depth of river, 

persistence of surface water throughout dry season, and numerous other vegetative, hydrologic, and 

geomorphic factors such as vegetation community type(s) in each area and depth of 

entrenchment/terracing. These environmental measurements will be included as independent variables, 

and bird density and diversity will be included as dependent variables, in different modeling efforts and 

are not utilized further in this report.

Resampling of Historical Surveys 

In order to investigate whether any changes in the bird community have occurred since the 

reintroduction of beaver, we revisited a subset (that is, the riparian portions) of two different studies in the 

SPRNCA, both of which were originally conducted prior to beaver establishment. David Krueper and 

associates at the BLM conducted Emlen line transects from 1986 to 1996 (Krueper and others, 2003), of 

which we repeated six riparian transects (Hereford, one transect north and another just south of U.S. 

Highway 90Highway 90, Gray Hawk Ranch, Curtis Windmill, and St. David Ditch). Arriana Brand 

conducted point counts at more than 260 stations throughout the upper San Pedro River from 1999 to 2001 

(Brand and others, 2006, 2010, for details), of which we re-visited 63 stations at 17 sites—essentially all 

of that study’s survey stations that were within riparian vegetation on the SPRNCA. The dataset resulting 

from resurveying these historical sites is being utilized for a before-after control-impact study (following 

methods recommended by McDonald and others, 2000) to test for differences in individual bird species 

densities and overall species diversity associated with the impacts of beaver activity.
59

Conservation Priority Index Ranking

In order to assess the upper San Pedro River study area for its conservation value to birds, and to 

inform management and research decisions regarding birds in the study area, we analyzed how the bird 

species we encountered in the United States and Mexico were listed or ranked by five different efforts to 

prioritize species for conservation action or concern. From these five efforts, we used 17 lists, ranging 

from critically imperiled species listed by the U.S. Fish and Wildlife Service (USFWS) through the 

Endangered Species Act (U.S. Fish and Wildlife Service, 2007) to birds whose populations are viable and 

stable, yet recommended for monitoring as “stewardship species” that can serve as environmental 

indicators (for example, Rich and others, 2004). We scanned the 17 lists and noted the rank, score, or 

other designation for all species we detected in the upper San Pedro River Basin. Many of the efforts used 

the Partners in Flight assessment criteria (Panjabi and others, 2005) as a starting point, but additional 

considerations relating to the different objectives of each organization yielded lists of conservation 

concern species for our study area that were different. This variation was expected as the five entities have 

different missions, and of course some of the lists were concerned with birds of the highest priority, while 

others are meant as a guide to encourage monitoring populations and not necessarily prompting immediate 

conservation action. The total number of the 17 lists that each species was ranked or listed on is reported 

in table 2-2 under “Conservation Priority Index.” Appendix 2-A contains a table of all species that were 

mentioned by at least one effort and their ranks/designations on each of the 17 lists, and follows with 

details and references regarding evaluation criteria used by each effort. 

Results 

For this report, we present abundance and distribution information for each bird species 

encountered in the United States and Mexico, and we then provide an analysis of how these species were 

ranked or listed throughout several conservation planning initiatives. Further analyses regarding specific 

variables of importance to bird communities on the San Pedro River, and the relative influence of beaver, 

are also provided. From 2005 to 2007, we established and repeatedly surveyed 315 avian point count 

stations in the United States and Mexico, and re-visited 63 point count stations and six transects from two 

historical studies (table 2-1). Repeated visits to these sites resulted in 3,235 individual surveys, distributed 

over 307 person-days by Glenn Johnson and field assistants (table 2-1). These efforts yielded 42,691 

individual bird detections from 126 species. In conducting these surveys, we sampled from over 102 linear 

kilometers of riparian habitat in the United States and Mexico, and re-visited 17 km of historical survey 

sites (which overlapped with the same riparian areas visited during the 7-minute point count sampling on 

the SPRNCA). Thus, our efforts spanned from the northern edge of the SPRNCA near St. David, Arizona, 

south and up-river to the international border, then south through 12 km of the main stem of the San Pedro 

River in Sonora, Mexico, and finally up to the slightly higher elevation riparian areas at Las Nutrias and 

Los Fresnos in Mexico. 

Breeding Birds of the Upper San Pedro River Riparian Areas 

The majority of our bird detections were associated with the 7-minute point count surveys that we 

established, while the historical survey efforts yielded a relatively small proportion of detections (table 2-

1). Moreover, there were no species detected on the historical surveys that were not encountered on the 7-

minute point count surveys, because of the more limited spatial and temporal extent of the historical 

surveys. We report the total detections and relative distribution (number of stations and occupancy as total 

percent of stations in each area) for 126 species encountered while conducting the (7-minute) point count 

sampling efforts in the United States, Mexico, and combined throughout the total study area (table 2-2,). 

Standard English and scientific names of birds in table 2-2 follow the American Ornithologist’s Union 

latest checklist for the bird species known from North America (American Ornithologists Union, 2007), 
60

which is the 7th edition checklist (American Ornithologists Union, 1998) but incorporates recent changes 

through the 48th supplement (Banks and others, 2007). Spanish names are from Kauffman (2005) and/or 

our collaborators in Mexico. 

Of the 126 species that we detected, at least 83 were actively breeding in the area (English name in 

table 2-2). We documented confirmatory evidence such as active nests, nest-building or carrying nest 

materials, carrying food (presumably to nestlings or potential mate) or fecal sacs, courtship displays in 

presence of female or other evidence of pairing, or feeding very recently fledged young that had limited 

mobility. Eighty-six species were neotropical migrants as defined by the U.S. Fish and Wildlife Service 

(2007) and Rappole (1995) (table 2-2).

Because the amount of time, spatial extent, and elevation varied between the United States and 

Mexico sampling efforts (table 2-1), our species list and results (table 2-2) are intended for summary 

purposes only and should not be construed as direct population comparisons among sites. For example, 

while the United States’ effort was essentially one long transect along the mid-elevation riparian habitat 

throughout the SPRNCA, in Mexico we sampled in three distinct areas—(1) along the main stem of the 

San Pedro on collectively owned Ejido-lands from border south 12 km, (2) on private land at Las Nutrias, 

and (3) on private land at Los Fresnos Land Trust, TNC-Mexico, the latter incorporating 3 km of high 

elevation (up to 1,600 m) riparian forest containing sycamores and surrounded by oak mixed with 

grasslands. This additional riparian forest type accounts for some of the differences in species’ abundance 

and distribution seen in Mexico, yet similar forest types exist in high elevation portions of the San Pedro 

River watershed in the United States; and one would find similar bird species [for example, abundant 

Dusky-capped Flycatcher (Myiarchus tuberculifer), Montezuma Quail (Cyrtonyx montezumae)] if one 

were to sample the biological community at the latter location.

Species of Conservation Concern

We found that 59 of 126 species were listed on at least one or more efforts to prioritize species for 

conservation actions (table 2-2, column 13). Our “Conservation Priority Index” is not weighted by the 

highest-priority species or threats to populations, and because passerine landbirds have been the subject of 

a greater number of evaluation efforts, our index score may be lower for more imperiled species [for 

example, federally listed Endangered birds such as the Least Tern (Sternula antillarum)]. Thus, our 

“index” should be used as a relative quantification of different conservation-consideration efforts, and not 

a tool for evaluating the relative conservation concerns of each species. Nonetheless, almost one-half the 

species we found along the upper San Pedro River are of some level of conservation concern, 

demonstrating the importance of these riparian areas for bird conservation at the local, regional, and binational scale (see appendix 2-A for detailed list of species and their designations, and each conservation 

initiative’s criteria under each list).

Incidental and Noteworthy Species, Excluded Observations 

Sixteen species (204 individuals) are included although they were incidental to our surveys, in that 

they were encountered between survey points or at riparian areas en route to survey areas (table 2-2). 

Several of these species are waterfowl or water-associated birds that were detected at Los Fresnos and Las 

Nutrias Reservoirs while scouting nearby riparian areas for the Mexican portion of the study. The 16 

species were included in this summary because their presence in the area is noteworthy from either a 

distributional and/or conservation perspective. Few accounts of species abundance and distribution are 

available for the upper San Pedro River watershed, and indeed for much of extreme northern Sonora 

(Flesch, 2008a), so we felt that including all information of potential interest was warranted. With this in 

mind, appendix 2-A contains detailed location and observation information for noteworthy species (table 

2-2) detected throughout the study area. 
61

We documented details for numerous species of interest in the region. In July 2005, we found an 

extreme southern nest attempt of the threatened, southwestern subspecies of Willow Flycatcher 

(Empidonax trailii, s.s. extimus) south of the Hereford Bridge on the SPRNCA. This nest attempt was 

monitored by E.E.C. Environmental Consultants of Tucson and the USFWS, and was ultimately 

unsuccessful. Importantly, this represented the first nesting record in the upper San Pedro area in 8 years, 

and an extreme southern record for Arizona, California, and New Mexico (see appendix 2-A).

Beginning in 2006, we encountered several species in the Mexican portion of the watershed for 

which there is little historical or contemporary documentation (see Russell and Monson, 1998), including 

breeding Abert’s Towhee (Melozone aberti), Gray Hawk (Buteo nitidus), Yellow-billed Cuckoo 

(Coccyzus americanus), and Bell’s Vireo (Vireo bellii). Although these records are not unexpected, they 

are an important addition to the distribution of these species. Our data on these and other riparian birds in 

the area also can be used to generate population estimates for riparian obligate birds for the areas we 

visited.

Several species that were documented in the study area were not included in these results for 

various reasons totaling approximately 21 observations. These included a few late wintering or migrant 

species that were not unexpected or noteworthy [Belted Kingfisher (Megaceryle alcyon), Green-tailed 

Towhee (Ipilo chlorurus), “Western” Flycatcher [Pacific Slope or Cordilleran], and Yellow-rumped 

Warbler (Setophaga coronata)], incidental sightings while off survey and not in a riparian area (for 

example, Ruddy Ground-Dove (Columbina talpacoti) at the San Pedro House bird feeders, Golden Eagle 

(Aquila chrysaetos) flying over Highway 82 near the Whetstone Mountains), or observed by a collaborator 

but reported elsewhere [Wood Duck (Aix sponsa) and Band-tailed Pigeon (Patagioenas fasciata), both 

reported at Los Fresnos in Flesch, 2008b).

Notes on Beaver Activity and Distribution 

Beavers on the upper San Pedro River are clustered, with groups comprised of one to five 

individuals, often a family group with one to two adults and one to three young. As of late 2007 in the 

United States, there were 15 clusters of activity, and at least three clusters in Sonora; thus, the estimated 

population in the upper San Pedro River watershed is approximately 55–85 individuals distributed along 

100 km of river. We assisted our Mexican collaborators (see section, “Additional Funding Received and 

Updated List of Project Partners”) in documenting beaver south of the border—remarkable in that it is the 

first time in more 100 years that this species has been present in the upper San Pedro River in Mexico (fig. 

2-1). Moreover, there appears to be a breeding colony of beavers at Los Fresnos (fig. 2-2) as of summer 

2008 (Jennifer Arnold, oral commun.). Wherever we encountered evidence of beaver activity in the 

United States or Mexico, we noted UTM coordinates and the intensity and age of beaver sign. Because we 

walked most of the upper basin’s riparian areas at a time when neither the BLM nor Mexican 

organizations were actively monitoring beaver, we have a fairly complete dataset for beaver establishment

and use for the period of 2005–2007. An unexpected find is that one of the densest areas of beaver activity 

is between Hereford Road Bridge on the south and Hunter Wash to the north. The riparian area there has 

burned twice since 1996 and there is reduced forest cover, especially of cottonwood, and to some extent 

willow. In this area, a larger proportion of beaver dams are made up of mud, rocks, and wood drifting 

from upstream, rather than wood cut from the immediate area (figs. 2-3 2-4, and 2-). The beaver here also 

appeared to be utilizing bull rush and cattail plants as a food resource, presumably because tree cambium 

was not as prevalent.
62

Modeling Bird and Beaver Associations Relative to Vegetation and Surface Water

Our preliminary analyses indicated that areas of beaver activity are associated with greater bird 

species richness, abundance of riparian bird species, and conservation concern species. We first eliminated 

highly correlated variables, and then used stepwise variable selection to build a multiple linear regression 

model that explained bird species richness within the riparian corridor, within 50 m of the station centers. 

Models incorporated perpendicular width of riparian vegetation, maximum percent cover of upper canopy 

layers, the presence and late-season persistence surface water, percent willow cover, and heterogeneity of 

habitat at several scales. After accounting for variation attributable to these sources, several beaver-related 

variables were chosen by stepwise variable selection as significantly influencing riparian bird species 

richness, including presence or absence of any beaver sign, intensity of beaver sign, and number of years 

within last 7 where a beaver dam was located within 250 m. Similarly, models comparing relative 

abundance of riparian obligate species or species of conservation concern (for example, Yellow Warbler) 

showed that beaver sign was correlated with higher abundance of these birds. These relationships held true 

even after controlling for the effect of water presence or depth, width of riparian vegetation area, canopy 

coverage, etc. The most recent analyses involve the use of density estimates for abundant species as 

derived from distance sampling methods, and comparing the historical data to those we collected at the 

same sites. 

Additional Funding Received and Updated List of Project Partners (In Bold)

Two Mexican partners new to the project who are actively involved in various conservation and 

avian management activities in the upper San Pedro River watershed of Sonora provided us with key 

logistical, translation, and field assistance as well as providing field accommodations. Beginning in 2006, 

staff from the Ajos-Bavispe National Forest Reserve (Cananea office) facilitated our efforts by 

providing vehicle transportation in the field, initial landowner contacts and securing permission to survey, 

as well as field accompaniment and assistance. This group is part of the Mexican federal land management 

agency Comisión Nacional de Áreas Naturales Protegidas (CONANP), which in turn is under the 

Secretaría de Medio Ambiente y Recursos Naturales (SEMARNAT), a cabinet level department of the 

federal executive branch that develops and implements environmental policy and legislation. In 2007, a 

second group, Biodiversidad y Desarrollo Armónico (BIDA), A.C. , a non-governmental organization 

with offices in Cananea and Hermosillo, Sonora, began assisting with logistical support and additional 

landowner contacts in the upper watershed. Both of these organizations provided field vehicle 

transportation for the study in Mexico, as well as salary for their respective individual employees while 

working with this project, for a total estimated contribution of $3,500 ($1,800 from CONANP in 2006 and 

2007, and $1,700 from BIDA in 2007). It is our understanding that an important part of the mission of 

both CONANP (SEMARNAT) and BIDA is to facilitate and direct conservation efforts, and specifically 

avian research, in the upper San Pedro watershed of Sonora. They were quite open to collaborating with us 

(including intensive training in bird and habitat survey techniques) as the activities were in-line with their 

priorities.

Individuals from CONANP included Eduardo Hinojosa Robles (2006), Luis Portillo (2006, 2007), 

Rosa Elena Jimenez Maldonado and Daniel Toyos Martinez (2007), as well as ecologist and director 

Elvira Rojero Diaz who authorized their participation. Individuals from BIDA greatly assisted the project 

in 2007 and mainly included Sergio Juàrez and Eduardo E. López Saavedra (director). These people were 

all crucial to the effort in Sonora and were a pleasure to work with. 
63

Additionally, individual researchers and/or graduate students from the University of Arizona 

collaborated on avian studies in the high-elevation portion of the watershed at Rancho Los Fresnos, a 

conservation land trust jointly owned and managed by The Nature Conservancy (Mexico), BIDA, and 

the Mexican conservation organization Naturalia. They participated in this study either through 

contributing their incidental bird or beaver observations and/or participating directly in the current study 

as contractor-surveyors. They include Aaron Flesch, Emilie Snell-Rood, Christine Lamanna, Greta 

Anderson, and Brad Boyle. 

Changes to Original Proposal - Interactions with People

The major change to the original Sonoran Joint Venture proposal was the extension of the work to 

include the Mexican portion of the watershed. This resulted in interactions with 12 biologists and natural 

resources agents that work in northern Sonora, 9 of whom attended the “Field Methods for Conducting 

Riparian Bird Surveys” intensive workshop on May 17–18, 2007 (fig. 2-1). We also facilitated the annual 

wet/dry surface water monitoring in 2006 and 2007 in the Mexican portion of the San Pedro River 

watershed along the main stem of river south of the border, where seven Mexican collaborators assisted. 

This partner program organized by TNC and the BLM in the United States now includes more Mexican 

collaborators in the upper watershed, in part due to our efforts. Eight individuals helped with that effort, 

which we integrated with some of our avian, vegetation and hydrology measurements. Additionally, three 

of the biologists mentioned in the “partners” section above accompanied surveyors at nearly all points 

while we were surveying in Sonora, and this was treated largely as an extended training session for bird 

identification and survey techniques. 

Conclusion

We conducted more than 2,000 individual surveys at the 315 point count stations, and at historical 

bird survey sites throughout the upper San Pedro River watershed in southeastern Arizona and northern 

Sonora. In total, we recorded well over 30,000 bird detections and much more environmental 

measurement data, which are being incorporated into modeling efforts. These survey data documented the 

abundance, distribution and breeding status of 126 species, nearly one-half of which are species 

recommended for monitoring or conservation concern. Little prior information was available from this 

area of northern Mexico for many of these species. We also documented beaver at numerous locations in 

the United States and Mexico, and make these data available to other researchers who would like to 

investigate beaver activity and distribution in the area. These data and preliminary analyses all indicate a 

positive correlation between beaver activity and greater species richness and abundance of riparian 

obligate birds. These riparian bird community metrics are of great interest to ecologists, conservation 

biologists, land managers and others interested in the conservation, restoration and management of 

riparian areas.

Largely because of Sonoran Joint Venture support, we worked with several important partners in 

the United States and Mexico that increased the breadth of our study and enhanced the overall impact on 

management activities. These management activities included intensive training with several Mexican 

biologists over the course of two survey seasons and a formal workshop in bird survey techniques, 

mapping surface-water distribution in Mexico, and providing information in scientific and public forums. 
64

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66

Figures

Figure 2-1 Scanning riparian vegetation for a bird heard during a training session lead by the investigator at Los 

Fresnos Land Trust, TNC~Mexico, in the Sonoran portion of the upper San Pedro River watershed, May 16, 2007.

Pictured are biologists from Comisión Nacional de Áreas Naturales Protegidas-Secretaría de Medio Ambiente y 

Recursos Naturales, Naturalia, and Biodiversidad y Desarrollo Armónico A.C. (BIDA A.C.). Photograph by Sergio 

Juàrez, BIDA, A.C.

Figure 2-2. Beaver (Castor canadensis) sign along upper San Pedro River 7.2 kilometers south of the U.S.-Mexico 

International border. The individuals responsible are presumed to have immigrated south from the recently 

reintroduced population in the United States, as there have been no beaver documented on the upper San Pedro of 

Sonora in over 100 years. Photograph by Sergio Juàrez, BIDA, A.C.
67

Figure 2-3: Dam in riparian area north of Hereford Bridge, Arizona, with recent fire history.

Figure 2-4. Beaver (Castor canadensis) dam over 1 meter in height, 1 kilometer south of Hunter Wash, north of 

Hereford, Arizona (8.5 x 11 inch clipboard for scale).
68

Figure 2-5. Beaver (Castor canadensis) dam (0.8 meter high dam), 3.7 kilometers north of U.S. Highway 90, Arizona, 

July 2006 (8.5 x 11 inch clipboard for scale).

Figure 2-6. Yellow-crowned Night-Heron (Nyctanassa violacea) observed 7 kilometers south of international border. 

See appendix 2-A for full details. Photograph by Sergio Juarez, Biodiversidad y Desarrollo Armónico A.C.
69

Tables

Table 2-1. Four components of the investigation of bird community dynamics in riparian areas in the upper San Pedro 

River watershed and details of survey technique, effort, total detections, and study design.

Study 

component

Number (time 

length) and type of 

survey, and spatial 

arrangement

Number 

of person days Years

Number 

of 

detections

Number of linear 

kilometers of riparian 

habitat sampled; 

elevation

Test/design 

considerations

Extensive 

systematic 

sampling 

throughout the 

SPRNCA, 

U.S.A.

245 (7-minute) 

point count 

stations, in one 

long transect along 

river, about 280 m 

apart

203 2005–

2007 32,475

69 km, cottonwoodwillow, some 

tamarisk, mesquite, 

early successional; 

1,130–1,310 m 

> 40 environmental 

variables measured, test 

for most important 

influences on birds as 

well as relative influence 

of beaver activity

Systematic 

sampling in 

Mexican 

portion of San 

Pedro River 

Watershed

70 (7-minute) point 

count stations, on 

seven routes along 

river and other 

riparian zones, 

about 280 m apart

21 2006–

2007 4,041

23 km, cottonwoodwillow, (including 

3km of sycamore/ 

cottonwood at highest 

elevation);

1,310–1,600m

24 environmental 

Variables measured, test 

for important factors in 

upper watershed, 

differences between U.S. 

and MX, and recent 

beaver activity.

Repeat of 

riparian 

portions of 

Brand and 

others 

(2006,2010)

studies

63 (5-minute) point 

count stations at 17 

sites, in riparian 

area along river, 

100 m apart

53

(1999–

2001);

2005–

2007

2,147

10 km, mostly 

cottonwood-willow, 

tamarisk, early 

successional; 

1,130–1,310m

Historical: Before/After to 

investigate if bird 

community different in 

areas where beaver have 

moved in.

Repeat of 

riparian 

portions of 

Krueper and 

others (2003) 

study

6 (30m/minute) 

Emlen line 

transects 1 to 1.5 

km in length, along 

river/riparian.

30

(1986–

1996);

2005–

2007

4,028

7 km, cottonwoodwillow and post fire.

1,150–1,275m

Historical: Before/After to 

investigate if bird 

community different in 

areas where beaver have 

moved in.
70

Table 2-2. Bird species encountered during summer surveys of riparian areas in upper San Pedro River watershed within the United States and Mexico. 

[English Name: θ, information found in table; θ, evidence of local breeding encountered. Scientific Name: *, neotropical migrants by U.S. Fish and Widlife Service 

(2007). Spanish name: Kaufman and others (2005). U.S. Birds, Mexico Birds Detected, Total Detections: No. U.S. Stations Where Detected: Number of U.S. 

stations where bird species encountered at least once. Conservation Priority Index: Number of times each species was ranked on 17 different conservation-related 

lists (from appendix 2-A). U.S. Stations Occupied (%): Percentange of stations where No. MX Stations Where Detected: Number of stations in Mexico where bird 

species found at least once. No. U.S. Stations Where Detected: Number of U.S. stations where bird species found at least once. MX Stations Occcupied (%): 

Percentage of bird species occupancy at 70 stations in Mexico. Total Stations Where Detected: Total stations where bird species recorded. Total Stations Occupied: 

Total stations where bird species occupancy recorded] 

English name 

(Bold=evidence of local breeding

encountered, θ=Info in Appdx D)

Scientific Name 

(Neotropical Migrants by 

USFWS (2007) noted with * )

Spanish name 

(Kauffmann and others 2005)

U.S. 

Birds 

Detected 

Mexico 

Birds 

Detected 

Total 

Detections 

No. U.S. 

Stations 

Where 

Detected 

U.S. 

Stations 

Occupied 

(%) 

No. MX 

Stations 

Where 

Detected 

MX 

Stations 

Occupied 

(%) 

Total 

Stations 

Where

Detected 

Total 

Stations 

Occupied

(%) 

Conservation 

Priority 

Index 

Black-bellied Whistling-Duck

θ

Dendrocygna 

autumnalis * Pijije de Ala Blanca 7 7 3 4.3 3 1

Mallard Anas platyrhynchos * Pato de Collar 196 24 220 85 34.7 12 17.1 97 30.8

Blue-winged Teal θ Anas discors * Cerceta alazul 4 4 1 1.4 1 0.3

Cinnamon Teal θ Anas cyanoptera * Cerceta Canela 1 1 i* i*

Ruddy Duck θ Oxyura jamaicensis * Pato Tepalcate 24 24 i* i*

Scaled Quail Callipepla squamata Codorniz Escamosa 10 2 12 9 3.7 2 2.9 11 3.5 4

Gambel’s Quail Callipepla gambelii Codorniz de Gambel 380 128 508 134 54.7 50 71.4 184 58.4 4

Montezuma Quail Cyrtonyx montezumae Codorniz Moctezuma 12 12 4 5.7 4 1.3 5

Pied-billed Grebe θ Podilymbus podiceps *

Zambullidor de Pico 

Grueso 2 2 2 2.9 2 0.6

Eared Grebe θ Podiceps nigricollis * Zambullidor Orejudo 6 6 i* i*

Western Grebe θ

Aechmophorus 

occidentalis *

Achichilique Picoamarillo 4 4 i* i*

Double-crested Cormorant

Phalacrocorax auritus 

* Cormorán Orejudo 4 4 1 1.4 1 0.3

Great Blue Heron Ardea herodias * Garzón Cenizo 146 21 167 78 31.8 16 22.9 94 29.8

Great Egret Ardea alba * Garza Blanca 10 1 11 8 3.3 1 1.4 9 2.9

Green Heron Butorides virescens * Garceta Verde 4 4 3 1.2 3 1

Yellow-crowned Night-Heron

θ (fig., 2-6) Nyctanassa violacea * Pedrete de Corona Clara 2 2 2 2.9 2 0.6

White-faced Ibis θ Plegadis chihi * Ibis Cara-blanca 7 7 i* i*

Turkey Vulture Cathartes aura * Zopilote Aura 135 9 144 35 14.3 6 8.6 41 13

White-tailed Kite Elanus leucurus Milano de Cola Blanca 8 8 6 2.4 6 1.9 1

Cooper’s Hawk Accipiter cooperii * Gavilán de Cooper 40 4 44 25 10.2 4 5.7 29 9.2 3

Gray Hawk θ Buteo nitidus Halcon (Aguililla) Gris 382 50 432 157 64.1 31 44.3 188 59.7 5
71

English name 

(Bold=evidence of local breeding

encountered, θ=Info in Appdx D)

Scientific Name 

(Neotropical Migrants by 

USFWS (2007) noted with * )

Spanish name 

(Kauffmann and others 2005)

U.S. 

Birds 

Detected 

Mexico 

Birds 

Detected 

Total 

Detections 

No. U.S. 

Stations 

Where 

Detected 

U.S. 

Stations 

Occupied 

(%) 

No. MX 

Stations 

Where 

Detected 

MX 

Stations 

Occupied 

(%) 

Total 

Stations 

Where

Detected 

Total 

Stations 

Occupied

(%) 

Conservation 

Priority 

Index 

Swainson’s Hawk θ Buteo swainsoni *

Halcon (Aguililla) de 

Swainson 5 5 10 5 2 5 7.1 10 3.2 7

Zone-tailed Hawk θ Buteo albonotatus * Aguililla cola roja 1 1 1 0.4 1 0.3 2

Red-tailed Hawk Buteo jamaicensis *

Halcon (Aguililla) Cola 

Roja 48 16 64 33 13.5 10 14.3 43 13.7

American Kestrel Falco sparverius Cernícalo Americano 46 20 66 32 13.1 15 21.4 47 14.9

Peregrine Falcon θ Falco peregrinus * Halcón peregrino 2 2 1 0.4 1 0.3 6

American Coot θ Fulica americana * Gallareta Americana 2 2 2 2.9 2 0.6

Killdeer Charadrius vociferus * Chorlo Tildío 27 43 70 12 4.9 20 28.6 32 10.2

Spotted Sandpiper θ Actitis macularius * Playero Alzacolita 4 1 5 4 1.6 1 1.4 5 1.6

Wilson’s Phalarope θ Phalaropus tricolor * Falaropo Picolargo 12 12 i* i*

Least Tern θ Sternula antillarum * Charrán Mínimo 1 1 i* i* 4

White-winged Dove Zenaida asiatica * Paloma de Ala Blanca 3113 123 3236 245 100 37 52.9 282 89.5

Mourning Dove Zenaida macroura * Paloma Huilota 856 151 1007 228 93.1 62 88.6 290 92.1

Common Ground-Dove Columbina passerina Tórtola Común 307 77 384 138 56.3 43 61.4 181 57.5 1

Yellow-billed Cuckoo Coccyzus americanus * Cuco de Pico Amarillo 208 49 257 118 48.2 29 41.4 147 46.7 7

Greater Roadrunner

Geococcyx 

californianus Correcaminos Norteño 20 24 44 19 7.8 17 24.3 36 11.4

Barn Owl θ Tyto alba Lechuza de Campanario 2 2 i* i* 1

Western Screech-Owl θ Megascops kennicottii Tecolote Occidental 4 3 7 2 0.8 2 2.9 4 1.3 2

Great Horned Owl Bubo virginianus Búho Cornudo Grande 44 13 57 29 11.8 9 12.9 38 12.1

Lesser Nighthawk θ

Chordeiles acutipennis 

* Chotocabras menor 35 5 40 24 9.8 3 4.3 27 8.6

Common Nighthawk θ Chordeiles minor * Chotacabras Zumbón 18 8 26 15 6.1 7 10 22 7

Common Poorwill θ

Phalaenoptilus nuttallii 

* Tapacaminos Tevíi 70 1 71 i* i* i* 2

Vaux’s Swift θ Chaetura vauxi * Vencejo de Vaux 1 1 1 0.4 1 0.3

Black-chinned Hummingbird Archilochus alexandri *

Colibrí Garganta 

Morada 133 8 141 93 38 6 8.6 99 31.4

Acorn Woodpecker 

Melanerpes 

formicivorus Carpintero Arlequín 32 32 14 20 15 4.8

Gila Woodpecker Melanerpes uropygialis Carpintero del Desierto 1496 150 1646 244 99.6 46 65.7 290 92.1 2

Ladder-backed Woodpecker Picoides scalaris Carpintero Mexicano 336 39 375 167 68.2 28 40 195 61.9

Northern Flicker Colaptes auratus 

Carpintero de Pechera 

Comun 257 103 360 137 55.9 47 67.1 184 58.4
72

English name 

(Bold=evidence of local breeding

encountered, θ=Info in Appdx D)

Scientific Name 

(Neotropical Migrants by 

USFWS (2007) noted with * )

Spanish name 

(Kauffmann and others 2005)

U.S. 

Birds 

Detected 

Mexico 

Birds 

Detected 

Total 

Detections 

No. U.S. 

Stations 

Where 

Detected 

U.S. 

Stations 

Occupied 

(%) 

No. MX 

Stations 

Where 

Detected 

MX 

Stations 

Occupied 

(%) 

Total 

Stations 

Where

Detected 

Total 

Stations 

Occupied

(%) 

Conservation 

Priority 

Index 

Northern BeardlessTyrannuletCamptostoma imberbe 

* Mosqero Lampino 218 36 254 114 46.5 28 40 142 45.1 4

Olive-sided Flycatcher θ Contopus cooperi * Pibí boreal 1 1 2 1 0.4 1 1.4 1 0.3 7

Western Wood-Pewee Contopus sordidulus * Pibí Occidental 663 48 711 181 73.9 31 44.3 212 67.3

Willow Flycatcher θ Empidonax traillii * Mosquero saucero 9 9 5 2 5 1.6 7

Black Phoebe Sayornis nigricans * Papamoscas Negro 120 31 151 73 29.8 20 28.6 93 29.5

Says Phoebe Sayornis saya * Papamoscas Llanero 6 11 17 6 2.4 7 10 13 4.1

Vermilion Flycatcher Pyrocephalus rubinus * Mosquero Cardenalito 902 167 1069 216 88.2 60 85.7 276 87.6

Dusky-capped Flycatcher θ

Myiarchus tuberculifer 

* Copetón Triste 4 28 32 2 0.8 13 18.6 15 4.8 1

Ash-throated Flycatcher

Myiarchus cinerascens 

* Copetón Cenizo 363 95 458 164 66.9 47 67.1 211 67 1

Brown-crested Flycatcher Myiarchus tyrannulus * Copetón Tirano 509 76 585 187 76.3 30 42.9 217 68.9 1

Tropical Kingbird θ

Tyrannus melancholicus 

* Tirano tropical 18 18 8 3.3 8 2.5 1

Cassins Kingbird Tyrannus vociferans * Tirano Gritón 1517 257 1774 235 95.9 63 90 298 94.6 4

Western Kingbird Tyrannus verticalis *

Tirano de Bordes 

Blancos 208 46 254 98 40 29 41.4 127 40.3

Loggerhead Shrike Lanius ludovicianus * Alcaudón Verdugo 4 8 12 1 0.4 2 2.9 3 1 7

Bell’s Vireo Vireo bellii * Vireo de Bell 744 37 781 187 76.3 20 28.6 207 65.7 8

Plumbeous Vireo Vireo plumbeus * Vireo Plomizo 15 2 17 9 3.7 2 2.9 11 3.5 3

Hutton’s Vireo θ Vireo huttoni Vireo Reyezuelo 1 1 1 0.4 1 0.3 1

Warbling Vireo θ Vireo gilvus * Vireo Gorjeador 1 1 1 1.4 1 0.3 1

Chihuahuan Raven Corvus cryptoleucus Cuervo Llanero 95 1 96 41 16.7 1 1.4 42 13.3

Common Raven Corvus corax Cuervo Comun 50 22 72 35 14.3 12 17.1 47 14.9

Horned Lark θ Eremophila alpestris Alondra Cornuda 60 60 i* i*

Violet-green Swallow

Tachycineta thalassina 

*

Golondrina Verde 

Tornasol 6 6 i* i* 1

North. Rough-winged 

Swallow

Stelgidopteryx 

serripennis*

Golondrina de Ala 

Aserrada 98 13 111 42 17.1 7 10 49 15.6

Cliff Swallow

Petrochelidon 

pyrrhonota * Golondrina Risquera 124 124 22 9 22 7

Barn Swallow Hirundo rustica * Golondrina Tijereta 10 6 16 6 2.4 1 1.4 7 2.2

Bridled Titmouse Baeolophus wollweberi Carbonero Embridado 17 4 21 9 3.7 1 1.4 10 3.2 3

Verdin Auriparus flaviceps Baloncillo 48 1 49 28 11.4 1 1.4 29 9.2 2
73

English name 

(Bold=evidence of local breeding

encountered, θ=Info in Appdx D)

Scientific Name 

(Neotropical Migrants by 

USFWS (2007) noted with * )

Spanish name 

(Kauffmann and others 2005)

U.S. 

Birds 

Detected 

Mexico 

Birds 

Detected 

Total 

Detections 

No. U.S. 

Stations 

Where 

Detected 

U.S. 

Stations 

Occupied 

(%) 

No. MX 

Stations 

Where 

Detected 

MX 

Stations 

Occupied 

(%) 

Total 

Stations 

Where

Detected 

Total 

Stations 

Occupied

(%) 

Conservation 

Priority 

Index 

Bushtit Psaltriparus minimus Sastrecillo 90 16 106 29 11.8 6 8.6 35 11.1 1

White-breasted Nuthatch Sitta carolinensis Sita de Pecho Blanco 443 49 492 188 76.7 32 45.7 220 69.8

Cactus Wren

Campylorhynchus 

brunneicapillus Salltapared Barranqueao 4 2 6 4 1.6 3 4.3 7 2.2 2

Rock Wren Salpinctes obsoletus Saltapared Roquero 2 8 10 2 0.8 4 5.7 6 1.9

Canyon Wren θ Catherpes mexicanus Saltapared barranqueño 25 25 14 5.7 14 4.4 3

Bewick’s Wren Thryomanes bewickii Saltapared de Bewick 1557 190 1747 241 98.4 61 87.1 302 95.9

House Wren θ Troglodytes aedon * Saltapared Continental 1 1 2 i* i* i*

Blue-gray Gnatcatcher θ Polioptila caerulea * Perlita Azul Gris 3 1 4 i* i* i*

Northern Mockingbird Mimus polyglottos Cenzontle Norteño 215 137 352 119 48.6 56 80 175 55.6

Curve-billed Thrasher Toxostoma curvirostre 

Cuitlacoche de Pico 

Curvo 14 19 33 14 5.7 15 21.4 29 9.2 1

Crissal Thrasher θ Toxostoma crissale Cuitlacoche crisal 39 39 31 12.7 31 9.8 9

European Starling Sturnus vulgaris Estornino Europeo 4 1 5 1 0.4 1 1.4 2 0.6

Phainopepla Phainopepla nitens Capulinero Negro 186 76 262 75 30.6 23 32.9 98 31.1 6

Orange-crowned Warbler θ Vermivora celata * Chipe Oliváceo 1 1 i* i* 1

Lucy’s Warbler Vermivora luciae * Chipe de Rabadilla Rufa 440 130 570 182 74.3 54 77.1 236 74.9 9

Northern Parula θ Parula americana * Parula Norteña 1 1 2 1 0.4 1 1.4 2 0.6

Yellow Warbler θ Dendroica petechia * Chipe Amarillo 3102 245 3347 242 98.8 58 82.9 300 95.2 1

Palm Warbler θ Dendroica palmarum * Chipe Playero 1 1 i* i*

American Redstart θ Setophaga ruticilla * Chipe flameante 1 1 1 0.4 1 0.3 1

Common Yellowthroat Geothlypis trichas * Mascarita Común 1053 68 1121 222 90.6 30 42.9 252 80

Wilson’s Warbler θ Wilsonia pusilla * Chipe de Corona Negra 6 4 10 6 2.4 4 5.7 10 3.2

Yellow-breasted Chat Icteria virens * Buscabrena 3036 120 3156 241 98.4 37 52.9 278 88.3

Summer Tanager Piranga rubra * Tangara Roja 984 120 1104 222 90.6 57 81.4 279 88.6 2

Western Tanager θ Piranga ludoviciana *

Tangara de Capucha 

Roja 7 6 13 6 2.4 5 7.1 11 3.5

Canyon Towhee Pipilo fuscus * Rascador (Toqui) Pardo 23 15 38 16 6.5 10 14.3 26 8.3 5

Abert’s Towhee θ Pipilo aberti 

Rascador (Toqui) de 

Abert 989 63 1052 234 95.5 35 50 269 85.4 5

Rufous-winged Sparrow Aimophila carpalis * Zacatonero de Ala Rufa 1 8 9 1 0.4 7 10 8 2.5 9

Cassin’s Sparrow Aimophila cassinii * Zacatonero de Cassin 30 11 41 21 8.6 5 7.1 26 8.3 7

Botteri’s Sparrow Aimophila botterii * Zacatonero de Botteri 88 31 119 53 21.6 19 27.1 72 22.9 6

Rufous-crowned Sparrow Aimophila ruficeps *

Zacatonero de Corona 

Rufa 1 23 24 1 0.4 15 21.4 16 5.1 2
74

English name 

(Bold=evidence of local breeding

encountered, θ=Info in Appdx D)

Scientific Name 

(Neotropical Migrants by 

USFWS (2007) noted with * )

Spanish name 

(Kauffmann and others 2005)

U.S. 

Birds 

Detected 

Mexico 

Birds 

Detected 

Total 

Detections 

No. U.S. 

Stations 

Where 

Detected 

U.S. 

Stations 

Occupied 

(%) 

No. MX 

Stations 

Where 

Detected 

MX 

Stations 

Occupied 

(%) 

Total 

Stations 

Where

Detected 

Total 

Stations 

Occupied

(%) 

Conservation 

Priority 

Index 

Lark Sparrow

Chondestes grammacus 

* Gorrión Arlequin 1 14 15 1 0.4 11 15.7 12 3.8 1

Black-throated Sparrow Amphispiza bilineata 

Gorrión de Gargante 

negra 42 5 47 18 7.3 4 5.7 22 7 4

Grasshopper Sparrow θ

Ammodramus 

savannarum * Gorrión Saltamontes 2 2 1 1.4 1 0.3 7

Song Sparrow Melospiza melodia Gorrión Cantor 1217 91 1308 206 84.1 35 50 241 76.5

Northern Cardinal Cardinalis cardinalis Cardinal Norteño 186 13 199 103 42 11 15.7 114 36.2

Pyrrhuloxia Cardinalis sinuatus Cardenal desértico 6 6 5 2 5 1.6 3

Rose-breasted Grosbeak θ

Pheucticus ludovicianus 

*

Picogordo de pecho 

rosa 1 1 1 0.4 1 0.3 1

Black-headed Grosbeak

Pheucticus 

melanocephalus * Picogordo Tigrillo 23 9 32 19 7.8 6 8.6 25 7.9

Blue Grosbeak Passerina caerulea * Picogordo Azul 382 102 484 175 71.4 53 75.7 228 72.4

Indigo Bunting θ Passerina cyanea * Colorín Azul 2 2 1 0.4 1 0.3 1

Varied Bunting Passerina versicolor * Colorín Morado 17 2 19 13 5.3 2 2.9 15 4.8 6

Red-winged Blackbird Agelaius phoeniceus * Tordo Sargento 119 18 137 33 13.5 9 12.9 42 13.3

Eastern Meadowlark Sturnella magna *

Pradero Tortilla con 

Chile 4 26 30 3 1.2 16 22.9 19 6 4

Great-tailed Grackle θ Quiscalus mexicanus Zanate mexicano 2 2 1 0.4 1 0.3

Brown-headed Cowbird Molothrus ater * Vaquero de Cabeza Café 1022 111 1133 232 94.7 50 71.4 282 89.5

Hooded Oriole Icterus cucullatus * Bolsero Enmascarado 20 1 21 16 6.5 1 1.4 17 5.4 4

Bullocks Oriole Icterus bullockii * Bolsero de Bullock 177 62 239 110 44.9 42 60 152 48.3

Scott’s Oriole Icterus parisorum * Bolsero Tunero 4 3 7 4 1.6 3 4.3 7 2.2 6

House Finch Carpodacus mexicanus Pinzón Mexicano 1037 100 1137 228 93.1 47 67.1 275 87.3

Lesser Goldfinch Carduelis psaltria *

Dominico de Dorso 

Oscuro 1802 114 1916 227 92.7 37 52.9 264 83.8

House Sparrow θ Passer domesticus Gorrión Doméstico 38 10 48 6 2.4 i* 6 1.9

TOTALS: Detections (includes incidental), and on stations only (d) and 

number stations possible (/) 32533 4187 36720

32475

(d)

/ 245 

sta.

4041

(d)

/ 70 

sta. 36516 (d) / 315 sta. 59 spp.
75

Appendix 2-A. Table of 59 species of conservation interest detected throughout study area along the San Pedro River and 

associated riparian areas in southeastern Arizona and northern Sonora, and designation under each of 17 lists intended to aid in the 

prioritization of species for conservation action. 

[Each list and/or ranking is from one of five separate (though often overlapping) national efforts on the part of government agencies and/or conservation organizations. 

See numbered notes below for citation and guide to each column’s corresponding list and codes used to designate different rankings]

English name

(in Bold if evidence of local 

breeding encountered)

Scientific Name

(Neotropical Migrants by 

USFWS (2007) noted with * )

AZ PIF 

Priority 

Score 

(≥20) 1 

AZPIF 

Priority 

Spp (by 

Hab) 2

BBS % 

Drop 

1966-

2003 3

Comn. 

Spp % 

Drop 

1967-

2007 4

Aud 

Watch 

List 

2007 5

PIF 

Cont. 

Watch 

List 6

PIF 

S.west 

Stwd 

Spp 7

PIF 

Wetland 

Spp 8

BCR 

34 

Reg 

Conc 

Spp 9

BCR 

34 

Cont 

Stwd 

Spp 10

BCR 

34 

Reg 

Stwd 

Spp 11

US 

FWS 

BCR 

34 12

US 

FWS 

Reg 2 

13

US 

FWS 

USA/

Nat 14

US 

FWS 

ESA 15

Mex. 

BCR 34 

SJV Reg 

Conc 

Spp 16

Mex 

BCR 

34

SJV

Stwd 

Spp 17

Conservation 

Priority 

Initiatives 

Index 18

Scaled Quail Callipepla squamata 

YLL

W 2 SW Y 4

Gambel’s Quail Callipepla gambelii SW ST Y Y 4

Montezuma Quail Cyrtonyx montezumae MPO

YLL

W 3 Y Y 5

White-tailed Kite Elanus leucurus 20 1

Cooper’s Hawk Accipiter cooperii * 21 Y 2 3

Gray Hawk Buteo nitidus 26 Y 1 1 3 5

Swainson’s Hawk Buteo swainsoni * 21 HG

YLL

W 2 WL Y 1 7

Zone-tailed Hawk Buteo albonotatus * 23 R 2

Peregrine Falcon Falco peregrinus * 21 1 1 1 DL DS 6

Least Tern Sternula antillarum * RED 1 1

LE, 

LT 4

Common Ground-Dove Columbina passerina 2 1

Yellow-billed Cuckoo Coccyzus americanus * 35 Rip. 1 1 1 C** 5 7

Barn Owl Tyto alba R,DS 1

Western Screech-Owl Megascops kennicottii Y 3 2

Common Poorwill Phalaenoptilus nuttallii * Y 3 2

Gila Woodpecker Melanerpes uropygialis 21 4 2

Northern BeardlessTyrannulet Camptostoma imberbe * 23 1 1 3 4

Olive-sided Flycatcher Contopus cooperi * 23 MC,P 73.2

YLL

W 2 WL 1 7

Willow Flycatcher (see Ap. A,

D) Empidonax traillii * 38 Rip.

YLL

W 2 WL LE ** 5 7

Dusky-capped Flycatcher Myiarchus tuberculifer * R 1

Ash-throated Flycatcher Myiarchus cinerascens * Y 1
76

English name

(in Bold if evidence of local 

breeding encountered)

Scientific Name

(Neotropical Migrants by 

USFWS (2007) noted with * )

AZ PIF 

Priority 

Score 

(≥20) 1 

AZPIF 

Priority 

Spp (by 

Hab) 2

BBS % 

Drop 

1966-

2003 3

Comn. 

Spp % 

Drop 

1967-

2007 4

Aud 

Watch 

List 

2007 5

PIF 

Cont. 

Watch 

List 6

PIF 

S.west 

Stwd 

Spp 7

PIF 

Wetland 

Spp 8

BCR 

34 

Reg 

Conc 

Spp 9

BCR 

34 

Cont 

Stwd 

Spp 10

BCR 

34 

Reg 

Stwd 

Spp 11

US 

FWS 

BCR 

34 12

US 

FWS 

Reg 2 

13

US 

FWS 

USA/

Nat 14

US 

FWS 

ESA 15

Mex. 

BCR 34 

SJV Reg 

Conc 

Spp 16

Mex 

BCR 

34

SJV

Stwd 

Spp 17

Conservation 

Priority 

Initiatives 

Index 18

Brown-crested Flycatcher Myiarchus tyrannulus * 21 1

Tropical Kingbird Tyrannus melancholicus * 20 1

Cassins Kingbird Tyrannus vociferans * 21 Y Y 5 4

Loggerhead Shrike Lanius ludovicianus * 22 77.1 71 Y 1 1 3 7

Bell’s Vireo Vireo bellii * 25 RED 2 WL Y 1 1 1 8

Plumbeous Vireo Vireo plumbeus * 22 Y R 3

Hutton’s Vireo Vireo huttoni 23 1

Warbling Vireo Vireo gilvus * 3 1

Violet-green Swallow Tachycineta thalassina * R 1

Bridled Titmouse Baeolophus wollweberi 20 Y R,PO 3

Verdin Auriparus flaviceps 85.6 SW 2

Bushtit Psaltriparus minimus 2 1

Cactus Wren

Campylorhynchus 

brunneicapillus SW Y 2

Canyon Wren Catherpes mexicanus 70 Y DS,R 3

Curve-billed Thrasher Toxostoma curvirostre SW 1

Crissal Thrasher Toxostoma crissale 20 SW ST Y Y 1 1 1 R 9

Phainopepla Phainopepla nitens SW ST Y Y Y 3 6

Orange-crowned Warbler Vermivora celata * 21 1

Lucy’s Warbler Vermivora luciae * 29 Rip.

YLL

W 3 SW WL Y Y Y 9

Yellow Warbler Dendroica petechia * 4 1

American Redstart Setophaga ruticilla * 25 1

Summer Tanager Piranga rubra * 22 4 2

Canyon Towhee Pipilo fuscus * SW Y Y Y 4 5

Abert’s Towhee Pipilo aberti 25

YLL

W 3 SW WL 5

Rufous-winged Sparrow Aimophila carpalis * 28 DS,DG

YLL

W 3 SW Y 1 1 1 9

Cassin’s Sparrow Aimophila cassinii * 28 DG SW Y 1 1 3 7

Botteri’s Sparrow Aimophila botterii * 28 DG 1 1 2 5

Rufous-crowned Sparrow Aimophila ruficeps * 21 Y 5 3

Lark Sparrow Chondestes grammacus * 63 1

Black-throated Sparrow Amphispiza bilineata 79.6 63 SW Y DS, G 5

Grasshopper Sparrow Ammodramus savannarum * 28 DG,HG 77.1 65 1 1 5 7
77

English name

(in Bold if evidence of local 

breeding encountered)

Scientific Name

(Neotropical Migrants by 

USFWS (2007) noted with * )

AZ PIF 

Priority 

Score 

(≥20) 1 

AZPIF 

Priority 

Spp (by 

Hab) 2

BBS % 

Drop 

1966-

2003 3

Comn. 

Spp % 

Drop 

1967-

2007 4

Aud 

Watch 

List 

2007 5

PIF 

Cont. 

Watch 

List 6

PIF 

S.west 

Stwd 

Spp 7

PIF 

Wetland 

Spp 8

BCR 

34 

Reg 

Conc 

Spp 9

BCR 

34 

Cont 

Stwd 

Spp 10

BCR 

34 

Reg 

Stwd 

Spp 11

US 

FWS 

BCR 

34 12

US 

FWS 

Reg 2 

13

US 

FWS 

USA/

Nat 14

US 

FWS 

ESA 15

Mex. 

BCR 34 

SJV Reg 

Conc 

Spp 16

Mex 

BCR 

34

SJV

Stwd 

Spp 17

Conservation 

Priority 

Initiatives 

Index 18

Pyrrhuloxia Cardinalis sinuatus 22 SW 3 3

Rose-breasted Grosbeak Pheucticus ludovicianus * 0 1

Indigo Bunting Passerina cyanea * 20 1

Varied Bunting Passerina versicolor * 22

YLL

W 2 WL 1 1 6

Eastern Meadowlark Sturnella magna * 25 72 Y 5 4

Hooded Oriole Icterus cucullatus * 23 Y 1 R 4

Scott’s Oriole Icterus parisorum * 21 SW ST Y Y DS, G 6

TOTAL NUMBER of San Pedro Species on each list  36 10 7 5 11 10 15 11 14 6 18 10 13 11 4 23 12 59
78

1

Arizona Partners in Flight (PIF), Potential Consideration for Priority Status (score of 20 or above).

2

Arizona Partners in Flight, Priority Species (listed by habitat). Latta and others (1999) used 11 criteria developed by Arizona 

Partner’s in Flight (AZPIF) to prioritize bird species most in need of conservation efforts in the state. Species were ranked 

higher for relative low abundance compared to species sharing similar habitats throughout the entire range of that species, 

relative low abundance as compared to other Arizona species in similar habitats, narrow distribution overall or within Arizona, 

more or graver threats on the breeding grounds or wintering grounds (over the species range or within Arizona), and if a high 

proportion of a species’ breeding range is in Arizona (See appendix 2-A and Latta and others, 1999, for more details). This 

ranking did not use Breeding Bird Survey (BBS) data because of inadequate coverage in Arizona. Latta and others (1999) 

assessed Arizona’s 280 species of landbirds, of which 234 that breed in with high scores of 38 (Willow Flycatcher) to low 

scores of 8 (Common Raven). Initially, the authors decided to consider any species with a score of 20 or higher for 

consideration as a species of conservation concern (thus, we include the score for all species with a score of 20 or higher as one 

of our conservation prioritization efforts, column 1). For choosing the final list of Arizona’s PIF priority bird species, the 

authors assigned all birds as belonging to one of more than 20 habitat types. From the pool of species with scores of 20 or 

higher resulting from the initial quantitative assessment, they chose the highest scoring species’ found within a habitat type as 

one of Arizona’s top 43 conservation concern species (thus we included this as another effort, by denoting their habitat type in 

column 2).

3

Breeding Bird Survey Percent Drop from 1966–2003, noted only if on Audubon Society’s list of “Sixteen species undergoing 

most severe population declines in North America” (Audubon.org, 2008a). This is the percentage of decline for continental 

populations, from an analysis of BBS route data from 1966 to 2003. 

4

Common Species percentDrop from 1967–2007, noted only if on Audubon Society’s list of “Common Birds in Decline: 

What's happening to birds we know and love?” (Audubon.org, 2008b). This is based on both Christmas Bird count and U.S. 

Geological Survey (USGS) breeding bird survey data.

5

Audubon Watch List 2007, Audubon and the American Bird Conservancy used two categories to delineate level of concern. 

RED=Declining rapidly and/or small populations, and face major conservation threats (typically of global conservation 

concern), whereas yellow YLLW=declining or rare (national conservation concern). This list utilized similar criteria to Panjabi 

and others (2005), but included water-associated birds as well, and included species such as Least Tern, and endangered, 

federally listed species not included on other lists (Audubon.org and American Bird Conservancy, 2007).

6

Partners in Flight Continental Watch List, from table 1 of the North American Landbird Conservation Plan: “Partners in Flight 

Species of Continental Importance for the U.S. and Canada," (table 1, p. 18–19) (Rich and others, 2004). Score=2 if a PIF 

“Continental Watch List” species that is moderately abundant or widespread but with declines or high threats (higher priority); 

3=if a Watch List Species that has a restricted distribution or low population size (slightly lower priority). There were no 

species in our area with the highest priority (multiple causes for concern across entire range and sharp declines). “Continental 

Watch List Species are those which are most vulnerable at the continental scale, because of a combination of small and 

declining populations, limited distributions, and high threats throughout their ranges. Some of these species are already 

recognized as threatened or endangered at federal levels.” (Panjabi and others, 2005). The Watch List is “comprised of species 

that have multiple reasons for conservation concern across their entire ranges” (Rich and others, 2004).

7

Partners in Flight Southwest Stewardship Species, from the North American Landbird Conservation Plan, table 1: “Partners in 

Flight Species of Continental Importance for the U.S. and Canada", (table 1, p. 20–21) (Rich and others, 2004). If species 

designated with “SW”=Southwest is of Global Importance to the species, or it is a "Stewardship Species" where the Southwest 

is listed as "Avifaunal Biome with Global Stewardship Responsibility." 

8

Partners in Flight Wetland-Associated Species, from Rich and others (2004), table C: “Species of Continental Importance 

associated with wetland habitats in all or part of their range.” If designated here with WL=it is a wetland associated species that 

is also on the PIF watch list; if designated with ST=it is a wetland associated species that is also a “Stewardship Species” in the 

Southwest. 

9

Bird Conservation Region 34 (Sierra Madre Occidental) Regional Concern Species. From North American Bird Conservation 

Initiative (NABCI) Committee (see North American Bird Conservation Initiative, 2008, for more information). Y=Species of 

Regional Concern in Bird Conservation Region 34 (BCR 34), based on Partners in Flight assessment efforts for landbirds 

(Partners in Flight, 2005 for data, Panjabi, 2005, for methodology). 
79

10Bird Conservation Region 34 (Sierra Madre Occidental) Continental Stewardship Species. From North American Bird 

Conservation Initiative (NABCI) Committee (see North American Bird Conservation Initiative, 2008, for more information). 

Y=Species is a “Continental Stewardship Species” for Bird Conservation Region (BCR) 34, from Partners in Flight assessment 

efforts (Partners in Flight, 2005, for data; Panjabi, 2005, for methodology). 

11Bird Conservation Region 34 (Sierra Madre Occidental) Regional Stewardship Species. From North American Bird 

Conservation Initiative (NABCI) Committee (see NABCI 2008 for more information). Y=Species is a “Regional Stewardship 

Species” for BCR 34, from Partners in Flight assessment efforts (Partners in Flight, 2005, for data, Panjabi, 2005, for 

methodology). 

12U.S. Fish and Wildlife Service (USFWS) BCR 34. 1=Birds listed as a priority within BCR 34 for USFWS planning and 

consideration, from U.S. Fish and Wildlife Service (2002) (table 34, p. 57). Listed here if scoring as a high priority on 

assessment methodologies. See page 7–9, U.S. Fish and Wildlife Service (2002) for more details on inclusion on this and next 

two lists. U.S. Fish and Wildlife Service (2002) lists birds species of conservation concern at three different scales, this and the 

next two lists moving from smaller to larger scales. Inclusion in “Bird Conservation Region (BCR) 34—The Sierra Madre 

Occidental” is based on a species importance at the eco-regional scale, meaning a high proportion of population (or geographic 

distribution) of a species is concentrated within a region bearing similar topography and vegetation communities. “By 

concentrating on the importance of these populations to the persistence of a species as a whole and then assessing the threats to 

these eco-regional populations, the Bird Conservation Region approach directs conservation actions that maintain or enhance 

these populations” (North American Bird Conservation Initiative, 2007, see Panjabi and others, 2005 for details on the 

assessment methodology). U.S. Fish and Wildlife Service (2002) also lists birds of importance at the scale of USFWS regional 

administrative Units (for example, Region 2, “Southwest”) and birds that are important to consider as a national priority for the 

USFWS across the United States. 

13U.S. Fish and Wildlife Service Region 2. 1=Birds as high priority for planning efforts for USFWS for Region 2 (Southwest 

administrative planning region for the agency). From USFWS (2002), table 42, p. 55–56. 

14U.S. Fish and Wildlife Service National-level Watch List. 1=Birds on USFWS National Watch List, of high priority at the 

national level. From USFWS (2002), table 43, p. 73–74. 

15U.S. Fish and Wildlife Service Endangered Species Act Relation of species to Endangered Species Act listings (United 

States), from USFWS (2008). DL=De-Listed, Cand=Candidate Species, LE=Listed Endangered, LT=Listed Threatened. 

16Mexican Highlands Portion of BCR 34 Sonoran Joint Venture Regional Concern Species. If a Mexican Highlands (BCR 34 in 

Mexico) Regional Concern Species, then Relative Density score is given from Ciacedo and others (2006), table 11, "Species of 

Regional Concern in the Mexican Highlands." From Ciacedo and others (2006): “Scores reflect the "magnitude of stewardship 

responsibility" that the BCR 34 in Mexico shares for the maintenance of the species, with 5 being the highest (populations are 

concentrated in this region and are under high threat) and 2 meaning lower responsibility for the persistence of the species (for 

example, populations of this species are more spread out and not concentrated in this region, but still these regional populations 

are still important to the species as a whole). These are species that have a combination of moderately high vulnerability, high 

regional threats, and declining population trends. High scoring subspecies only were included in table 11 and thus in this 

column. 

17Mexican Highlands Portion of BCR 34 Sonoran Joint Venture Stewardship Species. If a Mexican Highlands (BCR 34 in 

Mexico) Stewardship Species, then signature habitat is given from Ciacedo and others (2006), table 12: "Stewardship 

Responsibility Species for the Mexican Highlands". R=Riparian, DS=Desert Scrub, G=Grassland, PO=Pine Oak. “These are 

species for which the SJV has a "stewardship responsibility". They are species in which a large percentage of the population 

inhabits this Region during...either portion of their annual cycle. Concern level for them is not high at present, but it is the SJV's 

responsibility to ensure that they do not slip into the "concern" category. This usually means monitoring is an action and these 

species and their habitats need to be maintained and considered in long-term planning" (Ciacedo and others, 2006). 

Black-bellied Whistling-Duck (Dendrocygna autumnalis): Several presumably breeding at Las Nutrias reservoir, Mexico. Two 

sightings at Los Fresnos (lower riparian area as well as reservoir). 

Blue-winged Teal (Anas discors): Four seen in May 2007 at Las Nutrias. 

Cinnamon Teal (Anas cyanoptera): One at Las Nutrias, June 2007.
80

Ruddy Duck (Oxyura jamaicensis): Mexican reservoirs May-July 2006 and 2007.

Pied-billed Grebe (Podilymbus podiceps): Several presumably breeding at Los Fresnos and Las Nutrias reservoirs, Mexico.

Eared Grebe (Podiceps nigricollis): Six late migrants at Las Nutrias, May 2007. 

Western Grebe (Aechmophorus occidentalis): Four Las Nutrias, May 2007. 

Yellow-crowned Night-Heron (Nyctanassa violacea): Presumably, the same individual was seen at nearly the same location 

on May 19 and again on June 20, 2007 along the upper San Pedro River, Ejido San Pedro, Sonora, 7 km south and up-river 

from the international border with the United States (12 R 577985 3460074). Each time the bird was seen by two observers 

(Glenn Johnson and Sergio Juarez, or Aaron Flesch and Sky Jacobs), and a few pictures were obtained during the first sighting 

when the bird perched in a cottonwood tree across the river (fig. 6). All viewing was within 30 to 100 m. The long legs were 

very noticeable, extending well beyond the tail when observed in flight and when perched as well. It's obvious cream-yellow 

crown, longish-crown-plumes, pale cheek and dark eyeline, as well as brownish body plumage with small white streaks, were 

all noted after it landed across the river in the cottonwood. This combination of characters indicates that the bird was a second 

year in alternate plumage (within the variation described in Birds of North America species account #161). While perched, its 

long legs, neck, and upright stance helped further confirm the species ID (versus a Black Crowned Night-heron). We obtained 

several pictures of varying quality using a digital camera and binoculars that, at the very least, confirm the stance and crown 

color of the bird. 

By making this photo available to respected regional field ornithologists for scrutiny, our species identification was confirmed 

by several ornithologists in the region (Troy Corman, oral commun; Dave Stejskal, oral commun.). 

White-faced Ibis (Plegadis chihi): Flock migrating north high in sky, May 2007. 

Gray Hawk (Buteo nitidus): Common breeder in SPRNCA, at least eight territories on main stem south of border, 3–4 

territories on Los Fresnos. 

Swainson’s Hawk (Buteo swainsoni): Nesting on San Pedro Riparian National Conservation Area (SPRNCA, BLM) and one 

pair at Los Fresnos. 

Zone-tailed Hawk (Buteo albonotatus): Rare in riparian sites in U.S., one possibly nesting south of Boquillas Ranch (SPRNCA) 

in May 2007. 

Peregrine Falcon (Falco peregrinus): Adult and immature at Hereford, May 30, 2006. 

American Coot (Fulica americana): Several presumably breeding at Los Fresnos and Las Nutrias reservoirs, Mexico. 

Spotted Sandpiper (Actitis macularius): Infrequently seen in August throughout study area. 

Wilson’s Phalarope (Phalaropus tricolor): Twelve observed for one day at Las Nutrias reservoir, May 2007. 

Least Tern (Sternula antillarum): Two individuals observed over a half hour period by three observers on May 19, 2007.

Viewed from north side of reservoir at Las Nutrias Ranch reservoir, 

12 R 568430 3452278. One individual observed over a half hour period starting at 3 p.m., including several flights 30 m 

directly above the observer and many more within 100 m of the observers. The second individual joined the first, but was 

mostly seen from across the reservoir at approximate distances of 150–300 m. Observed the following distinguishing 

characters: very small size, fast and direct yet lilting/buoyant flight, dark leading edge and very faint dark tips to outer wing 

primaries, and especially the black crown and eye stripe with white forehead, yellow bill w/ black tip. Three observers: Glenn 

Johnson, Sergio Juarez, and Casey Fitzgerald.
81

In recent years there are regular reports of non-breeding Least Terns in May and early June from nearby southeastern Arizona, 

indicating that the species moves through the region during this time. For example, on May 20, 2005, June 6, 2006, and May 

12, 2007, at least one Least Tern was present at Cochise Lake (Twin Lakes) in Wilcox, Arizona (see 

http://listserv.arizona.edu/cgi-bin/wa?A2=ind0505c&L=birdwest&P=1962; http://www.rarebirds.com/listing/2599/; 

http://www.huachuca-audubon.org/NAMC.php). Another individual was seen at this site on May 19, 2007, the same day as our 

observations to the south (Dave Beaudette, oral commun.). The Cochise Lake site is 123 km at a bearing of 20 degrees from the 

Las Nutrias site, and both sites could be regular stopover points in migration to breeding sites farther to the northeast in the 

Midwestern United States in the Mississippi drainage, or even to coastal California populations. Other sites where Least Tern 

has been seen nearby include nearby Benson sewage ponds (84 km north of Las Nutrias) on May 9, 2006, Rio Rico area south 

of Tucson (83 km, 300 degrees) on May 10, 2006, Hereford/Sierra Vista (31.5 km, 214 degrees) on May 11, 2006 

(http://www.rarebirds.com/listing/2400/), and Gilbert Water Ranch south of Phoenix (292 km at 332 degrees) 

(http://www.azfo.org/gallery/lete_gilbert.html) on May 12, 2007. 

Krueper (2000) lists the Least Tern as a casual to rare vagrant to southern Arizona from May to September, with five detections 

in the Upper San Pedro River valley in the Sierra Vista area. 

Barn Owl (Tyto alba): Resident pair at Los Fresnos.

Western Screech-Owl (Megascops kennicottii): Commonly detected at night where trees present. 

Great Horned Owl (Bubo virginianus): Commonly Detected at night. 

Lesser Nighthawk (Chordeiles acutipennis): Uncommon at dusk and throughout night. 

Common Nighthawk (Chordeiles minor): Uncommon at dusk, night, and in early morning. 

Common Poorwill (Phalaenoptilus nuttallii): Commonly detected at night. 

Vaux’s Swift (Chaetura vauxi): Rare migrant. 

Olive-sided Flycatcher (Contopus cooperi): Late migrants observed in May at Boquillas (SPRNCA) and at Los Fresnos lower 

riparian area. 

Willow Flycatcher (Empidonax traillii): Migrants uncommonly seen in late May and early June. One nest found in July 2005, 

the southern most documented attempt for all southwestern states except Texas, where several historical nesting attempts 

occurred at more southerly latitudes. 

Dusky-capped Flycatcher (Myiarchus tuberculifer): Commonly detected in our study only in upper Los Fresnos in association 

with oak forests. Only two pairs noted in SPRNCA, one at Curtis Windmill and another just south of Green Kingfisher Pond 

(where appeared to be nesting in June–July 2007). 

Tropical Kingbird (Tyrannus melancholicus) : Common only within 2 km of U.S. Highway 90. 

Hutton’s Vireo (Vireo huttoni): One singing north of Charleston Bridge (SPRNCA), 2006. Otherwise, found commonly in oaks 

in upper portions of the watershed. 

Warbling Vireo (Vireo gilvus): Singing in May, presumably migrants. 

Horned Lark (Eremophila alpestris): Occasionally seen in grasslands between routes. 

Canyon Wren (Catherpes mexicanus): Only detected in area from 1 km north of U.S. Highway 90 to 2 km north of Charleston 

Bridge. 

House Wren (Troglodytes aedon): Two detected in late May, one 500 m. south of U.S. Highway 90, 2005, another at Los 

Fresnos, 2007. 

Blue-gray Gnatcatcher (Polioptila caerulea): Occasionally wander to riparian area in late summer. 
82

Crissal Thrasher (Toxostoma crissale): Detected numerous times in northern SPRNCA riparian areas only during 2005 season. 

Orange-crowned Warbler (Vermivora celata): August migrant at Los Fresnos, late-wintering/migrating individuals seen on 

SPRNCA in May as well. 

Northern Parula (Parula americana): Late (June 16) singing migrants, one in SPRNCA at Curtis Windmill in 2006, one at 

Los Fresnos 2007. Both detected by Glenn Johnson. On June 6, 2006, one individual was heard singing in the Curtis Windmill 

area of the SPRNCA in the U.S., although not seen. On May 18, 2007, one individual was heard singing and confirmed visually 

at Los Fresnos Nature Preserve, north of Cananea, Sonora, Mexico, and 7.5 km south of the international border with the 

United States, 12 R 557766 3458788. One singing male was detected singing, and then observed briefly, while conducting 

multi-species bird surveys on the ranch-preserve. The typical ascending, buzzy, “type A” song (for example, similar to Birds of 

North America Online, issue #215) was heard twice while conducting the survey, after which the bird was tracked for 

approximately 5 minutes and several brief yet good views were obtained. The bird was flying rapidly between gleaning from 

the outer tips of cottonwood (Populus freemontii) branches and briefly perching in yew-leaf willow (Salix taxifolia). It was not 

seen subsequently when several observers returned in the afternoon, or the next day, so it was assumed that the individual was a 

vagrant in migration. 

Yellow Warbler (Dendroica petechia): Very common throughout all Cottonwood/Willow Riparian forests. Some survey 

stations had as many as six singing males for much of the breeding season. Although some of these could have been singing 

migrants, this was obviously one of the most abundant breeding birds throughout the study area. 

Palm Warbler (Dendroica palmarum): One rare migrant seen by two observers, August 18, 2006, at Los Fresnos. Of the 

western race (Dendroica palmarum, s.s. palmarum) in migration at Los Fresnos Nature Preserve (north of Cananea, Sonora, 

Mexico, and 7.7 km south of the international border with the United States,12 R 558800 3459003). Christine Lamanna and 

Glenn Johnson observed the bird for several minutes from 30–40 m, foraging on the ground and perching on a corral fence post.

Clearly seen were its yellow under tail coverts, russet-red crown, white supercillium, darkish streaking on breast and flanks. It 

also had a slight yellowish wash on the lower flanks, but not as yellow as the under tail coverts. 

Although there are scattered winter records throughout Baja California and in Pacific lowlands of Mexico from Sonora south to

Oaxaca and on offshore islands (Howell and Webb, 1995 as cited in Birds of North America on-line, issue # 238), this is the 

first record we know of from Sonora in August. 

American Redstart (Setophaga ruticilla): One female south of Boquillas (SPRNCA), June 2006. 

Wilson’s Warbler (Wilsonia pusilla): Migrants often seen through May and occasionally in June. 

Western Tanager (Piranga ludoviciana): Late migrants in May, early migrants in July–August. 

Abert’s Towhee (Pipilo aberti): Common breeder throughout SPRNCA, and to a lesser extent along main stem San Pedro in 

Mexico just south of border. Only a few pairs, though, at Los Fresnos throughout riparian sections there. One individual 

appeared to be paired with a Canyon Towhee at Los Fresnos (A. Flesch, oral commun.. 

Grasshopper Sparrow (Ammodramus savannarum): Breeding at Los Fresnos, common in grasslands there in late summer. 

Rose-breasted Grosbeak (Pheucticus ludovicianus): One male detected, Hereford, May 2005. 

Indigo Bunting (Passerina cyanea): Probable breeding pair 500 m south of Gas line Bridge, St. David, 2007. 

Great-tailed Grackle (Quiscalus mexicanus): Rare visitor to riparian area. 

House Sparrow (Passer domesticus): Several breeding at ranch house at Los Fresnos. 
83

Chapter 3. Southwestern Willow Flycatcher Nest Records and Potential for Future 

Breeding along the Upper San Pedro River, Arizona

Abstract

We reviewed Southwestern Willow Flycatcher (Empidonax traillii extimus) nest records and 

investigated the potential for future breeding along the upper San Pedro River in southeastern Arizona, 

where in July 2005, we encountered the southernmost verifiable nest attempt for the species. We 

confirmed evidence of 20 other nests from the area, the majority (n=17) of which are historical records. 

Application of spatially explicit habitat models classified the riparian vegetation distributed along 64.12 

river km north of the United States-Mexico international border, and identified 7.7 percent (4.9 km) of the 

reach as having a greater than 80 percent likelihood of supporting nesting pairs of E. t.extimus. The active 

nest we found in 2005 was within the largest (a 910 m-reach) of several high-probability habitat patches 

identified by the model, and environmental measurements were similar to nest-site characteristics reported 

regionally. Continued conservation and management of the area’s riparian vegetation and surface water 

has potential to contribute additional breeding sites for this endangered subspecies. Given the nest record 

along the upper San Pedro River and the presence of high-density breeding sites to the north, the native 

cottonwood-willow forests of the upper San Pedro River could become increasingly important to E. t. 

extimus recovery, especially considering the anticipated effect of the tamarisk leaf beetle (Diorhabda 

carinulata) north of the region. 

Introduction

Conservation and recovery of endangered species necessitates data on current and historical 

distribution along with habitat use so that specific sites with high potential to support populations can be 

identified and managed. Although the exact historical distribution of Southwestern Willow Flycatchers 

(Empidonax traillii extimus) is poorly known, habitat losses and other factors led to declines across their 

range in the Southwestern United States in the 20th century (Phillips and others, 1964; Hunter and others, 

1987; Unitt, 1987) resulting in a listing as federally endangered by the U.S. Fish and Wildlife Service 

(1995). Southwestern Willow Flycatchers breed in dense riparian vegetation near surface water, often in 

forest environments comprised of native willow (Salix sp.) and Fremont cottonwood (Populus fremontii), 

and also in dense thickets of exotic salt cedar (Tamarix spp.) (Sogge and others, 1997; Sedgwick, 2000; 

Durst and others, 2007). Currently, E.t. extimus breed locally in widely scattered locations in six States 

where they are threatened by nest predation, nest-parasitism by Brown-headed Cowbirds (Molothrus ater), 

water diversion, and loss of riparian vegetation, which is already naturally limited in spatial extent 

throughout the arid, southwestern United States (U.S. Fish and Wildlife Service, 2002, 2005). 

After encountering a nesting pair of E.t. extimus in 2005 along the upper San Pedro River within 

the Bureau of Land Management-administered San Pedro Riparian National Conservation Area 

(SPRNCA) in the vicinity of Hereford, Arizona (reported in Engineering and Environmental Consultants, 

Inc., 2005; English and others, 2006; Ellis and others, 2008; and see appendix 3-A for details), we 

investigated the known nesting records and current habitat conditions in the area. Although unspecific 

reports of historical nesting at Charleston on the upper San Pedro River are mentioned by several authors 

(Phillips, 1948; Phillips and others, 1964; Krueper, 1999; Pardzick and Woodward, 2003), and surveys are 

conducted along several kilometers of the upper river corridor as recommended and reported in annual 

survey and recovery efforts (U.S. Fish and Wildlife Service, 2002; Ellis and others, 2008), no one has 

reviewed the nesting records there or compared these with other records along the southern extent of 

summer E.t. extimus breeding distribution. Whereas the lower (northern) portion of the San Pedro River, 

near its confluence with the Gila River, is well-known as an important complex of E.t. extimus breeding 

sites, the upper (southern) portion of the watershed is viewed in diverse ways: as an E.t. extimus migration 
84

corridor only, as containing only historical-but-not-extant breeding habitat, or conversely as containing 

high-value, suitable breeding habitat (Krueper, 1999; U.S. Fish and Wildlife Service, 2002, 2005; 

Pardzick and Woodward, 2003; Sogge and others, 2003; Steinitz and others, 2005; Durst and others, 2007; 

Ellis and others, 2008;). 

Correctly assessing the historical and current value of the habitat along the upper San Pedro is 

important, because it is likely that current resource management decisions throughout the upper San Pedro 

River Basin could potentially affect habitat that might exist along the upper river. For example, 

groundwater pumping of the aquifer-source of the upper river threatens to reduce surface water and 

vegetation cover that E.t. extimus and other riparian birds utilize (Stromberg and Tiller, 1996; Dougherty, 

2007; Webb and others, 2007; Brand and others, 2010; Davis, 2013). Also, the recent (1999–2001) 

reintroduction of beaver (Castor canadensis) could be viewed as either a threat or a potential benefit to E.

t. extimus breeding habitat (U.S. Fish and Wildlife Service, 1998 p. 30; Finch and others, 2000; U.S. Fish 

and Wildlife Service, 2002, p. 117; Longcore and others, 2007). Our objectives in this paper are to explore 

recovery potential for E.t. extimus in the upper San Pedro River through (1) a review of the historical and 

contemporary nesting record from the upper San Pedro River and nearby areas, (2) a comparison of the 

2005 Hereford nest site with regionally reported nest site characteristics, and (3) a comparison of the 

environmental characteristics at the 2005 nest site with nearby sites along the upper San Pedro River, 

including application of a spatially explicit geographic information systems (GIS) model which 

successfully predicted E.t. extimus breeding sites elsewhere. 

Methods

Historical and Contemporary Breeding Records 

We scanned regional bird distribution accounts, conducted literature searches using ISI/Web of 

Science and the Searchable Ornithological Research Archive (SORA; http://elibrary.unm.edu/sora/) 

hosted by the University of New Mexico, and initially accessed many of the museum records on March 

31, 2009, through two online holdings databases, the Ornithology Information System (ORNIS; 

http://olla.berkeley.edu/ornisnet/) and the Global Biodiversity Information Facility (GBIF; 

http://www.gbif.org/). We augmented this initial search by looking through individual institution’s 

specimen holdings as found on-line, by contacting collections and curatorial staff, and by attempting to 

match literature-reported observations with catalogued specimen(s) or field notes. We considered a record 

as “confirmed breeding” when we found egg sets and/or nests housed in museum collections, specimen 

skins with associated notes that unequivocally indicated the specimen was nesting and/or clearly described 

nesting events reported directly by the observer, where corroborating details, such as date, number of 

eggs, behaviors, etc., were included. 

Upper San Pedro River 2005 Nest–Site Characteristics Compared to Regional Measurements 

After the 2005 breeding attempt was over, we measured that nest-site’s characteristics using a 

standardized protocol (Rourke and others, 1999) to compare with other reported nest-sites in the region. 

We measured nest height above ground, nest-tree height and diameter at breast height (DBH), nest-stem 

diameter and branching, and nearest horizontal distance to foliage/canopy opening and surface water. We 

also estimated the percent canopy cover within 5 m of the nest in three height strata (<4.5, 4.5–7.5, and 

>7.5 m). Several general habitat features were documented within 50 m of the 2005 nest, including tree 

species and height range, proximity to beaver activity, and late-summer persistence of surface water, to 

compare with other nesting sites throughout breeding range of extimus (Sogge and Marshall, 2000; Sogge 

and others, 2003). 
85

Environmental Conditions Along the Upper San Pedro River

2005 Nest Site Compared to Local Survey Stations 

We compared the 2005 nest site with nearby riparian sites along 68.5 km of the upper San Pedro 

River, using environmental data from a separate study (Johnson, 2011) within the San Pedro Riparian 

National Conservation Area (SPRNCA). We compared vegetation and surface water persistence at a 

point-count survey station 2 m from the nest site with 239 other survey stations uniformly distributed at 

approximately 275 m intervals within the SPRNCA, from the southernmost point located less than 3 km 

north of the international border to the northernmost point near St. David, Arizona. We screened over 20 

measured variables, using descriptions of E.t. extimus breeding habitat from the literature to identify 

characteristics of the nest site, which might help to explain why E.t. extimus had settled there. These 

variables included total width of riparian vegetation perpendicular to the river, percent cover of each 

woody species within nine canopy-height strata (0–1, 1–3, 3–5, 5–7, 7–10, 10–15, 15–20, 20–25, >25 m), 

and a count of all plants/stems and their mean DBH all of which were measured within 30 m of the station 

centers. The nest was located 2 m from the center point of the avian survey station, so the 30 m 

measurements at that site were within 28–32 m of the nest. 

2005 Nest Site Compared to Local Application of Geographic Information Systems Model

We applied a spatially explicit GIS model originally designed to predict E.t. extimus breeding 

habitat in Arizona using vegetation data from remotely sensed Landsat Thematic Mapper imagery and 

landscape features from a Digital Elevation Model (Hatten and Paradzick, 2003). The model assigns 

riparian vegetation to one of five probability classes based on floodplain width and the metric Normalized 

Difference Vegetation Index, which is calculated from the reflectance values of the vegetation at a 30 m 

resolution. Although originally developed in central Arizona at the confluence of San Pedro and Gila 

Rivers and another site at Lake Roosevelt, this model has also successfully predicted the presence of E.t. 

extimus breeding territories in riparian vegetation at many sites across the subspecies’ range (Hatten and 

Paradzick, 2003; Dockens and others, 2004; Hatten and Sogge, 2007). 

Results

Historical and Contemporary Breeding Records 

In addition to the nest encountered in 2005 at Hereford, we found 20 other nest records along the 

upper San Pedro River (table 1), with a total of 4 contemporary and 17 historical (that is, pre-1920) 

records. We did not find verifiable evidence of any known breeding populations or single nesting attempts 

south in latitude from the upper San Pedro area, and apparently the upper San Pedro represents the 

southern extent of confirmed nesting for E.t. extimus and the breeding attempt we reported in 2005 is the 

southernmost verifiable nest record. Currently, the southernmost locations where E.t. extimus are 

consistently found breeding are 85–150 km to the north, along the middle and lower San Pedro, near the 

confluence with the Gila River (Durst and others, 2007; Ellis and others, 2008). 

Historical Nest Records 

The earliest record of breeding for the upper San Pedro is from an 1887 egg set collected at

unspecified location on Fort Huachuca by Lieutenant H.C. Benson (table 1), which contemporary U.S. 

Army workers believe may actually have been collected on the upper San Pedro River and not the Fort 

proper (Sheridan Stone, oral commun.). Swarth (1904) reported the species (“E. trailli”) as “a fairly 

common summer resident in suitable spots along the San Pedro River.” Although Swarth did not publish 

any more specific accounts of nesting sites or occurrences, it is obvious from his introductory chapter that 
86

his references to the San Pedro River were intended for the upper portion of the watershed where they 

conducted “considerable work along the valley of the San Pedro, but a few miles distant” from his main 

study area of the Huachuca Mountains. In June 1896, two partners in a scientific expedition Swarth was 

leading, O.W. Howard and W.B. Judson, each collected an adult female E.t. extimus during a couple days 

spent on the upper San Pedro River at Charleston (table 1), and in addition to egg sets of several other 

species, “They also took (egg) sets of Traill’s Flycatcher, and Yellow Warbler with Cowbird eggs” (see 

Swarth, 1896). In later work, Swarth again indicated the species (“E. trailli trailli”) as breeding along the 

river, and stated again that the early work from him and others was along the upper river adjacent to the 

Huachuca Mountains (Swarth, 1914), not along the lower San Pedro as cited elsewhere (that is, U.S. Fish 

and Wildlife Service, 2005). The majority of historical E. traillii nest documentation on the upper San 

Pedro occurred at or near Fairbank (32.6 km north of the 2005 nest site) by Fred C. Willard (table 1), who

lived in nearby Tombstone and visited Fairbank to collect eggs and nests of different species. Thus far, 

Willard (1912) represents the only historical nest account from the upper San Pedro River directly noted in 

the primary literature, where he related finding two nests at Fairbank, Arizona (one egg set of these is 

currently at SBNHM, table 1). Unitt (1987) used another nest from this site to determine nest 

measurements for the subspecies, but the collector or other details were not available (Phillip Unitt, oral

commun.). 

Contemporary Nest Records

Although E.t. extimus are regularly detected in migration along the upper San Pedro and singing 

males can persist in an area for a few days to 1 week or more (Krueper ,1999; U.S. Fish and Wildlife 

Service, 2002; Engineering and Environmental Consultants, Inc., only three contemporary nests were 

documented in the area prior to the 2005 nest (table 1). The last documented successful nest along the 

upper/southern portion of the San Pedro River was at Lewis Springs near U.S. Highway 90in 1977 (12 km 

north from the 2005 nest site), near where another pair built two nests in 1997 that failed because of

predation and Brown-headed Cowbird nest parasitism (Doug Danforth, oral commun.; Krueper, 1999; 

Paradzick and Woodward, 2003). The reach of river containing the 2005 nest site had been regularly 

surveyed for Southwestern Willow Flycatchers during several preceding years, although 2005 was the 

only time breeding activity had been documented (English and others, 2006). Other detections of 

individual birds include several near St. David in 1996, three singing males near U.S. Highway 90 in 1997 

(near the nesting pair), one near Green Kingfisher Pond and another at the Hereford Bridge in 1998, two at 

Green Kingfisher Pond in 1999, and at least 19 more “prospecting” singing males since 2001, all within 

the San Pedro Riparian Conservation Area in the upper river corridor (Krueper, 1999; U.S. Fish and 

Wildlife Service, 2002). 

Upper San Pedro 2005 Nest-Site Characteristics Compared to Regional Measurements 

The 2005 nest near Hereford was 5.4 m above ground in an 11.5 m tall Goodding’s willow (Salix 

gooddingii), with a 26.5 cm dbh main trunk, from which several smaller stems (10–18 cm diameter) arose 

at about 1.5–2 m above the ground. The nest was located along one of these stems, where a small branch 

(approximately 0.75 cm diameter) forked and split into three smaller, upright stems and was composed of 

small twigs, grass, spider webbing, and other unidentified materials that appeared typical of other extimus

nests in the region. The nest was 3.65 m from the nearest foliage edge/canopy opening and 4.35 m from 

the river’s edge, although late-summer floods at the end of the nest attempt resulted in flowing water 

directly below the nest approximately 1 m deep and extending 25 m inland past the initial river’s edge. We 

estimated percent canopy cover within a 5 m radius was 45 percent in the understory (under 4.5 m) and 70 

percent in the two upper canopy layers (4.5–7.5 m, and >7.5 m). Nest and canopy height, stem diameter, 

and distance from water were all well within the reported ranges reported from Arizona (Paradzick and 

Woodward, 2003; Smith and others, 2003; Corman and Wise-Gervais, 2005). 
87

General habitat features around the nest were similar to those identified in the E.t. extimus

ecological literature (for example, Sogge and others, 1997;), including tall Goodding’s willow (8–14 m) 

and Frémont cottonwood (≤24 m) within 50 m, nearby dense stands of seep willow, and close proximity to 

open water. Surface water was present and flowing at the site through the arid summer (that is, to the lateJuly monsoon-season), although it was a particularly dry year and most (64 percent) of the SPRNCA 

contained no surface water by late June (The Nature Conservancy, 2007). A small beaver dam (0.2 m 

high) constructed 35 m upstream was initiated 2 weeks before nest commencement, and a larger beaver 

dam (0.8 m high) was present all summer 495 m down-river, and had caused water to pond within 120 m 

of the nest. 

Environmental Conditions Along Upper San Pedro 

2005 Nest Site Compared to Local Survey Stations

Compared to the mean values from the 239 other sampling stations along the upper San Pedro 

River, values measured at the nest site were significantly greater (p<0.001, one-sample, one-sided t-tests) 

with respect to the following variables: Goodding’s willow percent cover at the 3–5m and 5–7m canopy 

layers (T239=-30.7 and T239=-24.4, respectively), total number of Goodding’s willow stems (T239=-78.6), 

total number of seep willow stems (T239=-167.7), and total number of all woody plant stems (T239=-48.4) 

(table 2). The width of the cottonwood-willow riparian vegetation as measured perpendicular to the river 

on both sides at the nest site was not greatly different (P>0.05) from the mean measurements taken from 

the other sites within the SPRNCA. 

2005 Nest Site Compared to Local Application of Geographic Information Systems Model

Application of the spatially explicit, GIS-based E.t. extimus-breeding site prediction model to our 

study area revealed that 11 (4.5 percent) of our 240 uniformly distributed point count stations in the upper 

San Pedro fell directly into an area of the highest probability class, defined as 80–100 percent probability 

of a supporting E.t. extimus nesting, and 35 (14.5 percent) stations fell within 180 m of an area of this 

highest probability. The nest site was located within the largest and most contiguous patch (approximately 

6.6 ha, 970 m in length along river) of the 80–100 percent probability class that was identified. Overall, 

the model was applied to 64.2 km of the upper San Pedro river corridor within the SPRNCA, and showed 

high-probability breeding sites along 4.8 km (7.7 percent) of this upper reach, as distributed in 15 patches 

and averaging 331.6 m in length (range: 90–970) along the upper river (McFarland and others 2012).

Discussion

Except for references to unspecified “historical nesting” at Charleston (Phillips, 1948; Phillips and 

others, 1964), the verifiable E.t. extimus nesting records from the upper San Pedro River have not received 

much attention by contemporary authors, including in recovery planning documents (U.S. Fish and 

Wildlife Service, 2002, 2005). The southernmost consistently active, current San Pedro River nesting sites 

for the subspecies are located along the middle and lower reaches, 85–185 km north of the 2005 Hereford 

nest site. The southernmost of these (“Three Links”) was established in 2004, and to our knowledge has 

been occupied consistently since that time (based on Ellis and others, 2008). The three most abundant 

currently active E.t. extimus breeding sites are 150 km or more north and northeast of the Hereford nest 

site, in Arizona near the confluence of the lower San Pedro and middle Gila Rivers and at Lake Roosevelt, 

and in New Mexico along the upper Gila River (Durst and others, 2007). 
88

We are not suggesting a viable breeding population in the upper San Pedro River that has gone 

undetected, as there are many keen observers in the area, including numerous bird watchers as well as 

regular E.t. extimus survey attempts on approximately 12 km of river each year. In extensive efforts 

conducting multiple species surveys from 2005 to 2008 along most of the length of river from St. David, 

Arizona to 12 km south of the Arizona-Sonora border, we encountered just the one nest in 2005 and 

scattered individuals that were either migrants or had temporarily occupied sites (Johnson and van Riper, 

2008), and other workers in the area also documented the species only intermittently (Krueper, 1999). 

There is, however, substantial evidence that E.t. extimus once bred consistently in the upper San Pedro 

Valley, and that individuals still migrate through, continue to regularly investigate, and indeed sometimes 

breed along the upper reaches of the river. The consistent frequency of detections, singing males, and 

occasional breeding attempts combined with detections in nearby drainages (including a nest in 2001 at 

Cienega Creek, Arizona; Ellis and others, 2008) and newly occupied breeding sites as close as 85 km 

north (“Three Links South”), all suggest the possibility that the subspecies may re-establish breeding sites 

in the upper portion of the San Pedro. 

The 2005 nest site was typical as compared to others regionally reported for E. t. extimus, and 

application of the GIS habitat-prediction model to a 64 km reach of upper San Pedro, comprising most of 

the SPRNCA, showed that nearly 8 percent of the reach appeared to contain suitable breeding habitat. The 

nest site’s relatively high percent cover of willow at the canopy height layers typical for E.t. extimus nests 

(3–7 m), combined with a high number of stems, but with a relatively low mean dbh (table 2), all indicate 

that this site is within the top percentiles of “suitable” sites we sampled along the upper San Pedro River, 

in terms of shrub-cover and density of woody plant stems. Thus, although suitable breeding sites on the 

upper San Pedro River do not appear to be consistently located throughout the predominantly native, 

cottonwood-willow habitat of the upper river corridor (contrary to assumptions in Steinitz and others, 

2005), there are at least some suitable sites present along portions of the upper river reach. 

Threats in other parts of the breeding range of E.t. extimus indicate that the native riparian 

vegetation and un-altered hydrology of the San Pedro River may become an important refugia habitat for 

future immigrants. The recent release of the tamarisk leaf beetle (Diorhabda carinulata), in an attempt at 

bio-control of the exotic and invasive salt cedar (Tamarix spp.), could, within a few years, reduce the 

suitability of many E.t. extimus breeding sites along the lower Colorado where salt cedar is an important 

habitat component (Sogge and others, 2008, 2013). The tamarisk leaf beetle is expected to continue to 

colonize new areas and become established along the middle and lower Colorado River in the next several 

years, and the warmer temperatures found farther south may in fact encourage the spread of the beetle, 

depending on the genetics and adaptation of the beetles (Tracy and Robbins, 2009). The majority of E.t.

extimus nests in Arizona occur in tamarisk (for example, 63 percent in 2002; Smith and others, 2003), and 

at sites with drastically altered hydrology (which facilitates tamarisk establishment). In contrast, the upper 

San Pedro is dominated by native cottonwood and willow vegetation and is free-flowing (Webb and 

others, 2007; Stromberg and others, 2010) and therefore is not as susceptible to threats from the tamarisk 

leaf beetle (for example, Colorado River sites) or inundation (for example, Roosevelt Lake) like many 

sites in other parts of the breeding range. 

The presence of beaver, especially the water impoundments and changes to vegetation created by 

dams that can persist long after abandonment, are associated with E.t. extimus presence in numerous 

locations throughout the subspecies’ range (Sogge and Marshall, 2000; Ellis and others, 2008). Beaver 

were common on the San Pedro well over a century ago (Webb and others, 2007), and a reintroduction 

program in 1999–2001 resulted in several consistently active beaver dams near the 2005 nest site (5 dams 

within 12 km in that year; Johnson, 2011). The two dams constructed in 2005 near the nest represent the 

first time, since at least the early 20th century, that beaver regularly utilized sites within 1 km of the 

Hereford nest site (M. Fredlake, Bureau of Land Management, unpub. data, 1999). Finch and others 

(2000) note that beaver reintroduction efforts should be monitored closely wherever breeding Willow 
89

Flycatchers have been found, to determine whether beaver activities are beneficial to E.t. extimus

populations or are detrimental (that is, decrease habitat through removal of willow trees). The decision of 

land managers to remove beaver from a site in California with endangered Willow Flycatchers and Least 

Bell’s Vireo (Vireo bellii subspecies pusillus) was strongly critiqued by Longcore and others (2007), who 

argued that the available data on the subject was more in support of leaving rather than removing beaver. 

Following beaver establishment along the lower San Pedro from 2002–2007, Ellis and others (2008) 

reported newly established E.t. extimus breeding sites, as well as increases in number of breeding 

territories at several existing breeding sites, in several locations where beavers had recently settled (these 

beaver were not emigrants from the reintroduction effort on the upper San Pedro, which were radio-tagged 

and monitored, but were suspected immigrants from the Gila River, as they were not radio-tagged; M. 

Fredlake, Bureau of Land Management, oral commun.). 

Beaver were historically quite abundant on the entire San Pedro (James Ohio Pattie named it 

“Beaver River” in 1825; Webb and others, 2007), and their extirpation from the area in the late 19th

century coincided generally with a region-wide episode of channel entrenchment and down-cutting that

resulted in riparian vegetation communities changing from marshy, cienega-like conditions to more 

coverage of cottonwood stringer and gallery forests present today (Webb and others, 2007). Although it is 

interesting that most of the breeding E.t. extimus records we found from the area were from the late 19th

century to the mid-1910s—that is, when beaver were present and just after being trapped out of the 

system—this period also coincided with the popular avocation of oology (Barrow, 2000), and we did not 

find ornithological efforts to document nesting in the area from the late 1910s to the early 1980s. So, while 

we cannot confidently infer that the extirpation of the beaver from the upper San Pedro precipitated the 

changing vegetation and subsequent extirpation of E.t. extimus, it is important to at least note that the two 

species coexisted previously in the locally beaver-modified environment. 

Conclusions and Management Implications

Although perhaps of little practical significance, the nest found in 2005 along the upper San Pedro 

was just south of historically occupied breeding habitat, and apparently is the southernmost confirmable 

breeding attempt ever recorded that is attributable to the E.t. extimus subspecies, or even the species E. 

traillii. This and other contemporary records from the upper and middle San Pedro are the only recently 

documented instances of breeding along the southernmost extent of historical E.t. extimus distribution, 

whereas E.t. extimus is apparently still extirpated at all other historical, southern breeding locations. While 

the “southernmost” aspects of the confirmed historical and contemporary breeding in the area may mainly 

be of interest to those studying bird distribution, range margins, etc., local land managers and regional 

biologists should be aware of the relatively strong historical record and persistence of occasionally 

occupied, potentially suitable habitat along the upper San Pedro. 

Contemporary recovery and planning documents (for example, U.S. Fish and Wildlife Service,

2002, p. 8, 31–32) promote the idea that earlier researchers (Willard, 1912; Swarth, 1914) documented 

nesting along the lower San Pedro where E.t. extimus currently nest, but the early ornithologists invoked 

actually worked on the upper portion of the river, and that is where the authors’ only records occurred 

(table 1), at least to our knowledge. Recently, the lower 126.2 km of the San Pedro River was designated 

as critical habitat (U.S. Fish and Wildlife Service, 2013) because of the important breeding sites closer to 

the confluence with the Gila River. The upper 120 km of river north of the international border was 

originally considered (U.S. Fish and Wildlife Service, 1997), and later the area was recommended for 

surveys and potential recovery (U.S. Fish and Wildlife Service, 1998, 2002), but this upper reach was 

eventually removed from explicit evaluation in final determinations of critical habitat (U.S. Fish and 

Wildlife Service, 2005, 2013). Perhaps the assumption that the historically documented breeding 

population was in the lower portion of the river corridor led to the upper San Pedro (where the historical 

nesting actually was documented) being excluded from consideration. Although eventually granted an 
90

exclusion under section 4(b)(2) of the Endangered Species Act because of pro-active, multi-species 

management agreements in the area, the lowest reach of the Colorado River in the U.S. was explicitly 

evaluated for critical habitat designation (U.S. Fish and Wildlife Service, 2005, 2013) because of historical

occupancy and contemporary records. This area is similar to the upper San Pedro, in that E.t. extimus has 

been mostly extirpated as a breeder, but currently individual birds migrate through or temporarily establish 

territories there. Future studies, recovery documents, and other planning efforts related to the upper San 

Pedro should endeavor to incorporate ours or other accurate assessments of the historical record and 

current extent of suitable habitat, in order to assess recovery potential in the area or investigate how 

changes in depth to groundwater and surface water may affect current and/or potential future habitat in the 

area (for example, Steinitz and others, 2005). 

The importance of the upper San Pedro as a potential recovery area for E.t. extimus is indicated by 

consistent breeding attempts, both historically and more recent (if intermittent) events such as the 2005 

nest we encountered, as well as environmental factors. Even though the 2005 nest did not produce young, 

the environmental characteristics of the site generally matched those at sites regionally reported as hosting 

successful nesting, and the existence of potentially suitable E.t. extimus breeding habitat on the upper San 

Pedro was further indicated by results of the GIS model application and our other analyses of site 

conditions along the upper San Pedro River. 

Continued survey and management efforts on the upper San Pedro are also warranted given the 

presence of an established, consistent breeding population from 81 to 182 km to the north-northwest at 

scattered sites along the same river and north of its confluence with the Gila, all of which are a likely 

source of immigrants because the populations probably migrate through the upper San Pedro. Although 

ours and other multi-species efforts in the upper San Pedro River (Krueper and others, 2003; Brand and 

others, 2008), and those of other biologists focused on E.t. extimus, do not indicate any substantial or 

consistent breeding population in the upper San Pedro River, it appears that individuals are regularly 

prospecting the area and are at least occasionally attempting to breed. Similar to state wildlife biologists 

considering potential E.t. extimus recovery in the region (Ellis and others, 2008), we recommend 

continued or even increased annual survey efforts in the SPRNCA and other areas south of the 

consistently occupied breeding sites to the north. Also, biologists should work with cooperators south of 

the border in Sonora where suitable habitat may exist in the portions of the upper San Pedro watershed 

(for example, especially the western inlet to Las Nutrias Reservoir). 

Even if only small groups of flycatchers eventually become established in the upper San Pedro 

River, these could contribute to recovery efforts, as most of the known E.t. extimus breeding sites contain 

five or fewer territories (Durst and others, 2007). In fact, encouragement of widely distributed, small 

populations can minimize the risk of simultaneous catastrophic loss and genetic isolation of populations at 

more abundant sites (U.S. Fish and Wildlife Service, 2002). Busch and others (2000) noted that small, 

well-distributed populations of E.t. extimus would not be genetically isolated, and would in fact act as a 

meta-population, especially for groups distributed along drainages and connected by riparian vegetation. 

The recently introduced beaver (Castor canadensis) population could have positive and/or 

negative effects on E.t. extimus habitat. Specifically along the upper San Pedro River, the BLM and other 

cooperators can sustain a program to monitor the effects of the reintroduced beaver populations in Arizona 

and adjacent areas where it has become recently established in Sonora, particularly in areas with high 

willow cover or otherwise classified by GIS modeling efforts as being the highest probability E.t. extimus

breeding habitat. It is also important that monitoring occur in any areas where beavers settle, even if not 

previously identified as high probability E.t. extimus breeding sites, because beaver activity has the 

potential to create favorable conditions for breeding E.t. extimus and other riparian bird species. Although 

it may be the case that E.t. extimus and beaver both just share a similar preference for sites with high 

willow cover (which could explain the association between the two and could actually result in loss of 

potential E.t. extimus habitat), it is also possible that the reintroduction of beaver may encourage E.t.
91

extimus recovery in the upper San Pedro River area by facilitating early-succession riparian vegetation 

(through removal of overstory canopy), increasing surface water, and availability of groundwater needed 

for the establishment, growth, and survival of riparian vegetation. 

Given historical use and indications of current suitable habitat in the upper and lower reaches of 

the San Pedro River, and regional and local threats to E.t. extimus populations and habitats, we 

recommend expanded E.t. extimus survey efforts and intentional management of the upper San Pedro 

River riparian vegetation as potential breeding habitat. Much of the upper reach is protected in the 

SPRNCA and managed for natural hydrological flow and vegetation dynamics conducive to E. t. extimus

habitat creation and maintenance. However, additional considerations such as beaver activity, groundwater 

depletion, and overgrazing by livestock, in areas where not currently excluded, warrant careful monitoring 

and intentional management decisions, as they may negatively impact riparian characteristics conducive to 

extimus breeding. Consideration of these issues along with expanded E.t. extimus surveys and monitoring 

in all appropriate areas will thus provide a tangible opportunity for recovery of the Southwestern Willow 

Flycatcher at the southernmost portion of its known breeding range. 

Acknowledgments

We would like to thank the following staff and institutions for verifying on-line specimen data, 

responding to requests to search for records, or otherwise providing invaluable assistance that made this 

project possible: James Dean, U.S. National Museum of Natural History (Smithsonian); Mike Westberg, 

Bell Museum of Natural History at University of Minnesota; Moe Flannery and Danielle Castronovo, 

California Academy of Sciences; Krista A. Fahy, Santa Barbara Museum of Natural History; Gregory J. 

Watkins-Colwell and Susan Hochgraf, Yale Peabody Museum of Natural History; Philip Unitt, San Diego 

Natural History Museum; Jean Woods, Delaware Museum of Natural History; Dave Willard, Field 

Museum of Natural History; Thomas Erdman, Richter Museum of Natural History, University of 

Wisconsin-Green Bay; and René Corado, Western Foundation of Vertebrate Zoology. Eben Paxton, Mark 

Stevenson, Doug Danforth, Sheridan Stone, and Dave Brown all took time to provide valuable 

information. Jennifer Arnold, Aaron Flesch, Eben Paxton, and Dave Krueper provided helpful comments 

on early drafts. Brian Bielfelt, Sonya Smith, Morgan Jackson, Christine Lamanna, Josh Brown, and 

Jessica Gist assisted with field surveys or measurements. Tiffany Harvey applied the GIS model and 

provided GIS-derived data from the spatially explicit habitat-prediction model, which was developed and 

provided by James Hatten. The Bureau of Land Management, U.S. Geological Survey, T & E., Inc, and 

the USFWS Sonoran Joint Venture provided funding or other support for this project. 
92

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95

Tables

Table 3-1. Records of breeding Southwestern Willow Flycatchers (E.t. extimus) within the upper San Pedro River watershed in Arizona, including: locality, 

number of nests documented, date(s) of record, sources used to find or investigate record (specimen information given in parentheses: institution

abbreviation*, catalog number, and collector), and further information and comments on the record.

[BMNH, Bell Museum of Natural History, University of Minnesota; DMNH, Delaware Museum of Natural History; FMNH, The Field Museum (Chicago); MVZ, Museum 

of Vertebrate Zoology at Berkeley; RMNH, Richter Museum of Natural History, University of Wisconsin-Green Bay; SBMNH, Santa Barbara Museum of Natural History; 

USNM, U.S. National Museum of Natural History (Smithsonian); YPMNH, Yale Peabody Museum of Natural History] 

Locality

Number

of nests Date(s) 

Sources for record, specimen 

information (institution, cat. no., 

collector)

Comments

Hereford 1 7/1-

7/30/2005

Originally reported in E.E.C. 2005, 

Ellis, 2008

2005 nest site near Hereford

Lewis Springs 1 1977 Krueper, 1999, D. Danforth oral

comm. 

Last documented successful nest on upper San Pedro. 

Lewis Springs 2 1997 Krueper, 1999; Paradzick and 

Woodward, 2003 

Two nests attempted in area, both depredated.

Fairbank 1 5/27/1906 ORNIS (SBMNH AV 23889 

Willard, F.)

"25 feet up in an alder tree.

Fairbank 1 6/17/1906 ORNIS (SBMNH AV 23890 

Willard, F.)

"About 10 feet up in a slender willow sapling in a thicket."

Fairbank 1 6/14/1908 ORNIS (SBMNH AV 23891 

Willard, F.)

3 eggs, "7 feet up among twigs against trunk of a small willow in a 

thicket."

Fairbank 1 6/5/1910 ORNIS (SBMNH AV 23892 

Willard, F.)

"8 feet up in some slender willow brush growing along an irrigating 

ditch. Made as usual." 

Fairbank 1 6/5/1910 GBIF.org, M. Westberg oral. comm. 

(BMNH 40678 Langevin)

Complete nest with 3 eggs (Mike Westberg at BMNH)

Fairbank 1 6/10/1910 DMNH (DMNH E64699 Willard, 

F.C.)

"Female flushed, 9 feet up in a slender willow; made as usual, nest with 

set; San Pedro River near Fairbank." 

Fairbank 1 6/29/1910 RMNH (RMNH uncataloged 

Willard, F.C.)

Record provided by Thomas Erdman, oral commun. No catalog number 

"A.O.U. set mark 466 s/3"Nest: 12 ft up in a slender willow growing in 

a thicket. Made as usual. Nest with set. Both birds present."
96

Locality

Number

of nests Date(s) 

Sources for record, specimen 

information (institution, cat. no., 

collector)

Comments

Fairbank 2 5/26/1911 Willard 1912, ORNIS (USNM 

B35081 Willard, F.)

Willard reported that he found two nests w/ 3 eggs each here ("upon 

arrival") in Fairbank; one set at USNM. 

Fairbank 1 7/16/1911 ORNIS (SBMNH AV 23893 

Willard, F.)

"5 feet up in a slender willow. Typical of the late nests of this species."

Fairbank 1 6/22/1913 ORNIS (SBMNH AV 23894 

Willard, F.)

"7 feet up in a thick growth of brush along an irrigating ditch. Made as 

usual."

Fairbank 1 6/14/1914 ORNIS (YPMNH 130701 Willard, 

F.C.)

3 egg set; No locality details except "Cochise County", assumed from 

Fairbank because Willard collected there often. 

Fairbank 1 6/11/1915 ORNIS (SBMNH AV 23895 

Willard, F.)

"7 feet up in some thick brush fringing an irrigating ditch. Made as 

usual." 

Fort Huachuca 1 6/15/1887 USNM, ORNIS (USNM B23156 

Benson, H.C.)

2 eggs collected, no more specific locality information available (oral

commun., J. Dean of USNM), probably collected along San Pedro River 

at Fairbank, where Fort H. property extended to river (Sheridan Stone, 

Fort Huachuca Wildlife Biologist, oral commun.). 

Charleston 2 6/15/1896 Swarth 1896, Phillips 1948, Phillips 

and others 1964, Krueper 1999, 

ORNIS, GBIF. (MVZ 33389 and 

10256, Howard, O.W., Judson, W.B.)

Historical nesting said to have occurred at Charleston by several 

authors, although no specific dates, specimens, or sources were 

referenced. The two female specimens (noted here) were breeding, as 

they were originally collected with associated egg sets as noted by H.S. 

Swarth in his 1896 Journal, although the institutional location of the egg 

sets are unknown.

Charleston 1 5/16/1904 FMNH Database at 

http://fm1.fieldmuseum.org/birds/ 

(FMNH 9377 Howard, O.W.)

3 eggs; Assuming extimus subspecies and Charleston as the locality 

based on Howard’s other records; "San Pedro River, Cochise County" is 

all information available (Dave Willard, FMNH). 
97

Table 3-2. Comparison of mean and range of values from 240 sites, value at nest site, and rank and percentile 

(= percent sites with lesser values) of nest site in relation to range of values for all stations, for each of seven

environmental variables: percent Goodding’s willow (Salix gooddingii) cover in 3–5 m and 5–7 m canopy layers, 

total number of Goodding’s willow, seep willow and all woody plant stems, mean diameter-at-breast-height (DBH) of 

all woody species’ main stems, and total width of riparian vegetation. 

Environmental Variable

Mean value 

from 240 

stations

Range of 

values at 240 

stations

Value 

measured at 

nest site 

Rank of nest 

site among 240 

stations

Percentile of 

nest site among 

240 stations 

Goodding’s willow percent 

cover, 3-5 m

6.9 0-25 17 9 96.2

Goodding’s willow 

percent cover, 5-7 m

5.8 0-27 14 23 89

Total number of Goodding’s

willow stems

21 0-195 143 2 99.5

Total number of 

seep willow stems

2 0-77 77 1 100

Total number of all woody 

stems 

75 4-278 220 5 98.3

Mean DBH of all woody 

main stems (cm)

16.6 2-64 7.85 213 10.9
98

Appendix 3-A. Discovery, Location, Subsequent Monitoring, and Fate of the 2005 Hereford Nest

The breeding pair of Southwestern Willow Flycatchers was detected after the distinctive “fitzbew!” song was heard on July 5, 2005 by Brian Bielfelt while conducting surveys for a multi-species bird 

study in the SPRNCA. The location was 200 m south of the Hereford Road Bridge over the San Pedro 

River, 15 km southeast of Sierra Vista, Arizona, and 11.6 km north of the international border (elevation 

1,266 m). The surveyor and G. Johnson visited the site later that day and confirmed the observation, 

noting the singing male had a silver USFWS aluminum band on one leg and was engaged in typical pairinteractions with an unbanded bird that was repeatedly flying to a patch of Goodding’s willow while 

carrying nest materials. After observing the unbanded bird building a nest, which appeared to be 

approximately one-third completed, we notified the BLM and USFWS. On several dates from July 7 to 

August 3, 2005, authorized personnel from Engineering and Environmental Consultants, Inc. observed 

nest completion, two eggs in the nest, female incubation and rotation of the eggs, and continued 

occupancy by the male. By August 3, 2005, the nest was determined a failure as the adults had vacated the 

area and apparently abandoned one remaining egg, possibly after a predation and/or one of several flood 

events (Engineering and Environmental Consultants, Inc., 2005). On July 20, USGS E. t. extimus

research-biologists captured both the male and female near the nest site using mist nets with an audio lure.

The male had originally been captured and banded with a USFWS aluminum band (number 2270-30306) 

as an adult in apparent non-breeding condition on July 11, 2002, at a BLM multi-species bird banding 

operation located 11.5 km downriver (at Green Kinfisher Pond) to the north of the nest site. After 

removing the old band from the male, both the male and female were fitted with new, unique colored 

aluminum band combinations (E. Paxton, oral commun.). To our knowledge, the pair has not been resighted since the breeding attempt ended. 
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Johnson and van Riper III—Effects of Reintroduced Beaver on Riparian Bird Community Structure along the Upper San Pedro River—Open-File Report 2014–1121

ISSN 2331-1258 (online)

http://dx.doi.org/10.3133/ofr20141121