U.S. Department of the Interior

U.S. Geological Survey

Open File-Report 2020-1107

Prepared in cooperation with East Contra Costa County Habitat Conservancy Science and 

Research Grant Program, East Bay Regional Parks District, Save Mount Diablo’s Mary 

Bowerman Science and Research Grant Program, and NextEra Energy

Distribution and Abundance of Aquila chrysaetos (Golden 

Eagles) in the East Contra Costa County Habitat Conservation 

Plan/Natural Community Conservation Plan Area, California
Cover: View of Round Valley, East Contra Costa County Habitat Conservation Plan/

Natural Community Conservation Plan Area, California. Photograph by Patrick Kolar, 

U.S. Geological Survey, 2016.
Distribution and Abundance of Aquila chrysaetos (Golden 

Eagles) in the East Contra Costa County Habitat 

Conservation Plan/Natural Community Conservation Plan 

Area, California

By J. David Wiens, Patrick S. Kolar, and Douglas A. Bell

Prepared in cooperation with East Contra Costa County Habitat Conservancy Science and 

Research Grant Program, East Bay Regional Parks District, Save Mount Diablo’s Mary 

Bowerman Science and Research Grant Program, and NextEra Energy

Open-File Report 2020-1107

U.S. Department of the Interior

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

DAVID BERNHARDT, Secretary

U.S. Geological Survey

James Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2020

For more information on the USGS—the Federal source for science about the Earth,

its natural and living resources, natural hazards, and the environment—visit

https://www.usgs.gov/ or call 1–888–ASK–USGS (1–888–275–8747).

For an overview of USGS information products, including maps, imagery, and publications,

visit https:/store.usgs.gov.

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

endorsement by the U.S. Government.

Although this information product, for the most part, is in the public domain, it also may

contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items

must be secured from the copyright owner.

Suggested citation:

Wiens, J.D., Kolar, P.S., and Bell, D.A., 2020, Distribution and abundance of Aquila chrysaetos (golden eagles) in 

East Contra Costa County Habitat Conservation Plan/Natural Community Conservation Plan area, California: U.S. 

Geological Survey Open-File Report 2020-1107, 11 p., https://doi.org/10.3133/ofr20201107. 

ISSN 2331-1258 (online)
iii

Contents

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

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

Study Area..................................................................................................................................................... 2

Methods......................................................................................................................................................... 3

Golden Eagle Surveys................................................................................................................................ 3

Data Analysis ............................................................................................................................................. 3

Results........................................................................................................................................................... 4

Site Occupancy and Abundance of Golden Eagles.................................................................................... 4

Detection Probability .................................................................................................................................. 5

Nesting Success and Reproductive Output................................................................................................ 6

Bald Eagle Observations............................................................................................................................ 6

Discussion ..................................................................................................................................................... 6

Acknowledgments ......................................................................................................................................... 7

References Cited........................................................................................................................................... 7

Appendix 1. Ranking of Candidate Occupancy and Abundance Models ..................................................... 11

Figures

1. Distribution of territorial pairs of golden eagles and survey sites in the East Contra Costa County

Habitat Conservation Plan/Natural Community Conservation Plan (HCP/NCCP) inventory area,

California, 2019 ............................................................................................................................................. 2

2. Probability of detecting 1 or more territorial pairs of golden eagles decreased with survey date

(January 1–July 24) in the East Contra Costa County Habitat Conservation Plan/Natural Community 

Conservation Plan area, California, 2019 ...................................................................................................... 5

Table

1. Parameter estimates from the best models used to characterize site occupancy and abundance of

territorial pairs of golden eagles in the East Contra Costa County Habitat Conservation Plan/Natural

Community Conservation Plan Inventory area, California, 2019.................................................................... 5
iv

Conversion Factors

International System of Units to U.S. customary units 

Multiply By To obtain

Length

kilometer (km) 0.6214 mile (mi)

Area

square kilometer (km2) 247.1 acre

square kilometer (km2) 0.3861 square mile (mi2)

Abbreviations

CI confidence interval

SE standard error

HCP/NCCP East Contra Costa County Habitat Conservation Plan/Natural Community Conservation Plan
1

Distribution and Abundance of Aquila chrysaetos

(Golden Eagles) in the East Contra Costa County 

Habitat Conservation Plan/Natural Community 

Conservation Plan Area, California

By J. David Wiens1, Patrick S. Kolar1, and Douglas A. Bell2

Abstract

The East Contra Costa County Habitat Conservation Plan/Natural Community 

Conservation Plan (HCP/NCCP) Preserve System was designed to protect and enhance 

ecological diversity and function in eastern Contra Costa County, California. Aquila chrysaetos

(golden eagle) is a special-status species expected to benefit from biological goals of the 

HCP/NCCP. As part of a broader study, we estimated site-occupancy, abundance, and 

reproduction of golden eagles in the HCP/NCCP inventory area in 2019. We completed 99 

surveys and recorded a total of 50 detections of territorial pairs of eagles at 20 (67 percent) of 30 

sites (13.9-square-kilometer [km2] plots). Detection probability of territorial pairs was highest in 

January and February (≥0.75) and lowest in mid-June to late July (<0.50). After correcting for 

imperfect detection, the expected probability of site-occupancy was 0.69 (standard error [SE] = 

0.09), and mean expected abundance was 0.76 pairs per site (SE = 0.16), or 27.4 pairs per 500 

km2

. We found evidence of successful nesting (≥1 young fledged) for 3 (14 percent) of 22 pairs 

of eagles monitored in 2019. Our study design and baseline results should be useful for future 

monitoring and conservation of golden eagles in the HCP/NCCP area. 

Introduction

The East Contra Costa County Habitat Conservation Plan/Natural Community 

Conservation Plan (HCP/NCCP; hereinafter, Plan) provides a framework to protect and enhance 

ecological diversity in the rapidly urbanizing region of eastern Contra Costa County, California

(Jones and Stokes, 2006). The Plan provides comprehensive species, wetlands, and ecosystem 

conservation and is intended to contribute to the recovery of sensitive and endangered species.

The Aquila chrysaetos (golden eagle) is a special-status species expected to benefit from 

biological goals of the Plan in grassland and oak-woodland habitats. Specifically, the 

HCP/NCCP is intended to “maintain or increase population size and distribution of golden eagles 

in the Plan area” (Jones and Stokes, table 5-1). Despite the importance of golden eagles to the 

biological objectives of the Plan, information on this species in the Plan area is limited (see Hunt 

2002; Hunt and Hunt, 2013; H.T. Harvey and Associates, 2018). 

1

 U.S. Geological Survey 

2 East Bay Regional Parks District
2

In 2019, we completed a comprehensive survey of golden eagles in the Plan area. 

Surveys were completed as part of a larger, concurrent study on population ecology of golden 

eagles in the vicinity of the Altamont Pass Wind Resource Area (Hunt and others, 2017; Wiens 

and others, 2015, 2018). Our goals were to (1) build upon our existing study design to complete 

site-occupancy surveys of golden eagles throughout the entire Plan area; and (2) provide baseline 

estimates of distribution, abundance, and reproduction of golden eagles. 

Study Area

The 704-km2 HCP/NCCP inventory area is located in eastern Contra Costa County, 

California (fig. 1). This area was delineated based on a combination of political, ecological, and 

hydrologic factors (Jones and Stokes, 2006). The northern and eastern parts of the Plan area are

dominated by developed, urban, and agricultural lands, whereas the western and southern parts

are mostly grassland with widely scattered stands of Quercus spp. (oak), Eucalyptus spp.

(eucalyptus), and conifer trees (fig. 1). The HCP/NCCP inventory area is located in the 

northeastern part of the larger Diablo Range golden eagle study area (fig. 1). This broader study 

used a random census-plot design to monitor golden eagles during 2014–19 (for details, see 

Wiens and others, 2015, 2018). The Altamont Pass Wind Resource Area overlaps with the 

southern boundary of the HCP/NCCP inventory area.

Figure 1. Distribution of territorial pairs of golden eagles and survey sites (13.9-square-kilometer hexagons)

in the East Contra Costa County Habitat Conservation Plan/Natural Community Conservation Plan

(HCP/NCCP) inventory area, California, 2019. Inset shows the location of the inventory area relative to 

sites monitored (highlighted hexagons) in the broader northern Diablo Range golden eagle study area. 
3

Methods

Golden Eagle Surveys 

Areas targeted for repeated surveys of golden eagles were based on a grid of 30 equalsized (13.9 km2

) hexagonal cells overlaid on the Plan area (total area surveyed = 443.2 km2; fig. 

1). We excluded densely populated and agricultural landscapes from our surveys because 

previous studies show that golden eagles rarely used these landscapes (Hunt, 2002; Hunt and 

others, 2017). The size of each survey hexagon corresponded to the estimated mean territory size 

of golden eagles (Wiens and others, 2015, 2018). Thus, under our sampling design a “site” was 

defined as a 13.9-km2 area that had the potential to be occupied by 1 or more (≥1) territorial pair 

of golden eagles during the breeding season. We modified survey protocols developed by 

Driscoll (2010) and Pagel and others (2010) to survey golden eagles. We partitioned surveys 

within years based on approximate transition dates for stages in the breeding cycle for eagles in 

our study area (Hunt, 2002): (1) courtship (December 15–February 28), (2) incubation (March 1–

April 30), (3) nestling (May 1–June 15), and (4) the fledging period (June 16–July 30). 

Whenever possible, we classified all eagles as juveniles, subadults, or adults based on visible 

plumage characteristics (Bloom and Clark, 2001; Kolar and Wiens, 2017). Further details on our 

survey protocols are provided in Wiens and others (2015, 2018). 

A sample site was considered to be occupied by a territorial pair of eagles if we observed 

(1) a male and female copulating, undulating, perching together, or attacking intruders; or (2) an 

incubating eagle in a nest, young nestlings in a nest, or a female observed with a brood patch. 

We attributed detections of territorial pairs to sample sites by mapping activity centers of pair 

observations (a used nest; locations of fledglings; or focal area of copulations, territorial

displays, and perches) (Wiens and others, 2018). We surveyed sites with pair detections during 

mid-June to late July to determine the number of young fledged after most nestlings were more 

than 80 percent of average fledging age (about 56 days old; Driscoll, 2010). A site was accorded 

successful reproduction if ≥1 nestlings of this age or fledglings were observed (Steenhof and 

Newton, 2007). During surveys we also recorded observations of Haliaeetus leucocephalus (bald 

eagle).

Data Analysis

We estimated site occupancy (ψ), abundance (λ), and detection probability (p) for 

territorial pairs of golden eagles with the “unmarked” package in R, version 3.5.0 (Fiske and 

Chandler, 2011; R Core Team, 2017). We estimated occupancy using a likelihood-based method 

(MacKenzie and others, 2002) and abundance using N-mixture models (Royle, 2004; Fiske and 

Chandler, 2011). For both data types, we used an information-theoretic approach (Burnham and 

Anderson, 2002) to rank competing sets of models representing different effects of primary land 

cover types or topographic conditions on the distribution of eagles. We used the Coastal Change 

Analysis Program land cover map of the National Oceanic and Atmospheric Administration to 

calculate the proportion of developed/urban, forest, grassland, and scrub/shrub cover types in 

each sample site (National Oceanic and Atmospheric Administration, 2010; fig. 1). We evaluated 

the effects of landscape topography on eagles by calculating the mean terrain ruggedness index 

for each site, calculated as the square root of the sum of the squared differences between the 

elevation in a cell and the elevation of its neighboring cells (Riley and others, 1999; Evans and 

others, 2014). 
4

We used the N-mixture model to estimate abundance (λ, the mean number of territorial 

pairs per site), which assumes that (1) there is within-plot closure during surveys across the 

breeding season, and (2) detection histories among sample sites are independent (Nichols and 

others, 2009; Barker and others, 2018). Our study design minimized the likelihood of violating 

of these assumptions, but our sample sites were not centered on actual territory locations. As a 

consequence, we interpreted λ as the mean number of breeding pairs with territories (defended 

areas) overlapping a focal survey site. Counts of territorial pairs during surveys were modeled 

using a Poisson distribution because preliminary analyses showed that this specification 

outperformed zero-inflated or negative binomial options. We summed site-specific estimates of 

mean abundance to yield total expected abundance of territorial pairs and used a parametric 

bootstrap approach to calculate associated 95-percent confidence intervals (Fiske and Chandler, 

2011). We also estimated density of territorial pairs based on pair detections and mapped 

locations of activity centers. This method was analogous to territory spot-mapping techniques 

often used to assess model-based methods of density estimation (Bibby and others, 2000; 

Chandler and others, 2011). When possible, we also mapped activity centers of territorial pairs in 

areas adjacent to our focal sample sites. This helped us ensure that counts of eagle pairs in the 

Plan area were accurate and that pairs were not double-counted. 

For occupancy and abundance data, we included the day of the survey season on which 

each site was surveyed (day 0 = January 1) as a covariate to account for within-season variation 

in detection (p) of eagle pairs. We assessed goodness of fit of occupancy models using methods 

outlined by MacKenzie and Bailey (2004). We assessed fit of N-mixture models using a 

parametric bootstrap approach and the Pearson chi-square test statistic (Chandler and others, 

2011). In both cases, we bootstrapped model estimates 500 times to obtain P-values (that is, 

evidence against the null hypothesis of no lack of model fit) for our most highly parameterized 

models. We also estimated a dispersion parameter (cĉ) as the ratio of the observed fit of the chisquare test statistic to the mean of the simulated distribution of the test statistic.

Results

Site Occupancy and Abundance of Golden Eagles 

We searched 30 of 32 hexagon survey sites overlapping the HCP/NCCP area up to 4 

times each (mean = 3.3 surveys per site) and detected 22 territorial pairs of eagles in 20 (67

percent) of these sites (fig. 1). Based on mapped locations of used nests and activity centers, 2 

(10 percent) of the 20 occupied sites were used by two pairs of eagles each. We also documented

two additional pairs of eagles with activity centers outside the boundary of the Plan area and our 

focal survey sites (fig. 1). These additional pairs were observed using areas within the Plan area 

but were not included in analyses of occupancy and abundance. 

Expected estimates of landscape occupancy (ψ�) and abundance (λ�) of territorial pairs of 

eagles estimated under the best models (ΔAICc = 0.003; appendix 1) are provided in table 1. 

Estimated total abundance (23 pairs) was similar to the naïve estimate (22 pairs) calculated using 

maximum plot counts and unadjusted for detection probability. We found some support for a 

negative effect of developed land use on expected site-occupancy (ΔAICc = 0.72; appendix 1), 

but the precision of this relationship was poor (ββ̂ = –3.67, SE = 2.77). Occupancy and abundance 

3

Difference between the Akaike’s Information Criterion for small sample size (AICc) value of each model and the 

lowest AICc model.
5

models including landscape and terrain covariates generally outperformed a null model without 

these effects, but 95-percent confidence intervals of covariate slope coefficients widely 

overlapped zero, indicating little to no support for these effects in our analyses. We found no 

strong evidence of lack of fit for occupancy (P = 0.13; cĉ = 1.30) or abundance (P = 0.97; cĉ= 

0.69) models, indicating adequate fit of our survey data to model assumptions. 

Table 1. Parameter estimates from the best models used to characterize site occupancy and abundance of 

territorial pairs of golden eagles in the East Contra Costa County Habitat Conservation Plan/Natural 

Community Conservation Plan Inventory area, California, 2019. 

Parameter Estimate Standard error

95-percent 

confidence interval

Expected site occupancy (ψ�) 0.69 0.09 0.55–0.78

Expected mean number of pairs per site (λλ̂) 0.76 0.16 0.49–1.16

Expected pair abundance 23 0.11 22–29

Detection Probability

Detection probability (p) of territorial pairs in the Plan area was negatively associated 

with survey date (ββ̂ = –0.01, SE = 0.01, fig. 2, appendix 1). Detection of pairs was highest during 

surveys conducted during the courtship phase of the breeding season in January and February 

(≥0.75), and lowest during the fledging-dependency period in mid-June to late-July (<0.50). This 

result was similar in both analyses of occupancy and abundance, and consistent with previous 

findings in the broader Diablo Range study area (Wiens and others, 2015, 2018). 

Figure 2. Probability of detecting 1 or more territorial pairs of golden eagles decreased with survey date 

(January 1–July 24) in the East Contra Costa County Habitat Conservation Plan/Natural Community 

Conservation Plan area, California, 2019. Solid black line represents expected values estimated under the 

best model of site occupancy; shading represents 95-percent confidence intervals.
6

Nesting Success and Reproductive Output

We found evidence of successful nesting (≥1 young fledged) for 3 (14 percent) of the 22 

pairs of eagles monitored in the Plan area in 2019. The mean number of young detected per 

successful pair was 1.33 (2 pairs fledged 1 young each and 1 pair fledged 2 young), and the mean 

number of young fledged per pair was 0.19. Three (50 percent) of 6 nesting attempts failed 

during the early incubation or nestling stages, based on evidence collected at used nests. An 

additional pair of golden eagles successfully fledged 1 young from a historical nest located on 

the Concord Naval Weapons Base near the northwest edge of the HCP/NCCP area (fig. 1). 

Bald Eagle Observations

While surveying golden eagles, we identified a previously undocumented pair of bald 

eagles that nested within the boundary of the Plan area and successfully fledged 2 young. The 

nest location was approximately 7 km from the closest body of water. 

Discussion

Our estimate of landscape occupancy by territorial pairs of golden eagles in the Plan area 

(0.69) in 2019 was similar to estimates reported for the broader northern Diablo Range study 

area during 2014–16 (0.68–0.79; Wiens and others, 2018). This result indicated that, on average, 

occurrence and distribution of eagles in the Plan area were comparable to that in the broader 

study region. Expected occupancy and abundance of eagle pairs in the Plan area was negatively 

associated with the amount of developed land use at survey sites, but that the precision of this 

relationship was poor. Indeed, landscape conditions known to strongly influence the distribution 

of eagles in the broader study region like the amount of open grassland or terrain ruggedness

were not well-supported in our analysis in the smaller Plan area. We attribute this inconsistency

to the relatively small sample of sites used in our single-season (2019) analysis of the Plan area 

(n = 30 sites) relative to estimates derived from multiple years of surveys in the broader study 

area (n = 138 sites; Wiens and others, 2018). Estimated abundance of territorial pairs in the Plan 

area was 0.76 pairs per 13.9 km2

 (or 27.4 pairs per 500 km2), which is substantially greater than 

reported elsewhere for this species (Watson, 2010). Prior to our study, we estimate that historical 

surveys documented 16 of the 22 pairs we detected in the Plan area (Hunt, 2002; 2017; H.T 

Harvey and Associates, 2018). We also identified an additional 2 pairs of eagles that were 

regularly observed using the Plan area but had activity centers (nests) located outside of the Plan 

area boundary (fig. 1). Thus, in total, we documented 24 pairs of eagles that were associated 

with, and regularly used, the Plan area, which fell within the 95-percent confidence interval of 

our N-mixture estimate of total expected abundance (22–29 pairs). Naïve and modeled estimates 

of abundance were similar because of a relatively high probability of detecting pairs during 

surveys (mean = 0.66 percent). 

Our findings were consistent with previous analyses (Wiens and others, 2018) that 

highlighted the importance of accounting for imperfect detection in estimates of landscape 

occupancy and abundance of golden eagles. We found that the detection of pairs decreased over 

the course of the breeding season. We attribute this pattern to seasonal changes in territorial 

behaviors (for example, undulation displays) that enhance an observer’s ability to detect 

territorial pairs. This finding emphasized how surveys conducted early in the breeding season 

(December–February) can optimize detection rates and help provide rapid assessments of site 

occupancy. 
7

Occupancy and N-mixture models can produce biased estimates when there are few sites, 

few repeated visits to each site, and few detections; or when heterogeneity in detectability and 

abundance is not accommodated (Barker and others, 2018, Duarte and others, 2018). Although 

the number of sites included in our analysis of the Plan area was relatively small, estimates of 

detection were relatively high across survey periods (>50-percent) and we used a covariate to 

account for within-season change in detectability of eagles. Moreover, the size of our survey 

sites matched the biological scale at which territorial pairs of eagles operate within the study 

region, and modeled estimates of pair abundance aligned well with naïve estimates based on 

maximum plot counts. As a consequence, we believe that our estimates of site-occupancy and 

abundance for the Plan area were accurate, robust to analytical assumptions, and repeatable. 

Accurate estimates of site-occupancy and abundance provide baseline information

essential to assess the effectiveness of conservation strategies in benefitting wildlife like golden 

eagles. Our findings on the spatial distribution and density of golden eagles in the HCP/NCCP

preserve system provide resource managers with an opportunity to prioritize site-specific 

conservation actions (for example, minimizing disturbance at used nests) that can help minimize 

the negative impacts of human-caused stressors (for example, fatalities caused by collisions with 

wind-turbines at the Altamont Pass Wind Resource Area). Because golden eagles are wideranging, long-lived apex predators, local actions that promote survival and productivity can also 

benefit regional populations and ecosystems. 

Acknowledgments

We thank Christine Carino, Colleen Lenihan, Jeff Smith, Misty Cain, Ben Dudek, Ode 

Bernstein, and volunteers from the Golden Eagle Monitoring Team of the East Bay Regional 

Parks District (EBRPD) for their assistance with surveys of golden eagles. We thank the many 

State, county, and private landowners who provided us with permission to access their lands 

during our study. Two reviewers provided helpful comments that improved this report.

References Cited

Barker, R.J., Schofield, M.R., Link, W.A., and Sauer, J.R., 2018, On the reliability of N-mixture 

models for count data: Biometrics, v. 74, no. 1., p. 369–377. 

Bibby, C.J., Burgess, N.D., Hill, D.A., and Mustoe, S., 2000, Bird census techniques (2d ed.): 

San Diego, California, Academic Press, 302 p. 

Burnham, K.P., and Anderson, D.R., 2002, Model selection and multimodel inference—A 

practical information-theoretic approach (2d ed.): New York, Springer-Verlag. 

Bloom, P.H., and Clark, W.S., 2001, Molt and sequence of plumages of golden eagles and a 

technique for in-hand ageing: North American Bird Bander, v. 26, p. 97–116. 

Chandler, R.B., Royle, J.A., and King, D.I., 2011, Inference about density and temporary 

emigration in unmarked populations: Ecology, v. 92, p. 1429–1435. 

Driscoll, D.E., 2010, Protocol for golden eagle occupancy, reproduction, and prey population 

assessment: Apache Junction, Arizona, American Eagle Research Institute, 55 p. 

Duarte, A., Adams, M.J., and Peterson, J.T., 2018, Fitting N-mixture models to count data with 

unmodeled heterogeneity—Bias, diagnostics, and alternative approaches: Ecological 

Modelling, v. 374, p. 51–59. 
8

Evans, J.S., Oakleaf, J., Cushman, S.A., and Theobald, D., 2014, An ArcGIS toolbox for surface 

gradient and geomorphometric modeling, Version 2.0.0, accessed August 30, 2019, at 

https://evansmurphy.wixsite.com/evansspatial/. 

Fiske, I., and Chandler, R. 2011. Unmarked—An R package for fitting hierarchical models of 

wildlife occurrence and abundance: Journal of Statistical Software, v. 43, p. 1–23. 

Hunt, T.L., and Hunt, W.G., 2013, Golden eagle territory occupancy and reproduction in the 

vicinity of the Altamont Pass Wind Resource Area—2013 survey results: Report to the East 

Bay Regional Parks District, Oakland, California.

Hunt, W.G., 2002, Golden eagles in a perilous landscape—Predicting the effects of mitigation 

for wind turbine blade-strike mortality: Consultant report to California Energy Commission 

under contract P500-02-043F, Public Interest Energy Research, California Energy 

Commission, Sacramento. 

Hunt, W.G., Wiens, J.D., Law, P.R., Fuller, M.R., Hunt, T.L., Driscoll, D.E., and Jackman, R.E., 

2017, Quantifying the demographic cost of human-related mortality to a raptor population: 

PLOS One, 12(2):e0172232, 22 p. [Also available at 

https://journals.plos.org/plosone/article/file?id=10.1371/journal.pone.0172232&type=printable

.] 

H.T. Harvey and Associates, 2018, Los Vaqueros Reservoir project annual golden eagle 

monitoring report 2018: Prepared for Contra Costa Water District, Concord, California.

Jones and Stokes, 2006, East Contra Costa County Habitat Conservation Plan and Natural 

Community Conservation Plan: Prepared by Jones and Stokes, San Jose, California, for East 

Contra Costa Habitat Conservation Plan Association [variously paged]. 

Kolar, P.S., and Wiens, J.D., 2017, Distribution, nesting activities, and age-class of territorial 

pairs of golden eagles at the Altamont Pass Wind Resource Area, California, 2014–16: U.S. 

Geological Survey Open-File Report 2017–1035, 18 p., https://doi.org/10.3133/ofr20171035. 

MacKenzie, D.I., and Bailey, L.L., 2004, Assessing the fit of site-occupancy models: Journal of 

Agricultural, Biological, and Environmental Statistics, v. 9, p. 300–318. 

MacKenzie, D.I., Nichols, J.D., Lachman, G.B., Droege, S., Royle, J.A., and Langtimm, C.A., 

2002, Estimating site occupancy rates when detection probabilities are less than one: Ecology, 

v. 83, p. 2248–2255, https://doi.org/10.1890/0012-9658(2002)083[2248:ESORWD]2.0.CO;2. 

National Oceanographic and Atmospheric Administration, 2010, Coastal Change Analysis 

Program (C-CAP) regional land cover database: Charleston, South Carolina, National Oceanic 

and Atmospheric Administration Coastal Services Center, accessed March 3, 2013, at

https://coast.noaa.gov/digitalcoast/training/ccap-land-cover-classifications.html. 

Nichols, J.D., Thomas, L., and Conn, P.B., 2009, Inferences about landbird abundance from 

count data—Recent advances and future directions, in Thomson, D.L., Cooch, E.G., and 

Conroy, M.J., eds., Modeling demographic processes in marked populations: New York, 

Springer, p. 201–235, https://doi.org/10.1007/978-0-387-78151-8_9. 

Pagel, J.E., Whittington, D.M., and Allen, G.T., 2010, Interim golden eagle inventory and 

monitoring protocols; and other recommendations: U.S. Fish and Wildlife Service, Division of 

Migratory Bird Management, Arlington, Virginia. 

R Core Team, 2017, R—A language and environment for statistical computing: Vienna, Austria, 

R Foundation for Statistical Computing, http://www.R-project.org/.

Royle, J.A., 2004, N-mixture models for estimating population size from spatially replicated 

counts: Biometrics, v. 60, p. 108–115, https://doi.org/10.1111/j.0006-341X.2004.00142.x. 
9

Steenhof, K., and Newton, I., 2007, Assessing nesting success and productivity, in Bird, D.M., 

and Bildstein, K.L., eds., Raptor research and management techniques: Blaine, Washington, 

Hancock House Publishers, p. 181–192. 

Watson, J., 2010, The golden eagle (2d. ed.): London, United Kingdom, T & AD. 

Wiens, J.D., Kolar, P.S., Fuller, M.R., Hunt, W.G., and Hunt, T., 2015, Estimation of occupancy, 

breeding success, and predicted abundance of golden eagles (Aquila chrysaetos) in the Diablo 

Range, California, 2014. U.S. Geological Survey Open-File Report 2015-1039, 23 p., 

http://dx.doi.org/10.3133/ofr20151039. 

Wiens, J.D., Kolar, P.S., Hunt, W.G., Hunt, T., Fuller, M.R., and Bell, D.A., 2018, Spatial 

patterns in occupancy and reproduction of golden eagles during drought—Prospects for 

conservation in changing environments. The Condor—Ornithological Applications, v. 120, p. 

106–124. 
10

This page intentionally left blank. 
11

Appendix 1. Ranking of Candidate Occupancy and Abundance Models

Table 1.1 Ranking of models used to estimate landscape occupancy (ψ) and abundance (λ) of territorial 

pairs of golden eagles in the East Contra Costa County Habitat Conservation Plan/Natural Community 

Conservation Plan area, California, 2019. 

[Model parameters include: days=survey date (number of days since Jan 1); developed = proportion of survey site 

with developed landscape; forest = proportion of survey site with forest cover; scrub = proportion of site with scrubshrubland type; grassland = proportion of survey site with grassland; TRI = terrain ruggedness index; AICc = 

Akaike’s Information Criterion for small sample size, ΔAICc = difference between the AICc value of each model 

and the lowest AICc model; AICc wt. = AICc weight of each model; K = the number of model parameters]. 

Models K AICc ΔAICc AICc wt.

Occupancy models

ψ(.) p(days) 3 121.7 0.00 0.25

ψ(developed) p(days) 4 122.4 0.72 0.17

ψ(.) p(days2) 4 122.9 1.24 0.13

ψ(forest) p(days) 4 123.0 1.31 0.13

ψ(scrubland) p(days) 4 124.1 2.47 0.07

ψ(TRI) p(days) 4 124.3 2.67 0.07

ψ(grassland) p(days) 4 124.3 2.67 0.06

ψ(forest2) p(days) 5 125.1 3.42 0.04

ψ(grass2) p(days) 5 126.7 5.04 0.02

Null model: ψ(.) p(.) 2 127.0 5.28 0.02

ψ(scrubland2) p(days) 5 127.0 5.31 0.02

ψ(TRI2) p(days) 5 127.2 5.48 0.02

Abundance models

λ(.) p(days) 3 145.14 0.00 0.35

λ(developed) p(days) 4 146.73 1.59 0.16

λ(scrubland) p(days) 4 147.46 2.33 0.11

λ(forest) p(days) 4 147.69 2.56 0.10

λ(grassland) p(days) 4 147.78 2.65 0.09

λ(TRI) p(days) 4 147.81 2.68 0.09

λ(grass2) p(days) 5 150.21 5.07 0.03

λ(forest2) p(days) 5 150.33 5.20 0.03

λ(scrubland2) p(days) 5 150.54 5.40 0.02

λ(TRI2) p(days) 5 150.56 5.42 0.02

Null model: λ(.) p(.) 2 154.55 9.41 0.00

λ(.) p(days2) 4 163.87 18.74 0.00

Publishing support provided by the U.S. Geological Survey

Science Publishing Network, Tacoma Publishing Service Center

For more information concerning the research in this report, contact the

Director, Forest and Rangeland Ecosystem Science Center

U.S. Geological Survey

777 NW 9th St., Suite 400

Corvallis, Oregon 97330

https://www.usgs.gov/centers/fresc/ 
Wiens and others—Golden Eagles in the East Contra Costa County Habitat Conservation Plan/Natural Community Conservation Plan Area, California—OFR 2020-1107

ISSN 2331-1258 (online)

https://doi.org/10.3133/ofr20201107