U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2015-1039

Estimation of Occupancy, Breeding Success, and 

Abundance of Golden Eagles (Aquila chrysaetos) 

in the Diablo Range, California, 2014
Cover: An adult golden eagle flying through the Altamont Pass Wind Resource Area, California. Photograph taken by 

Patrick Kolar in 2014 and used with permission.
Estimation of Occupancy, Breeding Success, and 

Abundance of Golden Eagles (Aquila chrysaetos) in the 

Diablo Range, California, 2014

By J. David Wiens, Patrick S. Kolar, Mark R. Fuller, W. Grainger Hunt, and Teresa Hunt

 

Open-File Report 2015-1039

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: 2015

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Suggested citation:

Wiens, J.D., Kolar, P.S., Fuller, M.R., Hunt, W.G., and Hunt, Teresa , 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.

ISSN 2331-1258 (online) 
iii

Contents

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

Introduction.................................................................................................................................................................... 2

Study Area..................................................................................................................................................................... 3

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

Sampling Design and Survey Protocol....................................................................................................................... 3

Potential Sources of Variation.................................................................................................................................... 5

Data Analysis............................................................................................................................................................. 6

Results........................................................................................................................................................................... 8

Detection and Breeding Classification of Territorial Pairs .......................................................................................... 9

Probability of Occupancy and Successful Reproduction...........................................................................................11

Total Area Occupied and Number of Territorial Pairs................................................................................................13

Discussion ....................................................................................................................................................................13

Detectability of Golden Eagles and Their Young .......................................................................................................14

Spatial Pattern of Occupancy and Breeding Success...............................................................................................15

Estimated Abundance of Territorial Pairs..................................................................................................................16

Applications for Monitoring and Conservation...........................................................................................................17

Acknowledgments ........................................................................................................................................................18

References Cited..........................................................................................................................................................18

Appendix A. A priori Models Used to Characterize Variation in Detection, Occupancy, and Successful 

Reproduction of Golden Eagles in the Diablo Range, California, 2014.........................................................................21

Appendix B. Multistate Occupancy Detection Histories of 133 Sample Sites Searched Repeatedly for 

Evidence of Occupancy and Successful Reproduction by Territorial Pairs of Golden Eagles in the 

Diablo Range, California, 2014.....................................................................................................................................23

Figures

Figure 1. Map showing Diablo Range study area of west-central California and distribution of 133 randomly 

selected survey plots (1,385-hectare hexagons highlighted in yellow) that were repeatedly searched for 

evidence of occupancy and breeding success of golden eagles in 2014....................................................................... 4

Figure 2. Graphs showing seasonal change in probability of (A) detecting 1 or more territorial pairs of 

golden eagles in the Diablo Range, California, with and without historical data available on locations of 

previously used nests and activity centers, and (B) misclassifying a site with successful reproduction as 

nonbreeding given the site was occupied in 2014 ....................................................................................................... 11

Figure 3. Graphs showing probabilities of occupancy (ψ1) and successful reproduction (ψ2) of territorial pairs of 

golden eagles as a function of vegetation cover and terrain ruggedness in the Diablo Range, California, 2014......... 12

Figure 4. Maps showing estimated spatial pattern of (A) occupancy (ψ1), and (B) occupancy with successful 

reproduction (ψ(breeding)) for territorial pairs of golden eagles in the Diablo Range study area, California, 2014.......... 16
iv

Tables

Table 1. Survey effort and detections of golden eagles and their young during multistate occupancy surveys 

conducted in the Diablo Range, California, 2014........................................................................................................... 9

Table 2. Ranking of single-season multistate occupancy models used to characterize variation in the probability of 

occupancy (ψ1), breeding success (ψ2), detection (p), and breeding classification (δ) for territorial pairs of golden 

eagles in the Diablo Range, California, 2014............................................................................................................... 10

Conversion Factors and Datum

Conversion Factors

SI to Inch/Pound

Multiply By To obtain

Length

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

hectare (ha) 2.471 acre

hectare (ha) 0.003861 square mile (mi2) 

square kilometer (km2) 247.1 acre

square kilometer (km2) 0.3861 square mile (mi2)

Datum

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83). 
1 

Estimation of Occupancy, Breeding Success, and 

Abundance of Golden Eagles (Aquila chrysaetos) in 

the Diablo Range, California, 2014

By J. David Wiens1, Patrick S. Kolar2, Mark R. Fuller1, W. Grainger Hunt2, and Teresa Hunt2

Abstract

We used a multistate occupancy sampling design to estimate occupancy, breeding 

success, and abundance of territorial pairs of golden eagles (Aquila chrysaetos) in the Diablo 

Range, California, in 2014. This method uses the spatial pattern of detections and non-detections 

over repeated visits to survey sites to estimate probabilities of occupancy and successful 

reproduction while accounting for imperfect detection of golden eagles and their young during 

surveys. The estimated probability of detecting territorial pairs of golden eagles and their young 

was less than 1 and varied with time of the breeding season, as did the probability of correctly 

classifying a pair’s breeding status. Imperfect detection and breeding classification led to a 

sizeable difference between the uncorrected, naïve estimate of the proportion of occupied sites 

where successful reproduction was observed (0.20) and the model-based estimate (0.30). The

analysis further indicated a relatively high overall probability of landscape occupancy by pairs of 

golden eagles (0.67, standard error = 0.06), but that areas with the greatest occupancy and 

reproductive potential were patchily distributed. We documented a total of 138 territorial pairs of 

golden eagles during surveys completed in the 2014 breeding season, which represented about 

one-half of the 280 pairs we estimated to occur in the broader 5,169-square kilometer region 

sampled. The study results emphasize the importance of accounting for imperfect detection and 

spatial heterogeneity in studies of site occupancy, breeding success, and abundance of golden 

eagles. 

 

1 U.S. Geological Survey.

2 The Peregrine Fund, Inc.
2 

Introduction

The northern Diablo Range of west-central California is hypothesized to support one of 

the densest known breeding populations of golden eagles (Aquila chrysaetos) in North America

(Hunt, 2002; Hunt and Hunt, 2006, 2013). This region includes the Altamont Pass Wind 

Resource Area (APWRA), where previous reports suggest that about 65 golden eagles were 

killed annually by collisions with wind turbines during 1998–2002 (Smallwood and Thelander, 

2008). Concerns about whether the local breeding population of golden eagles surrounding the 

APWRA could sustain such mortality prompted a demographic investigation (Hunt and others, 

1998; Hunt, 2002; Hunt and Hunt, 2013). One component of that study used radiotelemetry to 

examine life-stage-specific movements and survival of golden eagles in an approximately 7,000-

km2 aerial survey region encompassing APWRA, while another component focused on 

determining annual occupancy of nesting territories and reproduction within 30 km of APWRA. 

Tracking data indicated that many individuals captured near the wind-energy facility remained 

year-round residents of the broader region of the Diablo Range, and that collisions with wind 

turbines was the leading cause of death (Hunt and others, 1998; Hunt, 2002). Estimates of 

survival and reproduction were used to determine the potential rate of population change and 

total number of territorial pairs required to sustain 50 turbine blade-strike fatalities annually 

(about 165 pairs; Hunt and others, 1998; Hunt, 2002; Hunt and Hunt, 2006). Golden eagles have not 

been systematically surveyed in the broader region to which these demographic estimates apply, thus 

the approximate number of breeding pairs that actually occupy this remote and varied landscape 

remains unknown. 

Monitoring occupancy and reproduction of golden eagles at broad spatial scales can be 

logistically challenging and expensive. Recent advances in survey design and interpretation of 

distributional data have made species-level monitoring at broad spatial scales considerably more 

feasible and cost effective than in the past (Noon and others, 2012). Sampling designs associated 

with site occupancy models have been shown to be particularly useful in investigating the 

dynamics of territorial species occurrence and evaluating how landscape features or human land 

use might influence those dynamics (MacKenzie and others, 2006; Martin and others, 2009; 

Wiens and others, 2011). Individuals of many species may be difficult to detect, depending on 

physiographic conditions or period of the breeding cycle, so strong inference for site occupancy 

studies requires accounting for imperfect detection (that is, the inability of researchers to detect a 

species at a sample site with 100 percent certainty). Failing to account for imperfect detection 

can lead to inaccurate estimates of population parameters, such as the proportion of sites that are 

occupied, the proportion of sites with successful reproduction, or abundance (MacKenzie and 

others, 2006, 2009; Nichols and others, 2007). Extensions of basic site-occupancy models 

consider multiple biologically relevant states, such as breeding or nonbreeding, which increase

the applicability of these methods to address conservation of species of special concern. In the 

case of golden eagles, this approach can be used to estimate, map, and analyze spatial patterns of 

occupancy and reproduction over a range of spatial scales. Such information can help identify 

areas for conservation prioritization—a key requirement for conservation policy that aims to 

maximize breeding success and maintain stable or increasing breeding populations of golden 

eagles (U.S. Fish and Wildlife Service, 2013). 
3 

We used a multistate occupancy study design (Nichols and others, 2007; MacKenzie and 

others, 2009) to investigate detection, occupancy, and breeding success for the territorial 

population of golden eagles in the northern Diablo Range encompassing APWRA. We also 

explored how occupancy data could be used to infer abundance of territorial pairs in the Diablo 

Range study area. The surveys included all age classes of golden eagles (that is, juveniles, 

subadults, breeders, and nonbreeding adults), but the focus here is on estimation of detection, 

occupancy, and reproduction of golden eagles in 2014. Specific objectives of the study were to: 

1. Examine potential sources of variation in the probability of detection, occupancy, 

and reproduction of territorial pairs; 

2. Use estimates of landscape occupancy to estimate the total number of pairs in the 

Diablo Range study area; and

3. Provide recommendations to improve strategies for monitoring breeding and 

nonbreeding golden eagles exposed to renewable energy development. 

Here, we report results from an analysis of survey data collected during the 2014 breeding 

season. 

Study Area

The 5,170-km2 study area was located in the Diablo Range of west-central California 

(fig. 1). We delineated this area based on movements of golden eagles radio-marked near the 

APWRA during 1994–2000 (Hunt, 2002). The study area ranged in elevation from 0 to 1,333 m 

above sea level and was dominated by chaparral and California oak-woodland communities, with 

stands of conifer forest appearing at elevations greater than 1,200 m above sea level. The 

northern part of the study area was mostly grassland with occasional rocky outcroppings and 

widely scattered stands of oak (Quercus spp.), eucalyptus (Eucalyptus spp.), digger pine (Pinus 

sabiniana), bigcone pine (Pinus coulteri), and California buckeye (Aesculus californica).

Grasslands with scattered stands of oak (oak savannah) descend to the agricultural flatlands of 

the San Joaquin Valley to the east, and the study area was bounded to the west and southwest by 

highly developed urban areas extending from Berkeley and the San Francisco Bay southward to 

San Jose. Vegetation types were more diverse in the central part of the study area, which 

included a high density of oak woodlands interspersed with chaparral, sage-scrub, and open 

grassland. Land use in the study area included private ranchlands with livestock grazing, in 

addition to State and county lands characterized by varying levels of grazing and recreational 

use. The APWRA was located in the northeastern quadrant of the study area (fig. 1). We 

delineate this 17,345 ha area using a minimum convex polygon estimated from 5,425 windturbine locations (http://www.altamontsrc.org/alt_amp.html, accessed February 23, 2013). 

Further details on APWRA are provided by Hunt (2002) and Smallwood and Thelander (2008). 

Methods

Sampling Design and Survey Protocol

The study area included all areas in public or private ownership potentially occupied by 

territorial pairs of golden eagles. We excluded densely populated areas and agricultural fields of 

the San Joaquin Valley from the sampling scheme because golden eagles were rarely observed in 

these landscapes (Hunt, 2002, p. 24–25). Surveys could not be performed in all areas potentially 
4 

Figure 1. Map showing Diablo Range study area of west-central California and distribution of 133

randomly selected survey plots (1,385-hectare hexagons highlighted in yellow) that were repeatedly 

searched for evidence of occupancy and breeding success of golden eagles in 2014. 

occupied by eagles because of budget constraints, logistical considerations, and limited access to 

private lands. As a consequence, we used a probabilistic sampling approach to infer estimates of 

occupancy, reproduction, and number of territorial pairs of golden eagles. Specifically, we used a 

multistate occupancy sampling framework (Nichols and others, 2007; MacKenzie and others, 

2009), where randomly selected sites were surveyed repeatedly and classified as unoccupied 

(state = 0), occupied by one or more pairs with no production of young (state = 1), or occupied 

with successful reproduction (state = 2). Areas targeted for repeated surveys of golden eagles 

were selected randomly from a grid of 373 equal-sized (1,385 ha) hexagonal cells overlaid on the 

study area (total area = 516,844 ha; fig. 1). The size of each survey hexagon corresponded to the 

estimated mean size of the annual core-use area for territorial golden eagles radio-marked in the 

study area during 1994–1999 (about 1,385 ha; Wiens and Hunt, U.S. Geological Survey, unpub. 

data, 2012). Thus, in the sampling design, a “site” was defined as a 1,385-ha randomly placed 

survey hexagon that had the potential to be occupied by 1 or more pairs of golden eagles during 

the breeding season. 
5 

We followed general recommendations by Driscoll (2010) and Pagel and others (2010) 

for determining territory occupancy and reproduction of golden eagles during ground-based 

surveys. We partitioned surveys using four developmental stages of the breeding season, based 

on mean dates for stages in the breeding cycle of golden eagles in our study area (Hunt and 

others, 1996; Hunt, 2002): courtship (January 1–February 28), incubation (March 1–April 30), 

nestling (May 1–June 15), and the fledging period (June 16–July 30). Each two-month interval 

represented a survey occasion in our occupancy analysis. On each survey, observers established 

1–4 observation points on selected ridges and hilltops to provide complete coverage of the focal

sample site. Observers then searched for evidence of occupancy and reproduction by territorial 

pairs for up to a 4-hr observation period. The goal of each survey was to characterize the focal 

sample site as being in one of three possible observation states: (1) no territorial pair of golden 

eagles detected; (2) territorial pair detected but no evidence of nesting observed; or (3) territorial 

pair detected with evidence of successful reproduction found (1 or more fledgling produced). 

Some sites required more than one survey spaced less than 5 days apart to achieve complete 

coverage. When possible, eagles were classified as juveniles, subadults, or adults based on 

visible plumage characteristics (Bloom and Clark, 2001). We considered a sample site to be 

occupied by a territorial pair if we observed (1) a male and female copulating, undulating, or 

perching together, or attacking intruders; (2) an incubating eagle in a nest or a female with a 

brood patch; or (3) nestlings or fledglings in or near a nest (Steenhof and others, 1997; Kochert 

and others, 2002). We attributed detections of territorial pairs to sample sites by mapping activity 

centers of pairs (that is, a nest with young, focal area of copulations, territorial displays, fledged 

young, perches). Sample sites were surveyed with pair detections during mid-June to late July to 

determine number of young fledged, after most nestlings were greater than 80 percent of average 

fledging age (51 days old; Kochert and others, 2002). A site was classified with successful 

breeding if nestlings or fledglings of this age were observed (Steenhof and Newton, 2007).

Potential Sources of Variation

Anticipating heterogeneity in parameter estimates and minimizing its effects—through 

study design and by collecting data on relevant covariates to model existing variation—is 

essential for good performance of occupancy models (MacKenzie and others, 2006). 

Consequently, we developed a small set of a priori models to explore potential site- and surveyspecific sources of variation in probability of detection, occupancy, and reproduction of golden 

eagles. We anticipated spatial heterogeneity in the probability of detection because some of the 

sample sites (n = 33) overlapped areas previously surveyed by Hunt and Hunt (2013) and had a 

wealth of historical information available on locations of previously used activity centers, 

whereas other sites (n = 100) had never been surveyed and had no historical information on 

golden eagles. Observers often used historical information as a basis to locate eagles, so we 

anticipated that probability of detection may have been greater for sites where such information 

was available. To account for this source of heterogeneity in detection, we used a spatial 

covariate that allowed detection rates to vary between sample sites with and without historical 

information on golden eagles (for example, the covariate was 1 if historical data were available

for a site and 0 otherwise). We also predicted that the amount of forest cover could have a 

negative influence on the probability of detection because golden eagles in the study area nested 

almost exclusively in trees, which often obscured observations of adults and young during 

surveys, especially at sites with more contiguous patches of oak and conifer forest. 
6 

We hypothesized that landscape features including vegetation cover type and terrain 

could influence occupancy and reproduction of golden eagles because prey-detection and 

hunting success typically are associated with rugged, open landscapes (Marzluff and others, 

1997; Carrete and others, 2000; McIntyre and others, 2006). Accordingly, we used the 1996 

classification of the National Oceanic and Atmospheric Administration’s Coastal Change 

Analysis Program (C-CAP) regional land cover to characterize the distribution of primary 

vegetation types in our study area (National Oceanic and Atmospheric Administration, 2010).

The map included 29 categories of land cover (21 of which occurred within the study area) and 

was produced for the Pacific coastal region of California using 30-m resolution Landsat 

Thematic Mapper and Landsat Enhanced Thematic Mapper satellite imagery. Extensive field 

sampling, validation, and standard quality-control review procedures were used in developing 

the map (National Oceanic and Atmospheric Administration, 2010). For our analysis, we 

combined existing land-cover types into three general categories that we hypothesized would 

represent important sources of spatial variation in the study area—open grassland, scrub/shrub 

cover, and evergreen and deciduous forest cover. To evaluate how terrain features might 

influence landscape occupancy of golden eagles, we used ArcGISTM (v. 10.1) and a 10-m 

resolution digital elevation model (DEM) to estimate mean terrain ruggedness (TRI) for each 

sample site in the study area (Riley and others, 1999). Terrain ruggedness was calculated as the 

difference between the elevation value of each 10-m cell of the DEM and the mean of the 8-cell 

neighborhood surrounding the focal cell. We considered TRI as a measure of terrain 

heterogeneity among sample sites, where larger values indicated greater heterogeneity. 

Data Analysis

Detection, Occupancy, and Reproduction of Territorial Pairs.—We used singleseason multistate occupancy models (Nichols and others, 2007) in Program MARK (White and 

Burnham, 1999) to estimate detectability (p), occupancy (ψ1

), and breeding state of golden 

eagles (ψ2), where : 

ψi

1 was the probability that sample site i is occupied by 1 or more territorial pairs of 

golden eagles, regardless of reproductive state (state 1); 

ψi

2 was the conditional probability that young occurred, given that sample site i is 

occupied by a territorial pair (state 2); 

ψi (breeding)was the unconditional probability that sample site i is occupied with 

breeding, regardless of occupancy state (= ψi

1 ∗ ψi2); 

pit

1 was the probability that occupancy is detected for site i, survey occasion t, given 

that true state = 1; 

pit

2 was the probability that occupancy is detected for site i, survey occasion t, given 

that true state = 2; and 

δit was the probability that evidence of successful reproduction is found, given 

detection of occupancy at site i on occasion t, and true state = 2. 
7 

Primary assumptions of the multistate occupancy analysis were that (1) occupancy status 

of sites (1,385-ha sample hexagons) did not change during the breeding season, and (2) detection 

histories among sample sites were independent (Nichols and others, 2007; MacKenzie and 

others, 2009). We developed and tested models using a sequential approach to model selection

(appendix A). We began by developing a set of 10 a priori models where detection probabilities

(p) were modeled as constant over occupancy states and survey occasions (that is, breeding 

stages), varying between occupancy states but constant over survey occasions, constant between 

occupancy states but varying with survey occasion, and varying with occupancy state and survey 

occasion. Classification probabilities (δ) were modeled as either constant over survey occasions 

or time-dependent. We fixed δ1 = 0 in all models because it was not possible to observe 

successful reproduction during the first survey occasion (courtship period; January–February). 

During this initial analysis, the two occupancy parameters (ψi

1, ψi2) were allowed to vary as a 

function of grassland and terrain ruggedness covariates. Our intention was to use the most 

general model for ψi

1 and ψi2 while identifying the best model structure for detection and 

classification parameters. In the second step of the analysis, we fixed detection (p) and 

classification (δ) to the best model structure from the previous analysis step and evaluated the 

relative influence of spatial covariates on occupancy (ψi

1) and reproduction (ψi2). We compared 

all models to an intercept-only model, which represented the null hypothesis that probability of 

detection, occupancy, and reproduction probabilities remained spatially constant. 

We used a deviance-based goodness-of-fit statistic to compute the quasi-likelihoodadjusted, small-sample Akaike’s Information Criterion (QAICc) for model selection (Burnham 

and Anderson, 2002; Nichols and others, 2007). The deviance-based variance inflation factor (ĉ) 

estimated for our survey data was 1.60, which indicated minor lack of fit of the most general 

model. Lack of fit was not severe, and was anticipated because detections of the same territorial 

pairs in adjacent sample sites could lead to a minor lack of independence among detection 

histories. The use of ĉ, therefore, was biologically justified and provided a means to adjust model 

selection and variance estimation accordingly (Burnham and Anderson, 2002; Mackenzie and 

others, 2006). We ranked a total of 24 candidate models using QAICc; the model with the lowest 

QAICc value was selected as providing the best description of the survey data. We evaluated 

strength of evidence for each model using ΔQAICc and QAICc weights, and calculated modelaveraged parameter estimates and unconditional variances from top-model sets (Burnham and 

Anderson, 2002). We further evaluated the effect of covariates by evaluating 95-percent

confidence intervals (95% CI) of slope coefficients (β), based on the degree to which intervals 

overlapped zero. 
8 

Estimation of Total Area Occupied and Number of Territorial Pairs.—We used our 

top multistate occupancy model to infer estimates of occupancy and reproduction across the

entire study area. For sample sites that were not surveyed, we estimated unconditional occupancy 

and reproduction (that is, the probability that site i is occupied based on covariate values 

associated with the site; MacKenzie and others, 2006). For surveyed sites where 1 or more pairs 

of golden eagles were seen, the probability of occupancy was set to 1. For surveyed sites where 

pairs of golden eagles were not seen, conditional estimates of occupancy were calculated using

the site’s detection history and associated covariate values (Rich and others, 2013). To estimate 

the total area occupied by pairs of golden eagles in the study area, we multiplied site-specific 

estimates of occupancy by the respective size of each sample site (1,385 ha), and summed these 

values across all available sites (n = 373). This procedure was similar to that used to estimate

annual numbers of gray wolf (Canis lupus) territories in Montana (Rich and others, 2013), which 

assumed minimal overlap among occupied territories and minimal unoccupied space between 

territories. To estimate the total number of territorial pairs, we divided our estimate of the total 

area occupied by the estimated mean territory size for golden eagles in the study area (1,385 ha). 

We used this estimation procedure because it was repeatable, it accounted for spatial variation in 

pair occupancy, and it dealt with uncertainties associated with imperfect detection probabilities. 

Confidence intervals for total area occupied and number of territorial pairs were obtained using a 

parametric bootstrap procedure in program MARK (White and Burnham, 2001). Here, we 

repeatedly simulated encounter histories for each survey site using the estimated sampling 

uncertainty associated with parameter estimates in the top multistate occupancy model. We then 

calculated total area occupied and number of pairs for each bootstrap iteration, and obtained 95% 

confidence intervals for these values from the resulting distribution of simulated estimates. 

Results

We searched 133 sample sites over four occasions and recorded 899 detections of golden 

eagles—534 (59 percent) adults, 164 (18 percent) subadults, 67 (8 percent) juveniles (first-year 

birds), and 134 (15 percent) golden eagles of unknown age-class. We observed 98 territorial 

pairs of golden eagles at 87 sample sites (table 1). Based on mapped locations of used nests and 

pair activity centers, 77 (88.5 percent) of the 87 sample sites were occupied by one pair of 

golden eagles, 9 (10.3 percent) were occupied by two pairs, and 1 (1.1 percent) was occupied by 

three pairs (x̅ = 1.13 pairs per sample site). During surveys, we observed an additional 40 

territorial pairs of golden eagles with activity centers outside the boundary of the focal survey 

site. Thus, we identified a total of 138 territorial pairs of golden eagles, 98 of which were located 

in focal sample sites and included in occupancy analyses. No golden eagles were detected in 14 

(11 percent) of 133 sites surveyed, and subadults or juveniles, but no adults, were detected at 27 

(20 percent) sites. 
9 

Table 1. Survey effort and detections of golden eagles and their young during multistate occupancy 

surveys conducted in the Diablo Range, California, 2014. 

[Breeding stages were courtship (January 1–February 28), incubation (March 1–April 30), nestling (May 1–June 

15), and fledgling (June 16–July 31)] 

Detection and Breeding Classification of Territorial Pairs

Detection histories of sample sites that were surveyed at least once are shown in appendix 

B. Model selection results for detection and classification probabilities showed that the 

probability of detecting pairs of golden eagles was dependent on time of season (that is, stage of 

the reproductive cycle) and site-specific availability of historical data on previously used activity 

centers (table 2, fig. 2A). As predicted, availability of historical data during a survey had a 

positive effect on detection probability (β� = 0.840, 95% CI = 0.046–1.634). Models where 

detection probabilities differed between breeding and nonbreeding occupancy states were not 

well-supported (ΔQAICc ≥ 5.5; table 2). We found a weak negative effect of forest cover on the 

probability of detecting golden eagles and their young (β� = –0.629, 95% CI = –2.552–1.295), but 

the confidence interval of the beta coefficient included zero, and models with this effect received 

relatively weak support. The per-visit probability of misclassifying an occupied sample site with 

successful reproduction decreased from 0.91 for surveys conducted during the early nestling 

period to near 0 for surveys conducted late in the breeding season (fig. 2B). 

Survey occasion and 

breeding stage

Sites

surveyed

Number of sites

surveyed with 1 or more 

golden eagles detected

(percentage)

Number of sites

surveyed with 1 or 

more territorial pairs 

detected (percentage)

Number of occupied

sites with 1 or more 

young produced

(percentage)

1: courtship 111 95 (77.5) 64 (57.7)

2: incubation 123 90 (72.6) 64 (52.0) 2 (1.6)

3: nestling 113 80 (64.0) 39 (34.5) 11 (9.7)

4: fledgling 71 49 (76.6) 20 (28.2) 11 (15.5)

All visits combined 133 119 (89.5) 87 (65.4) 17 (19.5)
10

Table 2. Ranking of single-season multistate occupancy models used to characterize variation in the 

probability of occupancy (ψ1), breeding success (ψ2), detection (p), and breeding classification (δ) for 

territorial pairs of golden eagles in the Diablo Range, California, 2014.

[Model: Time effects modeled as constant (.) or varying with survey occasion (t). Spatial covariates included: 

sample site availability of historical data (HD), proportion of sample site with grassland, forest, or scrub cover, and 

terrain ruggedness (TRI). QAICc = quasi-likelihood Akaike’s Information Criterion for small sample size; ∆QAICc

= difference between the QAICc value of each model and the lowest QAICc model; wi = model QAICc weight]

Model

Number of

parameters QAICc ΔQAICc Weight, wi

Probability of detection and classification

ψ1(grassland + TRI) ψ2(grassland2) p(t + HD) δ(t) 14 376.9 0.0 0.63

ψ1(grassland + TRI) ψ2(grassland2) p(t) δ(t) 13 379.0 2.1 0.22

ψ1(grassland + TRI) ψ2(grassland2) p(t + forest) δ(t) 14 381.1 4.2 0.08

ψ1(grassland + TRI) ψ2(grassland2) p1(t) p2(t) δ(t) 17 382.9 5.9 0.03

ψ1(grassland + TRI) ψ2(grassland2) p1(.) p2(.) δ(t) 11 383.4 6.5 0.02

ψ1(grassland + TRI) ψ2(grassland2) p(.) δ(t) 10 384.4 7.4 0.02

ψ1(grassland + TRI) ψ2(grassland2) p1(t) p2(t) δ(.) 15 389.3 12.4 0.00

ψ1(grassland + TRI) ψ2(grassland2) p(t) δ(.) 11 390.1 13.1 0.00

ψ1(grassland + TRI) ψ2(grassland2) p1(.) p2(.) δ(.) 9 391.6 14.7 0.00

ψ1(grassland + TRI) ψ2(grassland2) p(.) δ(.) 8 395.7 18.7 0.00

Probability of occupancy and reproduction

ψ1(grassland + TRI) ψ2(scrub) p(t + HD) δ(t) 13 371.8 0.0 0.66

ψ1(grassland + TRI) ψ2(grass) p(t + HD) δ(t) 13 375.8 4.0 0.09

ψ1(grassland + TRI) ψ2(forest2) p(t + HD) δ(t) 13 376.6 4.8 0.06

ψ1(grassland + TRI) ψ2(.) p(t + HD) δ(t) 12 376.8 5.0 0.05

ψ1(grassland + TRI) ψ2(grass2) p(t + HD) δ(t) 14 376.9 5.1 0.05

ψ1(grassland + TRI) ψ2(forest) p(t + HD) δ(t) 13 377.9 6.1 0.03

ψ1(grassland) ψ2(.) p(t + HD) δ(t) 11 380.6 8.8 0.01

ψ1(grassland + TRI) ψ2(grass2) p(t + forest) δ(t) 14 381.1 9.3 0.01

ψ1(.) ψ2(.) p(t + HD) δ(t) 10 381.9 10.1 0.00

ψ1(grassland2) ψ2(.) p(t + HD) δ(t) 12 382.5 10.7 0.00

ψ1(forest) ψ2(.) p(t + HD) δ(t) 11 382.5 10.7 0.00

ψ1(forest + TRI) ψ2(.) p(t + HD) δ(t) 12 382.5 10.7 0.00

ψ1(grassland + TRI) ψ2(grass2) p1(t) p2(t) δ(t) 17 382.9 11.1 0.00

ψ1(TRI) ψ2(.) p(t + HD) δ(t) 11 384.1 12.3 0.00

ψ1(scrub) ψ2(.) p(t + HD) δ(t) 11 384.3 12.5 0.00

ψ1(forest2) ψ2(.) p(t + HD) δ(t) 12 384.3 12.5 0.00

ψ1(scrub + TRI) ψ2(.) p(t + HD) δ(t) 12 386.2 14.5 0.00
11

Figure 2. Graphs showing seasonal change in probability of (A) detecting 1 or more territorial pairs of 

golden eagles in the Diablo Range, California, with and without historical data available on locations of 

previously used nests and activity centers, and (B) misclassifying a site with successful reproduction as 

nonbreeding given the site was occupied in 2014. Vertical bar shows 95% confidence interval.

Probability of Occupancy and Successful Reproduction

We detected at least one pair of golden eagles in 87 of the 133 sites surveyed, yielding a 

naïve occupancy probability (that is, uncorrected for imperfect detection) of ψ�1

= 0.65. The 

model-averaged probability that a sample site was occupied by a territorial pair,ψ��1

, was 0.67 

(95% CI = 0.53–0.78), which was only slightly larger than the naïve estimate owing to the 

overall high probability of detecting territorial pairs that were present. Nesting (incubating eagle 

or young present) at was evident at 20 sample sites; 5 pairs fledged 2 young each, 12 pairs 

fledged 1 young each, and 3 pairs failed in their nesting attempt during the incubation stage for 

unknown reasons (mean number of fledglings per successful nest = 1.1, standard error [SE] = 

0.14). Successful reproduction (nestlings or fledglings present) was detected at 17 of the 87 

occupied sites, yielding a naïve probability of successful reproduction at an occupied site of ψ�2≈ 

0.20. The model-averaged probability that successful reproduction occurred in an occupied 

site,ψ��2

, was 0.30 (95% CI = 0.12–0.57). The model-averaged, unconditional probability that a 

given sample unit was occupied with successful breeding, ψ��(breeding), was 0.20 (95% CI = 0.08–

0.42), whereas the naïve estimate of the proportion of sites surveyed with successful 

reproduction was ψ�1*2 = 0.13. Thus, the occurrence of territorial pairs that successfully produced 

young was underestimated by about one-third when the uncorrected, naïve estimate was used for 

inference. 

A B
12

Models that incorporated the influence of spatial covariates on probability of occupancy 

and reproduction greatly outperformed models without these effects (table 2). Our overall top 

model included the additive and positive effects of grassland cover and mean terrain ruggedness 

on probability of occupancy (fig. 3). We also found support for a negative effect of scrub/shrub 

cover on the probability of successful reproduction (β = –9.83, 95% CI = –19.67–0.02), but the 

precision of this relationship was poor, likely owing in part to the low number of sites with 

breeding (n = 17). Models that considered quadratic effects of vegetation cover on occupancy 

parameters were not competitive relative to models without these effects (ΔQAICc >5.0). 

Figure 3. Graphs showing probabilities of occupancy (ψ1) and successful reproduction (ψ2) of territorial 

pairs of golden eagles as a function of vegetation cover and terrain ruggedness in the Diablo Range, 

California, 2014. Solid black lines represent predicted values estimated under the top multistate occupancy 

model (ΔQAICc = 0.00; table 2); dotted lines represent 95% confidence intervals.
13

Total Area Occupied and Number of Territorial Pairs

Using our best-fitting multistate occupancy model and the approach outlined by Rich and 

others (2013), we estimated that 388,217 ha (68.9 percent) of the 516,844-ha study area was 

occupied by territorial pairs of golden eagles in 2014. Thus, assuming an average territory size of 

1,385 ha, the estimated total number of territorial pairs of golden eagles in the entire study area 

was 280 (bootstrap SE = 12.2, 95% CI = 256–305 pairs of eagles).

Discussion

Three general findings characterized our investigation of detectability, occupancy, and 

reproduction of territorial pairs of golden eagles in the Diablo Range in 2014. First, the 

probability of detecting breeding golden eagles and their young during surveys was less than 1 

and varied with time of the breeding season, as did the probability of correctly classifying a 

pair’s breeding status. These two sources of uncertainty—imperfect detection and 

classification—led to a notable difference between the naïve estimate of the proportion of sites 

with successful reproduction (0.13) and our model-based estimate (0.20). Second, our multistate 

occupancy analysis provided strong support for non-random spatial distribution in occurrence

and breeding success of territorial pairs of golden eagles in the broader landscapes surrounding 

APWRA. Spatial variation in these parameters was best explained by broad-scale differences 

among survey sites in primary vegetation types and terrain conditions. Third, the relatively high 

probability of landscape occupancy (0.67) and number of territorial pairs we estimated to occur 

in the Diablo Range study area (about 280) supported the perception that the high density of 

breeding golden eagles observed near APWRA by Hunt and Hunt (2006, 2013) extends into

much of the broader surrounding region of the Diablo Range. 

Our results emphasize the importance of accounting for imperfect detection and spatial 

heterogeneity in broad-scale investigations of occupancy and reproduction of golden eagles. The 

analysis presented here was limited in that it was based on a single breeding season. Moreover, 

this study was conducted during a period of extreme drought conditions in central California, 

which could have influenced behavior and reproduction of golden eagles (Steenhof and others, 

1997; McIntyre and Schmidt, 2012). Additional years of survey data are required to investigate 

spatial dynamics of occupancy and reproduction, to evaluate predictive performance of models, 

and to capture a broader range of environmental conditions affecting golden eagles in the region. 

Despite these limitations, our sampling design was highly effective in estimating and detecting 

variation in landscape occupancy and breeding success of golden eagles while accounting for 

variable and imperfect detection of adults and their young during surveys. The multistate 

occupancy study design offers a promising technique for monitoring the spatial distribution of 

occupancy and reproduction of golden eagles at broad spatial scales when alternative methods 

may be limited by logistics, accessibility, or budget constraints. 
14

Detectability of Golden Eagles and Their Young

The probability of detecting 1 or more territorial pairs of golden eagles during groundbased surveys of an occupied sample site (mean = 0.80 over four survey occasions) was 

comparable to that estimated for golden eagles during aerial surveys of cliff nests (0.68; Booms 

and others, 2010) and during a combination of aerial- and ground-based surveys (about 0.90; 

Martin and others, 2009). Golden eagles are large, conspicuous birds with highly visible 

territorial displays, but our finding that detection probability during a single survey was less than 

1, regardless of breeding state, indicates that multiple surveys are required to reliably detect pairs 

of golden eagles that are present. In our study, detection probability was greatest during the early 

stages of breeding, when pairs were most heavily engaged in courtship behaviors and territorial 

displays that made them readily observable. Detection probability also increased when observers 

used historical data on previously used nest locations or activity centers to locate eagles during 

surveys. Golden eagles often re-use alternate nest structures within their territories (Kochert and 

others, 2012), so the ability to integrate historical data into our survey protocol and analysis of 

detection rates was a major advantage of our approach. Despite our predictions, we found only 

weak evidence that detection probability was negatively influenced by the amount of forest cover 

in a survey site. Our measure of forest cover, however, was coarse in that it did not account for 

finer-scale attributes of forest patches that could have influenced visibility of golden eagles and 

their young (for example, tree species, tree height, or canopy cover). Given the difficulties we 

experienced observing golden eagles and their young in heavily forested parts of the study area, 

we suspect that detection rates are more strongly influenced by forest conditions than what was 

supported by our analysis. Future analyses could examine this hypothesis by incorporating more 

detailed spatial covariates in modelling detection in forested areas. 

The importance of accounting for imperfect detection of nests and young during groundbased surveys of golden eagles was highlighted by our analysis. Successful breeding by 

territorial pairs was correctly identified only about one-half the time when the naïve, uncorrected 

survey data were used. Correct classification probabilities at sample sites where young were 

produced were very low during surveys conducted during early stages of nestling development 

(0.09–0.53), but increased to near 1.0 later in the breeding season as young eagles began to 

fledge from their tree nests and became more visible to observers. These sources of uncertainty 

encountered during surveys led to a considerable difference between naïve (0.13) and modelbased (0.24) estimates of the proportion of sites with successful reproduction. Our analysis was 

limited in that it was based on a single survey season with a low number of sites with successful 

breeding detected (n = 17). Nonetheless, these findings clearly illustrate the importance of 

appropriately accounting for uncertainty in detection and breeding classification of golden 

eagles. Given recent studies showing imperfect detection of cliff-nesting golden eagles 

inhabiting much more open landscapes than our study area (Booms and others, 2010), we 

suggest that estimation approaches such as those we used here be considered for future 

demographic analyses of golden eagles.
15

Spatial Pattern of Occupancy and Breeding Success

Golden eagles were nonrandomly distributed in the study area, with occupancy being 

greatest in sample sites with relatively rugged terrain and greater amounts of open grassland 

habitats. Our analysis indicated a relatively high probability of landscape occupancy by pairs of 

golden eagles (0.67), but that areas with the greatest predicted occupancy potential (>0.6) were 

patchily distributed. By projecting our occupancy model across the study area (fig. 4), we 

illustrated that the spatial pattern of pair occupancy and successful breeding was not uniform. 

Several studies have identified relationships between terrain conditions and foraging areas of 

golden eagles, and our results were consistent with these findings in that occupancy was 

positively associated with terrain ruggedness. Topography is an important component of the 

landscape that may interact with wind conditions and vegetation to influence how golden eagles 

use space (McLeod and others, 2002; Katzner and others, 2012). The coarse-scale spatial 

covariates we used to represent vegetation cover and terrain features likely missed many of the 

fine-scale features that influence occupancy and breeding success of golden eagles (for example, 

prey abundance), but the strong support for these effects in our analysis showed clear 

associations between landscape structure, distribution of territorial pairs, and reproductive 

success. This finding has important implications for conservation prioritization for golden eagles, 

as it highlights a potential linkage between landscape composition and population vital rates—a 

connection relevant to understanding fitness consequences of changes in site quality, source-sink 

dynamics, and other aspects of metapopulation dynamics (Pulliam, 1988; Runge and others, 

2006). We emphasize that dynamic occupancy models that incorporate multiple years of data and 

focus on characterizing how rates of local colonization and extinction vary spatially and over 

time hold the best promise for understanding and predicting such connections (Yackulic and 

others, 2015). 

Perhaps owing to the extreme drought conditions in central California in 2014, few

territorial pairs were documented nesting in the study area, and even fewer successfully produced 

young. Our naïve estimate of the probability of successful breeding at an occupied survey site

(0.20) was lower than that estimated from six previous annual reproduction surveys of golden 

eagles completed within 30 km of APWRA (range = 0.33–0.65; Hunt and Hunt, 2013). 

Predictions from our best-supported multistate occupancy model indicated a negative, albeit 

imprecise, association between proportion of scrubland vegetation cover at a site and breeding 

success. Large patches of scrubland, which dominated sites in the central part of the study area, 

included chaparral shrubs 2–5 m tall, tree shrubs, young trees in early successional state, and 

trees stunted from environmental conditions (National Oceanic and Atmospheric Administration, 

2013). We posit that these conditions limit prey availability and foraging opportunities for 

golden eagles. It is also possible that prey abundance was reduced in these areas because of the 

severe drought conditions in 2014. In many areas, for example, golden eagles nest more often 

and produce more fledglings when their primary prey is abundant and weather conditions are 

favorable (Steenhof and others, 1997; Kochert and others, 2002; McIntyre and Schmidt, 2012). 

Our analysis was based on a single breeding season, and we used simplistic land-cover 

categories to demonstrate the ecological applications of our sampling method and estimation 

techniques. A more detailed analysis would include additional years of data and the refinement 

of spatial and climatic covariates. 
16

A B

Figure 4. Maps showing estimated spatial pattern of (A) occupancy (ψ1), and (B) occupancy with 

successful reproduction (ψ(breeding)) for territorial pairs of golden eagles in the Diablo Range study area, 

California, 2014. We estimated predicted values under the top multistate occupancy model (ΔQAICc = 

0.00; table 2). Altamont Pass Wind Resource Area also is shown (black polygon).

Estimated Abundance of Territorial Pairs

We documented a total of 138 territorial pairs of golden eagles during surveys completed 

in the 2014 breeding season, which represented about one-half of the estimated 280 pairs in the 

study area. The method we used to estimate the number of territorial pairs of golden eagles from 

occupancy data was based on three key assumptions that warrant careful consideration. The first 

assumption was that our estimate of mean territory size (1,385 ha) was accurate. That estimate 

was based on the mean size of the core-use area estimated from adult golden eagles radiomarked in the study and on the sizes of previously studied, contiguous territories. The mean 

number of territorial pairs observed per 1,385-ha sample site (1.1 pairs) also did not deviate 

substantially from 1, which gave us further confidence that our sampling scale was appropriate

for the study population. We found 1 or more pairs of eagles in 9 of 133 surveyed sites, however, 

and our estimate of total number of pairs did not account for 1 or more pairs per site. As a 

consequence, our estimate of the total number of pairs in the study area should be viewed as 

conservative. The second assumption was that there was minimal overlap among territories of 

adjacent pairs of golden eagles. We do not believe there was a major violation of this assumption 

because the relatively high density of breeding pairs in the study area likely resulted in minimal 

overlap among territorial boundaries. The third assumption was that unoccupied space between 

territories was minimal. Some spatial separation among territory boundaries was likely, 

particularly in parts of the study area with lower probability of occupancy (fig. 4). By not 

accounting for inter-territory buffer space within unoccupied habitats, the estimate of the number 

of pairs would have been positively biased. A possible approach to correct estimates of 

abundance for inter-territory space would be to allow territory size to vary spatially in 

accordance with site-specific conditions. 
17

Applications for Monitoring and Conservation

The development of wind-power and solar facilities is expected to increase dramatically 

in habitats occupied by golden eagles the Western United States. A key objective of conservation 

plans for golden eagles exposed to renewable energy development includes monitoring 

population trends so that regulatory officials can determine the appropriate number of permits to 

issue for take requests by renewable energy projects while maintaining the goal of a stable or 

increasing breeding population (U.S. Fish and Wildlife Service, 2013). The monitoring of 

changes in vital rates of a wide-ranging, long-lived species such as the golden eagle, however, is 

strongly limited by the cost and logistical challenges associated with implementing traditional 

sampling methods and mark-recapture studies at broad spatial scales. Our findings suggest that 

estimation of breeding state and associated detection probabilities, as defined by Nichols and 

others (2007) and MacKenzie and others (2009), may be an especially effective tool for 

identifying and monitoring spatial patterns of occupancy and reproduction of golden eagles 

exposed to renewable energy development. Under this framework, failure to detect young at a 

site does not necessarily indicate absence of successful reproduction, but instead admits the 

uncertainty associated with classifying the breeding state of a site. In our study, undetected nests 

and young during surveys led to a negative bias in determining breeding success at sites 

occupied by territorial pairs. The magnitude of this bias further suggested that estimates of 

fecundity and population trends derived from data collected during remote surveys of golden 

eagles also would be biased low. We base this conclusion on well-established sampling theory 

and repeatable modeling techniques that provide population estimates accompanied by measures 

of uncertainty. We have carefully described our sampling design, and the underlying 

assumptions of our models used to estimate occupancy, reproduction, and the number of 

territorial pairs occupying a given area, such that these assumptions can be critically evaluated 

by future work. Accordingly, we encourage the development of multi-season occupancy models 

that directly account for imperfect detection of breeding and nonbreeding golden eagles in 

estimates of occupancy and nesting success. Indeed, this adaptable approach promises a clearer 

understanding of the dynamic spatial mechanisms influencing populations of golden eagles (also 

see Martin and others, 2009). 

Conservation policy often is aimed at managing landscape change, either through 

restoration, mitigation, or protection. Identifying linkages between landscapes, climate, and 

population vital rates, therefore, is a critical step in the development of effective conservation 

policy aimed at managing species sensitive to changes in human land use and climate (Lawler

and others, 2008; Hole and others, 2011). This study provides quantitative information on spatial 

patterns of occupancy and reproduction of golden eagles near a large wind-energy facility during 

a period of extreme drought. Additional years of survey data are required to confirm good

performance of the sampling design and to capture a broader range of environmental conditions 

that may influence breeding success of golden eagles in the region. We also note that the analysis

presented here was focused on detections of territorial pairs of golden eagles, but that our 

surveys included all golden eagles (that is, juveniles, subadults, breeding and nonbreeding 

adults). Surveys focused solely on the territorial segment of golden eagle populations do not 

account for nonbreeding, floater individuals of a golden eagle population that can quickly replace 

breeder mortality and complicate estimates of population trends (Hunt, 2002; Hunt and Hunt, 

2006. Future research could evaluate the estimation methods we present here in identifying

spatial patterns of occurrence of the nonbreeding segment of golden eagle populations. 
18

Acknowledgments

This study was jointly funded by the U.S. Fish and Wildlife Service (Science Support 

Partnership, Region 8), the U.S. Geological Survey, the East Bay Regional Park District, and The 

Peregrine Fund, Inc. We are indebted to Step Wilson and Craig Himmelwright for their 

assistance with surveys of golden eagles. Colleen Lenihan and Eric Jepsen generously 

volunteered their assistance with survey logistics. We thank Doug Bell, Joe DiDonato, Harv 

Wilson, and their team of volunteers for providing logistical support and historical information 

on locations of golden eagles. Our work would not have been possible without support from the 

East Bay Regional Park District, California Department of Fish and Wildlife, California State 

Parks, San Francisco Water Department, Santa Clara County Parks, The Nature Conservancy, 

NextEra Energy, Inc., and the many other landowners that provided us with permission to access 

their lands to survey golden eagles. J.D. Nichols and 1 anonymous reviewer provided helpful 

comments that improved our manuscript. 

References Cited

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. 

Booms, T.L., Schempf, P.F., McCaffery, B.J., Lindberg, M.S., and Fuller, M.R., 2010, Detection 

probability of cliff-nesting raptors during helicopter and fixed-wing aircraft surveys in western 

Alaska: Journal of Raptor Research, v. 44, p. 175–187. 

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

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

Carrete, M., Sánchez-Zapata, J.A., and Calvo, J.F., 2000, Breeding densities and habitat 

attributes of golden eagles in southeastern Spain: Journal of Raptor Research v. 34, p. 48–52. 

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

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

Hole, D.G., Huntley, B., Arinaitwe, J., Butchart, S.H.M., Collingham, Y.C., Fishpool, L.D.C., 

Pain, D.J., and Willis, S.G., 2011, Toward a management framework for network of protected 

areas in the face of climate change: Conservation Biology, v. 25, p. 305–315. 

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

vicinity of the Altamont Pass Wind Energy Resource Area—2013 survey results: Oakland, 

California, Report to East Bay Regional Parks District.

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

for wind turbine blade-strike mortality: Sacramento, California, Consultant Report to 

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

72 p. 

Hunt, W.G., and Hunt, T.L., 2006, The trend of golden eagle territory occupancy in the vicinity 

of the Altamont Pass Wind Resource Area—2005 survey: California Energy Commission, 

PIER Energy-Related Environmental Research. CEC-500-2006-256, 11 p. 

Hunt, W.G., Jackman, R.E., Brown, T.L., and Culp, L., 1998, A population study of golden 

eagles in the Altamont Pass Wind Resource Area—Population trend analysis 1994–1997: 

Santa Cruz, California, University of California, Report to National Renewable Energy 

Laboratory, Subcontracts XAT-5-15174-01, XAT-6-16459-01 to the Predatory Bird Research 

Group. 
19

Hunt, W.G., Jackman, R.E., Brown, T.L., Driscoll, D.E., and Culp, L., 1996, A population study 

of golden eagles in the Altamont Pass Wind Resource Area—Second-year progress report: 

Santa Cruz, California, University of California, Report to National Renewable Energy 

Laboratory, Subcontracts XAT-5-15174-01 and XAT-6-16459-01 to the Predatory Bird 

Research Group. 

Katzner, T.E., Brandes, D., Miller, T., Lanzone, M., Maisonneuve, C., Tremblay, J.A., Mulvihill, 

R., and Merovich, G.T., Jr., 2012, Topography drives migratory flight altitude of golden 

eagles: implications for on-shore wind energy development: Journal of Applied Ecology v. 49, 

p. 1178–1186. 

Kochert, M.N., and Steenhof, K., 2012, Frequency of nest use by golden eagles in southwestern 

Idaho: Journal of Raptor Research, v. 46, p. 239–247. 

Kochert, M.N., Steenhof, K., McIntyre, C.L., and Craig, E.H., 2002, Golden eagle (Aquila 

chrysaetos), in Poole, A., and Gill, F. (eds.), The Birds of North America, no. 684, 

Philadelphia, The Birds of North America Inc., p. 1–44. 

Lawler, J.J., Tear, T.H., Pyke, C., Shaw, M.R., Gonzalez, P., Kareiva, P., Hansen, L., Hannah, 

L., Klausmeyer, K., and Aldous, A., 2008, Resource management in a changing and uncertain 

climate: Frontiers in Ecology and the Environment, v. 8, p. 35–43. 

MacKenzie, D.I., Nichols, J.D., Royle, J.A., Pollock, K.H., Bailey, L.A., and Hines, J.E., 2006, 

Occupancy estimation and modeling: San Diego, California, Academic Press. 

MacKenzie, D.I., Nichols, J.D., Seamans, M.E., and Gutiérrez, R.J., 2009, Modeling species 

occurrence dynamics with multiple states and imperfect detection: Ecology, v. 90, p. 823–835. 

Martin, J., McIntyre, C.L., Hines, J.E., Nichols, J.D., Schmutz, J.A., and MacCluskie, M.C., 

2009, Dynamic multistate site occupancy models to evaluate hypotheses relevant to 

conservation of golden eagles in Denali National Park, Alaska: Biological Conservation v. 

142, p. 2,726–2,731. 

Marzluff, J.M., Knick, S.T., Vekasy, M.S., Schueck, L.S., and Zarriello, T.J., 1997, Spatial use 

and habitat selection of golden eagles in southwestern Idaho: The Auk, v. 114, p. 673–687. 

McIntyre, C.L., Collopy, M.W., Kidd, J.G., and Stickney, A.A., 2006, Characteristics of the 

landscape surrounding golden eagle nest sites in Denali National Park and Preserve, Alaska: 

Journal of Raptor Research, v. 40, p. 46–51. 

McIntyre, C.L., and Schmidt, J.H., 2012, Ecological and environmental correlates of territory 

occupancy and breeding performance of migratory golden eagles Aquila chrysaetos in interior 

Alaska: Ibis, v. 154, p. 124–135. 

McLeod, D.R.A., Whitfield, D.P., and McGrady, M.J., 2002, Improving prediction of golden 

eagle (Aquila chrysaetos) ranging in western Scotland using GIS and terrain modeling: Journal 

of Raptor Research, v. 36, p. 70–77. 

Nichols, J.D., Hines, J.E., MacKenzie, D.I., Seamans, M.E., and Gutiérrez, R.J., 2007, 

Occupancy estimation and modeling with multiple states and state uncertainty: Ecology, v. 88, 

p. 1,395–1,400. 

National Oceanic and Atmospheric Administration, 2010, Coastal Change Analysis Program (CCAP) Regional Land Cover Database: Charleston, South Carolina, National Oceanic and 

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

www.csc.noaa.gov/landcover. 

Noon, B.R., Bailey, L.L., Sisk, T.D., and McKelvey, K.S., 2012, Efficient species‐level 

monitoring at the landscape scale: Conservation Biology, v. 26, p. 432–441. 
20

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

monitoring protocols and other recommendations: Division of Migratory Bird Management, 

U.S. Fish and Wildlife Service. 

Pulliam, H.R., 1988, Sources, sinks, and population regulation: American Naturalist, v. 132, p. 

652–661. 

Rich, L.N., Russell, R.E., Glenn, E.M., Mitchell, M.S., Gude, J.A., Podruzny, K.M., Sime, C.A., 

Laudon, K., Ausband, D.E., and Nichols, J.D., 2013, Estimating occupancy and predicting 

numbers of gray wolf packs in Montana using hunter surveys: Journal of Wildlife Management 

v. 77, p. 1,280–1,289. 

Riley, S.J., DeGloria, S.D., and Elliot, R., 1999, A terrain ruggedness index that quantifies 

topographic heterogeneity: Intermountain Journal of Sciences, v. 5, p. 23–27. 

Runge, J.P., Runge, M.C., and Nichols, J.D., 2006, The role of local populations within a 

landscape context: defining and classifying sources and sinks: American Naturalist, v. 167, p. 

925–938. 

Smallwood, K.S., and Thelander, C., 2008, Bird mortality in the Altamont Pass Wind Resource 

Area, California: Journal of Wildlife Management, v. 72, p. 215–223. 

Steenhof, K., Kochert, M.N., and McDonald, T.L., 1997, Interactive effects of prey and weather 

on golden eagle reproduction: Journal of Animal Ecology, v. 66, p. 350–362. 

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

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

Hancock House Publishers, p. 181–192. 

White, G.C., and Burnham, K.P., 1999, Program MARK: survival estimation from populations 

of marked animals: Bird Study, v. 46, p.120–139. 

White, G.C., Burnham, K.P., and Anderson, D.R., 2001, Advanced features of program MARK, 

Wildlife, land, and people—Priorities for the 21st century, in The Second International Wildlife 

Management Congress, Proceedings: Bethesda, Maryland, The Wildlife Society. 

Wiens, J.D., Anthony, R.G., and Forsman, E.D., 2011, Barred owl occupancy surveys within the 

range of the northern spotted owl: Journal of Wildlife Management, v. 75, p. 531–538. 

Yackulic, C.B., Nichols J.D., Reid, Janice, and Der, Ricky, 2015, To predict the niche, model 

colonization and extinction: Ecology, v. 96, no. 1, p. 16-23, http://dx.doi.org/10.1890/14-

1361.1. 

U.S. Fish and Wildlife Service, 2013, Eagle conservation plan guidance: module 1—Land-based 

wind energy, version 2: U.S. Department of the Interior, Division of Migratory Bird 

Management. 
21

Appendix A. A priori Models Used to Characterize Variation in Detection, 

Occupancy, and Successful Reproduction of Golden Eagles in the Diablo 

Range, California, 2014

[Model: Time effects modeled as constant (.) or varying with survey occasion (t). Spatial covariates included 

sample site availability of historical data (HD); and proportion of sample site with grassland, forest, or scrub cover, 

and terrain ruggedness (TRI). We used the most general covariate structure for ψ1 and ψ2 when modeling 

probabilities of detection and classification]

Model Description of model structure

Probability of detection (p1, p2) and breeding classification (δ)

1 ψ1

(grassland + TRI) ψ2(grassland2)

p1(t) p2(t) δ(t) - General model

Detection varies by occupancy state and occasion, 

classification varies with occasion

2 ψ1

(grassland + TRI) ψ2(grassland2)

p1(.) p2(.) δ(t) 

Detection varies by occupancy state but not occasion, 

classification varies with occasion

3 ψ1

(grassland + TRI) ψ2(grassland2) p(t) 

δ(t) 

Detection constant between states but varies with occasion, 

classification varies with occasion

4 ψ1

(grassland + TRI) ψ2(grassland2) p(.) 

δ(t) 

Detection is constant between states and over occasions, 

classification varies with occasion

5 ψ1

(grassland + TRI) ψ2(grassland2)

p1(t) p2(t) δ(.)

Detection varies by occupancy state and survey occasion, 

classification is constant

6 ψ1

(grassland + TRI) ψ2(grassland2)

p1(.) p2(.) δ(.)

Detection varies by occupancy state but not occasion, 

classification is constant

7 ψ1

(grassland + TRI) ψ2(grassland2) p(t) 

δ(.)

Detection constant between states but varies with occasion, 

classification is constant

8 ψ1

(grassland + TRI) ψ2(grassland2) p(.) 

δ(.) - Null model

Detection is constant between states and over survey 

occasion, classifications is constant

9 ψ1

(grassland + TRI) ψ2(grassland2)

p(best structure from 1—8 + HD) δ(best 

structure from 1—8)

Detection probability is dependent on availability of preexisting historical data on eagle locations

10 ψ1

(grassland + TRI) ψ2(grassland2)

p(best structure from 1—8 + forest) 

δ(best structure from 1—8)

Detection probability is dependent on forest cover
22

Model Description of model structure

Probability of occupancy (ψ1) and breeding success (ψ2) 

11 ψ1

(forest) ψ2 p(best structure from 1—10) 

δ(best structure from 1—10)

Occupancy is dependent on amount of forest

12 ψ1

(forest2) ψ2 p(best structure from 1—10) 

δ(best structure from 1—10)

Nonlinear, quadratic effect of amount of forest on occupancy

13 ψ1

(grassland) ψ2 p(best structure from 1—

10) δ(best structure from 1—10)

Occupancy is dependent on amount of grassland

14 ψ1

(grassland2) ψ2 p(best structure from 1—

10) δ(best structure from 1—10)

Nonlinear, quadratic effect of amount of grassland on 

occupancy

15 ψ1

(scrub) ψ2 p(best structure from 1—10) 

δ(best structure from 1—10) 

Occupancy is dependent on amount of scrub/shrub cover

16 ψ1

(TRI) ψ2 p(best structure from 1—10) 

δ(best structure from 1—10) 

Occupancy is dependent on terrain ruggedness (TRI)

17 ψ1(forest + TRI) ψ2 p(best structure from 1 

- 10) δ(best structure from 1—10) 

Additive effects of forest and TRI on occupancy

18 ψ1(grassland + TRI) ψ2 p(best structure 

from 1—10) δ(best structure from 1—

10) 

Additive effects of grassland and TRI on occupancy

19 ψ1(scrub + TRI) ψ2 p(best structure from 

1—10) δ(best structure from 1—10) 

Additive effects of scrub/shrub cover and TRI on occupancy

20 ψ1

(best structure from 11—19) ψ2(forest) 

p(best structure from 1—10) δ(best 

structure from 1—10) 

Reproduction is dependent on amount of forest

21 ψ1

(best structure from 11—19) ψ2(forest2) 

p(best structure from 1—10) δ(best 

structure from 1 - 10)

Nonlinear, quadratic effect of amount of forest on 

reproduction

22 ψ1

(best structure from 11—19)

ψ2(grassland) p(best structure from 1—

10) δ(best structure from 1—10) 

Reproduction is dependent on amount of grassland

23 ψ1

(best structure from 11—19)

ψ2(grassland2) p(best structure from 1—

10) δ(best structure from 1—10) 

Nonlinear, quadratic effect of amount of grassland on 

reproduction

24 ψ1

(best structure from 11—19) ψ2(scrub) 

p(best structure from 1—10) δ(best 

structure from 1—10) 

Reproduction is dependent on amount of scrub/shrub cover

. 
23

Appendix B. Multistate Occupancy Detection Histories of 133 Sample Sites

Searched Repeatedly for Evidence of Occupancy and Successful 

Reproduction by Territorial Pairs of Golden Eagles in the Diablo Range, 

California, 2014

Detection histories (formatted for program MARK) show whether a particular site was 

classified as unknown occupancy status (0), occupied by a territorial pair (1), or occupied by a 

territorial pair with young present (2) on each survey occasion. A “.” denotes a missing 

observation (see details in Nichols and others, 2007).

1.1. 1;

100. 1;

.1.0 1;

1110 1;

1002 1;

1010 1;

1.11 1;

111. 1;

.10. 1;

1110 1;

111. 1;

000. 1;

1111 1;

112. 1;

00.0 1;

110. 1;

1111 1;

.00. 1;

000. 1;

1111 1;

1010 1;

1100 1;

000. 1;

000. 1;

000. 1;

1110 1;

.110 1;

1101 1;

.111 1;

1111 1;

000. 1;

110. 1;

000. 1;

1.0. 1;

.00. 1;

00.0 1;

1000 1;

1.10 1;

00.0 1;

.100 1;

.000 1;

100. 1;

11.1 1;

100. 1;

1101 1;

110. 1;

1100 1;

0110 1;

1120 1;

.0.0 1;

00.. 1;

00.0 1;

.0.0 1;

1111 1;

1100 1;

010. 1;

000. 1;

01.0 1;

011. 1;

1012 1;

1111 1;

0110 1;

..11 1;

1101 1;

..00 1;

.010 1;

10.. 1;

000. 1;

11.. 1;

.112 1;

0111 1;

1101 1;

1111 1;

112. 1;

1112 1;

.110 1;

.0.. 1;

1001 1;

.000 1;

1112 1;

1000 1;

0101 1;

.12. 1;

1122 1;

1110 1;

1101 1;

1110 1;

.1.1 1;

00.0 1;

01.0 1;

1.22 1;

122. 1;

00.0 1;

0..0 1;

000. 1;

012. 1;

000. 1;

000. 1;

1..2 1;

110. 1;

100. 1;

000. 1;

0.0. 1;

111. 1;

110. 1;

111. 1;

111. 1;

112. 1;

0000 1;

000. 1;

1110 1;

110. 1;

1022 1;

1222 1;

000. 1;

000. 1;

000. 1;

0112 1;

000. 1;

0000 1;

000. 1;

11.. 1;

1100 1;

000. 1;

.110 1;

000. 1;

000. 1;

000. 1;

000. 1;

.00. 1;

000. 1;

.00. 1;

011. 1;

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

http://fresc.usgs.gov/
Wiens and others—Estimation of Occupancy, and Abundance of Golden Eagles in the Diablo Range, California, 2014—Open-File Report 2015-1039

ISSN 2331-1258 (online)

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