Golden Eagle (Aquila chrysaetos)

About this Indicator

The golden eagle (Aquila chrysaetos), an apex predator with a long lifespan, is widely distributed throughout the Northern Hemisphere. As one of the largest predatory birds in the world, it is an iconic species throughout the Unites States and elsewhere within its Holarctic range. While the global population of golden eagles is considered relatively stable, long-term trends show declines of breeding populations at local and regional scales in some areas of North America (Katzner et al. 2020). The birds are considered a Fully Protected species by the California Department of Fish and Wildlife, and both federal and state regulations (i.e., the Bald and Golden Eagle Protection Act) prohibit “take,” defined as killing, harassing, or disturbing individuals or nests.

Why is This Resource Included?

The golden eagle is recognized as an indicator of ecosystem health throughout its geographic range (Katzner et al. 2020). Breeding pairs have been found to be especially abundant in the northern Diablo Range of west-central California, where they nest in higher densities relative to other parts of their range in California and North America (Hunt et al. 2017). Historical and current studies of golden eagles in the northern Diablo Range have spatially overlapped with the three subregions of the East Bay Stewardship Network (Network), thereby providing a rich source of information for this species in the area of focus. Thus, the golden eagle was selected as an important indicator species within the area of focus for this NatureCheck ecological health assessment.

Desired Condition and Trend

Conservation policy for the golden eagle aims to maximize reproductive success and maintain stable to increasing breeding populations (U.S. Fish and Wildlife Service [USFWS] 2013, 2016). Based on these conservation goals, historical studies, and current research on golden eagles (Hunt et al. 2017, Wiens et al. 2018 and 2021), the desired condition and trend for golden eagles in the area of focus are to:

1. Maintain or improve site occupancy (i.e., the probability of a survey site being occupied by at least one territorial pair of golden eagles during the breeding season). Here and below, a ‘site’ is defined as a 1,385-hectare hexagonal cell within the area of focus. In 2021, site occupancy was estimated to be 0.66 (standard error [SE] = 0.06, 95% confidence interval [CI] = 0.54–0.77) across all subregions.
2. Maximize reproductive rate (i.e., the probability of a survey site being occupied by at least one territorial pair of golden eagles that fledge at least one young during the breeding season). In 2021, the reproductive rate was estimated to be 0.36 (SE = 0.08, 95% CI = 0.23–0.52) across all subregions.
3. Minimize the occurrence of territorial subadults (i.e., territorial pair members observed with subadult plumage, or birds typically aged two to four years old). An increasing occurrence of territorial subadults can indicate that nonterritorial adult birds are unavailable to fill territory vacancies arising from local increases in the mortality rate of territory holders (Hunt et al. 215 2017, Kolar and Wiens 2017). In 2021, the estimated percentage of pairs monitored with at least one member in subadult plumage ranged from a low of 4% in the East Hills subregion to a high of 12% in the Mt. Diablo subregion.

Current Condition and Trend

The northern Diablo Range, which includes all three subregions of the area of focus, supports one of the densest known breeding populations of golden eagles in the world (Wiens et al. 2015, Hunt et al. 2017, Katzner et al. 2020). The analyses of site occupancy and reproductive rate of territorial pairs specific to those parts of the northern Diablo Range that fall within the area of focus indicated that the local breeding population was unchanging (stable) during 2014–2021 (i.e., no evidence of increase or decline in these population metrics). However, a consistently high proportion of territorial subadults has been observed at breeding territories near the Altamont Pass Wind Resource Area (APWRA, 22%–35%) relative to territories in surrounding regions (~3%; Wiens et al. 2021). The heightened occurrence of territorial subadults suggested an increased mortality rate for territorial eagles occupying the Mt. Diablo Range subregion that overlapped with the APWRA. The portion of the Mt. Diablo Range subregion was also where a consistently high fatality rate of golden eagles due to collisions with wind turbines was documented. Thus, while no trends were detected in Metric 1 (site occupancy) or Metric 2 (reproductive rate), caution is warranted based on Metric 3 (occurrence of territorial subadults).

Stressors

Additional Resources

Other Metrics Considered but Not Included 

We considered analyzing the survey data based on territory locations of golden eagle pairs instead of randomly selected, hexagonal survey plots. This approach was not used, however, because: 1) it would have limited inferences to used territories rather than the entire landscape of interest, 2) the grid-based approach we used to identify focal plots for surveys allowed statistical inferences on site occupancy and reproduction to be extended to sites not surveyed within the defined sampling frame (e.g., the area of focus), and 3) the design we used still provided territory-level information on the distribution and reproductive status of territorial pairs within sample sites (hexagons).

Data Gaps and Data Collection/Management Needs 

• Currently, results from the USGS golden eagle study are limited to years of severe and prolonged drought conditions in west-central California (2014–2021; surveys were not conducted in 2017, which was an exceptionally wet, intervening year). Severe drought in California can negatively affect the reproductive rate of golden eagles, as shown by Wiens et al. (2018), and more recently by Smith et al. (2020). Additional research is needed to capture years with more normal precipitation levels and to quantify how drought conditions impact the local eagle population.
• Several thousand molted golden eagle feathers have been collected from breeding territories and roosting locations in the area of focus over the past seven years. Genetic analyses of molted feathers can be used to estimate site-specific adult survival and turnover rates of breeding territories, in addition to survival and abundance of non-breeding subadult and adult golden eagles. Such information can provide an unprecedented evaluation of the population dynamics of golden eagles in the area of focus, in addition to the anticipated impacts of multiple interacting stressors within this region (including wind-energy production, drought, wildfire, and increasing development/urbanization of the landscapes used by golden eagles).
• We lack information on territory occupancy and productivity of golden eagles in areas immediately adjacent to the northern Diablo Range, such as the southern Diablo Range and the extensive Inner Coast Range across from the Central Valley river systems immediately north. Both regions no doubt harbor golden eagle populations and are well within the dispersal distances of juveniles. Applying Wiens et al. (2015) survey methods and occupancy modelling to these regions would provide a more complete picture of the health of the regional golden eagle population and contribute to a better understanding of the species in California.
• The volunteer Golden Eagle Monitoring Team (GMT, East Bay Regional Park District [EBRPD]) uses the same survey methodology as the Wiens et al. (2015, 2018) studies to augment USGS occupancy modeling in the northern Diablo Range, but also conducts surveys of known territorial pairs for site-specific management purposes. The GMT uses trained volunteers to perform the surveys. Organization, training, and volunteers (who may have a variety of skill levels) require significant supervision and a commitment of time that may limit its application.
• Focused studies of eagle nest sites have been performed by Sunol resident Hans Peeters. Additional studies by Bell (EBRPD) and DiDonato (Wildlife Consulting and Photography, Loleta, California), with assistance from the American Eagle Research Institute (AERIE, Apache Junction, Arizona), have included trapping and banding as well as attaching Global Positioning System/Global System for Mobile Communication (GPS/GSM) satellite transmitters (Cellular Tracking Technologies, https://celltracktech.com) to eagles and monitoring their movements, especially in relation to impacts from wind turbines in the APWRA (see Bell and Wilson 2016, Bell 2017a, 2017b). In addition, satellite telemetry data have been analyzed in conjunction with researchers investigating region-wide movements of eagles and flight behavior in relation to topography (Brown et al. 2017, Duerr et al. 2019, Sur et al. 2020). Further analysis of eagle dispersal, home range, and age-cohort-specific movements in relation to the APWRA and other stressors in the northern Diablo Range could help identify specific areas where eagles of different age classes may be most susceptible to human-caused disturbance or mortality.
• Smallwood et al. (2009, 2016, 2017) have been observing and analyzing inter- and intraspecific eagle behavior and flight patterns in the APWRA for many years. These data, along with satellite telemetry information, have been analyzed in a Digital Elevation Model framework to create eagle-wind turbine collision hazard maps. The latter are used by some companies to inform wind turbine placement in the APWRA during repowering to potentially lessen impacts to golden eagles. Further analyses are needed to determine if and how eagles react in flight to spinning and stationary wind turbine rotors, the meteorological conditions that may contribute to chances of collisions, and the relative roles of prey hunting/searching behavior versus social interactions in causing eagle-wind turbine collisions.
• Preliminary work on the degree to which local versus non-local golden eagles are killed by APWRA wind projects has been completed (Katzner et al. 2017), but more detailed information on impacts of the APWRA to regional eagle populations is needed. Kolar and Bell have been collecting molted golden eagle feathers from territories and communal roost sites, and Bell has been collecting blood samples from trapped eagles. In addition, feather and blood samples are acquired from wounded golden eagles picked up in the APWRA and taken to local wildlife rehabilitation facilities (e.g., Lindsay Wildlife Hospital), and carcasses of golden eagles killed in the APWRA are sent to the California Department of Fish and Wildlife’s Wildlife Investigations Laboratory for necropsy and sampling. These efforts have resulted in the compilation of an extensive set of samples of local and non-local eagles that are available for genetic and isotopic analysis.
• Determining the mortality rate of golden eagles resulting from wind project operations in the APWRA and relating it to the vital rates of golden eagles in the region is key to determining broader impacts of the AWPRA as a potential population sink for golden eagles. Operating since the early 1980s with a rated capacity of 580 megawatts (MW), the APWRA at one time consisted of more than 5,000 old-generation wind turbines that ranged in size from 40 to 400 kilowatts, which collectively represented its “pre-repowered” condition (Smallwood 2008). Because of various settlement agreements (e.g., see Alameda County Community Development Agency 2014), by the mid-2000s project operators in the APWRA began replacing the numerous lower-capacity, old-generation wind turbines with fewer, highercapacity (> 1MW) wind turbines, an ongoing process known as “repowering.” Golden eagle fatality estimates for the pre-repowered APWRA average approximately 55 to 66 eagles per year (Smallwood and Thelander 2008, Smallwood and Karas 2009, ICF International 2016). Hunt et al. (2017) estimated that it would take the reproductive output of 216 to 255 breeding pairs of golden eagles to compensate for this loss and have a self-sustaining population. Wiens et al. (2015) estimated the Diablo Range population of golden eagles at 280 pairs, so possibly large enough to sustain APWRA-related fatalities. However, Wiens et al. (2018) reported exceptionally low productivity of golden eagles during the drought years 2014–2016, which would potentially tip the APWRA into population sink status. More recently, the APWRA has been in the process of repowering (i.e., replacing many smaller, old-generation wind turbines [< 1 MW] with fewer, larger turbines [up to 5MW]. Results from post-monitoring studies of repowered wind farms suggest that cumulative golden eagle fatality rates were not substantially reduced (H. T. Harvey and Associates 2021, 2022a, 2022b). As a consequence, fatality rates of golden eagles at APWRA currently remain a significant management concern.
• Precise estimates of golden eagle fatality rates are key in understanding the possible broader, population-level impacts of the APWRA. However, fatality estimates are prone to many biases (Smallwood 2007). For example, independently derived information on the persistence and scavenging rates of raptors is typically used to correct for these factors in fatality estimates. Information on persistence and scavenging rates for golden eagles is essentially non-existent, however, so the degree to which these factors have affected fatality estimates of golden eagles at APWRA is largely unknown.
• Information on the impacts of human recreation on eagles (e.g., camping, hiking, mountain biking, equestrian activities, and use of off-highway vehicles), and the effectiveness of protective buffers around used eagle nests to alleviate these impacts, would help inform management actions within the area of focus. For example, analyses based upon long-term data on reproductive output of eagle pairs in territories with and without impacts from recreational use could be used to determine impacts, and thus identify the best protective measures near nesting sites during the nesting season.
• Understand nest-site selection by golden eagles in oak/woodland savanna landscapes of the area of focus, and how recent large wildfires have impacted these conditions, would provide information needed to identify and manage specific physiographic conditions that promote golden eagle nesting activities.
• The effects of range management practices on prey species used by golden eagles, particularly those that can either negatively affect or promote ground squirrel populations as a prey source, require further study (e.g., see Smallwood et al. 2008). For example, many private landowners at the APWRA and surrounding landscapes regularly use rodenticides to control ground squirrel populations. Studies of golden eagle demographic performance in areas with and without rodenticide exposure can help determine the ecosystem-level consequences of such actions, leading to more informed land-management decisions.
• The effectiveness of established USFWS mitigation measures for the take of golden eagles at renewable energy facilities (e.g., retrofitting of high-risk power-poles) could be assessed relative to other potential mitigation measures (e.g., maintaining or enhancing prey habitat) to determine the most effective use of limited resources available for these actions.
• Existing historical records, such as golden eagle egg and other specimen collections housed in various scientific institutions (e.g., the Museum of Vertebrate Zoology, University of California, Berkeley), are accessible through web-based Arctos collections (http://arctos.database.museum/home.cfm). This is a rich source of information that could be compiled and analyzed in a GIS framework to better understand the historical distribution of golden eagles in California – an important source of information in determining possible changes in the status and distribution of this species.
• The effect of newly discovered wildlife diseases on golden eagle populations (e.g., mange [Mete et al. 2014]) is largely unknown. Data on emergent disease in golden eagles is needed to better understand how this stressor may impact golden eagles in the area of focus.
• The golden eagle’s role in the culture of the region’s indigenous peoples is poorly understood by the scientific community. A better understanding of that role would benefit land managers’ understanding and protection of culturally significant resources and areas.

Past and Current Management

Golden eagles in California and the area of focus are primarily managed under legal protections afforded under state and federal regulations. The USFWS provides a framework for preventing the net loss of golden eagles by incidental take resulting from development activities, such as wind farm projects, using a permit system tied to the Bald and Golden Eagle Protection Act and protocols outlined in the USFWS (2013) Eagle Conservation Plan Guidance. This wholly voluntary permit system provides guidance to, first, avoid and minimize the impact of human activities from individual projects to the maximum degree practical, and second, to offset losses that cannot be avoided by reducing eagle mortality and/or increasing their productivity elsewhere (Allison et al. 2017). Implementation of this voluntary permit system in the APWRA exposes energy companies that do not apply for and receive a take permit to potential enforcement action for take of golden eagles.

For eagles nesting in the APWRA, permitting guidelines have provided recommended distance buffers to avoid incidental take of breeding eagles and prevent disturbance near nests until young have fledged (USFWS 2013). The size of these avoidance buffers varies depending upon intervening topography, or the type and extent of activity, but are recommended to be up to half the mean species-specific inter-nest distance (approximately two miles in the area of focus [Wiens and Kolar, 2021]) from large-scale developments like wind energy facilities (USFWS 2013). One Network partner agency in the area of focus, Contra Costa Water District (CCWD), uses smaller buffers to close recreational trails 0.5 miles of known nesting areas prior to incubation and until either nest failure is confirmed, or 10 days after fledging (H. T. Harvey and Associates, 2020b). Another partner (EBRPD) closes trails on as as-needed basis based on nest monitoring results from its volunteer Golden Eagle Monitoring Team. Such efforts could be expanded to include all Network partner agencies.

Golden eagles are also a species of conservation value in local and regional conservation planning measures, such as in the East Contra Costa County Habitat Conservation Plan/Natural Community Conservation Plan Area (Wiens et al. 2021) and the volunteer East Alameda County Conservation Strategy (http://www.eastalco-conservation.org/). Under USFWS guidance and regional conservation 236 plans, local governments, land agencies, and developers may be required to implement measures to maintain stable golden eagle populations. This process can involve the creation of long-term monitoring plans for local breeding territories, or purchasing lands suitable for breeding and foraging requirements to mitigate development impacts such as those from the construction and operation of wind energy projects, the construction and expansion of reservoirs, and others.

Potential Future Actions 

• Continue monitoring of golden eagles within the area of focus. Continuation of the established golden eagle demographic study in the area of focus would provide the most direct comparisons with established baseline conditions outlined in this assessment. The established framework facilitates future conservation and management options by identify specific areas within the area of focus were conservation actions may be most beneficial to golden eagles.
• Expand and implement volunteer Golden Eagle Management Team efforts among Network partner agencies. These efforts have been effectively used as part of the aforementioned USGS monitoring program for golden eagles and have provided a means to reduce project costs while also promoting educational and outreach opportunities to East Bay communities.
• Genetic and isotopic analysis of extensive eagle feather and blood sample collections from the APWRA and USGS study could be used to better understand key population parameters of interest, such as individual turnover rates at breeding territories exposed to disturbances and annual survival of breeding and non-breeding golden eagles.
• An assessment of the impacts of human disturbance and recreational activities on nesting success of golden eagles could be used to identify and prioritize actions to mitigate those impacts and other stressors.
• More detailed spatial data on land-use within the area of focus could be used to address hypothesized relationships between livestock grazing, range-management practices, status of golden eagle prey populations (e.g., ground squirrels), and dynamics in site occupancy and reproduction of golden eagles.
• The effectiveness of established mitigation measures on reducing or compensating for take of golden eagles at the APWRA (e.g., retrofitting of power poles) could be examined relative to site-specific alternatives (e.g., curtailment of wind-turbines near used nests during the breeding season).
• Broaden regional partnerships with cooperating private landowners to implement all the above actions.

Key Literature and Data Sources 

For additional information about this indicator including key literature and data sources see NatureCheck