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Golden eagles generally have large home ranges and can move great distances during dispersal and between breeding and non-breeding seasons. As a result, they can be exposed to numerous hazards such as electrocution, lead poisoning, and collisions with wind turbines across wide geographic areas. Understanding the relative magnitude of a hazard and its distribution in relation to eagle use of the landscape is an important precursor to effective conservation and management.
Risk can be defined as the relative threat to individual golden eagles, or populations, of having reduced survival or reproductive success caused by a specific hazard. Within a given landscape, risk can be estimated as the combination of hazard, exposure, and vulnerability, and risk analysis is a formal evaluation that takes into account two or more of these components.
Hazards include natural or anthropogenic objects, conditions, or events that could cause the death or significant reduction of fitness of individuals in a population of golden eagles.
Exposure is the degree of opportunity to encounter hazards, in terms of spatial and temporal overlap, and spatial exposure can be estimated by the relative density of golden eagles occurring in a particular area.
Vulnerability is a function of factors such as weather, season, age class, sex (morphology) and behavior that influence the likelihood and magnitude of effect at a given level of hazard and exposure. Because these factors are highly variable and difficult to quantify or predict, our risk analyses made the simplifying assumption that vulnerability was constant across the gradient of hazard and exposure.
Map of predicted electrocution risk for golden eagles occupying breeding habitat in the Northern Great Basin. By prioritizing retrofitting efforts in areas with higher risk (darker purples), utilities and other responsible entities can maximize their program's effectiveness and conservation benefit to golden eagles. View Full Screen.
Vulnerability is the likelihood and magnitude of effect to individual golden eagles or a population upon exposure. Vulnerability to a hazard may vary temporally according to multiple factors such as weather, age class and foraging behaviors, and is therefore difficult to quantify or predict. For example, large numbers of eagles may migrate through an area with dense electrical infrastructure (high hazard + high exposure) but if they rarely stop to perch on power poles vulnerability is low. Vulnerability may increase, however, if inclement weather causes the eagles to halt migration and seek shelter. For this reason, our risk analyses are focused on quantifiable measures of exposure and hazard.
To address variation in golden eagle exposure to risk throughout their annual cycle, the Western Golden Eagle Team (WGET) worked with collaborators to develop regional-scale, predictive models of golden eagle distribution and movements throughout the year. Spatial prioritization of relative risk across the landscape was determined by the overlap between golden eagle habitat suitability models and models or indices of hazards, including electrocution, wind energy development, oil and gas development, wildfire, and an index of lead hazard based on ungulate harvest rates. Our risk analyses can be used to inform conservation planning, targeted mitigation, land acquisition, energy development, etc. at regional scales. Relative risk maps, areal calculations, and a discussion on applying these products for spatial prioritization of management actions are provided in a risk analysis report and incorporated into WGET’s Ecoregional Conservation Strategies.
Electrocution risk is a function of hazard (power lines and equipment), exposure (opportunity to encounter), and vulnerability (possibility of encounter causing electrocution mortality). For example, large numbers of eagles can migrate through an area with large amounts of dangerous electrical infrastructure (high hazard + high exposure), but if they rarely stop to perch on poles, then vulnerability to electrocution is low. Retrofitting distribution power poles is the primary method of minimizing electrocution risk and is commonly used as a compensatory mitigation measure.
A utility crew works to retrofit a power pole. Credit: EDM International, Inc. View Full Screen.
However, the large number of power poles within golden eagle habitats and the expense of retrofitting limit the number of poles that can be retrofitted each year. It is therefore important to develop regional retrofitting plans that prioritize retrofitting efforts on poles that pose the highest risk to golden eagles and other raptors. Currently, retrofitting is planned primarily at the scale of individual utilities, which may not focus mitigation where conservation efforts are most needed, allowing high-risk poles to persist on the landscape.
The Western Golden Eagle Team (WGET) and partners have produced planning tools that 1) guide retrofit prioritization at the landscape scale by combining spatial models of power pole density (hazard) with predictive models of golden eagle relative density (exposure); and 2) support the rapid assessment of relative electrocution risk of individual power poles within a defined local area. These tools enable land managers and utility companies to identify areas of elevated electrocution risk, which will help prioritize their retrofitting investment, thereby increasing the effectiveness of their mitigation programs. Companies may also use the predictive models of golden eagle relative density to avoid high-risk areas and inform the routing of new distribution lines.
The number of wind energy facilities in the western United States continues to expand, and their placement often coincides with areas that provide habitat for golden eagles. Because of this overlap, collision with the blades of wind turbines is a substantial source of mortality for golden eagles, often affecting individuals from a broad area around wind energy facilities and potentially leading to population-level impacts to the species.
Hatch-year female golden eagle with a wind energy installation in the background. Credit: Mike Lockhart. View Full Screen.
In an effort to help make wind facilities compatible with eagle conservation and the laws and regulations that protect eagles, the Service developed an Eagle Conservation Plan Guidance (ECPG) module for land-based wind energy that provides a staged approach for siting, constructing and operating wind facilities.
To support regional-scale identification of potential wind facility locations with manageable risk to eagles (ECPG Stage 1), the Western Golden Eagle Team and collaborators conducted regional-scale spatial assessments of relative risk for wind energy development. These assessments were accomplished by combining models of golden eagle density with indices of wind development potential. The results of these analyses are summarized in a risk analysis report, including overlap among golden eagle seasonal habitat exposure and wind development hazard, areal calculations, and maps of relative risk for each assessment region.
The relative risk maps highlight broad landscapes with wind speeds suitable for development, including areas that pose relatively low risk to golden eagles and others where conflicts between golden eagles and wind energy development are more likely. This approach allows developers and land managers to more effectively screen potential sites for wind facilities before proceeding to the more time- and cost-intensive stages of Eagle Conservation Plan development.
Ingestion of lead bullet fragments from hunting and recreational shooting is a recognized source of lead poisoning for golden eagles that feed on carcasses and offal. Lead is a neurotoxicant, and lead toxicosis can present a wide range of physiological and neurological responses including mortality. Lead exposure can also increase the risk to golden eagles of accidental trauma and the likelihood of a fatal collision with vehicles or structures. Carcasses and gut-piles of hunter-killed game animals are a substantial and wide-spread source of lead for eagles in the western United States, as indicated by substantial increases in reported cases of lead toxicity in eagles during fall and early-winter hunting season. The temporal specificity of this exposure pathway offers an opportunity for targeted conservation actions to avoid and minimize lead exposure.
In order to identify and prioritize areas for conservation actions aimed at reducing lead exposure, such as gut-pile removal and non-lead ammunition distribution programs, the Western Golden Eagle Team (WGET) and collaborators compiled hunter harvest data for big game species from State management agencies. This information is summarized in a report and geospatial data set, and was used in combination with WGET’s predictive models of golden eagle distribution to develop regional-scale Risk Analyses for lead contamination. Information and literature review on lead exposure from recreational shooting of other species, such as ground squirrels, is provided in a series of prey accounts and publications. WGET has or is currently supporting a number of studies and subsequent publications on lead and other contaminants.
Greater sage-grouse (Centrocercus urophasianus) have been the focus of conservation attention across the western U.S., and landscape-scale conservation actions taken to protect greater sage-grouse are thought to provide an “umbrella” of conservation for many other species. Golden eagles inhabit much of the sagebrush biome where greater sage-grouse occur, and there may be substantial spatial overlap between the habitats of the two species. Because both species face similar hazards in parts of their range–including habitat loss due to anthropogenic disturbance, invasive annual grasses, and altered fire regimes–conservation actions undertaken for greater sage-grouse are likely to benefit golden eagles.
To support development of habitat and prey enhancement for mitigation, conservation banking, and conservation planning, the Western Golden Eagle Team (WGET) and its partners developed a spatially-explicit risk analysis framework for calculating risk to golden eagles from habitat-mediated hazards, as well as quantifying the potential reduction in risk that eagles face in greater sage-grouse conservation areas. This framework has been adapted to quantify risk for a variety of landscape-scale hazards and draw comparisons among areas of conservation or development interest (see Risk Analysis Report and Spatial Data at top of page). Furthermore, these products can be used to target management actions using information on prey-habitat associations, prey response to habitat modification, and spatial distribution of habitat-mediated risk to golden eagles.