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ForewordAlthough the use of wind energy as an alternative electric generation source is now a viable choice, concern is still raised over the possible impacts of wind farms on birds. The concern includes two primary areas: the effect of avian mortality on bird populations, and possible litigation over the killing of even one bird if it is protected by the Migratory Bird Treaty Act, the Endangered Species Act, or both.The activities of the avian research program at the National Renewable Energy Laboratory (NREL) focuses on working towards minimizing the effects of wind turbines on birds and bird populations. NREL, funded by the U.S. Department of Energy, conducts research that (1) contributes to the refinement of the methods developed to assess impacts on the avian populations within wind farms, (2) seeks to understand how birds behave in and around wind turbines in different environments, (3) will try to identify how birds recognize wind turbines and develop recommendations for increasing the conspicuity of the turbines, and (4) will soon conduct acoustical research to develop acoustical deterrents where needed.The habitat surrounding the Altamont Pass Wind Resource Area (WRA) supports a substantial resident population of golden eagles (Aquila chrysaetos), perhaps one of the most dense ever recorded. Each year, the wind industry reports 28-43 golden eagles killed by turbine blade strikes in the Altamont WRA.NREL has supported the Santa Cruz Predatory Bird Research Group at the University of California, Santa Cruz, in conducting a golden eagle population study in the Altamont Pass WRA. The research, conducted over a four-year period, was designed to assess the impact of golden eagle deaths in the wind farm on the species' population survival and reproduction. This final report provides a detailed discussion of the data and research findings. Much has been learned during the first four years of this study. However, because the golden eagle is a long-lived species, the findings contained in this document are considered preliminary. Additional data collection over the next few years should provide a better understanding of how the golden eagle population is being impacted in the Altamont Pass WRA. EXECUTIVE SUMMARYThe Predatory Bird Research Group (PBRG), University of California, Santa Cruz, is conducting a long-term field study of the ecology of golden eagles (Aquila chrysaetos) in the vicinity of the Altamont Pass Wind Resource Area (WRA). The facility lies just east of San Francisco Bay in California and contains about 6,500 wind turbines on 190 km 2 of rolling grassland. Each year, the wind industry reports 28-43 turbine blade strike casualties of golden eagles in the WRA, and many more carcasses doubtless go unnoticed. Because golden eagles are naturally slow to mature and reproduce, their populations are sensitive to changes in adult and subadult survival rates. The U.S. Fish and Wildlife Service and the California Department of Fish and Game have therefore expressed concern that the fatalities might have an...

In the United States, the Bald and Golden Eagle Protection Act prohibits take of golden eagles (Aquila chrysaetos) unless authorized by permit, and stipulates that all permitted take must be sustainable. Golden eagles are unintentionally killed in conjunction with many lawful activities (e.g., electrocution on power poles, collision with wind turbines). Managers who issue permits for incidental take of golden eagles must determine allowable take levels and manage permitted take accordingly. To aid managers in making these decisions in the western United States, we used an integrated population model to obtain estimates of golden eagle vital rates and population size, and then used those estimates in a prescribed take level (PTL) model to estimate the allowable take level. Estimated mean annual survival rates for golden eagles ranged from 0.70 (95% credible interval = 0.66–0.74) for first‐year birds to 0.90 (0.88–0.91) for adults. Models suggested a high proportion of adult female golden eagles attempted to breed and breeding pairs fledged a mean of 0.53 (0.39–0.72) young annually. Population size in the coterminous western United States has averaged ~31,800 individuals for several decades, with λ = 1.0 (0.96–1.05). The PTL model estimated a median allowable take limit of ~2227 (708–4182) individuals annually given a management objective of maintaining a stable population. We estimate that take averaged 2572 out of 4373 (59%) deaths annually, based on a representative sample of transmitter‐tagged golden eagles. For the subset of golden eagles that were recovered and a cause of death determined, anthropogenic mortality accounted for an average of 74% of deaths after their first year; leading forms of take over all age classes were shooting (~670 per year), collisions (~611), electrocutions (~506), and poisoning (~427). Although observed take overlapped the credible interval of our allowable take estimate and the population overall has been stable, our findings indicate that additional take, unless mitigated for, may not be sustainable. Our analysis demonstrates the utility of the joint application of integrated population and prescribed take level models to management of incidental take of a protected species.

Raptors are exposed to a wide variety of human-related mortality agents, and yet population-level effects are rarely quantified. Doing so requires modeling vital rates in the context of species life-history, behavior, and population dynamics theory. In this paper, we explore the details of such an analysis by focusing on the demography of a resident, tree-nesting population of golden eagles (Aquila chrysaetos) in the vicinity of an extensive (142 km2) windfarm in California. During 1994–2000, we tracked the fates of >250 radio-marked individuals of four life-stages and conducted five annual surveys of territory occupancy and reproduction. Collisions with wind turbines accounted for 41% of 88 uncensored fatalities, most of which were subadults and nonbreeding adults (floaters). A consistent overall male preponderance in the population meant that females were the limiting sex in this territorial, monogamous species. Estimates of potential population growth rate and associated variance indicated a stable breeding population, but one for which any further decrease in vital rates would require immigrant floaters to fill territory vacancies. Occupancy surveys 5 and 13 years later (2005 and 2013) showed that the nesting population remained intact, and no upward trend was apparent in the proportion of subadult eagles as pair members, a condition that would have suggested a deficit of adult replacements. However, the number of golden eagle pairs required to support windfarm mortality was large. We estimated that the entire annual reproductive output of 216–255 breeding pairs would have been necessary to support published estimates of 55–65 turbine blade-strike fatalities per year. Although the vital rates forming the basis for these calculations may have changed since the data were collected, our approach should be useful for gaining a clearer understanding of how anthropogenic mortality affects the health of raptor populations, particularly those species with delayed maturity and naturally low reproductive rates.

Lead poisoning occurs worldwide in populations of predatory birds, but exposure rates and population impacts are known only from regional studies. We evaluated the lead exposure of 1210 bald and golden eagles from 38 US states across North America, including 620 live eagles. We detected unexpectedly high frequencies of lead poisoning of eagles, both chronic (46 to 47% of bald and golden eagles, as measured in bone) and acute (27 to 33% of bald eagles and 7 to 35% of golden eagles, as measured in liver, blood, and feathers). Frequency of lead poisoning was influenced by age and, for bald eagles, by region and season. Continent-wide demographic modeling suggests that poisoning at this level suppresses population growth rates for bald eagles by 3.8% (95% confidence interval: 2.5%, 5.4%) and for golden eagles by 0.8% (0.7%, 0.9%). Lead poisoning is an underappreciated but important constraint on continent-wide populations of these iconic protected species.