The Predatory Bird Research Group, University of California, Santa Cruz, has been conducting a long-term study of golden eagles (Aquila chrysaetos) in the Diablo Mountains of west-central California. The initial work (1994-1997), funded by the wind industry and by the National Renewable Energy Laboratory (NREL), used aerial tracking of radio-tagged eagles to address the question of whether eagle deaths resulting from wind turbine blade strikes at the Altamont Pass Wind Resource Area (WRA) were seriously affecting the population. Estimates are that wind turbines kill 40-60 subadult and adult golden eagles each year, on average. Golden eagles, being naturally slow to reproduce, are particularly sensitive to changes in adult and subadult survival rates. For this reason, and because of its popularity, the species is afforded special protection by both federal and state governments. There is no legal provision for the killing of golden eagles.
Wind turbine blades also kill other protected species in the WRA, including several hundred red-tailed hawks (Buteo jamaicensis) and American kestrels (Falco sparvarius) each year. The fatalities have caused adverse public perception of wind power plants, and the threat of fines and lawsuits has delayed, modified, or even stopped wind energy development in some states, including California. Alameda County, for example, has imposed a moratorium on increase over current electrical production (~580 MW) until progress is made toward resolving the birdstrike issue. To address the problem, research must determine whether the fatalities threaten the birds on a population basis, what kinds of turbine/tower configurations are most destructive, and what management actions could reduce the number of fatalities.
We began the current investigation in June of 1998 under the support of the California Energy Commission’s Public Interest Energy Research (PIER) Program. At that time, extensive repowering appeared imminent in the WRA. Of particular interest was the intended replacement of some 1300 turbines with a larger and possibly more benign type, at an approximate ratio of seven removed for every one replaced. Our objectives were to increase the samples of radio-tagged eagles and to continue monitoring them for the purpose of (1) further understanding the demographics, (2) tracking the net result of repowering, and (3) exploring other measures that might effectively reduce the incidence of golden eagle mortality. As time passed, it became apparent that difficulties within the wind industry would delay the repowering process beyond the scope of the study. We therefore focused upon eagle deaths relative to existing turbine configurations in an attempt to identify the factors contributing most to blade-strike mortality. This approach, with its emphasis on radio-telemetry, a technique with virtually no distributional bias, offered a measure of prediction regarding the efficacy of expected changes in the WRA.