Since January 1994, the Predatory Bird Research Group, University of California, Santa Cruz, has been conducting a field investigation of the ecology of golden eagles (Aquila chrysaetos) in the vicinity of the Altamont Pass Wind Resource Area (WRA). The 190 h2 facility lies just east of San Francisco Bay in California and contains about 6,500 wind turbines. Grassland and oak savanna habitats surrounding the WRA support a substantial resident population of golden eagles. Each year, the U.S. Fish and Wildlife Service receives reports from the wind industry of about 30 golden eagle casualties occurring at the WRA, and it is probable that many more carcasses go unnoticed (see Orloff and Flannery 1992). Over 90 percent of the casualties are attributed to collisions with wind turbines. The main purpose of this study is to estimate the effect of turbine-related mortality on the golden eagle population of the area.
Assessing the impact of the WRA kills on the population requires quantification of both survival and reproduction. To estimate survival rates of both territorial and non-territorial golden eagles, we tagged 179 individuals with radio-telemetry transmitters expected to function for about four years and equipped with mortality sensors. Population segments represented in the tagged sample include 79 juveniles, 45 subadults, 17 floaters (non-territorial adults), and 38 breeders. Effective sample sizes in the older segments increase as younger eagles mature or become territorial.
Since the beginning of the study, we have conducted weekly roll-call surveys by airplane to locate the tagged eagles in relation to the WRA and to monitor their survival. The surveyed area extends from the Oakland Hills southeast through the Diablo Mountain Range to San Luis Reservoir about 75 h southeast of the WRA. The surveys show that breeding eagles rarely enter the WRA while the non-territorial eagles tend to move about freely throughout the study area and often visit the WRA.
Through 12 August 1996, we have detected 26 fatalities among the radio-tagged sample, 7 of which involved fledglings around the time of first flights from the nest. Of the 19 fatalities of free-ranging eagles, 9 (47%) were struck by turbine blades, 4 were electrocuted (or struck electric wires), 1 was shot, 2 were victims of lead poisoning, 1 was probably poisoned, 1 was killed by another eagle, and 1 died of unknown causes.
In 1996 we observed 74 territorial pairs of eagles in the study area and were able to determine the reproductive outcome of 57 of these pairings from the incubation period onwards. The 57 pairs produced 37 young to fledging age, for an average of 0.65 young per territorial pair (S.E. = 0.10). In our final report to NREL due in summer 1997, we will incorporate this estimate and those of survival to predict the effect of WRA mortality on population age structure.