During 1994, the Predatory Bird Research Group, University of California, Santa Cruz, conducted a preliminary field investigation of the ecology of golden eagles (Aquila chrymem) in the vicinity of the Altamont Pass Wind Resource Area (WRA). The facility contains about 6,500 wind turbines on 189 km2 just east of San Francisco Bay in California. Grassland and oak savanna habitats surrounding the WRA support a substantial 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, with over 90 percent attributed to collisions with wind turbines. The purpose of this pilot study was to clarify and set in motion a research program that will identify (1) the effect of turbine-related mortality on the golden eagle population, (2) factors attracting eagles to the WRA, and (3) conditions that increase the risk of turbine strikes.
To show the distribution of the population potentially impacted and to obtain survival data, we radio-tagged 31 adult and subadult eagles in the WRA vicinity during January and February 1994 and an additional 25 juveniles from nests surrounding the WRA in spring. Each transmitter contained a mortality sensor and was expected to last five years. Throughout 1994, we determined the location and status of each eagle in twice-weekly airplane roll-call surveys.
Of the 31 winter-tagged eagles, 5 were members of pairs breeding near the WRA. Of the remaining 26 birds, 19 were subadults and seven were non-breeding adults (floaters). Over the course of the year, four (15%) of the 26 non-breeders either departed the region or their radios failed. The remainder traveled within the northern Diablo Range which extends from the Qakland Hills southeast to San Luis Reservoir, an area of about 3,500 km2, for which we developed a digitized (GIS) vegetation map from satellite photographs. Our findings suggest that at least three-quarters of the golden eagles frequenting the WRA vicinity in winter are year-round residents of the Diablo Range.
Three (11%) of the 26 itinerant subadults and floaters died during 1994: one from lead poisoning and two from collisions with wind turbines. One of the five breeders was killed in its territory by another eagle. We recorded no fatalities among the tagged sample of 25 juveniles, but we censored two transmitters early on; one detached from the eagle and the other malfunctioned, leaving a sample of 23 tagged juveniles. Of these, two left the study area and another may have had a failed transmitter. The remaining 20 birds were alive in or near the study region in December 1994, most within 30 km of the WRA.
Determining the population significance of turbine-related mortality will require an increased sample of radio-tagged eagles, several more years of monitoring survival, and knowledge of the reproductive rate. To the latter end, we conducted a nest search in the area within 30 km of the WRA boundary. In all, we observed pairs at 54 locations, but some were on private property and could not be closely observed. We were able to locate and record the activities of eagles at 37 nesting territories. Eagles laid eggs at 32 (86%) of these. Three pairs failed in the egg stage, and the remaining 29 pairs fledged 47 young. Rates of reproductive success, brood size, and productivity per occupied nest compared favorably with those of ten populations of golden eagles studied elsewhere in North America and Europe. In an 820 km2 section of oak savanna where we were given access to conduct a detailed survey, we calculated a density of one pair per 22 km2, a value among the highest recorded for the species.
We identified 339 prey items from collections made at golden eagle nests in the study area. The only prey species common to every pair was the California ground squirrel (Spermuphilus beecheyii). It represented 69 percent of prey numbers and 64 percent of prey biomass identified from remains. The second most important species was the black-tailed jackrabbit (Lepus californicus) at eight percent biomass; the third was the black-tailed deer (Odocoileus hemionus) at six percent. In all, mammals accounted for 92 percent of prey biomass, followed by seven percent birds, and one percent reptiles. These proportions reflect very closely the food habits of breeding eagles studied in other parts of North America.
To investigate the occurrence and behavior of golden eagles within the WRA we conducted weekly ground-based surveys from late May through November during which we recorded all sightings and activities of eagles. Routine examination of 4,543 turbine towers of 21 types yielded 249 sightings of golden eagles of which 155 were flying and 94 were perched; 23 of these were on turbine towers, all of the lattice-type. The ground-based data suggest that an average of 14-17 eagles occupied the WRA during the months of survey, while the aerial telemetry surveys point to a considerable flux in individual tenure. Both tagged and untagged eagles were most commonly recorded in the northwestern and southern extremes of the WRA. A comparable ground survey conducted on an adjacent area of grassland without turbines revealed a higher average density of eagles.