Abstract
This report details the results of a survey in spring 2005 of golden eagle (Aquila chrysaetos) breeding territory occupancy in the vicinity of the Altamont Pass Wind Resource Area (WRA) where numerous eagles from a dense local population are killed each year by wind turbine blade strikes. A demographic investigation conducted during 1994-2000, and reported to the California Energy Commission in 2002, indicated that the blade-strike mortality prevented the maintenance of substantial reserves of nonbreeding adults characteristic of healthy populations elsewhere, suggesting the possibility of an eventual decline in the breeding population. The unanswered question of population trend prompted the present study, which was designed to detect any change in the local breeding population since the last survey. Within a sample of 58 territories, results showed that all territories occupied by eagle pairs in 2000 were occupied by pairs in 2005. No upward trend was apparent in the proportion of subadult eagles as pair members, a condition that would have suggested an insufficiency of non-breeding adults to replace annual deaths among breeders. However, the number of eagle pairs required to support estimated levels of blade-strike mortality is large. The authors estimate, for example, that to maintain a stable population, the young of 167 breeding pairs are necessary to support a blade-strike mortality of 50 eagles per year. Such mortality is likely additive with other lethal agents in influencing population health. The most effective way to minimize an eventual decline of the breeding population associated with the rapid expansion of human development in the region is to mitigate sources of current mortality and to preserve foraging areas for nonbreeders.