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Golden Eagles in a Changing Landscape

Golden EagleThe slopes and valleys of the Diablo Mountains near San Francisco Bay support the highest known density of golden eagle nesting territories in the world. In one 820 square kilometer section of oak savanna near Livermore, there are at least 44 occupied breeding territories, a pair every 19 square kilometers. The open grassland habitats, upslope winds funneling from the Bay to the Central Valley, the scattering of oaks suitable for nesting, and an abundance of California ground squirrels create ideal conditions for this highly-regarded predator.

Adjacent to habitat occupied by these breeding eagles in Alameda, Contra Costa, and Santa Clara counties is a rapidly growing metropolitan complex. During the 1980s, the population of these three counties increased by over 17 percent, and by 2015, is expected to approach 4.5 million.

Beginning in 1994, Dr. Grainger Hunt  monitored the activities of more than 200 radio-tagged eagles representing a variety of age classes. This study, one of the first and largest of its kind, uncovered fascinating details of the demography and natural history of golden eagles. Hunt flew weekly surveys to detect fatalities, each of which was recovered and the cause of death determined [see Tracking].

The project began in response to reports that at least 40 eagles per year were colliding with the wind turbines at nearby Altamont Pass where a large-scale wind energy generation plant has been in operation since the mid-1980s. The telemetry study showed that turbine blade strikes are the most frequent cause of eagle death, followed by electrocutions on utility lines. These two factors account for more than one-half of the recorded deaths, and, overall, at least three-quarters of all eagle fatalities detected during the study were human-related. Data on mortality rates among the eagle life stages, together with estimates of the annual reproductive rate, provided the basis for estimating the overall trend of the population. Funding for the project was provided by the National Renewable Energy Laboratory, the California Energy Commission, and the wind industry.

Golden eagle nestlingHunt also focused field studies on determining the factors influencing nest occurrence and success. The work called for an understanding of current and projected effects of human activities on the eagles and on determining ways to conserve their nesting and foraging habitat. His primary assumption was that society prefers to accommodate the golden eagle near peopled landscapes. Because of its vast range throughout the northern hemisphere, the elements of its conservation may provide a standard for open-spaces management.