The Trend of Golden Eagle Territory Occupancy in the Vicinity of the Altamont Pass Wind Resource Area: 2005 Survey

Publication Number: CEC-500-2006-056
Publication Date: September 2006
PIER Program Area: Energy-Related Environmental Research

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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.

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Executive Summary

Introduction

The area surrounding the Altamont Pass Wind Resource Area (WRA) near Livermore, California contains a higher reported density of nesting golden eagles than anywhere else in the world. The Predatory Bird Research Group (PBRG) has been studying aspects of golden eagle ecology in the region since 1994 to determine if wind turbine blade strikes in the WRA are causing the population to decline (Figures 1, 2). Estimates of the number of eagles annually killed by turbines in the WRA range from 40 to more than 100. Because golden eagles mature and reproduce slowly, and their populations are particularly sensitive to changes in adult and subadult survival rates, state and federal wildlife agencies have been concerned for the welfare of the population.

Purpose

The PBRG's seven-year study (1994-2000) was based on aerial surveys of survival among a large sample of radio-tagged eagles and annual reproduction of about 60 pairs in the vicinity of the WRA. The telemetry study suggests that most of the eagles killed in the WRA derive locally from the Diablo Mountains. Demographic analysis of the survival and reproduction data, reported by PBRG to the California Energy Commission in 2002 in Golden Eagles in a Perilous Landscape: Predicting the Effects of Mitigation for Energy-related Mortality, produced a point estimate indicating no change in population size from year to year, but the variance of the estimate fell more or less equally into scenarios of increase and decrease. The implication of the point estimate was that the studied population was failing to maintain a healthy contingent of non-breeding adults (floaters) that would normally buffer the breeding sector by filling vacancies within breeding territories. The question therefore remained of the fate of the population, and the PBRG recommended that golden eagle breeding territories within 30 km (19 miles) of the WRA be resurveyed every few years.

Project Objectives

The present report, which the reader should regard as an addendum to the 2002 report, details the results of a survey in spring 2005 of 58 golden eagle breeding territories within 30 km (19 miles) of the WRA boundary, each of which was occupied by a pair of eagles in 2000. The work was designed to test the hypothesis that sufficient numbers of floaters exist within the area to fill all breeding vacancies as they occur. The PBRG further noted the age-class of each territory-holder because an increase in the proportion of young eagles (subadults) would suggest an unhealthy scarcity of floaters in the population.

Project Outcomes

The results showed that within the surveyed sample of territories, all 58 that were occupied by pairs in 2000 were occupied by pairs in 2005, and there was no trend of increase of subadult pair members that might suggest a decline in the floater buffer.

Conclusions

These findings indicate that the breeding population of golden eagles within the vicinity of Altamont Pass remains intact. This does not mean, however, that the impact of WRA fatalities is benign. The WRA kills more eagles than can be produced by the studied sample of 58 pairs—that sample necessarily being part of a larger population that has provided an influx of recruits to fill nesting territory vacancies. The authors estimate, for example, that 167 breeding pairs are required to sustain 50 blade-strike fatalities per year. Moreover, golden eagle survival can be expected to decrease with the projected expansion of the human population in the region, i.e., human-induced mortality is likely additive in its effect on the eagle population.

Recommendations

Current knowledge suggests that the most effective way to minimize an eventual decline is to mitigate the primary agents of human-related mortality and to preserve open grasslands for the benefit of nonbreeders.

Benefits to California

This report increases public knowledge and understanding of the conservation implications of golden eagle mortality at the Altamont Pass Wind Resource Area.

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Table of Contents

Preface
Abstract
Executive Summary
1. Introduction
2. Methods 5
3. Results
4. Conclusion
5. References


List of Figures

Figure 1. Two golden eagles at the Altamont Pass Wind Resource Area
Figure 2. Golden eagle flying near turbine blades at Altamont Pass
Figure 3. Golden eagle habitat in the Diablo Mountains east of Dublin, California
Figure 4. Golden eagle territories within 30 km (19 miles) of the Altamont Pass Wind Resource Area (WRA)


List of Tables

Table 1. Ages of breeding golden eagles at territories within 30 km (19 miles) of the Altamont Pass Wind Resource Area during 1996–2005