thumbnail

Golden Eagle Population Surveys in the Vicinity of the Altamont Pass Wind Resource Area, California, 2014–21

Open-File Report 2021-1107
Prepared in cooperation with the U.S. Fish and Wildlife Service and East Bay Regional Park District
By:  and 
https://doi.org/10.3133/ofr20211107

Links

Abstract

Golden Eagles (Aquila chrysaetos) are among the highest profile species killed by collisions with wind turbines at the Altamont Pass Wind Resource Area (APWRA) in the northern Diablo Range of west-central California. Understanding the distribution, site occupancy, and nesting status of eagles near the APWRA is needed to identify and minimize possible population-level impacts. We established a broad-scale survey design across a 5,185-km square-kilometer region of the Diablo Range, including the APWRA, to monitor site occupancy, abundance, and reproduction of Golden Eagles. During the study period we identified as many as 230 territorial pairs of Golden Eagles in the study area, up to 21 of which overlapped with the APWRA in any given year. On average, we detected a similar density of pairs at sites surveyed in the APWRA (1 pair per 17.8 km2 surveyed) relative to sites surveyed in the surrounding region (1 pair per 20.3 km2 surveyed). In 2020 and 2021, estimates of the proportion of pairs that successfully fledged at least one young (nesting success) were 0.27 and 0.26, respectively, which were above the seven-year average (0.22). On average, eagle pairs monitored in the APWRA had similar reproductive output (0.37 young fledged per pair) relative to pairs monitored outside of the APWRA (0.30 young fledged per pair). We observed a substantially higher proportion of territorial subadult pair members at the APWRA (mean = 29 percent of 16 pairs aged) relative to pairs monitored and aged in the surrounding region (mean = 3 percent of 122 pairs), indicating potentially higher rates of adult mortality or displacement at territories overlapping with the APWRA. Emergent threats to eagles in the study region, including a severe wildfire that impacted over 60 historical breeding territories in 2020, may interact with existing threats to affect population status and thus warrants further investigation. Our study provides wind energy developers, land managers, and regulatory agencies with key information on the spatial distribution, habitat quality, and population status of Golden Eagles needed to promote compatible wind energy production and long-term conservation of this federally protected wildlife species.

Suggested Citation

Wiens, J.D., and Kolar, P.S., 2021, Golden eagle population surveys in the vicinity of the Altamont Pass Wind Resource Area, California, 2014–21: U.S. Geological Survey Open-File Report 2021–1107, 18 p., https://doi.org/10.3133/ofr20211107.

ISSN: 2331-1258 (online)

Study Area

Table of Contents

  • Abstract
  • Background and Research Objectives
  • Methods
  • Results
  • Summary
  • Acknowledgments
  • References Cited
  • Appendixes 1–2
Publication type Report
Publication Subtype USGS Numbered Series
Title Golden eagle population surveys in the vicinity of the Altamont Pass Wind Resource Area, California, 2014–21
Series title Open-File Report
Series number 2021-1107
DOI 10.3133/ofr20211107
Year Published 2021
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Forest and Rangeland Ecosystem Science Center
Description iv, 17 p.
Country United States
State California
Other Geospatial Altamont Pass Wind Resource Area
Online Only (Y/N) Y
Google Analytic Metrics Metrics page
Additional publication details