A Survey of Golden Eagles (Aquila chrysaetos) in the Western US: 2006-2015

Western EcoSystems Technology, Inc. (Laramie WY); Nielson, Ryan; DiDonato, Guy; Mcdonald, Lyman

A report for the U.S. Fish & Wildlife Service that provides region-wide transect survey data for Golden Eagles in the Western USA during 2015 and discusses trends over the period 2006-2015. We flew aerial line transect surveys using distance sampling and mark-recapture procedures to estimate Golden Eagle (Aquila chrysaetos) abundance in 4 Bird Conservation Regions (BCRs) in the western United States between 15 August and 15 September, 2015. The study area consisted of BCRs 9 (Great Basin), 10 (Northern Rockies), 16 (Southern Rockies / Colorado Plateau), and 17 (Badlands and Prairies). In 2015, we flew 220 transects totaling 17,586 km. We observed 144 Golden Eagle groups within 1,000 m of 72 transects for a total of 179 individuals: 31 juveniles, 28 sub-adults, 50 adults, 4 unknown immatures (not adults), and 66 unknown adults (not juveniles). We estimated that a total of 6,431 Golden Eagles (90% confidence interval: 4,206 to 9,351) were within the Great Basin (9) during the survey period, a total of 4,884 (90% confidence interval: 2,901 to 7,512) were within the Northern Rockies (10), a total of 2,535 (90% confidence interval: 1,224 to 3,242) were within the Southern Rockies / Colorado Plateau (16), and a total of 6,877 (90% confidence interval: 4,384 to 9,964) were within the Badlands and Prairies (17). These estimates do not include Golden Eagles that were occupying military lands, elevations above 3,048 m (10,000 ft), large water bodies, or large urban areas. We used a Bayesian hierarchical model to estimate trends in individual BCRs and the entire study area. We did not find evidence of trends in total eagle numbers in most individual BCRs or the entire study area during 2006–2015, but we did detect a statistically significant positive trend in the Great Basin (9) over the 10-year period. The total number of Golden Eagles classified as juveniles during the survey in individual BCRs and the entire study area did not show evidence of trends during 2006-2015.