A Population Estimate for Golden Eagles in the Western United States

Abstract

Researchers have suggested golden eagle (Aquila chrysaetos) populations may be declining in portions of their range. However, there are few baseline data describing golden eagle populations across their range in the western United States. We used aerial line transect distance methodology with a double‐observer modification to estimate golden eagle population numbers in 4 bird conservation regions of the western United States. We conducted surveys from 16 August to 8 September 2003, after most golden eagles had fledged and before fall migration. The goal of our sampling strategy was to provide >80% power (α = 0.1) to detect an annual rate of total population change >3% per year over a 20‐year period. We observed 172 golden eagles across 148 transects and estimated 27,392 golden eagles (90% CI: 21,352‐35,140) occurred in the study area during the late summer and early fall of 2003. Following the surveys, we used Monte Carlo simulation to determine the statistical power to detect trends in the golden eagle populations if yearly surveys were continued over a 20‐year monitoring period. The simulation indicated the desired power could be achieved under the current methodology and sample size. The methods utilized in this study can be implemented for other raptor species when population estimates that include nonbreeding members of a population are needed. The results of this study can be utilized by professionals to help manage golden eagle populations and to develop conservation strategies.

Golden eagle population trends in the western United States: 1968–2010

“…Rich et al () estimated about 30,000 golden eagles occurred in parts of the United States and Canada sampled by BBS routes. Good et al () estimated 27,392 golden eagles (90% CI: 21,352–35,140) occurred in the WGES area in 2003. Nielson et al () updated the estimate of Good et al () for the WGES area for the years 2006–2010; annual estimates of total population size ranged from a low of 19,286 (90% CI: 15,802–23,349) in 2008 to a high of 24,933 (90% CI: 20,296–30,664) in 2007.…”

“…The WGES was initiated in 2003 as a pilot study, and was designed to estimate population size of golden eagles. Adjustments were made following the pilot study and the survey has been conducted annually using a consistent protocol and sample of survey transects since 2006 by WEST with funding from the Service (Good et al , Nielson et al ). This aerial transect‐based survey focuses on late summer, post‐breeding golden eagles in the Great Basin (BCR 9), Northern Rockies (BCR 10), Southern Rockies/Colorado Plateau (BCR 16), and Badlands and Prairies (BCR 17) BCRs, which collectively cover about 80% of the golden eagle's range in the conterminous western United States (U.S.…”

Computer-automated bird detection and counts in high-resolution aerial images: a review

“…Aerial surveys are particularly useful for rapid coverage of large areas, counting birds that are difficult to see from the ground, and accessing remote or otherwise challenging habitats such as roadless areas or wetland and marine environments (Kadlec and Drury 1968, Leonard and Fish 1974, Certain and Bretagnolle 2008, Kingsford and Porter 2009). Waterbirds, including waterfowl, seabirds, shorebirds, and wading birds, are the most frequent subjects of aerial surveys (Chabot and Bird 2015), although upland game birds (Rusk et al 2007, Butler et al 2008) and raptors (Good et al 2007, Henny et al 2008 have also been surveyed from the air.…”

An approach for using off-the-shelf object-based image analysis software to detect and count birds in large volumes of aerial imagery

“…They are commonly used to survey waterbirds (Kingsford and Porter 2009), as well as raptors (Good et al 2007) and upland game birds (Butler et al 2007). Bird identification and counts are performed either in real time by airborne observers or using on-board cameras to collect aerial imagery that is reviewed and analyzed later.…”