Researchers are using a new method to model the distribution and abundance of golden eagles across the western U.S. The new estimates could help inform management of the species, according to Ryan Nielson, a senior biometrician at Western Ecosystems Technology Inc. (WEST) who collaborated with the U.S. Fish and Wildlife Service on the recent study.
The researchers used two common ecology techniques for estimating the quantity and density of animal populations – distance sampling and visual mark-recapture – to assess golden eagle abundance from aerial surveys conducted across approximately 2 million square kilometers of the western U.S. The authors then modeled counts of golden eagle observations based on land cover and other environmental factors.
Most studies focus on breeding pairs of golden eagles when researching the species’ distribution and density; however, Nielson’s study was broader. Golden eagles migrate across the western U.S. and southwestern Canada late fall and early spring. Many adult eagles choose to be permanent residents of a particular region, but young birds will often pass through territories while migrating or looking for a place to inhabit. Nielson and his team decided to include these juvenile ‘floaters,’ and not limit the study to breeding status or age.
Their findings, published in PLOS ONE, revealed that the eagles were more abundant in open, elevated areas with high wind speeds instead of developed and forested areas.
“Over many years of monitoring since 2006, we were surprised that elevation and wind potential had such a strong association with golden eagle distribution during late summer,” Nielson said. Higher, windier conditions may help the eagles hunt for prey and fly more efficiently as opposed to forested areas where it may be harder for the animals to forage.
Since the new model has never before been used for golden eagles in the U.S., the researchers can make only limited comparisons to previous studies. Nielson notes, “The main limitations to our study were that we were focused on one time period (mid-August through mid-September; post fledging), and we could only consider landscape-level predictor variables that were available for our large area.”
Although there are limits to what the study can reveal about land use and distribution of the golden eagle, Nielson and his colleagues stress that the model is intended for larger geographic regions and could help the U.S. Fish and Wildlife Service prioritize certain landscapes for conservation efforts.
“Our study could help identify where mitigation may be most helpful, and where additional research and monitoring of the species may be more valuable for future efforts,” Nielson said.
Research Article: Nielson RM, Murphy RK, Millsap BA, Howe WH, Gardner G (2016) Modeling Late-Summer Distribution of Golden Eagles (Aquila chrysaetos) in the Western United States. PLoS ONE 11(8): e0159271. doi:10.1371/journal.pone.0159271