Date Published: November 21, 2018
Publisher: Public Library of Science
Author(s): Bryan E. Bedrosian, Robert Domenech, Adam Shreading, Matthew M. Hayes, Travis L. Booms, Christopher R. Barger, Mark S. Boyce.
There has been increasing concern for Golden Eagle (Aquila chrysaetos) populations in North America due to current and future projections of mortality risk and habitat loss from anthropogenic sources. Identification of high-use movement corridors and bottlenecks for the migratory portion of the eagle population in western North America is an important first step to help habitat conservation and management efforts to reduce the risk of eagle mortality. We used dynamic Brownian Bridge movement models to estimate utilization distributions of adult eagles migrating across the western North America and identified high-use areas by calculating the overlap of individuals on population and regional levels. On a population level, the Rocky Mountain Front from east-central British Columbia to central Montana and southwestern Yukon encompassed the most used migration corridors with our study extent for both spring and fall. Regional analysis on a 100 x 200 km scale revealed additional moderate and high-level use corridors in the central British Columbia plateaus. Eagles were more dispersed in the spring until their routes converged in southern Alberta. High-use fall corridors extended farther south into central Wyoming. Knowledge of these high-use areas can aid in conservation and site planning to help maintain and enhance migratory Golden Eagle populations in western North America.
Conservation and management of raptors requires knowledge of ecology and demographics within the breeding, wintering, and migratory periods across life stages [1, 2]. For long-lived raptors occupying large landscapes, such as the Golden Eagle (Aquila chrysaetos), a thorough understanding of these parameters across all life stages can be extremely difficult to achieve. An increasing amount of attention has been paid to the management of Golden Eagles in North America due to apparent population declines [2–5] and because of the juxtaposition of development and protections afforded by the Migratory Bird Treaty Act (United States and Canada), the Bald and Golden Eagle Protection Act (United States) and the Species at Risk Act (Canada). Though recent data suggest that populations in the western United States are currently stable [6, 7], Golden Eagles still are considered at risk due to low reproductive potential in combination with habitat loss and increasing risks of direct fatality [8–12].
Adult Golden Eagles were captured within six study areas as part of different but concurrent studies. We captured 16 over-wintering eagles within the “MPG Ranch study area,” which occurs within the Bitterroot Valley near Florence, MT (Fig 1). We tagged 16 actively migrating eagles at the “Nora Ridge” study site during September and October on the Continental Divide of the Rocky Mountains near Lincoln, Montana. Twenty-six eagles were tagged in the “Alaska” study area while they were on northward (i.e., spring) migration through southcentral Alaska. Finally, we captured five overwintering eagles across the Great Plains in Montana (“Eastern MT” study area). Additional data from two eagles were provided by the USFWS from the “4-Corners study” being conducted in the southwest US, and data from one eagle tagged in southeastern Wyoming were provided by the “FWS-Region 6 study.” Eagles from MPG Ranch, Eastern MT, Alaska, and 4-Corners were captured using net launchers (Trapping Innovations, LLC, Jackson, WY or Coda Enterprises, Mesa, AZ) baited with carrion. Eagles from the Nora Ridge study were captured using bow-nets with Rock Doves (Columbia livia) as bait. One eagle from the 4-Corners study was struck by a vehicle, rehabilitated, and released with a transmitter, and the eagle for the FWS-Region 6 study was captured using a carrion baited leg-hold trap. Methodologies used in this study were approved by Montana’s and Alaska’s Animal Care and Use Committees and conform to the Guidelines to the Use of Wild Birds in Research .
We identified Golden Eagle population level migration corridors for the spring and fall using the summation of all individual UDs  in both spring and fall (Fig 2). Using the population level utilization distribution summation, a maximum of 41% of our sample’s utilization distributions overlapped during the in the spring (n = 18) and 43% in the fall (n = 17). During the spring, eagles were more dispersed within the conterminous United States until they entered Canada where utilization distributions converged. Spring routes were slightly east on the Rocky Mountain Front and west on the interior route as compared to the fall. The clearest population level migration corridor in the fall was concentrated in southern Alaska through southwest Yukon then dispersed until north-central British Columbia where it became concentrated again along the Rocky Mountain Front from northern British Columbia into central Montana. Some individuals migrated farther west in central British Columbia in the Fraser and Thompson Plateaus (Level III Ecoregions ). The importance of the Rocky Mountain Front was similar in the spring, but routes were more concentrated within the western path.
Our analysis takes an initial step toward identifying high-use spring and fall migration corridors of adult Golden Eagles in western NA. Identification of specific high-use areas and migration bottlenecks can help inform conservation strategies, initial development site planning, and provide a base for future studies. Use of these data to help inform siting of new developments, such as energy extraction, outside identified high-use migration areas may reduce eagle take and compensatory mitigation needs from these areas. Prioritizing conservation efforts within high-use areas may help direct limited resources and maximize gains.
Knowledge of these key migration routes has obvious conservation implications in light of increased industrial wind development across western North America and the accompanying potential hazards for Golden Eagles. Some of the best predicted wind resources and commercial wind development in the United States lie at the ecotone of the Rocky Mountains and the Great Plains , and there are clear conflicts with wind production and Golden Eagle survival (e.g., [9, 11, 12]). Similarly, southern Wyoming hosts some of the best wind potential in the United States , as well as the highest development potential areas , which may intersect high-use Golden Eagle migration corridors.