Applying citizen-science data and mark-recapture models to estimate numbers of migrant Golden Eagles in an Important Bird Area in eastern North America

Estimates of population abundance are important to wildlife management and conservation. However, it can be difficult to characterize the numbers of broadly distributed, low-density, and elusive bird species. Although Golden Eagles (Aquila chrysaetos) are rare, difficult to detect, and broadly distributed, they are concentrated during their autumn migration at monitoring sites in eastern North America. We used hawk-count data collected by citizen scientists in a virtual mark-recapture modeling analysis to estimate the numbers of Golden Eagles that migrate in autumn along Kittatinny Ridge, an Important Bird Area in Pennsylvania, USA. In order to evaluate the sensitivity of our abundance estimates to variation in eagle capture histories, we applied candidate models to 8 different sets of capture histories, constructed with or without age-class information and using known mean flight speeds +/- 1, 2, 4, or 6 SE for eagles to travel between hawk-count sites. Although some abundance estimates were produced by models that poorly fitted the data ((c) over cap > 3.0), 2 sets of population estimates were produced by acceptably performing models ((c) over cap <= 3.0). Application of these models to count data from November, 2002-2011, suggested a mean population abundance of 1,354 +/- 117 SE (range: 873-1,938). We found that Golden Eagles left the ridgeline at different rates and in different places along the route, and that typically < 50% of individuals were detected at the hawk-count sites. Our study demonstrates a useful technique for estimating population abundance that may be applicable to other migrant species that are repeatedly detected at multiple monitoring sites along a topographic diversion or leading line.