Characterizing Habitat Associations and the Effects of Competition on Bald Eagle and Osprey Recovery in the Conterminous United States and Wisconsin

Abstract

As species recover from bottlenecks, during which population size is very low, a swath of new habitat becomes available as the population expands. With increased niche availability, expanding populations may alter their habitat associations leading to the question are habitat associations at population lows good predictors of habitat associations at population highs. I examined occurrences and habitat associations of Bald Eagles (Haliaeetus leucocephalus) and Osprey (Pandion haliaetus) from their near extinction in the conterminous U.S. in 1980 to successful recovery by 2017 to (1) determine if occurrences and colonizations were predictable throughout the recovery (2) examine if their relationships to habitat associations throughout recovery, (3) determine if habitat associations at newly colonized sites were similar to habitat associations at the population low points in the 1980s. I analyzed occurrences of both species from 1980-2017 on 4202 North American Breeding Bird Survey routes. I also calculated the minimum distance to conspecifics in the previous decade. I parameterized logistic regressions for each species to characterize habitat association and evaluated them using the True Skill Statistic. I also attempted to use these models to predict colonizations in each decade and evaluated performance based on Area Under the Curve. Both species expanded their 1 2 distribution in the US considerably since the 1980s. My models of Bald Eagles and Osprey habitat associations for each decade had minimum TSS of 0.3. Bald Eagles altered their habitat associations and by 2017 were occupying areas with less water and more grassland, while Osprey habitat associations remained unchanged. In 2010-2017, Bald Eagles colonized routes with less water (-11%) and more shrubland (+9%) than the routes they occupied in 1980. In the same period, Osprey colonized routes with more agriculture (+4%) than routes they occupied in 1980s. The colonization AUC decreased over four decades from 0.81 to 0.67 for Bald Eagles, and from 0.81 to 0.73 for Osprey. I was able to characterize occurrences for both species, but unable to reliably predict where colonizations would occur. My work highlights how the process of population recovery varies between species with similar ecological niches and showcases the ability of rare species to expand into habitat that is unexpected based on habitat associations apparent during a population low.