GPS telemetry reveals social structure and movement patterns of territorial and floater Haliaeetus leucocephalus (Bald Eagle) and Aquila chrysaetos (Golden Eagle)

Life-history theory posits that species evolve strategies to maximize lifetime fitness in response to environmental variability. For example, many species evolve complex social systems with adults that do not hold territories, a class of individuals labeled floaters. Populations of Haliaeetus leucocephalus (Bald Eagle) and Aquila chrysaetos (Golden Eagle) often have floaters whose behavior and demography are poorly understood. To differentiate between different behavioral states, we applied a random forest algorithm to GPS movement metrics generated from 43 adult H. leucocephalus and 94 adult A. chrysaetos, tracked over 164 and 274 breeding seasons, respectively. Using these movement metrics, we accurately predicted behavioral states within breeding seasons and individual eagles, providing an objective approach for state classification. For H. leucocephalus, we documented 101 (62%) territorial and 63 (38%) floater seasons. Within seasons when H. leucocephalus were territorial, 66% nested and 34% did not nest. For A. chrysaetos, we identified 178 (65%) territorial and 96 (35%) floater seasons. Within seasons when A. chrysaetos were territorial, 68% nested and 32% did not nest. Across individual eagles (as opposed to eagle seasons), 49% of monitored H. leucocephalus and 58% of A. chrysaetos were exclusively territorial, 37% and 33% were exclusively floaters, and 19% and 14% showed both behaviors. Sex was not a strong predictor, although more male A. chrysaetos were floaters than females, hinting at underlying demographic or behavioral differences between sexes. Importantly, our findings emphasize that a substantial and dynamic portion of the adult eagle population exists in the floater state. Understanding the behavioral states of long-lived raptors can contribute to accurate assessment of their population dynamics as well as conservation and management. Recognizing the prevalence of floaters in population assessments, for eagles and other long-lived species, likely will have critical implications for monitoring and conservation strategies.