Abstract
Exploring the causes of differential migration, or variation in migration distance, has increased our understanding of the remarkable variation in migratory behavior exhibited by birds more generally. However, considerable uncertainty exists regarding the mechanisms underlying differential migration distance in birds despite it being a common phenomenon. We leveraged migration distances from GPS-tracked rough-legged hawks Buteo lagopus to test predictions deduced from seven hypotheses proposed to explain the underlying cause(s) of differential migration distance. We provide the first empirical evidence in support of a social dominance food maximization hypothesis whereby dominant individuals migrate to higher quality nonbreeding locations with respect to foraging efficiency regardless of migration distance. Within females, larger more dominant individuals migrated intermediate distances while smaller, subordinate individuals migrated both longer and shorter distances. We also found support for the social dominance distance minimization hypothesis because more aggressive females migrated shorter distances, although increased aggression at shorter distances may be a consequence of poor body condition. Within males, we found some support for the fasting endurance and thermal tolerance hypotheses because body size was negatively correlated with migration distance. Body size was also negatively correlated with food availability and winter minimum temperatures within both sexes, providing additional mechanistic support for the fasting endurance and thermal tolerance hypotheses. Overall, our results suggest differential migration distance within rough-legged hawks is caused by a combination of competition for nonbreeding resources and constraints on fasting endurance or thermal tolerance, but is unrelated to competition for breeding opportunities, dietary preferences, or flight efficiency.