Correlates of alternative migratory strategies in western bluebirds

Partial migration occurs when only some animals in a population migrate. While evidence suggests that migratory strategies are partially controlled by genes, individual and environmental conditions which alter the cost‐benefit trade‐off of migration among individuals are also likely to play a role. Three hypotheses have been advanced to explain condition‐dependent partial migration: the arrival time, dominance, and body size hypotheses. In this study, we asked whether these hypotheses explained differences in migratory strategy among individuals in a partially migratory population of western bluebirds (Sialia mexicana) breeding in southern British Columbia, Canada. We used stable hydrogen isotope signatures in claw tissue to determine migratory strategy of individual bluebirds, and examined patterns of migration at both individual and population levels. The proportion of resident bluebirds varied significantly over the three years of the study, and across study sites. Several migrants switched to the resident strategy between years; however, we found no evidence of strategy switching in the opposite direction. Young birds were significantly more likely to be resident than older birds, a pattern which could arise if early arrival is particularly important for birds obtaining a territory for the first time. Furthermore, young females were the most likely of all sex‐age classes to be resident, which may reflect a survival advantage of residency for young females. Finally, birds mated assortatively by migratory strategy and isotopic evidence suggests that members of a pair often wintered in the same place. Our results provided no support for the dominance or body size hypotheses, and only limited support for the arrival time hypothesis in bluebirds. However, taken together, we suggest that our findings indicate that social factors may influence migratory strategies in this system.