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.
The costs and benefits associated with migration differ depending on individual decisions regarding the distance to migrate and where to winter. These decisions affect an individual’s winter experience, which may in turn impact reproductive success the following breeding season. We investigated whether winter location influences performance on the breeding grounds in a short-distance migrant, the Ipswich Sparrow ( Passerculus sandwichensis princeps Maynard, 1872), endemic to Sable Island, Nova Scotia, Canada. We used stable hydrogen isotope analysis to infer the winter location of individuals breeding on Sable Island, and investigated whether body condition, timing of breeding initiation, and number and condition of nestlings varied with winter location. Males wintered farther north than females, and males from the northern end of the wintering range were in better condition, established territories earlier, and tended to produce heavier nestlings than those wintering farther south. Conversely, females that wintered farther south had significantly earlier first egg dates than those that wintered farther north. The results of this study suggest that individual decisions about migration distance can impact reproductive success and that optimal winter locations differ between male and female Ipswich Sparrows.
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