For birds, a migratory lifestyle confers several benefits including avoidance of harsh winters and increased access to food resources during the breeding season. However, migration is energetically costly and elevates oxidative stress, which may contribute to increased mortality during migration. Oxidative stress is known to shorten telomeres, which are protective DNA regions on the ends of chromosomes. Thus, one consequence of migration may be accelerated telomere attrition. A migratory lifestyle may also increase telomere shortening via reduced investment into self-maintenance, as high mortality during migration may cause migrants to invest more in reproduction than residents. We therefore hypothesized that greater telomere attrition may reflect a long-term cost of a migratory life history strategy. We predicted that, among individuals of the same age, migrants would have shorter telomeres as compared to residents. We compared first-year individuals in an overwintering population of Slate-colored Dark-eyed Juncos (Junco hyemalis) that included both a migratory (J. h. hyemalis) and resident (J. h. carolinensis) subspecies in western Virginia. As predicted, first-year migrants had shorter telomeres than first-year residents. Since members of both subspecies experienced the same winter conditions and had not yet bred, differences in telomere lengths are likely due to migration-related costs as this was the only energetically expensive life history stage that differed between subspecies. Telomere length differences between subspecies could also be due to differences in initial telomere lengths or loss during growth, which could reflect relative investment into self-maintenance. These results are consistent with the idea that accelerated telomere shortening is a potential cost of a migratory life history strategy. This consequence could be the result of direct metabolic costs of migration and/or a life history strategy that places less emphasis on self-maintenance.
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