Mechanisms matching timing to resources: comparisons of closely related seasonally sympatric, migratory and non‐migratory populations

As day length increases in spring, birds prepare to migrate and breed, relying on timing mechanisms shaped by selection to match their behavior and physiology to ecological conditions suitable for reproduction. As the climate changes these mechanisms will determine how successful birds will be in keeping up. In this contribution, we review studies comparing photoperiodic thresholds, endocrine profiles of testosterone and corticosterone, and gene expression during pre‐breeding in seasonally sympatric migratory and resident populations of a songbird, the dark‐eyed junco Junco hyemalis . Elevation of testosterone in response to GnRH served as a proxy for gonadal development, visible fat served as a proxy for migratory state, and stable isotopes in feathers and claws served as a proxy for breeding and non‐breeding latitudes. Living in the same pre‐breeding environment, migrants prepared to migrate by fattening and delaying gonadal development, while residents initiated gonadal development while not fattening. Within migrants, estimated latitude of origin co‐varied positively with fattening and negatively with gonadal development. Together these mechanisms likely serve to match timing of migration and reproduction to the future appearance of favorable environments where breeding will occur. Differences observed in the wild persisted in a common environment, suggesting genetic divergence and local adaptation, though the possibility of early developmental effects on timing remain. As the climate warms and resources to support reproduction appear earlier, locally adapted dispersing immigrants from lower latitudes may bring along their earlier timing thus providing genetic or developmental rescue. Future research on mechanisms responsible for variation in timing among populations will allow better predictions of how adaptation to climate change will unfold. Keywords: common garden, dark‐eyed junco, gonad, hormone, local adaptation, migration, phenology, photoperiodic threshold, stable isotope