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 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 free-living and captive migratory and resident populations of a songbird, the dark-eyed junco, as they diverge in behavior and physiology during pre-breeding (spring) despite co-occurrence in the same environment. We report on differences in mechanisms related to timing including photoperiodic thresholds, endocrine profiles of testosterone and corticosterone, and gene expression. We also provide evidence for genetic differences in timing among populations based on sustained differences in timing when populations were held in a common environment. We conclude that existing variation in pre-breeding physiology likely serves to match timing of migration and reproduction to the appearance of favorable environments where breeding occurs. Greater knowledge of mechanisms responsible for variation in timing will allow better predictions of future adaptation.