Migratory strategy explains differences in timing of female reproductive development in seasonally sympatric songbirds

1. Divergent migratory strategies among populations can result in population-level differences in timing of reproduction (allochrony) and local adaptation. However, the mechanisms underlying among-population variation in timing are insufficiently understood, particularly in females. 2. We studied differences in reproductive development and its related mechanisms along the hypothalamic-pituitary-gonadal axis (HPG) in closely related migratory and sedentary (i.e., resident) female dark-eyed juncos (Junco hyemalis) living together in sympatry during early spring. Despite exposure to the same environmental cues in early spring, residents initiate breeding prior to the departure of migrants for their breeding grounds. We investigated whether residents would be more reproductively developed than migrants based on their behavioral differences. Alternatively, females could exhibit similar reproductive development in response to the same environmental cues despite differences in migratory behav...