Abigail A. Kimmitt, Jack W. Hardman, Craig A. Stricker, Ellen D. Ketterson
Many animals exhibit seasonal behaviors such as migration and reproduction. Species, populations or individuals can differ in seasonal migratory behavior, from non-migratory to obligate migratory. Populations that differ in migratory strategy might also differ in when they breed and whether they overlap in geographic space for only part of the year. We studied migratory and non-migratory (i.e., resident) dark-eyed juncos, which are small North American songbirds that live together only during winter and early spring before migrants depart for their summer breeding grounds. We investigated how migrants and residents differ in timing and the physiological differences between them. Under one hypothesis, resident females would be more reproductively advanced than migrant females owing to genetic or developmental divergence. Alternatively, migrant and resident females would be equal in reproductive readiness because they experience the same environment in early spring. To compare reproductive development in migrants and residents in early spring, we collected ovaries from migrants and residents to compare gene expression related to reproductive timing. Based on the known isotopic structure of North American precipitation, we can work out where birds were when they molted because of hydrogen deposited in the feathers; therefore we estimated breeding latitude and migratory distance of migrant females using stable hydrogen isotopes in feathers. We found that residents had heavier ovaries and higher expression of two genes related to reproductive timing as compared to migrants. These results suggest that despite being exposed to the same environment, residents are more reproductively ready than migrants in early spring. Additionally, migrant females that had a shorter distance to travel were more reproductively ready than migrant females that had a longer distance to travel. In sum, we learned about underlying mechanisms for population-level differences in when females become ready to reproduce, which could enable us to predict how seasonal animals will respond to a changing world.