Spring and fall migration phenology of an Arctic-breeding passerine

Understanding patterns in avian migration phenology and the proximate mechanisms for such patterns is important for assessing behavioural responses of individuals or populations to climate change. Among songbirds, protandry in spring is a common pattern; phenology in fall is less well described. Using tracking data collected from geolocators deployed at a breeding site, and capture data from banding stations, we assessed fall and spring migration phenology of an Arctic-breeding passerine, the Snow Bunting (Plectrophenax nivalis), by sex and age. We measured migration timing, speed, and distance, as well as duration of migration stopovers to test proximate mechanisms for observed sex and age differences in spring and fall migration phenology. During fall migration, hatch-year birds preceded adults, and adult males tended to precede adult females; however, there remained extensive variation by year. Males and females tracked directly arrived at winter sites at approximately the same time. During early spring migration, Snow Buntings exhibited moderate protandry, where after-second-year males preceded all other age-sex classes by ~6 days, on average. Surprisingly, protandry was not apparent at late spring migration or at breeding arrival. Instead, arrival dates by sex and age appeared highly variable between years. The winter site arrival date was predicted by fall migration departure date, total number of stopover days, migration speed, and migration distance. The breeding site arrival date was similarly predicted by spring migration departure date, total stopover days, and migration speed. Our results provide key baseline data for monitoring ongoing changes in migration phenology of this important Arctic-breeding songbird, as climate change effects become more pronounced across temperate and Arctic regions.