Data from: Seasonal changes in the altitudinal distribution of nocturnally migrating birds during autumn migration

Wind plays a significant role in the flight altitudes selected by nocturnally migrating birds. At mid-latitudes in the Northern Hemisphere, atmospheric conditions are dictated by the polar-front jet stream, whose amplitude increases in the autumn, resulting in stronger and more persistent westerly winds at lower migration altitudes. We expect this seasonality to result in migrants occupying progressively lower flight altitudes, which we test using density estimates of nocturnal migrants at 100-m altitudinal intervals from 12 weather surveillance radar stations located in the northeastern USA. Median migration altitudes deviated little across the season, whereas the variance was lower during the middle of the season and higher during the beginning and especially the end of the season. Early season migrants included small to intermediate sized long-distance migrants in the Orders Charadriiformes and Passeriformes, and late season migrants included large-bodied and intermediate-distance migrants in the Order Anseriformes. Therefore, seasonality in the composition of migratory species, and related variation in migration strategies and behaviors, resulted in a convex-concave bounded distribution of migration altitudes. Our results provide a basis for assessing the implications for migratory bird populations of changes in mid-latitude atmospheric conditions likely occurring under global climate change.