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. One consequence for migratory birds is that the region's prevailing westerly winds become progressively stronger at higher migration altitudes. We expect this seasonality in wind speed 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. Contrary to our expectations, median migration altitudes deviated little across the season, and 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 behaviours, 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 probably occurring under global climate change.STEM_probability-of-occurrenceWeekly estimates of probability of occurrence for 142 migratory species within 37.5 km radius circles centered on 12 weather surveillance radar stations located in the northeastern USA . Estimates are derived from spatiotemporal exploratory models (STEM) with occurrence information from the eBird citizen science dataset for the combined period 2004 to 2011.WSR_station_reflectivityDaily total effective scattering area (reflectivity) of birds (square centimeters) per unit volume (cubic km) estimated at 12 weather surveillance radar (WSR) stations in the northeastern USA during the autumn of 2010 and 2011 within 37.5 km radius of each WSR station and within 100-m altitudinal bins