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.