title: Seasonal changes in the altitudinal distribution of nocturnally migrating birds during autumn migration.
abstract: 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.
authors.affiliation: Cornell Laboratory of Ornithology Cornell University Ithaca, NY 14850, USA.
authors.forename: Frank A
authors.initials: FA
authors.lastname: La Sorte
authors.affiliation: Cornell Laboratory of Ornithology Cornell University Ithaca, NY 14850, USA.
authors.forename: Wesley M
authors.initials: WM
authors.lastname: Hochachka
authors.affiliation: Cornell Laboratory of Ornithology Cornell University Ithaca, NY 14850, USA.
authors.forename: Andrew
authors.initials: A
authors.lastname: Farnsworth
authors.affiliation: College of Information and Computer SciencesUniversity of MassachusettsAmherst, MA 01003, USA; Department of Computer ScienceMount Holyoke CollegeSouth Hadley, MA 01075, USA.
authors.forename: Daniel
authors.initials: D
authors.lastname: Sheldon
authors.affiliation: Cornell Laboratory of Ornithology Cornell University Ithaca, NY 14850, USA.
authors.forename: Benjamin M
authors.initials: BM
authors.lastname: Van Doren
authors.affiliation: Cornell Laboratory of Ornithology Cornell University Ithaca, NY 14850, USA.
authors.forename: Daniel
authors.initials: D
authors.lastname: Fink
authors.affiliation: Cornell Laboratory of Ornithology Cornell University Ithaca, NY 14850, USA.
authors.forename: Steve
authors.initials: S
authors.lastname: Kelling
keywords: flight altitude; mid-latitudes; polar-front jet stream; seasonal avian migration; weather surveillance radar; wind