Cornell Laboratory of Ornithology Cornell University Ithaca, NY 14850, USA.
College of Information and Computer SciencesUniversity of MassachusettsAmherst, MA 01003, USA; Department of Computer ScienceMount Holyoke CollegeSouth Hadley, MA 01075, USA.
R Soc Open Sci. 2015 Dec 9;2(12):150347. doi: 10.1098/rsos.150347. eCollection 2015 Dec.
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.
风在夜间迁徙鸟类选择的飞行高度中起着重要作用。在北半球中纬度地区，大气条件受极锋急流的支配，其振幅在秋季增加。这对候鸟的一个影响是，该地区盛行的西风在更高的迁徙高度上逐渐增强。我们预计风速的季节性变化将导致候鸟占据逐渐较低的飞行高度，我们使用位于美国东北部的 12 个天气监视雷达站在 100 米海拔间隔处对夜间候鸟的密度估计来检验这一点。与我们的预期相反，中值迁徙高度在整个季节变化不大，方差在季节中期较低，在季节开始和特别是结束时较高。早期迁徙的鸟类包括小到中型长距离迁徙鸟类，属于鸻形目和雀形目，而晚期迁徙的鸟类包括大尺寸和中型距离迁徙鸟类，属于雁形目。因此，迁徙物种组成的季节性变化，以及相关的迁徙策略和行为的变化，导致了迁徙高度的凸凹有界分布。我们的研究结果为评估在全球气候变化下可能发生的中纬度大气条件变化对候鸟种群的影响提供了依据。
