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.

风对夜间迁徙鸟类选择的飞行高度具有显著影响。在北半球中纬度地区，大气状况由极锋急流（polar-front jet stream）主导，其振幅在秋季逐渐增大。对于迁徙鸟类而言，这带来的一项后果是该区域的盛行西风随迁徙海拔升高而逐步增强。我们推测，风速的这种季节变化将导致迁徙鸟类的飞行高度逐步降低，并通过利用美国东北部12个天气监视雷达站（weather surveillance radar stations）获取的、以100米海拔间隔划分的夜间迁徙鸟类密度估算值开展检验。但与预期相反，整个迁徙季的迁徙高度中位数变化极小，且海拔方差在季中较低，在季初尤其是季末更高。季初的迁徙种群以鸻形目（Charadriiformes）和雀形目（Passeriformes）中小型至中型的长距离迁徙物种为主，季末的迁徙种群则以雁形目（Anseriformes）的大型和中距离迁徙物种为主。因此，迁徙物种种群组成的季节变化，以及与之相关的迁徙策略与行为差异，共同造就了迁徙高度的凸凹有界分布。本研究结果为评估全球气候变化背景下中纬度大气状况变化对迁徙鸟类种群的潜在影响提供了依据。