Seasonal effects of wind conditions on migration patterns of soaring American white pelican

Energy and time expenditures are determinants of bird migration strategies. Soaring birds have developed migration strategies to minimize these costs, optimizing the use of all the available resources to facilitate their displacement. We analysed the effects of different wind factors (tailwind, turbulence, vertical updrafts) on the migratory flying strategies adopted by 24 satellite-tracked American white pelicans (Pelecanus erythrorhynchos) throughout spring and autumn in North America. We hypothesize that different wind conditions encountered along migration routes between spring and autumn induce pelicans to adopt different flying strategies and use of these wind resources. Using quantile regression and fine-scale atmospheric data, we found that the pelicans optimized the use of available wind resources, flying faster and more direct routes in spring than in autumn. They actively selected tailwinds in both spring and autumn displacements but relied on available updrafts predominantly in their spring migration, when they needed to arrive at the breeding regions. These effects varied depending on the flying speed of the pelicans. We found significant directional correlations between the pelican migration flights and wind direction. In light of our results, we suggest plasticity of migratory flight strategies by pelicans is likely to enhance their ability to cope with the effects of ongoing climate change and the alteration of wind regimes. Here, we also demonstrate the usefulness and applicability of quantile regression techniques to investigate complex ecological processes such as variable effects of atmospheric conditions on soaring migration.

能量与时间消耗是决定鸟类迁徙策略的核心因素。翱翔性鸟类演化出适配的迁徙策略，以最小化此类成本，优化利用一切可用资源以助力其迁徙行进。本研究分析了不同风况因子（顺风(tailwind)、湍流(turbulence)、垂直上升气流(vertical updrafts)）对24只经卫星追踪(satellite-tracked)的美洲白鹈鹕（Pelecanus erythrorhynchos）在北美春秋两季的迁徙飞行策略的影响。我们提出如下假设：春秋迁徙沿途遭遇的不同风况，会诱导鹈鹕采取差异化的飞行策略，并对上述风资源的利用方式产生差异。本研究借助分位数回归(quantile regression)与精细尺度大气数据开展分析，结果显示美洲白鹈鹕会优化利用可用风资源：春季迁徙时的飞行速度更快、路径更直接，相较于秋季更为显著。无论是春季还是秋季迁徙，美洲白鹈鹕均会主动选择顺风飞行；但在春季迁徙阶段（此时它们需要抵达繁殖区域），它们会更多依赖可用的上升气流。上述风况因子的影响效应随鹈鹕的飞行速度而异。本研究还发现，鹈鹕的迁徙飞行方向与风向之间存在显著的相关性。基于本研究结果，我们认为美洲白鹈鹕的迁徙飞行策略具有可塑性，这或可提升其应对当前气候变化及风况改变的能力。此外，本研究还证实了分位数回归技术在探究复杂生态过程（如大气条件对翱翔迁徙的可变影响）中的实用性与适用性。