Extreme altitudes during diurnal flights in a nocturnal songbird migrant

Billions of nocturnally migrating songbirds fly across oceans and deserts on their annual journeys. Using multisensor dataloggers, we show that great reed warblers (Acrocephalus arundinaceus) regularly prolong their otherwise strictly nocturnal flights into daytime when crossing the Mediterranean Sea and the Sahara Desert. Intriguingly, when prolonging their flights, they climbed steeply at dawn, from mean 2,394 m asl to reach extreme cruising altitudes (mean 5,367 m asl, max 6,267 m asl) during daytime flights. This previously unknown behavior of using exceedingly high flight altitudes when migrating during daytime could be caused by diel variation in ambient temperature, winds, predation, vision range and solar radiation. Our finding of this striking behavioral adaptation provides new perspectives on constraints in bird flight and might help explain the evolution of nocturnal migration.

Methods
See https://doi.org/10.1126/science.abe7291 for details on data collection and compilation.

数十亿只夜间迁徙的鸣禽在年度迁徙旅程中飞越海洋与荒漠。本研究借助多传感器数据记录器（multisensor dataloggers）发现，大苇莺（*Acrocephalus arundinaceus*）在飞越地中海与撒哈拉沙漠时，会将原本严格限定于夜间的飞行时长延长至日间。尤为引人关注的是，在延长飞行的过程中，它们会于黎明时分急剧爬升：从平均海拔2394米的高度起步，最终在日间飞行阶段达到极高的巡航高度（平均5367米，最高可达6267米）。这种此前尚未被发现的日间迁徙时采用超高飞行高度的行为，可能受到环境温度、风速、捕食压力、可视范围以及太阳辐射的昼夜变化影响。本研究关于这一显著行为适应性的发现，为鸟类飞行限制因素的研究提供了全新视角，或有助于阐释夜间迁徙行为的演化历程。

研究方法
详见https://doi.org/10.1126/science.abe7291，以获取数据采集与整理的详细细节。