Billions of nocturnally migrating songbirds fly across oceans and deserts on their annual journeys. Using multisensor data loggers, 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. Unexpectedly, when prolonging their flights, they climbed steeply at dawn, from a mean of 2394 meters above sea level to reach extreme cruising altitudes (mean 5367 and maximum 6267 meters above sea level) 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 notable behavior provides new perspectives on constraints in bird flight and might help to explain the evolution of nocturnal migration.

数十亿只夜行性鸣禽在每年的旅行中飞越海洋和沙漠。使用多传感器数据记录仪，我们证明了，很棒的芦苇莺（Acrocephalus arundinaceus）在穿越地中海和撒哈拉沙漠时，定期将原本严格夜间活动的航班延至白天。出乎意料的是，延长飞行时间时，它们在黎明时急剧爬升，从平均海拔2394米到白天飞行时达到的极限巡航高度（平均5367米，最大海拔6267米）。白天在迁移时使用极高的飞行高度的这种先前未知的行为可能是由于环境温度，风，掠食，视距和太阳辐射的变化引起的。我们对这一显着行为的发现为鸟类飞行的限制提供了新的视角，并可能有助于解释夜间迁徙的演变。