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.