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.

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

Activity_and_barometric_data.txt: Data file in plain text containing detailed accelerometer, pressure and temperature data for the different great reed warblers. First column indicates individual and the second column indicates flight ID. Third column is a timestamp. Act0 means number of sequences that scored 0 during the preceding hour (12 measurement sequences). Next column Act1 to Act5 follow the same way. Column “check” is the sum of all activity counts and should be 12 if sampling worked as intended. “P” gives barometric pressure in mbar and is only given if the bird is flying at the sampling event (same as the timestamp).

Light_data.txt: Data file in plain text containing light data for the 7 great reed warblers included with positions in the manuscript. First column indicates date, second column indicates time, third column gives light in lux and the fourth column indicates individual.