Extreme altitudes during diurnal flights in a nocturnal songbird migrant
Sissel Sjöberg, Gintaras Malmiga, Andreas Nord, Arne Andersson, Johan Bäckman, Maja Tarka, Mikkel Willemoes, Kasper Thorup, Bengt Hansson, Thomas Alerstam, Dennis Hasselquist
Abstract
) 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.