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Extreme altitudes during diurnal flights in a nocturnal songbird migrant

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Science  07 May 2021:
Vol. 372, Issue 6542, pp. 646-648
DOI: 10.1126/science.abe7291

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High fliers

Migrating from hemisphere to hemisphere is a global strategy for many bird species. Despite allowing birds to track productivity, these long-distance movements bring them in contact with inhospitable regions such as deserts and oceans. Sjöberg et al. used geolocators to monitor flight in great reed warblers (Acrocephalus arundinaceus) and found that when over these types of regions, this normally nocturnal migrating species flew both day and night. During the day, the birds increased the altitudes at which they flew, rising to more than 5000 meters. Such behavior may allow them to avoid heat stress or other daytime threats during migration.

Science, this issue p. 646

Abstract

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.

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