Extreme altitudes during diurnal flights in a nocturnal songbird migrant

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.

Flight altitude dynamics of migrating European nightjars across regions and seasons

“…The tracked nightjars reached altitudes just below 5000 masl, which is similar to recordings of another trans-Saharan migrant, the Eurasian hoopoe, Upupa epops , but considerably lower than the maximum flight altitudes above 6000 masl of great reed warblers, Acrocephalus arundiaceus ( Liechti et al, 2018 ; Sjöberg et al, 2021 ), and great snipes, Gallinago media (above 8000 masl; Lindström et al, 2021 ). The nightjars flew at the highest altitudes during the spring crossing of the Sahara desert, which conforms well with the general pattern of altitude distribution of avian migrants based on radar observations in Europe and west-Saharan Africa (reviewed in Bruderer et al, 2018 ).…”

Far eastern curlew and whimbrel prefer flying low - wind support and good visibility appear only secondary factors in determining migratory flight altitude

“…It should be stressed that, despite the strong preference for staying low, birds in this and many other studies [ 3 , 8 , 10 , 14 , 17 , 25 , 52 ] occasionally climb to great altitudes. Indeed, our conditional logistic mixed effect modelling evaluating the contribution of the conditions aloft and flight altitude explained 67% and 61% of the variation in far eastern curlew and whimbrel respectively.…”

“…However, empirical studies based on radar observations have found mixed support for this hypothesis [ 6 , 19 , 20 ]. Tracking of great reed warblers and black-tailed godwits showed that their high altitude flights were associated with high air temperature at low altitude, suggesting they were potentially avoiding overheating [ 14 , 25 ] and conserving their water balance. Our finding showed that altitude at which both far eastern curlew and whimbrel chose to fly were neither directly related to air temperature nor relative humidity.…”

“…Most studies on flight altitude selection in migratory birds have been based on radar observations and suggest that migratory flights typically occur up to 1500 m above ground level (a.g.l. ), although much higher altitudes are also occasionally recorded [ 1 , 2 , 9 , 14 ]. Wind, which may provide assistance to migrating birds, can vary greatly in speed and direction with altitude [ 15 ] and studies in the trade wind zone (i.e.…”

“…and daytime (5367 m a.s.l.) flight altitude in migrating great reed warblers ( Acrocephalus arundinaceus ) crossing the Mediterranean Sea and Sahara Desert, a call was made to study diel variation in ambient temperature, wind, predation, vision range, and solar radiation as potential drivers for this spectacular phenomenon [ 14 ].…”