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Confronting the winds: orientation and flight behaviour of roosting swifts, Apus apus

Author:
  • Johan Bäckman
  • Thomas Alerstam
Publishing year: 2001
Language: English
Pages: 1081-1087
Publication/Series: Royal Society of London. Proceedings B. Biological Sciences
Volume: 268
Issue: 1471
Document type: Journal article
Publisher: Royal Society

Abstract english

Swifts, Apus apus, spend the night aloft and this offers an opportunity to test the degree of adaptability of bird orientation and flight to different ecological situations. We predicted the swifts' behaviour by assuming that they are adapted to minimize energy expenditure during the nocturnal flight and during a compensatory homing flight if they become displaced by wind. We tested the predictions by recording the swifts' altitudes, speeds and directions under different wind conditions with tracking radar; we found an agreement between predictions and observations for orientation behaviour, but not for altitude and speed regulation. The swifts orientated consistently into the head wind, with angular concentration increasing with increasing wind speed. However, contrary to our predictions, they did not select altitudes with slow or moderate winds, nor did they increase their airspeed distinctly when flying into strong head winds. A possible explanation is that their head-wind orientation is sufficient to keep nocturnal displacement from their home area within tolerable limits, leaving flight altitude to be determined by other factors (correlated with temperature), and airspeed to show only a marginal increase in strong winds. The swifts were often moving 'backwards: heading straight into the wind but being overpowered by wind speeds exceeding their airspeed. The regular occurrence of such flights is probably uniquely associated with the swifts' remarkable habit of roosting on the wing.

Keywords

  • Biological Sciences

Other

Published
  • ISSN: 1471-2954
Thomas Alerstam
E-mail: thomas [dot] alerstam [at] biol [dot] lu [dot] se

Professor emeritus

Evolutionary ecology

+46 46 222 37 85

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Centre for Animal Movement Research
Evolutionary Ecology, Department of Biology
Ecology building S-223 62 Lund Sweden