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First published online October 30, 2009
Journal of Experimental Biology 212, 3633-3642 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.031435

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Adjustments of wingbeat frequency and air speed to air density in free-flying migratory birds

H. Schmaljohann^1,2,* and F. Liechti¹

¹ Swiss Ornithological Institute, Luzernerstrasse 6, 6204 Sempach, Switzerland
² Institute of Avian Research `Vogelwarte Helgoland', An der Vogelwarte 21, 26386 Wilhelmshaven, Germany

^* Author for correspondence (heiko.schmaljohann{at}ifv-vogelwarte.de)

Accepted 11 August 2009

Birds adjust their flight behaviour to the physical properties of the air. Lift and drag, the two major properties in aerodynamics, are highly dependent on air density. With decreasing air density drag is reduced and lift per wingbeat decreases. According to flight mechanical theory, wingbeat frequency and air speed should increase with decreasing air density, i.e. increasing flight altitude. Although wind tunnel experiments have shed light on many aspects of avian flight, the effect of air density remained ambiguous, because air density could not be adjusted in wind tunnels, until now. By means of radar we recorded tracks of several thousand free-flying individual birds during nocturnal migration. From these tracks we derived wingbeat frequencies and air speeds covering air densities from 0.84 kg m^–3 to 1.13 kg m^–3, corresponding to an altitudinal range of about 3000 m. We demonstrate here with this sample of nocturnal migrants that: (1) wingbeat frequency decreases with air density (which corresponds to an increase in flap-gliding flyers by 0.4 Hz km^–1 and in bounding flyers by 1.1 Hz km^–1), (2) reducing wingbeat frequency to equivalent sea level values did not abolish the dependency on air density, as expected by flight mechanical theory, and (3) bounding flyers show a higher response in their flight behavioural adjustments to changes in air density than flap-gliding flyers. With respect to air speed flap-gliding flyers increase their air speed by 1.0 m s^–1 km^–1 and bounding flyers by 1.4 m s^–1 km^–1.

Key words: flight, bird, wingbeat frequency, air speed, air density, altitude

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