Assessing the aerodynamic effects of tail elongations in the house martin (Delichon urbica): implications for the initial selection pressures in hirundines |
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Authors: | K J Park M R Evans K L Buchanan |
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Institution: | (1) Department of Biological Sciences, University of Stirling, Stirling, FK9 4LA, UK e-mail: k.j.park@stir.ac.uk Tel.: +44-1786-467799, Fax: +44-1786-464994, GB |
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Abstract: | Of the three species of hirundine that breed sympatrically across the U.K., one, the barn swallow, has outer tail feathers
elongated into streamers, whereas the other two species, the house martin and the sand martin, do not. The tail streamer of
the barn swallow is regarded as a classic example of a sexually selected trait. Recent evidence, however, has suggested that
streamers may have evolved largely through natural selection for enhanced flight performance and increased maneuverability.
We tested the hypotheses that small streamers (1) increase performance in turning flight, but (2) decrease performance in
flight variables related to velocity. We manipulated the lengths of house martin outer tail feathers and measured changes
in their free-flight performance, using stereo-video to reconstruct the birds" three-dimensional flight paths. Five flight
variables were found to best describe individual variation in flight performance. Of these five, the three variables determining
maneuverability predicted that flight performance would be optimized by a 6- to 10-mm increase in the length of the outer
tail feathers. In contrast, for mean velocity and mean acceleration, extension of the outer tail feathers appears to have
a detrimental effect on flight performance. We suggest that the initial selection pressure for streamers in ancestral short-tailed
"barn swallows" was via natural selection for increased maneuverability. In addition, we propose that the benefits of increased
maneuverability have differed between hirundines in the past, such that the cost of increasing the length of the outer tail
feather has, to date, outweighed the benefits of doing so in streamerless hirundines.
Received: 14 February 2000 / Revised: 2 July 2000 / Accepted 18 July 2000 |
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Keywords: | Flight performance Hirundines Natural selection Sexual selection Tail streamers |
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