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The effect of tail streamer length on aerodynamic performance in the barn swallow

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posted on 01.03.2000, 00:00 authored by Kate BuchananKate Buchanan, M R Evans
The elongated tail of the male barn swallow (Hirundo rustica) is regarded as one of the classic examples of a male trait exaggerated by female choice. However, recently a hypothesis has been proposed suggesting that the streamers, or elongated outer tail feathers, may aid aerodynamic performance through the Norberg mechanism, providing lift at slow speeds and high angles of attack when the tail is fully spread. The possibility exists that the tail streamer has evolved under natural selection, sexual selection, or a combination of both selection pressures. We tested these three hypotheses by reducing the streamer length of free-flying swallows and measuring their aerodynamic performance, using stereo-video. Measurements of flight performance were made from the digitized three-dimensional flight paths. Five flight variables best described the individual variation in flight performance. Four of these five parameters - mean velocity, mean curvature, maximum agility, and mean rate change of curvature in the XY plane - had significant second-order polynomial relationships with tail streamer manipulation. The first and second principal components (from principal components analysis of the flight variables) also showed similar relationships with streamer manipulation. The combination of a curvilinear relationship between flight performance and streamer length and an aerodynamic optimum between 0 and 20 mm reduction is only predicted if both natural and sexual selection have been acting on streamer morphology. Our data therefore suggest that sexual selection has extended streamer length by around 10 mm beyond its aerodynamic optimum. We suggest that both natural and sexual selection have been important in shaping tail morphology in the barn swallow, and the relative importance of both selection pressures is discussed.

History

Journal

Behavioral ecology

Volume

11

Issue

2

Pagination

228 - 238

Publisher

Oxford University Press

Location

Oxford, Eng

ISSN

1045-2249

Language

eng

Publication classification

C1.1 Refereed article in a scholarly journal

Copyright notice

2000, International Society for Behavioral Ecology