ON AVIAN ASYMMETRY - EVIDENCE OF NATURAL-SELECTION FOR SYMMETRICAL TAILS AND WINGS IN BIRDS

Fluctuating asymmetry (FA), or minor deviations from perfect bilateral symmetry in otherwise symmetrical structures, may reflect exposure to environmental or genetic stresses during trait development. Consequen tly, several authors have recently suggested that FA in elaborate seco ndary sexual characters such as the long tails of some birds could pla y an important role in sexual selection, perhaps by being used as a re liable signal of phenotypic quality during mate choice. Here we examin e an alternative possibility: that the extent of FA in long tails and wings of birds is constrained primarily by natural selection for aerod ynamic efficiency. Predictions developed from aerodynamic theory were tested by using comparative data from 63 long-tailed species. Data on FA in wing lengths confirmed that aerodynamic requirements may underli e patterns of FA in lifting surfaces: species that spend much of their time flying and those that migrate had more symmetrical wings than ot her birds. There was also substantial variation in the extent of tail FA. As predicted, long-tailed species with aerodynamically functional forked tails had more symmetrical outer tail feathers than those with other long tail types in which feather symmetry has less impact on fli ght performance. However, in common with several other recent studies, our data failed to support the predictions of the sexual selection hy pothesis. In sexually dimorphic species, there was no evidence that ta il asymmetry was greater in the longer-tailed sex, nor was tail FA neg atively correlated with tail length. Our results therefore suggest tha t patterns Of FA in long tails and wings may often be better understoo d in a context of natural rather than sexual selection.