Aerodynamic costs of long tails in male barn swallows Hirundo rustica and the evolution of sexual size dimorphism

Exaggerated tail feathers of birds constitute a standard example of evolution of extravagant characters due to sexual selection. Such secondary sexual traits are assumed to be costly to produce and maintain, and they usually are accompanied by morphological adaptations that tend to reduce their costs. The aerodynamic costs for male barn swallows Hirundo rustica of having long tails were quantified using aerodynamics theory applied to morphological data from seven European populations. Latitudinal differences in tail length were positively correlated with differences in flight costs predicted by aerodynamics theory. A positive relationship between aerodynamic costs of long tails and the degree of sexual size dimorphism was found among populations. Latitudinal differences in foraging costs may result in tail length being relatively similar in males and females in southern populations, whereas the low foraging costs for males in northern populations may allow them to cope with higher aerodynamic costs, giving rise to large sexual size dimorphism. Enlargement of wingspan in males can alleviate but not eliminate the costs of tail exaggeration, and therefore differences in aerodynamic costs of male ornaments were maintained among populations. Sexual size dimorphism in the barn swallow arises as a consequence of latitudinal differences in the advantages of sexual selection for males and the costs of long tails for males and females.