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 evoluti on 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 cost s. The aerodynamic costs for male barn swallows Hirundo rustica of having l ong tails were quantified using aerodynamics theory applied to morphologica l data from seven European populations. Latitudinal differences in tail len gth were positively correlated with differences in flight costs predicted b y aerodynamics theory. A positive relationship between aerodynamic costs of long tails and the degree of sexual size dimorphism was found among popula tions. Latitudinal differences in foraging costs may result in tail length being relatively similar in males and females in southern populations, wher eas the low foraging costs for males in northern populations may allow them to cope with higher aerodynamic costs, giving rise to large sexual size di morphism. Enlargement of wingspan in males can alleviate but not eliminate the costs of tail exaggeration, and therefore differences in aerodynamic co sts of male ornaments were maintained among populations. Sexual size dimorp hism in the barn swallow arises as a consequence of latitudinal differences in the advantages of sexual selection for males and the costs of long tail s for males and females.