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.