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 exampleofevolution of extravagant characters due to sexual selection.Such secondarysexual traits are assumed to be costly to produceand maintain, and theyusually are accompanied by morphologicaladaptations that tend to reduce theircosts. The aerodynamiccosts for male barn swallows Hirundo rusticaof having longtails were quantified using aerodynamics theory applied tomorphologicaldata from seven European populations. Latitudinal differencesintail length were positively correlated with differences inflight costspredicted by aerodynamics theory. A positive relationshipbetween aerodynamiccosts of long tails and the degree of sexualsize dimorphism was found amongpopulations. Latitudinal differencesin foraging costs may result in taillength being relativelysimilar in males and females in southern populations,whereasthe low foraging costs for males in northern populations mayallowthem to cope with higher aerodynamic costs, giving riseto large sexual sizedimorphism. Enlargement of wingspan inmales can alleviate but not eliminatethe costs of tail exaggeration,and therefore differences in aerodynamic costsof male ornamentswere maintained among populations. Sexual size dimorphism inthebarn swallow arises as a consequence of latitudinal differencesin theadvantages of sexual selection for males and the costsof long tails for malesand females.