AERODYNAMICS AND THE EVOLUTION OF LONG TAILS IN BIRDS

TWO problems limit the interpretation of recent experiments1-3 supporting Darwin's4 suggestion that female choice for ornate males may account for the evolution of long tails in birds. First. in some species tail elongation may have been favoured by natural rather than sexual selection. Second, it is unclear how female preferences for elaborate males have evolved, because current tests of competing models are often inconclusive5-7. We have integrated aerodynamics theory with comparative data on sexual dimorphism in tail length to evaluate the flight costs of different forms of tail elongation. We report here that long tails with shallow forks are aerodynamically optimal, exhibit correspondingly low sexual dimorphism and may therefore have evolved under natural selection. Other long-tail types impair flight and show greater sexual dimorphism, but variation in their initial evolutionary cost suggests differences in how female preferences for them may have evolved.