Female choice selects for male sexual tail ornaments in the monogamous swallow

Male secondary sexual ornaments are widespread among polygynous animals¹ where they apparently arise through female choice and differential mating success of males^2-5. Darwin¹ and Fisher² suggested that female choice should also select for extravagant male ornaments in monogamous birds because female preference for ornamented males should result in earlier breeding which would increase male reproductive success by enhancing the quality and/or the quantity of offspring raised each year. In such cases female reproductive success should be positively related to body condition⁶ and a heritable component of the time of breeding should be affected by stabilizing selection⁷. Female preferences for orna-mented males would be favoured by sexual selection because such choosy females would have a greater probability of bearing orna-mented sons with an increased chance of pairing up early. I report here that in the monogamous swallow, Hirundo rustica, males with experimentally elpngated tail ornaments obtain mates more quickly than males with shorter tails, and enjoy increased reproductive output in one breeding season. Such males are also preferred by females seeking extra-pair-bond copulations. Thus male sexual ornaments may also be maintained by female mate choice in monogamous species.