Shared genetic architecture and sex-specific fitness consequences of forked tails in the Alpine swift Tachymarptis melba

The evolution of deeply forked tails in highly aerial birds has been suggested to be driven by sexual and viability selection. Using over 20 years of data on the Alpine swift, we investigated the sex-specific predictors and fitness consequences of deeply forked tails, as well as the role of genetics in shaping the evolutionary trajectory of this trait. In the Alpine swift, fork length shows moderate sexual dimorphism, being 7% longer in males than in females. Longer forks were associated with increased reproductive success in males and larger eggs in females, suggesting that this trait may be mutually selected and signal aspects of individual condition in both sexes. In addition, fork length was heritable and strongly genetically correlated between the sexes, with effectively the same genetic architecture expressed in both sexes. However, genetic constraints between the sexes are unlikely to explain the changes in fork length observed over the course of the study, as fork length increased more in females than in males. Further studies will be required to understand why male fork length has not became further elaborated, despite positive directional selection on this heritable trait.