Late burst of fork-tail evolution in hirundines (Aves: Hirundininae)

The evolutionary patterns of trait diversification provide insights into the function of the trait. Early burst of trait evolution is often associated with adaptive radiation, rapidly diversifying the trait in response to vacant niches followed by the slowdown of the diversification with niche filling, whereas late burst is more likely to be associated with sexual selection, possibly contributing to reproductive barriers between closely related species. Here, we studied the diversification of tail fork depth through time in hirundines to infer its function, which remains unclear due to the competing two alternative hypotheses: the sexual selection hypothesis, which is a classic explanation of deeply forked tails, proposed that this trait has evolved via sexual selection, which was then challenged by the viability selection hypothesis, which proposed that deeply forked tails have mainly evolved via viability selection for enhancing aerodynamic performance during aerial foraging on large prey. We found a late burst of tail fork depth, but not of bill length, i.e., an index of prey size. The observed pattern is consistent with the sexual selection hypothesis but not with the viability selection hypothesis.