Shorter-legged males start breeding earlier in the barn swallow

Many sexual selection studies focus on conspicuous ornaments, such as long tails, but neglect to investigate inconspicuous traits. Here, we studied a well-known sexual selection mechanism, the Darwin–Fisher mechanism (i.e., breeding date selection), in relation to tarsus length in male barn swallows Hirundo rustica , a model species for sexual selection. When controlling for body size, we found that males with shorter tarsi started breeding earlier than those with longer tarsi. The current study indicates a selective advantage of short-legged males, explaining the reversed sexual dimorphism in tarsus length in this short-legged species.