The sexual signals of the East-Mediterranean barn swallow: A different swallow tale

The differential functions of distinct morphological traits as sexual signals among closely related populations are associated with a potential role of sexual selection in population divergence and speciation. The cosmopolitan barn swallow Hirundo rustica complex consists of 6 subspecies, which differ substantially in tail streamer length and ventral coloration. Two of these subspecies - the European and North American subspecies - have been extensively studied. Though they are closely related, differentially exaggerated traits in these two subspecies appear to be the result of stronger sexual selection on tail length in Europe and ventral color in North America. The nonmigratory East-Mediterranean subspecies H. r. transitiva possesses both elongated long tail streamers and dark (brown-red) ventral coloration. We explored whether the expression of both traits is related to their potential role as sexual signals. In males, dark ventral coloration was the strongest predictor of breeding success, whereas tail streamer length was related to early breeding, and the likelihood of having multiple broods within a season. Older males, which typically have long streamers, were also less likely to have extrapair young in their nests. Tail streamer length and not ventral coloration predicted the breeding success of females. Considered in concert with earlier work on other populations, these findings show how patterns of phenotypic variation in the barn swallow species complex are underlain by differential sexual selection on tail length and ventral color, traits used variably among populations in mate choice and paternity decisions.