Negative interplay of tail and throat ornaments at pair formation in male barn swallows

Pair formation is indispensable for breeding in monogamous species, generating selection for male traits that increase the probability of pairing success. Male ornamentation is one such trait, and several empirical studies have shown the importance of each of multiple ornaments. Still, it remains unclear how multiple ornaments in combination affect the probability of pair formation. Using the Japanese barn swallow,Hirundo rustica gutturalis, we studied the interplay of two sexually selected male traits, tail length and throat coloration, during pair formation. Independent of other morphological, ornamental and abiotic variables, the probability of pair formation was predicted by the negative interplay between tail length and plumage colour saturation: males possessing more-colourful plumage with shorter tails or males possessing less-colourful plumage with longer tails had a higher probability of pair formation than others. The current findings may explain spatiotemporal variation in ornamentation and sexual selection in this model species.