Structural coloration and sexual selection in the barn swallow Hirundo rustica

Abstract

Structural coloration has been hypothesized to play a role in sexual selection, and we tested whether this was the case in a field study of the barn swallow Hirundo rustica. The dorsal iridescent plumage of barn swallows has a strong reflectance in the ultraviolet (UV) region, with adult males on average reflecting 8-9% more than adult females, as revealed by a 2-year study in southwestern Spain. The correlation between structural coloration (described by the reflectance in the UV part of the spectrum, UV chroma and blue chroma) and three other secondary sexual characters significantly associated with male mating success (tail length, tail asymmetry, and red facial coloration) was weak and generally nonsignificant. Nor was there a significant relationship between color parameters and body condition. We tested for an association between structural coloration of the dorsal plumage and sexual selection in a number of independent tests. Arrival date of males was not significantly related to color; there was no significant relationship between coloration and probability of survival or age; mated males did not have stronger reflectance than unmated males; and the duration of the premating period was not significantly related to color. Reproductive success was not significantly correlated with plumage coloration in males, nor was the feeding rate of offspring by brightly colored males higher than that of males with less bright plumage. Given that sample sizes were large, and the power of statistical tests high, we conclude that current sexual selection on the coloration of the dorsal plumage in the barn swallow is, at best, weak.