The influence of iridescent coloration directionality on male tree swallows’ reproductive success at different breeding densities

Abstract

Abstract: Conspicuous coloration in males of numerous bird species has been related to sexual selection. However, iridescent coloration is a complex trait that has received little attention despite its potential importance as a signal of individual condition. Directionality, the change in brightness when the animal moves under direct sunlight, has never been linked to fitness although being an integral part of iridescent coloration. Using 214 male tree swallows (Tachycineta bicolor) from a 2-year study, we investigated how different components of iridescent coloration were related to four measures of fitness (number of extra-pair and within-pair young produced, proportion of within-pair young in the nest and annual reproductive success) across different breeding densities. In addition to classical components of coloration (hue, brightness, saturation), we used directionality—measured as an angular breadth of color reflection—to quantify the iridescent coloration of male crown and back. Our results showed that all components of iridescent coloration we studied were related to at least one of the four measures of fitness. A negative association was detected between directionality and the number of within-pair young produced, suggesting that this plumage characteristic could be a signal of paternal care. Crown and back coloration were associated to different fitness components, and density modulated the slope of some of these relationships. Altogether, our results showed that different selective pressures could act on the multiple components of iridescent coloration via both within-pair and extra-pair components of reproductive success. Also, environmental conditions like breeding density are important factors influencing sexual selection processes. Significance statement: The idea that colorful plumage in males is the result of sexual selection is widely recognized. However, evidence of sexual selection on iridescent plumage is scarce. In birds displaying iridescent plumage, the brightness change occurring when the bird moves, called directionality, is striking and could be informative for females when choosing a mate. Using 214 male tree swallows, we found that directionality was related to the number of young produced by a male. As nests were distributed in 40 different farms, a high variability of breeding densities occurred in the study system. Breeding density highly influenced the relationship between color components of iridescent plumage and the number of young produced by a male. This study emphasizes the importance to quantify the angular properties of iridescent plumage as well as to take into account breeding density when studying sexual selection.