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

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.