Intrasexual competition in zebra finches, the role of beak colour and body size

Colourful ornaments are typically thought to be selected through female choice (intersexual selection), but often they also function as signals of dominance in intrasexual competition. The zebra finch, Taeniopygia guttata, is a well-studied model species for the evolution of colourful ornaments through female choice, and most research interest has focused on the red beak of males. However, results have been mixed, and in our population, we have found no female preference for males with redder beaks. To investigate whether beak colour is instead used as a signal in intrasexual competition, we studied aggressive behaviour of 160 male and 133 female zebra finches in experimentally staged encounters. Beak coloration was sexually dimorphic in the red and orange, as well as the ultraviolet (UV) part of the spectrum, where females showed a more pronounced UV peak. There was a trend for females with less male-like UV reflectance to be more aggressive, but otherwise, beak colour explained little, if any, of the variation in aggressiveness within sexes during competition for a potential partner. Hence beak colour is unlikely to be a signal of dominance, at least under our experimental conditions. Body size often influences the outcome of competition, and we found a weak but significant correlation between body size and aggressiveness that differed between the sexes. Large males were more aggressive than small males whereas small females were more aggressive than large females. Generally, the level of aggressiveness was highly repeatable within individuals, but beak colour and body size explained only little of the between-individual variation.