Female–female aggressiveness is influenced by female coloration in blue tits

Social and intrasexual competition for access to food, territory and reproduction is a major selection force acting on male phenotypes. In contrast, its evolutionary consequences for female phenotypes, especially female ornamentation, have been overlooked. Because fighting is costly for individuals, physical indicators of status that are correlated with aggressiveness or fighting ability are predicted to evolve. Under the badge of status hypothesis, individuals are predicted to attack unknown rivals that present signals of similar size or intensity more vigorously than dissimilar rivals. We used blue tits, Cyanistes caeruleus, to test whether UV/blue and yellow coloration mediates female aggression. We exposed territorial birds to two female blue tit decoys during nest building: one had a UV/blue crown with an enhanced UV signal (UV+) and the other had a reduced UV signal (UV−). We found that resident females with higher UV signal more often landed on the cage containing the UV+ decoys whereas females with a lower UV signal more often landed on the cage containing the UV− decoys. This result suggests that UV/blue crown coloration is a badge of status and supports the hypothesis that female–female competition can generate or maintain female ornamentation. Resident females with darker yellow chests also attacked the decoys with greater intensity. Previous studies suggest that brighter yellow females invest more in reproduction and consequently could be more sexually attractive. So, this last result may be explained by a higher motivation of darker yellow females or a trade-off between aggressiveness and female investment in reproduction.