Ultraviolet plumage does not signal social status in free-living blue tits; an experimental test

Ultraviolet (UV) signals are suggested to be sexually selected in a wide range of taxa. Most research, however, has focused on the role of UV signals in mate choice, whereas possible functions in intraspecific competition remain largely untested. Studies on other colors indicate that ornaments preferred by females can also function as signals of social status in competitive interactions between individuals. Whereas these colors are mainly pigment based, UV reflectance is generally caused by selective reflectance of light from surface structures. Here we test experimentally whether the structurally based UV-reflective crown plumage in the blue tit (Cyanistes caeruleus) serves as a signal of status in interindividual competition. We reduced the crown UV reflectance of free-living blue tits in winter and compared their probability of winning conflicts over food at a feeding table with control-treated and untreated individuals. Although we controlled for effects of sex, age, and distance from territory, we found no effect of reduced UV reflectance on the probability of winning nor were conflicts involving UV-reduced individuals more likely to escalate. Therefore, we conclude that the UV reflectance of the blue tit's crown does not serve as a signal of status in competition over food in winter. We suggest that the observed site-dependent dominance structure may constrain the opportunity for a status signal to evolve and that enhancing attractiveness in mate choice may be the sole function of the crown's UV reflectance.