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

Ultraviolet (UV) signals are suggested to be sexually selectedin a wide range of taxa. Most research, however, has focusedon the role of UV signals in mate choice, whereas possible functionsin intraspecific competition remain largely untested. Studieson other colors indicate that ornaments preferred by femalescan also function as signals of social status in competitiveinteractions between individuals. Whereas these colors are mainlypigment based, UV reflectance is generally caused by selectivereflectance of light from surface structures. Here we test experimentallywhether the structurally based UV-reflective crown plumage inthe blue tit (Cyanistes caeruleus) serves as a signal of statusin interindividual competition. We reduced the crown UV reflectanceof free-living blue tits in winter and compared their probabilityof winning conflicts over food at a feeding table with control-treatedand untreated individuals. Although we controlled for effectsof sex, age, and distance from territory, we found no effectof reduced UV reflectance on the probability of winning norwere conflicts involving UV-reduced individuals more likelyto escalate. Therefore, we conclude that the UV reflectanceof the blue tit's crown does not serve as a signal of statusin competition over food in winter. We suggest that the observedsite-dependent dominance structure may constrain the opportunityfor a status signal to evolve and that enhancing attractivenessin mate choice may be the sole function of the crown's UV reflectance.