Ultraviolet reflectance affects male-male interactions in the blue tit (Parus caeruleus ultramarinus)

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Abstract
Several animal species have been shown to use phenotypic traits to assess the competitive ability of opponents and adjust their aggressiveness depending on the likelihood to win the contest. In birds, these phenotypic traits usually involve patches of colored feathers. The benefit to harbor honest signals of male quality is the avoidance of wasteful aggressive interactions. Recent work has shown that ultraviolet (UV) plumage reflectance is an important signal used by females during mate choice. Surprisingly, however, the role of UV signaling on intrasexual selection has been neglected. In the present study, we aimed to test whether UV reflectance of crown feathers was used as a signal of male competitive ability during male-male interactions. Breeding male blue tits (Parus caeruleus ultramarinus) were exposed during the female egg-laying period to blue tit taxidermic mounts with either control or reduced UV reflectance of crown feathers. In agreement with the prediction that UV reflectance advertises male quality, we found that breeding blue tits behaved less aggressively toward the UV-reduced decoy. To our knowledge, this is the first experimental evidence suggesting a role for UV signaling on intrasexual selection.