Intersexual and intrasexual consequences of epaulet colour in male red-winged blackbirds: an experimental approach

We conducted experiments on the intersexual and intrasexual consequences of epaulet colour in male red-winged blackbirds, Agelaius phoeniceus. In the female choice experiment, we gave captive females a choice between males with normal or dulled epaulets, and between males with normal or reddened epaulets. Females tended to associate more with normal males than with dulled males, but were equally likely to perform precopulatory displays to the two types of males. Females associated significantly more with redder-than-normal males and were more likely to perform precopulatory displays to reddened males. In the male–male competition experiment, we presented free-living territorial males with a male model to which we could attach wings with dulled, normal or reddened epaulets. Presence of the model with reddened epaulets resulted in significant increases in display rates by territory owners, but the increase in display rates was probably in response to males other than the territory owner flying through and trespassing rather than to the reddened epaulet model per se. Normal epaulets produced a significant decrease in song rates and approach distances of owners. Responses of territorial and other males did not differ significantly during normal and dulled epaulet presentations. These results support the hypothesis that epaulets of male red-winged blackbirds are salient inter- and intrasexual signals: unusually red epaulets were attractive to females and attracted more attention and aggression from male conspecifics than normal epaulets. Epaulet colour may thus be an example of intrasexual selection opposing intersexual selection.