Testosterone increases UV reflectance of sexually selected crown plumage in male blue tits

A central assumption of models of sexual selection, including the immunocompetence handicap hypothesis, is that the male sex hormone testosterone (T) is responsible for the expression of male sexual signaling; however, this has been questioned for colorful avian plumage. In this experiment, we manipulated T in juvenile male blue tits (Cyanistes caeruleus) during the molt and measured crown ultraviolet (UV) chroma (a sexually selected trait) immediately after molt and in the following spring during the breeding season, as well as recording preening behavior during spring. We found that males that were implanted with T during the molt had higher crown UV chroma than control males (C-males) in the subsequent breeding season but not immediately after molt. We also found that testosterone-treated males preened more than C-males during the spring but not during the preceding molt. These results suggest not only that T influences plumage coloration during the mate attraction period, possibly by increasing preening behavior, but also that exogenous T administered during the juvenile molt may have organizational effects in the subsequent breeding season. Because our study supports the assumption that T enhances the expression of male sexually selected plumage coloration, the results indicate that T could enforce costliness, and therefore honesty, of male plumage color as a signal of quality to females.