| Androgens regulate male reproductive behaviour and may be a mechanistic link between sexual signals and physiological condition. However, the role of these hormones in regulating pre-nuptial moult and male plumage signals of passerine birds is unclear. In the red-backed fairy-wren ( Malurus melanocephalus), plumage colour is a sexually selected trait and males exhibit three reproductive phenotypes. Males can breed in red and black plumage, breed in brown plumage, or assume brown plumage and act as non-breeding auxiliaries: each phenotype differs in parental and reproductive behaviour. We found that both plasma androgen concentrations and body condition differed significantly among male phenotypes, with red/black breeding males having the highest androgen levels and best condition and auxiliaries the lowest levels and poorest condition. These differences were present during the pre-nuptial moult when nuptial plumage is acquired and androgen concentrations were positively correlated with body condition at that time.;While androgens appear to be involved in regulating these alternative reproductive phenotypes, their function may be mediated by the avian glucocorticoid stress hormone, corticosterone (CORT). We tested the hypothesis that CORT regulates breeding status by suppressing breeding in auxiliary males and mediating the relationship between body condition, testosterone, and production of plumage signals. Contrary to our predictions, subordinate auxiliary males and dominant red/black breeding males had similar levels of baseline CORT. Additionally, we found no correlation between CORT and body condition and a positive rather than negative association between molting male CORT and androgens. Thus, honesty of male red-backed fairy-wren plumage signals may be enforced by the relationship between body condition and androgens without the mediating influence of CORT.;Finally, we experimentally tested the hypothesis that red/black plumage color is testosterone (T: androgen) dependent using T, control, and anti-T implants. We found that, relative to controls, both T and anti-T treatment stimulated early onset of the pre-nuptial molt, lead to the production of bright plumage, influenced feather length, and differentially effected body morphology. Across all combined characters, anti-T males produced an intermediate phenotype to T and control males. Thus, bright plumage production is T dependent but T may not be the only hormonal mechanism functioning. |
