In males, testosterone plays a key role in ornament production and linking ornamentation with reproductive behaviors and other traits to produce an integrated phenotype. Less is known about whether females couple testosterone, ornamentation, and aggressive behaviors to achieve female-specific combinations of traits. Ornamentation in females may be the result of correlated expression with male ornamentation, or female traits could arise as the result of sex specific selection pressures. Resolving between these alternatives is necessary to understand the degree to which selection acts on female traits. The White-shouldered Fairywren (Malurus alboscapulatus) provides a useful context to address these questions because populations vary in degree of female ornamentation, a derived trait, whereas male ornamentation is constant across both populations. We found that ornamented females have higher levels of circulating testosterone and respond more aggressively to experimental territorial intrusions than do unornamented females. These findings are consistent with the idea that, among female White-shouldered Fairywrens, testosterone may mechanistically link plumage and behavioral traits to produce an integrated competitive phenotype, as has been reported for males of closely related species. In contrast, circulating testosterone in males did not differ significantly between populations. More broadly, our findings are consistent with ongoing selection on the mechanisms underlying female ornaments, likely via social selection.