Honesty of a dynamic female aggressive status signal: baseline testosterone relates to bill color in female American goldfinches

Status signals are linked to fi ghting ability and enable competitors to gain access to resources without risking injury in aggressive combat. Th e relationship between testosterone (T), a hormone that mediates aggression, and signals of status is well studied in males, but little is known about the relationship between T and female signals of status. Female and male American goldfi nches Spinus tristis express a dynamic carotenoid-based orange bill color during the breeding season and previous work has demonstrated that females use orange bill color to communicate competitive ability during intrasexual competition. We test the hypothesis that female bill color refl ects baseline T, which would allow receivers to directly assess a competitor ’ s aggressive potential. We found a positive relationship between T and bill coloration in females, indicating that bill color has the ability to signal female competitive status. Th is fi nding is consistent with the hypothesis that female bill color is a reliable signal of fi ghting ability, and indicates that females, like males, may use coloration to signal their hormonally mediated aggressive potential.