Female American goldfinches use carotenoid-based bill coloration to signal status

Abstract

Interest in female ornamentation has burgeoned recently, and evidence suggests that carotenoid-based female coloration may function as a mate-choice signal. However, the possibility that females may signal status with coloration has been all but ignored. Bill coloration of female American goldfinches (Spinus tristis) changes seasonally, from dull gray in winter to bright orange in the breeding season. We conducted a series of aviary experiments in the breeding season to examine the signaling role of female bill color during both intra- and intersexual contests as well as during male mate choice. We tested for status signaling by examining whether caged females and males avoided feeding adjacent to female taxidermic models as a function of the model’s bill color, which was experimentally augmented or dulled. We tested for a mate signaling function by giving captive males a choice between 2 live females with experimentally altered bill colors. Females avoided feeding near model females with colorful bills, but males showed neither avoidance of nor preference for females with more colorful bills. These results indicate that the female’s carotenoid-based bill coloration signals status during competitive interactions and suggest that female bill color does not function as a mate-choice signal. This represents the first experimental evidence that a carotenoid-based coloration of females functions to mediate contest competition over food. Key words: American goldfinch, bill color, carotenoid signal, intrasexual selection, ornamental female, social selection, Spinus (Carduelis) tristis, status signaling. [Behav Ecol]