Carotenoid-based ornamentation and status signaling in the house finch

The status signaling hypothesis (SSH) was devised primarily to explain the adaptive significance of avian ornamental coloration during the nonbreeding season. It proposes that conspicuous male plumage serves as an honest signal of social status within a population of birds. However, to date this hypothesis has been well tested and supported for only one type of plumage coloration, melanin-based coloration. Carotenoid-based pigmentation is known to positively reveal male health and condition during molt in a variety of species, but it is poorly understood whether this ornament type can also function as a status signal during the winter. We tested the SSH in male house finches (Carpodacus mexicanus) by manipulating the carotenoid-based plumage brightness of first-year males and then pairing unfamiliar birds of differing coloration in a series of dominance trials in captivity. Manipulated plumage color was unrelated to win/loss outcome in these trials, similarly, the natural pigmentation of males was a poor predictor of winter dominance; as in other studies with this species, we found only a weak tendency for naturally drab males to dominate naturally bright males. These results suggest that carotenoid-based coloration is not a reliable indicator of social status in male house finches during the nonbreeding season. In fact, carotenoid-based coloration may function only in mate choice in this species, and it may be retained throughout the year either because time constraints preclude a second plumage molt or because it aids in pair formation that begins in late winter.