Winter plumage coloration in male American goldfinches: Do reduced ornaments serve signaling functions in the non-breeding season?

摘要

The signaling role of sexual ornaments that are displayed during the mating season is well known for many species, but dimorphisms that occur in the non-breeding season have received much less attention, particularly when individuals only partially express their breeding condition during reproductively inactive periods. I assessed variation in the expression of colorful breeding and non-breeding plumages in male American goldfinches (Carduelis tristis), a species in which males molt out of their colorful breeding ornaments in the fall but still display reduced carotenoid- and melanin-based sexual dichromatism during the winter. I found that variability in the saturation of carotenoid-based plumage pigmentation did not differ significantly between the breeding and non-breeding seasons. Moreover, the area of melanin coloration in the cap was more variable in winter than when it is fully displayed during breeding. I also detected a significant positive correlation between the extent of melanin coloration during the winter and the saturation of non-breeding carotenoid-based plumage. Because of such variation in and correlated expression of these two color ornaments in winter, it is conceivable that male goldfinches display these hints of non-breeding coloration for use as conspecific social or sexual signals. Natural selection pressures like predation and energetic demands are traditionally thought of as factors that restrict sex ornaments to breeding times alone, but this should not preclude animals from simply reducing their exaggerated features during winter and finding an expression optimum that balances signal costs and value. Such 'remnant' winter ornaments might be expected to evolve not only in animals that live in large non-breeding groups (e.g. status-signaling systems) but also in those where mates begin associating before breeding onset.