Widespread polychromatism in female sunangel hummingbirds (Heliangelus: Trochilidae)

Abstract

In hummingbirds, a showy gorget of iridescent throat feathers is a characteristic male ornament in many sexually dichromatic species. Given that polygynous breeding systems are the only ones observed in hummingbirds, and that polygyny is often associated with dramatic male ornamentation and sexual dimorphism, it is not surprising that gorgets are usually interpreted as sexual display characters. This proposition is challenged by exceptional ‘polychromatic’ species in which adult females diner in their development of a male-like gorget. This analysis focuses on determining the prevalence of polychromatic variation in the Andean genus Heliangelus. In eight of nine species, females' gorgets range from a dull plumage characteristic of most female hummingbirds (female-like), to a bright iridescent plumage that matches that of the males (male-like). Analysis of museum study-skins reveals that individual variation in gorget colour is limited to females, and that it is independent of month and year of collection. Only H. exortis and H. amethysticollis exhibit significant geographic variation in the frequencies of different colour forms, though most male-like female H. amethysticollis occur in southern Ecuador-northern Peru. Some species-specific features of the male-like females' gorget appear to be linked to qualities of the male gorget (presence/absence, colour and maximum size). However, non-adaptive hypotheses, including genetic correlation between the sexes, hybridization and neutral variation, cannot account for the plumage variation among females. The absence of any association between female gorget colour and breeding condition suggests that gorget colour has a non-sexual function. I propose that gorgets can evolve solely as a non-sexual signal associated with different foraging behaviours.