Reversed plumage ontogeny in a female hummingbird: implications for the evolution of iridescent colours and sexual dichromatism

Abstract

In polygynous birds, bright plumage is typically more extensive in the sexually competitive males and develops at or after sexual maturity. These patterns, coupled with the importance of male plumage in sexual displays, fostered the traditional hypothesis that bright plumages and sexual dichromatism develop through the actions of sexual selection on males. This view remains problematic for hummingbirds, all of which are polygynous, because their bright iridescent plumages are also important non-sexual signals associated with dominance at floral nectar sources. Here I show that female amethyst-throated sunangels [Heliangelus amethysticollis (d'Orbigny & Lafresnaye)], moult from an immature plumage with an iridescent gorget to an adult plumage with a non-iridescent gorget. This ‘reversed’ ontogeny contradicts the notion that iridescent plumage has a sexual function because sexual selection in polygynous birds should be lowest among non-reproductive immature females. Moreover, loss of iridescent plumage in adult females indicates that adult sexual dichromatism in H. amethysticollis is due in large part to changes in female ontogeny. I suggest that both the ontogeny and sexual dichromatism evolved in response to competition for nectar.