Body size and climate as predictors of plumage colouration and sexual dichromatism in parrots

Psittaciformes (parrots, cockatoos and lorikeets) comprise one of the most colourful clades of birds. Their unique pigments and cavity nesting habits are two potential explanations for their colourful character. However, plumage colour varies substantially between parrot species and sometimes also between males and females of the same species. Here, we use comparative analyses to evaluate what factors correlate with colour elaboration, colour diversity and sexual dichromatism. Specifically, we test the association between different aspects of parrot colouration and (1) the intensity of sexual selection and social interactions, (2) variation along the slow-fast life-history continuum and (3) climatic variation. We show that larger species and species that live in warm environments display more elaborated colours, yet smaller species have higher levels of sexual dichromatism. Larger parrots tend to have darker and more blue and red colours. Parrots that live in humid environments are darker and redder, whilst species inhabiting warm regions have more blue plumage colours. In general, the variables we considered explain small to moderate amounts of variation in parrot colouration (up to 20%). Our data suggest that sexual selection may be acting more strongly on males in small, short-lived parrots leading to sexual dichromatism. More elaborate colouration in both males and females of the larger, long-lived species with slow tropical life-histories suggests that mutual mate choice and reduced selection for crypsis may be important in these species, as has been shown for passerines.