Data from: The effect of colour-producing mechanisms on plumage sexual dichromatism in passerines and parrots

Sexual dichromatism (SD) often reflects intense sexual selection on males. It has been hypothesized that sexual selection should favour the elaboration of those male colours that honestly signal quality and that such colours should therefore show higher SD. Costliness of colours is expected to vary according to their production mechanism (pigment type, feather microstructure and combinations thereof). Carotenoid-based colours, due to their dietary origin and competing functions of carotenoid pigments, are the best documented costly colours, while endogenous pigments and structural colours are expected to be less costly. However, how SD varies with colour-producing mechanism has not been systematically addressed. Here, we test the link between SD and mechanisms of colour production across nearly all Australian passerines (n = 302) and parrots (n = 48), using reflectance spectrometry and visual models. We quantified SD [chromatic (hue and saturation) and achromatic (light to dark)] for individual plumage patches as well as for the whole plumage. We then compared whether these SD estimates vary for plumage produced by different mechanisms of colour production. Not unexpectedly, individual plumage patches were most sexually dichromatic when males and females have different mechanisms of colour production. In most cases however, both sexes share the same mechanism. Among these, red carotenoid-based plumage in passerines showed the highest SD, while in passerines as well as in parrots, melanin-based plumage and white plumage have the lowest levels of SD. Average SD within species is weakly predicted by the proportion of the plumage produced by different mechanisms. This effect is strongest for red carotenoids, but equally so for males and females. In contrast, effects of structural and pheomelanin-based colours are different between sexes, highlighting the links between high SD and the evolution of different mechanisms of colour production in males and females. While our analysis showed that colours based on red carotenoids have higher levels of SD, the differences were small, and most mechanisms of colour production were able to produce a similar range of SD. These results do not support the hypothesis that sexual selection should preferentially lead to the elaboration of some types of colours over others