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 i...