Sexual dichromatism in birds independent of diet, parasites and androgens

Sexual selection may explain why male animals are typically more colourful than females. Females may choose brightly coloured males for mating because colour is a reliable signal of a male's genetic resistance to parasites, or that he can bear the cost of the immunosuppressant effects of androgens. Bright yellows, oranges and reds are the product of carotenoid pigments, which are known to have significant health benefits. Therefore, bright colours may be indicative of a bird's quality because it shows access to a superior diet or superior foraging ability. We maintained populations of American kestrels and loggerhead shrikes in captivity that were largely free of parasites and fed a uniform diet. Male kestrels were more brightly coloured than females in the colour of their ceres, lores and legs, and there were pronounced age- and gender-specific patterns to concentrations of carotenoids in their plasma. Even though shrikes do not show any carotenoid-based colouration, the sexes had pronounced differences in plasma carotenoids. Carotenoids in kestrels were unrelated to androgen levels, but the correlation between carotenoids and plasma proteins suggest colour may be a condition-dependent trait. These results suggest that neglected physiological processes may regulate carotenoids, and hence some colour variation need not be explained by parasites, androgens or diet.