SEXUAL DICHROMATISM IN BIRDS INDEPENDENT OF DIET, PARASITES AND ANDROGENS

Sexual selection may explain why male animals are typically more colou rful than females. Females may choose brightly coloured males for mati ng because colour is a reliable signal of a male's genetic resistance to parasites, or that he can bear the cost of the immunosuppressant ef fects of androgens. Bright yellows, oranges and reds are the product o f carotenoid pigments, which are known to have significant health bene fits. Therefore, bright colours may be indicative of a bird's quality because it shows access to a superior diet or superior foraging abilit y. We maintained populations of American kestrels and loggerhead shrik es in captivity that were largely free of parasites and fed a uniform diet. Male kestrels were more brightly coloured than females in the co lour of their ceres, lores and legs, and there were pronounced age- an d gender-specific patterns to concentrations of carotenoids in their p lasma. Even though shrikes do not show any carotenoid-based colouratio n, the sexes had pronounced differences in plasma carotenoids. Caroten oids in kestrels were unrelated to androgen levels, but the correlatio n between carotenoids and plasma proteins suggest colour may be a cond ition-dependent trait. These results suggest that neglected physiologi cal processes may regulate carotenoids, and hence some colour variatio n need not be explained by parasites, androgens or diet.