INFLUENCE OF DIETARY CAROTENOIDS ON PLASMA AND PLUMAGE COLOR IN THE HOUSE FINCH - INTRASEXUAL AND INTERSEXUAL VARIATION

1. Although carotenoid pigments cause much of the red, orange, yellow and violet coloration in feathers, the processes by which carotenoids are transported from the diet to those feathers are poorly understood. 2. Male house finches (Carpodacus mexicanus) display substantial variation in expression of carotenoid-based plumage coloration, an ornamental trait known to influence female choice. 3. To test the hypothesis that variation in carotenoid pigmentation of feathers is caused by processes that occur before the uptake of carotenoids by follicle cells during moult, we compared the hues of blood plasma and growing feathers in three populations (two in California and one in Mexico) of house finches that differed in median male plumage brightness. 4. In all three populations, there was a significant positive correlation between the hues of plasma and plumage in males, but there were no significant correlations between the hues of plasma and plumage in females. 5. In all three populations, the plasma hue of adult males was significantly brighter red than that of adult females; the plasma hue of males from the two brightly plumaged populations was also significantly brighter red than that of males from the drabber population. 6. These results suggest that sex-specific differences in the expression of carotenoid pigmentation and intraspecific variation in the expression of male plumage coloration is the result of processes that occur before carotenoid uptake by follicle cells during feather growth (e.g. due to the influence of foraging behaviour, gut parasites or food quality).