Carotenoid pigments are a widespread source of ornamental coloration in vertebrates and expression of carotenoid-based colour displays has been shown to serve as an important criterion in female mate choice in birds and fishes. Unlike other integumentary pigments, carotenoids cannot be synthesized; they must be ingested. Carotenoid-based coloration is condition-dependent and has been shown to be affected by both parasites and nutritional condition. A controversial hypothesis is that the expression of carotenoidbased coloration in wild vertebrates is also affected by the amount and types of carotenoid pigments that are ingested. We tested this carotenoid-limitation hypothesis by sampling the gut contents of moulting house finches and comparing the concentration of carotenoid pigments in their gut contents with the colour of growing feathers. We found a positive association: males that ingested food with a higher concentration of carotenoid pigments grew brighter ornamental plumage. We also compared the concentration of carotenoids in the gut contents of males from two subspecies of house finches with small and large patches of carotenoid-based coloration. Consistent with the hypothesis that carotenoid access drives the evolution of carotenoid-based colour displays, males from the population with limited ornamentation had much lower concentrations of carotenoids in their gut contents than males from the population with extensive ornamentation. These observations support the idea that carotenoid intake plays a part in determining the plumage brightness of male house finches.

Like males of many bird species, male House Finches (Carpodacus mexicanus) have patches of feathers with ornamental coloration that are due to carotenoid pigments. Within populations, male House Finches vary in expression of ornamental coloration from pale yellow to bright red, which previous research suggested was the result of variation in types and amounts of carotenoid pigments deposited in feathers. Here we used improved analytical techniques to describe types and amounts of carotenoid pigments present in that plumage. We then used those data to make comparisons of carotenoid composition of feathers of male House Finches at three levels: among individual males with different plumage hue and saturation, between age groups of males from the same population, and between males from two subspecies that differ in extent of ventral carotenoid pigmentation (patch size): large-patched C. m. frontalis from coastal California and small-patched C. m. griscomi from Guerrero, Mexico. In all age groups and populations, the ornamental plumage coloration of male House Finches resulted from the same 13 carotenoid pigments, with 3-hydroxy echinenone and lutein being the most abundant carotenoid pigments. The composition of carotenoids in feathers suggested that House Finches are capable of metabolic transformation of dietary forms of carotenoids. The hue of male plumage depended on component carotenoids, their relative concentrations, and total concentration of all carotenoids. Most 4-keto (red) carotenoids were positively correlated with plumage redness, and most yellow carotenoid pigments were negatively associated with plumage redness, although the strength of the relationship for specific carotenoid pigments varied among age groups and subspecies. Using age and subspecies as factors and concentration of each component carotenoid as dependent variables in a MANOVA, we found a distinctive pigment profile for each age group within each subspecies. Among frontalis males, hatch-year birds did not differ from adults in mean plumage hue, but they had a significantly lower proportion of red pigments in their plumage, and significantly lower levels of the red piments adonirubin and astaxanthin, but significantly higher levels of the yellow pigment zeaxanthin, than adult males. Among griscomi males, hatch-year birds differed from adults in plumage hue but not significantly in pigment composition, though in general their feathers had lower concentrations of red pigments and higher concentrations of yellow pigments than adult males. Both adult and hatch-year frontalis males differed from griscomi males in having significantly higher levels of most yellow carotenoid pigments and significantly lower levels of most red carotenoid pigments. Variation in pigment profiles of subspecies and age classes may reflect differences among the groups in carotenoid metabolism, in dietary access to carotenoids, or in exposure to environmental factors, such as parasites, that may affect pigmentation.

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).

No abstract