Carotenoid pigments in male house finch plumage in relation to age, subspecies, and ornamental coloration
ABSTRACT.-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. Received 18 January 1999, accepted 11 June 2001.
CAROTENOID PIGMENTS ARE responsible for the bright red, orange, and yellow coloration of plumage. Birds obtain those carotenoids exclusively through their diet. No animal has been shown unequivocally to be capable of in vivo synthesis of carotenoids (Goodwin 1984, 1986; Schiedt 1990). In birds, dietary carotenoids may either be deposited directly into feathers or chemically changed fromingested forms prior to pigment deposition, typically by addition or elimination of oxygen groups to one or both end rings of the molecule (Davies 1985, Goodwin 1986, Tyzckowski and Hamilton 1986a, b; Brush 1990, Schiedt 1990).
The House Finch (Carpodacus mexicanus) is a sexually dichromatic passerine bird species in which males display bright, carotenoid-based patches of color on their crowns, throats, breasts, and rumps, and male House Finches vary in expression of that ornamental coloration from a bright red to a dull yellow (Michener and Michener 1931, Hill 1990, 1993a). The carotenoid pigments responsible for colorful plumage in the House Finch and the pigmentary basis for variation among males in expression of that coloration were first studied by Brush and Power (1976). They attributed plumage color variation to differences in constituent carotenoids in feathers. Red birds had the most complex assemblage of pigments, consisting of beta-carotene, a group of unidentified mixed xanthophylls, orange isocryptoxanthin, and red echinenone; orange birds had the same subset of carotenoids without echinenone; and yellow birds lacked both echinenone and isocryptoxanthin. Recent analyses of several congeneric finch species of the Palearctic Carduelinae done by Stradi et al. (1995a, b; 1996, 1997), using new analytical techniques, revealed a more complex pattern that differed substantially from that described by Brush and Power (1976).
The proximate basis of variation in carotenoid-based plumage coloration in House Finches is of interest beyond improved understanding of the physiological control of avian pigmentation. Plumage redness in House Finches has been shown to be a primary criterion used by females in choosing mates (Hill 1990, 1991, 1994a). In addition, plumage brightness in male House Finches is correlated with overwinter survival (Hill 1991), nutritional condition during molt (Hill and Montgomerie 1994), parasite load (Thompson et al. 1997, Brawner et al. 2000), and provisioning of females during incubation (Hill 1991). It has been proposed that male plumage brightness is an honest signal of male condition, because carotenoids may be scarce resources in the environment and carotenoid-based color displays may be costly to produce (Hill 1994b, 1996a, 2002). A thorough understanding of the signal content of carotenoid-based ornamental displays can only be achieved, however, through an understanding of the proximate control of variation among males in expression of these displays (Hill 1992, 1996a, 2002).