THE PROXIMATE BASIS OF INTER-POPULATION AND INTRA-POPULATION VARIATION IN FEMALE PLUMAGE COLORATION IN THE HOUSE FINCH

As in many sexually dichromatic species in which males are brightly colored, female House Finches (Carpodacus mexicanus) show a subdued expression of the same coloration as males. I quantified the carotenoid plumage coloration of females from the subspecies C m. frontalis in Michigan, New York, Hawaii Island, and two sites in California, and from the subspecies C m. griscomi in Guerrero, Mexico. The proportion of females with detectable carotenoid pigmentation differed significantly among populations, as did the median plumage brightness of colorful females. In Michigan, but not California, yearling females tended to be more colorful than older females. Among C. m. frontalis populations, there was a significant positive correlation between the plumage brightness of females and males, but in the C m. griscomi population males were brightly colored while females were drab. In aviary experiments, females of all ages and from all populations converged on a similar plumage brightness after molt when fed a common diet. Moreover, females from all populations showed maximum color expression when provided with abundant red carotenoid pigments. These observations suggest that local and regional variation in the plumage brightness of females reflects local and regional variation in the availability of dietary carotenoid pigments and that female House Finches do not actively forage for carotenoids.