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Starting in spring and continuing through much of summer, male goldfinches are a bright lemon yellow with black foreheads, black wings and white markings above and beneath their tails. (Females are olive above and dull yellow beneath.) Come winter, both sexes turn a drab brown—so dull they often are mistaken for sparrows. The change occurs through the process of molting. The American goldfinch is the only finch species that molts its body feathers twice a year, once in late winter and again in late summer.
Late Breeders
Frequent molting is both time-consuming and physiologically taxing for the birds. Some scientists suggest this may be the reason goldfinches breed so late in the season—rarely beginning in earnest until mid-July. Another possibility is that the birds wait to nest until thistle, milkweed and other plants have produced fibrous seeds, which goldfinches not only eat but also use to build their nests.
Goldfinches are among the strictest vegetarians in the bird world, selecting an entirely vegetable diet and only inadvertently swallowing an occasional insect. This diet helps protect goldfinches from cowbirds: Though the parasitic birds may lay eggs in goldfinch nests, the hatchlings cannot survive long on an all-seed diet.
Color and Fitness
Choosy female goldfinches prefer to mate with males that have the brightest plumage. “Color matters”.
Why? One possibility stems from the fact that the male’s color comes from carotenoid pigments (the chemicals that make carrots orange) that he acquires through his diet. By selecting the brightest males, females also may be getting the best food providers for their offspring.
Autumn 