Editor's Note: Recent mitochondrial genetic data indicate that Carduelis is polyphyletic and that Spinus spp. belong to different clade. See the 50th supplement to the AOU Check-list of North American Birds for details. Future revisions of this account will reflect this change.
The American Goldfinch is an abundant and widely distributed species in temperate North America, common in summer in weedy fields, river flood plains, early second growth forest, and orchards and suburban gardens-habitats where they find their major foods and suitable nesting sites. As the breeding season wanes, flocks form as the birds enter the autumn (Prebasic) molt and prepare to move to winter habitats. Many northern populations migrate, with the occurrence and extent of migration varying with sex, age, and latitude. Wintering flocks are nomadic, their movements closely tied to food supply. During the winter months the species is common at bird feeders.
The American Goldfinch is both sexually and seasonally dimorphic. The males in their bright yellow summer plumage are a familiar sight, but the less brightly colored females are often overlooked. Both sexes are frequently misidentified in their muted winter plumages. The difference between winter and summer plumages is the most striking of any of the cardueline finches and results from a spring (Prealternate) body molt, unique among carduelines.
This goldfinch is also unusual because it is one of the latest breeders of all temperate zone passerines. In the East, it normally waits to nest until late June or early July. Although the cause of this delay is not well understood, there is a close relationship between the flowering of thistles (Cynareae), an important food plant, and the start of nest building. In addition, the physiological effects of spring molt may prohibit early nesting.
This goldfinch's nesting season is a short one. In the East, the last eggs are laid in mid-Aug. As a result, most pairs have time to produce only one brood in a season, although experienced breeders frequently produce two broods if eggs are laid early and the first brood is successful. To permit such second nestings, a female abandons the first brood to her mate, and then leaves to find another mate.
The American Goldfinch is almost exclusively granivorous. It consumes little insect matter, even when feeding nestlings, suggesting that the species is well adapted to obtaining its protein requirements from a seed diet. This diet may explain why the Brown-headed Cowbird (Molothrus ater) fails to survive in goldfinch nests. Even though cowbirds hatch successfully, their growth is retarded and almost all die before they can leave the nest.
Recent interest in this species has centered on the control and function of its striking yellow plumage and orange beak coloration. These colors are derived from carotenoid pigments, which birds and all other vertebrates acquire from their diet. Females prefer to mate with males that exhibit the brightest colors and thus may acquire the most skilled foragers in doing so. The American Goldfinch is also well-known for its susceptibility to the mycoplasmal conjunctivitis outbreak, which has infected and killed many members of its cardueline finch relative-the House Finch (Carpodacus mexicanus)-in the Eastern United States but has had relatively few other wild bird hosts. Finally, it has become a model species for studies of physiological responses to cold tolerance and of grassland-bird sensitivity to habitat disturbance and pesticide use.