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, 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 wintering habitats. Many northern populations migrate, with the occurrence and extent of migration varying by 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 in plumage. The males in their bright yellow breeding plumage are a familiar sight, but the less brightly colored females are often overlooked. Both sexes are frequently misidentified in their muted nonbreeding plumages. The difference between nonbreeding and breeding plumages is the most striking of any of the cardueline finches and results from a Prealternate body molt during spring, 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-August. 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, the 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 mycoplasmal conjunctivitis, which has infected and killed many House Finches (Haemorhous 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 sensitivity to habitat disturbance and pesticide use.