The American goldfinch is a small but one of the most widespread birds in North America. These colorful birds undergo a molt in the spring and autumn. Males are vibrant yellow in the summer and olive color during the winter, while females are dull yellow-brown shade which brightens only slightly during the summer. The autumn plumage is almost identical in both sexes, but males have yellow shoulder patches. In some winter ranges, American goldfinches lose all traces of yellow, becoming a predominantly medium tan-gray color with an olive tinge evident only on close viewing.
American goldfinches breed across North America from coast to coast. Their range is bounded on the north by Saskatchewan and stretches south across North America to North Carolina on the east coast, and northern California on the west coast. American goldfinches are short-distance migrants, moving south in response to colder weather and lessened food supply. Their winter range includes southern Canada and stretches south through the United States to parts of Mexico. American goldfinches prefer open countries such as fields, meadows, flood plains, as well as roadsides, orchards, and gardens. They may also be found in open deciduous and riparian woodlands and areas of secondary growth. In winter, in the northern part of their range, these birds may move nearer to feeders if they are available. In southern ranges, during winter, they remain in areas similar to the fields and flood plains where they live during the summer months.
American goldfinches fly in a distinctive undulating pattern, creating a wave-shaped path. This normally consists of a series of wing beats to lift the bird, then folding in the wings and gliding in an arc before repeating the pattern. Birds often vocalize during the flight producing "per-twee-twee-twee", or "ti-di-di-di" calls, punctuated by the silent periods. American goldfinches are gregarious during the non-breeding season and are often found in large flocks, usually with other finches. During the breeding season, they live in loose colonies, however, during the nest construction breeding pairs become aggressive, driving intruders away. American goldfinches express aggression through multiple displays. The head-up display, where the neck and legs are slightly extended, shows mild aggression. At higher intensities, the neck is lowered, the beak is pointed at the opponent, and one or both wings are raised. In extreme cases, the neck is retracted, the bill opened, the body feathers sleeked, and the tail is fanned and raised slightly. Aggression is also displayed by showing the front of the body to another individual. Attacks include pecking at feathers, supplanting the opponent by landing next to it, and flying vertically with legs and feet extended, beaks open, and necks extended. American goldfinches are diurnal feeders; they frequently hang from seedheads while feeding in order to reach the seeds more easily. In the spring, these birds feed on the catkins hanging from birches and alders by pulling one up with their beak and using their toes to hold the catkin still against the branch.
American goldfinches are mainly granivores, but will occasionally eat insects, which they also feed to their chicks to provide protein. The diet of these little birds consists of seeds from a wide variety of plants and trees. However, they also consume tree buds, maple sap, and berries.
American goldfinches are serially monogamous and form pair bonds every breeding season. However, females may mate with more than one mate. The breeding usually starts in late July with courtship displays. The courtship rituals include aerial maneuvers and singing by males. The flight displays begin as the male pursues the female, who flies in zigzagging evasive patterns. If a female accepts the male as a mate, the pair will fly in wide circles, with the male singing throughout the flight. Once a male has found a mate, he selects a territory, marking the boundaries by warbling as he flies from perch to perch. Two or three pairs may group their territories together in a loose colony, perhaps to aid in defense against predators. The nest is built by the female in the branches of a deciduous shrub or tree at a height of up to 10 m (33 ft). The outer shell of the nest is built of bark, weeds, vines, and grass. The cup is lined with plant down from milkweed, thistle, or cattail. The female lays 4 to 6 bluish-white eggs, which are roughly the size of a peanut and are incubated by the female alone within 12-14 days. The chicks are altricial; they are hatched naked, with reddish bodies, pale grey down, and closed eyes. The mother feeds her young regurgitated seeds and insects as they grow. The hatchlings develop quickly, opening their eyes after 3 days, and completing the growth of olive-brown juvenile plumage after 11-15 days; at this time they begin to practice short flights close to the nest. For up to 3 weeks after fledging, they are still fed by the male, who locates them by listening for their fledging call. The young become reproductively mature at 11 months of age.
There are no major threats to American goldfinches at present. However, in some areas of their range, these little birds suffer from diseases and can be easily poisoned by chemicals. They are also often killed by collisions with vehicles.
According to the What Bird resource, the total population size of the American goldfinch is around 24 million individuals. According to the All About Birds resource, the total breeding population size of the species is 42 million birds. Overall, currently, American goldfinches are classified as Least Concern (LC) on the IUCN Red List and their numbers today are increasing.
Due to their seed-eating preferences, American goldfinches help to disperse seeds in the ecosystem they live in.