The brightly colored male American goldfinch is especially recognizable. The American regularly visits seed feeders, particularly in the east. It is often very gregarious, especially during the nonbreeding season, when it flocks to roadsides and brushy fields to feed on thistle and sunflowers. It is often heard in flight, giving distinct flight calls. Polytypic (4 named ssp.; differences slight). Length 5" (13 cm).
The American goldfinch flies 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 flapping phase of the pattern and then go silent during the coasting phase. The call made during flight is "per-twee-twee-twee", or "ti-di-di-di", punctuated by the silent periods. The American goldfinch is gregarious during the non-breeding season, when it is often found in large flocks, usually with other finches. During the breeding season, it lives in loose colonies. While the nest is being constructed, the male will act aggressively toward other males who intrude into his territory, driving them away, and the female reacts in the same way toward other females. This aggressiveness subsides once the eggs have been laid.
A relatively large carduelid. Breeding male: unmistakable. Body entirely bright lemon yellow with white undertail coverts. Jet black cap. Black wings with yellow lesser coverts and narrow white tips to greater coverts, forming 2 white wing bars along with white edging to the tertials. White inner webs to most of the tail feathers. Pink, conical bill. Breeding female: very different from male. Underparts very yellow with white undertail coverts, while upperparts, including head, olive green. Lower wing bar buffy and quite wide. Tail feathers with white tips and inner webs. Bill pinkish. Winter male: cinnamon brown above and on breast and flanks, with white lower belly and undertail coverts, yellowish wash on throat and face, and muted black on forehead. Wings more boldly patterned. Yellow lesser coverts. Wide, whitish lower wing bar. Bill darker than in breeding season. Winter female: mostly drab gray body with black wings and 2 bold buffy wing bars. White undertail coverts and edging to tail feathers. Dark bill. Immature male: black on forehead reduced or lacking. Lesser coverts duller. Juvenile: resembles adult female. Unstreaked.
[source: National Geographic]