|American Goldfinch on teasel. May 2, 2012, Forest Grove, Oregon.|
The American Goldfinch has a spring molt from the tan basic (non-breeding) plumage in the fall to the bright yellow alternate (breeding) plumage. This molt strategy is called Complex Alternate. (The "complex" part means that after the juvenile plumage there is another molt not repeated in later molt cycles.)
A 1977 article in Condor says that "the prenuptial molt of the body plumage is unique to the American Goldfinch." It explains: "other cardueline species acquire the breeding aspect through abrasion."
What this means is that all other cardueline species--finches, other goldfinches and siskins, rosy-finches, and crossbills, evening and pine grosbeaks (but not rose-breasted, black-headed, or blue grosbeaks, which are in the cardinal family)--have only one molt per year, in the autumn. This molt strategy is called Complex Basic. Every fall they molt from basic to basic with no change in plumage pattern or color, other than fresh new feathers.
From reading Steve Howell's book on molt, though, some Lesser Goldfinches also have a Complex Alternate molt strategy--at least in the SW deserts.
American Goldfinches, and Lesser Goldfinches in the SW deserts, delay breeding until August. Thus they have enough time to have a prealternate molt before the breeding season.
An explanation of the process is a bit complicated, but the results are spectacular!