Molt is the process of feather replacement. All birds do it; they have to grow new feathers once or twice a year to stay warm, dry, and airborne, and in many cases they grow differently colored feathers at different seasons to match their surroundings or to impress others of their species.
Among the small songbirds, virtually all species have a complete molt (replacing all of their feathers) in late summer, and in addition many species have a partial molt (replacing some of the body feathers but not the wing or tail feathers) in the spring.
American Goldfinch follows this pattern. Beginning in September, and continuing for six to eight weeks, they molt all of their feathers, ending up with a completely new and pristine set of feathers (and drab colors) as they head into the winter. In the spring, as they grow new body feathers the males especially transform into bright yellow breeding plumage, but the wing and tail feathers remain from the previous fall. As these wing feathers get older the pale buff edges fade to white and disintegrate, so that by the end of the summer the wings look essentially all black. And in September another complete molt begins.
Watch the slideshow, or click on the main image to advance, and see what other patterns you can notice.