Why does my finch lose its feathers?

In spring the birds moult all but their black wing and tail feathers, and the bills of both sexes turn orange. The male assumes brilliant canary yellow plumage and a striking jet black cap; he has a very pleasing call song. In flight, a white rump contrasts with the black tail. The common winter plumage is a duller olive-brown with some yellow still showing on the head. In some winter ranges the birds lose all traces of yellow, becoming predominantly medium tan-grey banded colour, with an olive cast evident only on close viewing.