Posted by s alessi on February 11, 2002

Q: how do goldfinches go from summer yellow to winter drab? new feathers? if so, why dont we see piles of yellow feathers? is the change driven by light, temp or something else?

A: Feathers are not permanent things — they are are shed and replaced on a regular basis. The process of shedding and replacing feathers is called "molting". Most birds molt all of their wing, tail and body feathers late in summer and undergo a second, more limited molt of just their body feathers in early spring.

The timing and number of yearly molts varies from species to species, but American Goldfinches molt twice yearly: in early fall they molt all of their feathers and replace them with a drab winter plumage. In the spring their dull body feathers are replaced with bright yellow ones. Bright yellow plumage is a good thing during the summer because males use this "breeding" plumage to attract females. Drab plumage is an advantage during the winter when the birds aren't breeding because it makes them less obvious to predators. Females remain duller than males year round, probably for a similar reason: females may spend several weeks sitting on a nest where it is advantageous to blend in to their surroundings.

By late summer a bird's feathers can be very worn, so the autumn molt gives them a fresh set just before they migrate. In the temperate zone the cue for molt initiation is day length, which has an effect on the hormone levels that ultimately control molt progression. The reason you don't see piles of feathers is probably because birds shed them slowly over a long period, dropping a feather here and there. Shed feathers are small and delicate, but if you look closely you can find them almost any time of year.

Solon Morse ( February 12, 2002)