The answer has to be that birds replace a certain percentage of their
feathers all the time as a normal part of living much as older hairs fall
out of our skins as the skin layers age, die and rise to the surface.
Newer (if fewer) hairs replace them, although I would hope a bird doesn't
wind up with progressively fewer feathers as this would predispose it to
hypothermia and wetting.

Like David Wilbur's chickadee, the goldfinch I see in my yard "yellow up"
one feather at a time beginning in February, and gradually go from green
to yellow over a two month period. Similarly, a bird showing albinistic
feathers might change them literally one at a time depending on many
factors like nutrition, age, etc., without undergoing a well-defined molt.
Thus the same bird could appear quite differently within a two or so month
period.


Charles Easterberg
University of Washington
easterbg at u.washington.edu