Subject: Re: Color Change
Date: Oct 25 13:29:36 1994
From: Dennis Paulson - dpaulson at ups.edu


>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

Much as I hate to be contentious on such a benign forum as Tweeters, I must
take issue with Charles. Birds replace feathers that fall out by accident
(i.e., pulled out by a cat during an escape [haven't you seen part-tailed
birds in your yard?] or perhaps pulled out during an encounter with a spiny
plant), but otherwise they have a very well-defined annual molt during
which the entire plumage is replaced--in the autumn in our area. Species in
which there is a plumage change during the year (e.g., Yellow-rumped
Warbler and *American Goldfinch*) of course molt their body plumage twice a
year, the other time in late winter or early spring. There are plenty of
exceptions to this, but mostly in large birds, in which partial or complete
wing or tail molt may take place at other times of year (for example, some
loons molt their wings in spring, some young shorebirds their tail feathers
in spring, western grebes appear to molt head and neck feathers through a
long period in winter and spring, immature ducks may be molting all through
the winter as they move from one plumage into another). These exceptions in
no way controvert the "rule" that in virtually all birds there is a
complete molt every autumn and a general lack of molt through the remainder
of the year.

Picture the difference between an evergreen and a deciduous tree. The
evergreen loses needles (most conifers are evergreens) throughout the year
as they senesce and die, while the deciduous tree has been programmed
evolutionarily to hold its leaves all summer and drop them all at once in
autumn. Humans are evergreen (except some older men), chickadees deciduous
(except of course they regrow their "leaves" immediately, unlike deciduous
trees). Charles was right when he said that the need for insulation was a
constraint on molting, which may be why molt is programmed for relatively
benign seasons. No bird molts actively during the winter in cold climates.

Thus there is nothing in the normal black-capped chickadee molt scenario to
account for a white-tailed chickadee becoming gray-tailed through the
winter. And if there were regular molt, it should be obvious on a bird that
you watched every day, with short gray tail feathers coming in where white
ones were previously present. And it would only happen under very unusual
conditions (as in a sick bird?). It's very interesting that this doesn't
seem to be occurring in the wings, and it's hard to avoid the conclusion
that it is limited to the dorsal surface of the bird, thus an environmental
factor coming from above. As I said, it's too far out for me to understand.


Dennis Paulson phone: (206) 756-3798
Slater Museum of Natural History fax: (206) 756-3352
University of Puget Sound email: dpaulson at ups.edu
Tacoma, WA 98416