>On Thu, 20 Oct 1994, Dennis Paulson wrote:
>
>> Albinistic mutations are common in birds and are
>> usually taken by predators.
>
>Now _that's_ a fascinating statement! Why should this happen? It's
>obvious that some pure white birds will be more visible to predators than
>their patterned relatives, but when I think about the chickadees hopping
>around in the trees outside my window and compare them to chickadees with
>a little more white, for instance, I can't see why one would be more
>likely to be caught than another. You're really saying that predation is
>a strong selective force in maintaining the plumage pattern of a species,
>and that it's not just due to continuing sexual selection or some other
>factor. Biologists sometimes ask about clear cases of natural selection,
>and I don't think I've ever heard of anyone else making this particular
>connection.
>
>Burt Guttman guttmanb at elwha.evergreen.edu
>The Evergreen State College Voice: 206-866-6000, x. 6755
>Olympia, WA 98505 FAX: 206-866-6794

I should have commented further that the more albinistic a bird is, the
more it stands out to a predator, indeed a case of conspicuousness rather
than merely "difference." A bird with a few white feathers probably
wouldn't fit this criterion, although surely a merlin could more easily
follow a dunlin in a big flock if it had something to make it at all
different from the others. And in a flock of chickadees scattering from a
sharp-shin (as happened in my yard just a couple of days ago), I'll bet the
sharp-shin could focus more easily on the one with scattered white feathers
on its back. There are good data showing that goshawks preferentially crop
white or partly white pigeons from flocks, for example. A lot of people
have commented that the absence of avian predators in Hawaii is probably
why white pigeons are so common there.

The amount of white on the chickadees in my yard make them very conspicuous
indeed.

And, having said all this, I can categorically say we scarcely have an idea
why most birds are colored like they are and how these colors and patterns
are maintained by natural selection. In other words, the specifics of why a
mountain chickadee has a white supercilium and a black-capped doesn't. Too
bad feather colors don't fossilize. I can only hope there are still natural
history museums around in 10,000 years with some of the same specimens
we're preserving now, so future biologists can actually see the changes
that have taken place and perhaps gain some insights.

And Lisa, I'll comment on museum specimens one of these days.....


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