Dennis Paulson wrote, "I certainly think that the yellow rump on a
yellow-rump is a flight/flocking signal, appropriate for such a gregarious
warbler." Edwin Way Teale (in _North with the Spring_, I think--at least
one of the four "American seasons" books) suggested that the yellow rump
patch is for predator confusion. He pointed out that it's very prominent
when the bird is flying, so a predator might use it as a search image, but
then it disappears when the bird lands, leading to predator confusion.
This is supposed to be similar to the effect in morpho butterflies (I'm
told--I've never seen them in the wild): the brilliant blue color is very
attractive while the insect flies, but then it disappears when the insect
alights and folds its wings, so a predator that had been homing in on the
obvious attractant suddenly finds the image gone.

Of course, the obvious question is, "Why do the birds, or butterflies,
have the attractive feature in the first place?" Maybe, as Dennis
suggests, it's useful for a purpose such as flocking. So maybe the
principle here is, "If you evolve a feature that's attractive to
predators, evolve a way to hide it sometimes."

Let me make an important point that I've made on Tweeters before: Every
feature of an organism does not necessarily have a function. It's easy
to look at a crest on the head or a spot on the wing and ask, "What's
that good for?" The answer may be, "Nuthin!" It just happened. This
population happened to get the genes for long feathers on the head or a
white spot on the wing, and nothing in their lives selected _against_ it,
so there it is, for all to enjoy. Maybe it becomes one of the little
signals that individuals use for finding mates or something like that,
but not necessarily. Every function has to have a form, but not every
form has a function. (General biology lesson for the day, courtesy of
your friendly neighborhood state employee. No charge.)

Burt Guttman guttmanb at elwha.evergreen.edu
The Evergreen State College Voice: 360-866-6000, x. 6755
Olympia, WA 98505 FAX: 360-866-6794