Subject: Re: the significance of a wing bar
Date: Apr 27 09:31:53 1995
From: Dennis Paulson - dpaulson at ups.edu


Chris Hill wrote:

"So when you look for meaning in the coloration of the third under scapular
rectrix covert, I think you have to ask yourself 'Is this just an accident,
like _Conus_ shells?'"

I can only point out that wing bars *are* exposed, unlike the shells of
cones or cowries (the latter are not venomous and thus provide evidence
against the "warning-coloration" hypothesis for cone shell color, by the
way). Thus wing bars *could* be explained by something relating to their
appearance. I've been toying with hypotheses as purely speculative as
whether wear on a wing bar (that is, its disappearance) could provide
visual feedback to stimulate a bird to molt. Stranger things occur in
nature. And, by the way, this hypothesis could be tested experimentally.

The contrasty edgings on the upperparts of juvenile shorebirds make them
better camouflaged from predators on their vegetated breeding grounds, and,
as the edgings wear off, the birds become more uniformly colored and better
camouflaged on their mud and sand migration stopovers. Then they molt into
a more adultlike plumage and are perfectly camouflaged for their wintering
grounds. It probably doesn't matter exactly what sort of edging evolves,
the idea is to try to come up with an explanation for edgings as a class.
Same with wing bars, eye rings, and the like.

I'd also be perfectly happy if someone comes up with a mechanistic
(developmental) hypothesis to explain wing bars, etc.; my plea would merely
be not to dismiss them as "uninteresting," because no one has been able to
explain them. And I wonder if a developmental biologist would call the
pattern on a cone shell "accidental." It's obviously a different
perspective, and one could say that natural selection has brought about
patterns of development, even though the resulting color pattern is not
adaptive. How about an alternative hypothesis that these gastropods have
evolved pretty shells so shell collectors will love them?

And David Wright wrote:

" Suppose the common ancestor of warblers and
vireos had wing bars. If one species of vireo and one species of warbler
lost wingbars, all the descendant species of those ancestral populations
would lack wingbars. Explaining two losses of wingbars as resulting from
drift sounds a lot more plausible than trying to explain independent
losses in dozens of species, which is the assumption implicit in raw
comparisons of how many have them vs. how many do not. And those that do
have wingbars? That may require only one event -- the acquisition of
wingbars in the common ancestor."

But in fact vireos and warblers didn't have a common ancestor, and, if you
look at books on birds of different parts of the world, you will see again
and again that some species of a family have wing bars, and some don't.
Common ancestry can't stand up as a hypothesis here at all, as far as I can
see. I was using vireos and warblers as an easily understood example, but
examples abound, many independent gains and/or losses of wing bars. Anyone
interested in bird coloration should sit down with a half-dozen books with
colored illustrations of birds from North America, Europe, South America,
southeast Asia, Africa, and Australia. See how many color patterns are
shared by unrelated species, as well as how much diversity there is. It's
mind-boggling.

Sure, a lot of the bright colors of birds are probably a response to sexual
selection, but I suspect in fact that the little markings like eye lines,
eye rings, and wing bars *aren't.* It's obvious that a Marsh Wren is brown
to be cryptic in the cattails, and a male Red-winged Blackbird is black and
red to show up in the same marsh, but there are many other variations on
the theme of bird coloration that need to have the bright beam of
speculation turned on them. Speculation can do no harm, and in science it
has to be matched up with evidence to mean very much. To some (I guess
weird) people it's as much fun as birding.

David, further:

"So crests in birds such as jays may well be the product of sexual
selection on males, even though they are present in both sexes. (Along
the lines of Burt Guttman's suggestion, if these crests incur little cost
in females, there will be little pressure for females to "lose" them, and
that slight negative selection will be swamped by sexual selection for the
crest in males, and they could be retained by females indefinitely despite
having no function in females)"

But studies on Steller's Jays have shown that *both* sexes use them in
signalling "moods." That seems the more parsimonious explanation to me.
David, can you think of a bird species in which males use their crest but
females don't?

I'm merely making a plea for thought, nothing more, and I really appreciate
David's and Chris's contribution to the discussion. It's actually about
birds!

Send in those cards and letters, folks.

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