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.
"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