I'd like to contribute to this thread, which has long interested me.

1) I've seen Heermann's Gulls with white wing patches in WA several times,
and I'm sure they occur throughout the range of the species.

2) I agree entirely that it's a genetically based "morph." These birds have
persisted in the species for many decades since they were first described
in the literature. I've never seen much written about their frequency, but
in my experience it is considerably less than the 10% mentioned by Jane
Westervelt, more like 1%.

3) I have speculated that the white wing patches are actually advantageous
in making a kleptoparasitic, all dark gull look more like a jaeger. Jaegers
are surely recognized by gulls and terns, which may be more likely to
quickly relinquish a fish to an oncoming "jaeger." Total speculation, with
no evidence whatsoever, but it would be interesting to test if someone
could watch a large enough sample of white-winged Heermann's (and normal
birds for controls, of course).

4) I think the white patches are more typical of adults than immatures.
I've never seen one on an immature.

5) I agree wholeheartedly with Jane that it is counterproductive to try to
assign some human-related environmental problem as the cause of every odd
thing we see in nature!

Dennis Paulson, Director phone 253-756-3798
Slater Museum of Natural History fax 253-756-3352
University of Puget Sound e-mail dpaulson at ups.edu
Tacoma, WA 98416
http://www.ups.edu/biology/museum/museum.html