Posted by Bill Smith (18.104.22.168) on April 17, 2001 at 18:42:27:
I see a few birds like the Marin gull every spring about this time along the outer Washington coast. On my monitor, the Marin gull's bill is pink and black (not yellowish as David Fix apparently sees). To me, the pink suggests Glaucous parentage. But, the more extensive black along the lower mandible, extending much closer to the base of the bill than on the top, is not characteristic of 'pure' Glaucous, as far as I know. Also, the bird looks too petite, too high crowned, to be 'pure' Glaucous.
These differences seem pronounced when a bird like this is together in the field with a more typical Glaucous in similar plumage, as I've seen here a few times recently. I think the bill color in itself rules out various eastern-arctic-breeding species like Iceland or Thayer's, although not necessarily their hybrids, I suppose.
I don't know that we know enough about arctic gulleries to know for sure where what interbreeds with what, and how offspring of first generation hybrids progress in plumage and then backcross when adult. All large white-headed gulls are very similar genetically, so fertile hybridization is to be expected. It's easy to infer that the Marin gull may have Glaucous-winged genes because a cross between that species and Glaucous is perhaps the closest possibility, but birds I've seen which seem more obviously that hybrid have much deeper gonydeal angles, like Glaucous-wingeds usually do. But, it needn't be a first generation hybrid.
One other comment is that birds more-or-less like this seem to be picked out somewhere out-of-range at this particular season, and I think their true characters may be obfuscated by fading, wear, and perhaps hormonal activity. Along the outer Washington coast right now, we have lots of strikingly snowy-white immature gulls with jet black bills with the thickness and deep gonydeal angle of Glaucous-winged or our local Glaucous-winged - Western hybrid swarm. If one of these birds appeared much further away, I can't help but wonder what people would think it was.
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