The quick answer is Glaucous-winged Gulls tend to exploit a wider range of habitats than Westerns, especially out of nesting season. They are far more common inland. and also common farther out to sea,than Westerns.
IMO birders tend not to appreciate the amount of variation in GW Gull wingtips. If you read Doug Bell's papers frpm a couple of decades ago, this should become clearer. Per Bell, the contact zone has waxed and waned as populations expanded and contracted, for millennia. Thus it is not unlikely that darker primary tips, particularly in birds otherwise looking like Glaucous-wings might be persistent vestiges or earlier contacts, not evidence of recent mixed ancestry.
On 10/6/2017 8:12:30 PM, Rusty Scalf <rscalf...> wrote:
I've been in Oceanside, Tillamook for a couple weeks visiting family.
On the coast are many gulls, the great majority of which are Western. Some California, fewer Ring-billed, and a few Heermann's and Glaucus-winged. The GW I see generally show evidence of blending with Western. Two weeks and I've seen just a single really pure looking GW.
Nine miles east in the town of Tillamook are a good number of gulls, 100% of them so far are very 'pure' looking Glaucus-winged. I mean of the sort one might see in Alaska. Most of these are adult. I have yet to see a different species. There were half dozen in the Safeway Parking lot; all GW. This is an inland location (though granted not far from Tillamook Bay and Netarts Bay waters).
I always presumed Glaucus-winged are as Marine as Western.
Just the luck of the draw?