Thursday, November 6, 2014

“Olympic" Gull

About half-way across Puget Sound, as if on cue, several Western Gulls few up to the Bremerton/Seattle ferry. At the same time, several human teenagers came out on deck, despite the brisk temperatures. and threw popcorn to feed the gulls. I dutifully entered “Western Gull” into my BirdLog eBird app, which indicated that, if a Western Gull was to be found in Puget Sound, it would be rare.

Here is the problem. In Puget Sound, and along the coast from British Columbia to central Oregon, Western Gulls from the south massively hybridize with Glaucous-winged Gulls from the north. The center of this hybrid swarm is Gray’s Harbor, Washington, and the extent of the swarm is increasing, both north and south. The first generation hybrids are usually darker than the northern birds, and lighter than Western Gulls. Backcrosses can be indistinguishable from pure Westerns. These hybrids migrate south in the winter, making identification of Western Gulls difficult south into California (Backyardbider).

Birds of the hybrid swarm are sometimes called “Olympic" or "Puget Sound" Gulls. I do not understand why Western and Glaucous-winged gulls do not constitute a single species. Apparently the two are usually not differentiated by local birders. It may be noteworthy, on my photos, that the first bird appears to have darker gray upper wings than the other two gulls, but all may be hybrids. According to Haywood and Verbeek (2008), hybrids, within the area of hybridization, show superior survival to non-hybirds.

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