Tuesday, July 12, 2011

Western Gull

Western Gulls are encountered along the west coast of the United States and Baja California.  They are easily identified--a white bodied, pink-legged, dark-backed gull along the Pacific Coast is a Western Gull.

Western Gulls are, never-the-less, perplexing. Despite their being easy to see, they have a relatively small population size--only about 40,000 pairs nest at fewer than 200 colony sites within their range (Pierotti and Annett 1995). According to these authors, this gull is threatened by El Nino oscillations, oil spills, and pesticides.

In Puget Sound, Western Gulls hybridize with Glaucous-winged Gulls (see photo below), which breed from the Sound on north to Alaska. As many of 75% of birds in Puget Sound are hybrids, often called Puget Sound or Olympic gulls. The northern 300 kilometers of the Western Gull's range is a hybrid zone (Pierotti and Annett 1995). Local birders only identify birds at the extremes of the resulting variation. With such a great amount of genetic flow between Western and Glaucous-winged gulls, I wonder why they are not classified as races of a single species.

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