The Western Gull is a large white-headed gull that inhabits the Pacific Coast of North America, breeding from central Baja California north to Washington. In winter, this gull may be found throughout its breeding range, north to Vancouver I., south into Baja California, and in adjacent offshore waters of all these areas.
The Western Gull is currently divided into two subspecies based on geographic distribution. The Yellow-footed Gull (Larus livens) from the Gulf of California was formerly considered a third subspecies (L. o. livens). The behavior and ecology of both subspecies are well studied. Favored nesting sites appear to be islands, offshore rocks, and abandoned piers where birds are safe from terrestrial predation.
Although a familiar and well-known species on the Pacific Coast, the Western Gull is limited in distribution and has a smaller population size than most other North American gulls, with a total population of only about 40,000 pairs nesting at fewer than 200 colony sites. More than 30 percent of the total world population nests on Southeast Farallon Island, California ( Sowls et al. 1980 , Penniman et al. 1990 ). Western Gulls hybridize extensively with Glaucous-winged Gulls (Larus glaucescens), rendering the northern 300 kilometers of the Western Gull's range a hybrid zone ( Bell 1992 ). Low population numbers, recurrent El Niño Southern Oscillation-like (ENSO) conditions in the eastern Pacific Ocean, threats from oil spills, effects of pesticides on reproduction, extensive hybridization, and indifference among ornithologists to its fate render the Western Gull a species of conservation concern.