The western gull (Larus occidentalis) is a large white-headed gull that lives on the west coast of North America. It was previously considered conspecific with the yellow-footed gull (Larus livens) of the Gulf of California. The western gull ranges from British Columbia, Canada to Baja California, Mexico, and, because of its convenient colonies on the coast of California, it is well studied. Despite being a well-known bird species on the West Coast of the US, it is of some slight conservation concern given its restricted range (for a gull).
The western gull is a large gull that can measure 55 to 68 cm (22 to 27 in) in total length, spans 130 to 144 cm (51 to 57 in) across the wings, and weighs 800 to 1,400 g (1.8 to 3.1 lb). The average mass among a survey of 48 gulls of the species was 1,011 g (2.229 lb). Among standard measurements, the wing chord is 38 to 44.8 cm (15.0 to 17.6 in), the bill is 4.7 to 6.2 cm (1.9 to 2.4 in) and the tarsus is 5.8 to 7.5 cm (2.3 to 3.0 in). The western gull has a white head and body, and gray wings. It has a yellow bill with a red subterminal spot (this is the small spot near the end of the bill that chicks peck in order to stimulate feeding). It closely resembles the slaty-backed gull (Larus schistisagus). In the north of its range it forms a hybrid zone with its close relative the glaucous-winged gull (Larus glaucescens). Western gulls take approximately four years to reach their full plumage, their layer of feathers and the patterns and colors on the feathers. The western gull typically lives about 15 years, but can live up to 25 years.The largest western gull colony is on the Farallon Islands, located about 26 mi (40 km) west of San Francisco, California; an estimated 30,000 gulls live in the San Francisco Bay area.