In the north of its range it forms a hybrid zone with its close relative the glaucous-winged gull (Larus glaucescens).

In adult plumage, The largest western gull colony is on the Farallon Islands, located about 26 mi (40 km) west of San Francisco, California;[7] an estimated 30,000 gulls live in the San Francisco Bay area.

It is migratory, moving to northern Washington, British Columbia, and Baja California Sur to spend the nonbreeding season.

On land they feed on seal and sea lion carcasses and roadkill, as well as cockles, starfish, limpets and snails in the intertidal zone.

Western gulls, including one who lived at Oakland's Lake Merritt are known for killing and eating pigeons (rock doves).

[11] In the central part of the hybrid zone, clutch size was larger among pairs with hybrid males, many of which established breeding grounds in more vegetative cover than pure western gull males, which preferred sand habitat resulting in heavier predation.

Numbers were greatly reduced in the 19th century by the taking of seabird eggs for the growing city of San Francisco.

Western gull colonies also suffered from disturbance where they were turned into lighthouse stations, or, in the case of Alcatraz, a prison.

The automation of lighthouses and the closing of Alcatraz Prison allowed the species to reclaim parts of its range.

Thousands of gulls fly over Oracle Park in San Francisco during late innings of games.

Federal law prohibits shooting the birds, and hiring a falconer would cost the Giants $8000 a game.

[8] The western gull was one of the antagonists in Alfred Hitchcock's famous movie The Birds which was filmed in Bodega Bay, California.