Western gull

Appearance

The western gull is a large gull that can measure 55 to 68 cm in total length, spans 130 to 144 cm across the wings, and weighs 800 to 1,400 g. The average mass among a survey of 48 gulls of the species was 1,011 g. Among standard measurements, the wing chord is 38 to 44.8 cm, the bill is 4.7 to 6.2 cm and the tarsus is 5.8 to 7.5 cm. The western gull has a white head and body, and upperparts or mantle is dark grey. The head generally remains white year-round, developing little to no streaking in northern populations. It has a large and bulbous-tipped yellow bill with a red subterminal spot. The eye colour varies, averaging pale yellow in southern populations and darker in northern populations. It closely resembles the slaty-backed gull of Asia, but the latter species has paler eyes and deeper pink legs and eye orbitals. In the north of its range it forms a hybrid zone with its close relative the glaucous-winged gull. Western gulls take approximately four years to reach their full plumage, their layer of feathers and the patterns and colors on the feathers. In adult plumage, The largest western gull colony is on the Farallon Islands, located about 26 mi west of San Francisco, California; an estimated 30,000 gulls live in the San Francisco Bay area. Western gulls also live in the Oregon Coast.

Two subspecies are recognized, differentiated by the mantle and eye colouration. The northern subspecies "L. o. occidentalis" is found between Central Washington and Central California, has dark grey upperparts. The southern subspecies "L. o. wymani" is found between central and southern California has a darker mantle and has paler eyes on average. "wymani" has more advanced plumage development than "occidentalis", and generally attains adult plumage by the third year.The call of the Western gull is bright, piercing, and repetitive.

Distribution

The western gull is a year-round resident in California, Oregon, Baja California, and southern Washington. It is migratory, moving to northern Washington, British Columbia, and Baja California Sur to spend the nonbreeding season.

Behavior

The western gull rarely ventures more than approximately 100 miles inland, almost never very far from the ocean; it is almost an exclusively marine gull. It nests on offshore islands and rocks along the coast, and on islands inside estuaries. A colony also exists on Ano Nuevo Island, then Anacapa Island in Channel Islands National Park, and Alcatraz Island in San Francisco Bay. In the colonies, long term pairs aggressively defend territories whose borders may shift slightly from year to year, but are maintained for the life of the male.

Habitat

The western gull is a year-round resident in California, Oregon, Baja California, and southern Washington. It is migratory, moving to northern Washington, British Columbia, and Baja California Sur to spend the nonbreeding season.

Reproduction

A nest of vegetation is constructed inside the parent's territory and 3 eggs are laid. These eggs are incubated for a month. The chicks, once hatched, remain inside the territory until they have fledged. Chicks straying into the territory of another gull are liable to be killed by that territory's pair. Chick mortality is high, with on average one chick surviving to fledging. On occasion, abandoned chicks will be adopted by other gulls.

Food

Western gulls feed in pelagic environments and in intertidal environments. At sea, they take fish and invertebrates like krill, squid and jellyfish. They are unable to dive and feed exclusively on the surface of the water. On land they feed on seal and sea lion carcasses and roadkill, as well as cockles, starfish, limpets and snails in the intertidal zone. They also feed on human food refuse, in human-altered habitats including landfills, and take food given to them, or stolen from people at marinas, beaches and parks. Western gulls are known to be predatory, killing and eating the young of other birds, especially ducklings, and even the adults of some smaller bird species. Western gulls, including one who lived at Oakland's Lake Merritt are known for killing and eating pigeons. They will also snatch fish from a cormorant's or pelican's mouth before it is swallowed. The bird has also been observed directly pilfering milk from the elephant seal's teat.