Western Gull Larus occidentalis Scientific name definitions

The Western Gull is a large white-headed gull native to the Pacific Coast of North America. It breeds from Washington State, United States, south to central Baja California, Mexico, and winters along the coast and in offshore waters from Vancouver Island, Canada, south to the tip of the Baja California Peninsula, Mexico. Although it is a familiar and well-known West Coast species, it has a limited distribution and a smaller population size than most other North American gulls, with a total population of approximately 50,000 pairs nesting at fewer than 200 colonies.

There are two subspecies currently recognized: the Northern Western Gull, Larus occidentalis occidentalis, and the Southern Western Gull, L. o. wymani. The Yellow-footed Gull (Larus livens) of the Gulf of California was formerly considered a third subspecies (L. o. livens) but is now recognized as a separate species. The behavior and ecology of both subspecies are well studied, although there are more data available for the northern subspecies due to long-term studies on major breeding colonies on Southeast Farallon, Alcatraz, and Año Nuevo islands (California).

Colonies are sited on islands, offshore rocks, abandoned piers, and other areas where birds are safe from terrestrial predation. Until recently, more than 30% of the world’s population nested on Southeast Farallon Island, California; however, this population has been declining during the 21st century, likely due to climate change and its impacts on marine ecosystems.

The Western Gull has not been a major target for conservation measures; however, various factors have rendered it more vulnerable to population loss than is commonly acknowledged. The marine fish stocks on which it depends are now increasingly variable due to recurring El Niño Southern Oscillation (ENSO) events, heightened temperature fluctuations within the California Current Ecosystem (CCE), and other consequences of climate change. This, coupled with low population numbers, a small number of breeding colonies within an already restricted geographic range, extensive hybridization with Glaucous-winged Gull (Larus glaucescens) in the north of its range, the looming threat of oil spills, the adverse effects of pesticides and other contaminants on bird health and reproduction, and a general indifference among ornithologists, make the fate of the Western Gull particularly precarious.

The prospect of losing this iconic West Coast species should be taken seriously. Changing our attitudes to perceive the Western Gull as an intelligent, adaptable, and vital component of the marine ecosystem is imperative to reversing current declines.