Like most gulls, the Western Gull is an opportunistic feeder, capturing its own live prey, scavenging refuse, or stealing food from seals and other gulls. It is known to steal milk from lactating female seals while they lie on their backs sleeping on the beach.
The Western Gull hybridizes extensively with the Glaucous-winged Gull, with the hybrids being the most common form in Washington. The hybrids can be similar to the parent adult forms, but usually have intermediate back and wingtip coloring. With the medium-gray back, dark upper surface to wingtips, frosty white undersurface to wingtips and a darkish eye, a hybrid may closely resemble a robust Thayer's Gull. The flatter and larger head of the hybrid, and especially the thick bill with a pronounced angle on the bottom, should help distinguish it from the smaller, slimmer Thayer's Gull.
The Yellow-footed Gull of the Gulf of California formerly was regarded as a race of the Western Gull. But its legs are a different color, and it also takes only three, not four, years to reach adult plumage.
In colonies with many more females than males present, two females may establish a pair bond. Each lays eggs, and then takes care of the double-sized brood. The female-biased sex ratio of some Western Gull colonies may have been the result of pollution by pesticides that acted like estrogen and made some male embryos develop as females.
The oldest recorded Western Gull was at least 33 years, 11 months old. It was banded in 1973 in California, and sighted in the same state in 2007 and identified by its band.