The identification of large gulls is among the most difficult and contentious issues in North American field ornithology. A significant part of this problem is due to hybridization among various taxa, a phenomenon especially prevalent in western North America. In particular, Western Gulls (Larus occidentalis) and Glaucous-winged Gulls (L. glaucescens) hybridize extensively from Puget Sound south to central Oregon (Scott 1971, Bell 1996), while Glaucous-winged Gulls and Herring Gulls (L. argentatus smithsonianus) hybridize commonly from southwestern Alaska to British Columbia (Williamson and Peyton 1963, Merilees 1974, Patten 1980).