Adult Herring Gulls are similar to Ring-billed Gulls but are much larger, have pinkish legs, and a much thicker yellow bill with more pronounced gonys. First-winter Herring Gulls are much browner, but second and third-winter birds can be confusing since soft part colors are variable and third-year Herring Gull often show a ring around the bill. Such birds are most easily distinguished by the larger size and larger bill of Herring Gull. California Gulls are more similar in size and shape but are still larger and bigger-billed. Adults are darker-backed than Ring-billeds, with a brown eye, heavier head streaking, and a different bill pattern. Immatures are browner overall and show less contrast between the upperwing coverts and the secondaries. Adult Mew Gulls have unmarked bills, a darker gray back and are smaller. Juvenile Mew Gulls are much darker. First and second-winter Mew Gulls have a darker gray back and a daintier appearance. Common Black-headed and Bonaparte's Gulls are somewhat smaller in winter but have dark auriculars and very different wing markings in flight. Winter adult Black-legged Kittiwake has a plain yellow bill and entirely black wingtips.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.