Adult Mew Gulls are similar to adult Ring-billed Gulls but have darker mantles, dark eyes, unmarked yellow bills, rounder heads, and have more white in the primaries. First-winter Mew Gulls have darker gray backs, rounder heads, and different tail patterns-US birds have all-dark tails while European bird have white tails and upper and undertail coverts. Ring-billed Gulls have tails similar to European birds but have markings on the upper and undertail coverts. The secondary covert pattern is distinctive for each species, with Ring-billed showing dark, arrowhead-shaped internal markings versus the crisp pale border to the dark covert feathers of the Mew. Herring Gulls are similar to Mew Gulls but are much larger, paler-backed, and have thicker bills with a more pronounced gonys. First-winter Herring Gulls are much browner. California Gulls are similar in mantle color and shape but are larger and bigger-billed. Immatures are browner overall and show less contrast between the upperwing coverts and the secondaries. Adult Black-legged Kittiwake has entirely dark primary tips without white spots.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.