Thayer's Gulls are quite difficult to identify. Their plumage is between that of Herring Gull and Iceland Gull. They have the rounded head and slim bill of the Iceland Gull but the darker plumage of the Herring Gull. As adults, they can be told from both Iceland and Herring Gulls by their dark eyes. They have more extensive dark in the primaries than Iceland Gulls but less than Herring Gulls (especially when viewed from below). First-winter birds can be considerably paler than Herring Gulls without the darker outer primaries, and have rounded heads and thinner bills. Thayer's Gulls typically have brown secondaries and a brown tail band which are equal in color and duller than the body plumage. Iceland Gull rarely shares this, and onHerring Gull the tail band is always darker than the secondaries. Adult Thayer's Gulls are similar to California Gulls but are larger, have pinkish legs, and paler backs. First winter Thayer's Gulls are generally paler than young California Gulls and have entirely black bills. Immature birds can be distinguished by back color when it is acquired. Adult Thayer's Gulls are similar to adult Ring-billed Gulls but are much larger with larger bills. Ring-billed Gulls have yellow legs and a complete black ring on the bill. Immature Thayer's Gulls show less contrast between the upperwing coverts and the secondaries than immature Ring-billed Gulls. Adult Lesser Black-backed Gulls are much darker-backed and have yellow legs, while first-year birds are very similar but have whiter rumps, darker wing coverts, and underparts that are spotted with brown. Western Gull is quite similar in subadult plumages but has a white rump that contrasts with back in first-year plumages, a larger bill, and shows the distinctive dark back color by the second winter. Glaucous-winged Gull has a larger bill and primary tips the same color as the mantle. Glaucous-winged x Western Gull hybrids are typically larger-billed than Thayer's Gulls, and show paler primaries as immatures and a single white mirror on primaries as adults.