Adult Herring Gulls are similar to California Gulls but are larger, have pinkish legs, a yellow iris, paler backs, and a slightly thicker yellow bill with more pronounced gonys. First-winter Herring Gulls are extremely similar to young California Gulls but usually have entirely black bills and only show contrasting secondaries in flight, and lack contrasting greater secondary coverts. Immature birds can be distinguished by back color when it is acquired but are otherwise probably best separated by the subtleties of size, shape, and bill shape. Adult Herring Gulls are similar to adult Ring-billed Gulls but are much larger with larger bills and a more pronounced gonydeal angle. Ring-billed Gulls have yellow legs and a complete black ring on the bill. Immatures Herring Gulls are browner overall than immature Ring-billed Gulls and show less contrast between the upperwing coverts and the secondaries. 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, entirely black bills through their first summer, and paler heads 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. Immature Glaucous-winged Gulls lack the contrast between the upperwing coverts and primaries shown by all Herring Gulls. Thayer' Gull differs from Herring Gull primarily by morphological features. Glaucous-winged x Western Gull hybrids are typically larger-billed than Herring Gulls, and show paler primaries as immatures and a single white mirror on primaries as adults.