I never could get a good spread-wing shot from the back, but this shot does show the white, lightly-marked uppertail coverts as it preens. With a spread wing shot, these gulls lack the window shown by Herring's, and also have a double dark bar on the trailing edge of the wing formed by the secondaries and greater coverts. California Gull has a similar wing pattern, but lacks the contasting white uppertail.