Thayer's Gulls tend to be more round-headed than Herring Gulls, and tend to have a more slender-profiled bill. The overall frosty-ness of the Thayer's Gull can be seen in this shot. The primary tips aren't extremely dark (just slightly darker than the birds' backside), and the tertials (feathers that rest on the primaries when the bird is at rest) are also pale. On the Herring Gull on the left, note the higher contrast between the tertials & primaries, and the backside.