#1 Glaucous-winged Gull
The first thing you notice about this bird is the lack of contrast. The pattern is soft and blurry, and the primaries and tertials are the same color as the rest of the bird. The big head and substantial bill are also good for this species.
#2 Thayer’s Gull
The pattern on this bird is very crisp, almost checkered. The tertials and primaries are darker than the rest of the bird. The primaries are two-toned; a dark leading edge and a pale trailing edge. This is the classic mark for Thayer’s Gull. The round head and thin bill are also important marks.
#3 Herring Gull
This bird’s pattern is fairly dark, but not as crisp as the Thayer’s. The tertials and primaries are blackish, and mostly solid. Note the sloping forehead and two-toned bill (heavier than Thayer’s but still fairly thin).
So now go to your local parking lot or beach with a few cat treats and take another look at your local gulls. There is great beauty to be found in subtleties.