Note the limited amount of black on the primaries (or abundance of gray near the wing end, depending on how you look at it). The amount of black quickly near the wing-ends becomes limited to the primary tips, as you look towards the inner wing. Herring Gulls have a larger amount of continuous dark primaries going towards the inner wing. In addition to the limited amount of black on the primaries, the black-banded yellow bill, dark eye, white tail with black band, and mostly gray back with specks of light brown scattered throughout the backside make the bird on the left a 3rd cycle Thayer's Gull.