Among the Herring Gulls in this photo, look for the gull with black eyes, a round, smooth-looking head, and a slightly more-slender, bright yellow bill. This bird is the adult Thayer's Gull I found this morning.

Also, for identifying Thayer's Gulls in flight (next picture shown), take a look at the Herring Gull's open wings and wing tips in this photo (and note how much black there is throughout the outer wing). There is also only a single mirror on this gull's wings (mirrors are the white inset spots, located near the ends of the outermost flight feathers).

Now in the next photo, notice the amount of black on the wingtip, and the increase in number of mirrors in the Thayer's Gull's wing...