For this bird, notice how the windows (blocky white inner regions of the primaries look). A Herring Gull can show a good-sized window on the outermost primary (also known as the P10 feather). However on P9 (one feather inward), the window size decreases significantly to petite white mark (for Herring Gull). The bird in this photograph shows a continuation of the amount of white as the windows 'move' through the length of the bird's wings. This is what gives Thayer's Gulls a pale-wing-tipped appearance. Also notice how the bird's bill is vibranty-yellow. The adults tend to show this, as well as a evenly hooded head, such as the bird shown. Thayer's Gulls are also slightly smaller than Herring Gulls, but this is a field mark which is difficult to see in the field, and can be noted a little easier in photographs.