For this bird, the overall roundness of a Thayer's Gull is apparent. Herring Gulls tend to be on the slightly bulkier side, but this is hard to see in the field unless you see a side-by-side comparison in a photograph. Note the round head. Also, and most importantly, note the primaries. There is a lot of white in the primaries! The ratio of white to black seen on this bird's primaries is about 50% white and 50% black. Herring gull's folded primaries are dominated by black coloration. On a side note, the dark eye and vibrant yellow bill are apparent.