Once considered to be part of the same same species, Thayer’s and Herring Gulls can appear quite similar at first glance.The pattern on the spread wing is very different between the two, but that doesn’t help you with birds at rest.
Here is a Thayer’s Gull on the left, and a Herring Gull on the right. In direct comparison, we can see that the Thayer’s has a slightly darker mantle, rounder head, dark eyes (usually), and a bill that is greenish-gray at the base and yellow at the tip.
Another Herring Gull, probably a female. This bird has a smaller bill and a slightly more rounded head than the bird in the first photo, giving this individual a more Thayer’s-like quality. Note the all-yellow bill, the pale eye, and the light gray mantle.