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.
Here is a closer look at the Thayer’s Gull. This is probably a male, given the long bill and slightly sloping forehead. Note the color pattern on the bill, typical of Thayer’s Gulls in winter.
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.
This Herring Gull, probably a male, has a larger bill and a more sloping forehead.
This is a first-cycle Thayer’s Gull. Note the round head and small bill. The overall color is rather pale, with fine markings.
First-cycle Herring Gulls are darker overall, with heavier markings. This is most apparent on the greater wing coverts. Note the sloping forehead and slightly larger bill.