Thayer's Gulls have dark eyes throughout their lives, whereas Herring Gulls have dark eyes during their juvenile months of life, and sometimes into their first winter. After that, they'll have yellow eyes. Also, the Thayer's Gulls have a more smooth, round-headed appearance. The books don't mention this, but from the adult Thayer's Gulls which I have observed, I feel that these birds tend to have a brighter, more sunflower-yellow-colored bill than the slightly duller billed Herring Gulls. Many adult Herrings show a bright yellow bill, but when seen next to an adult Thayer's Gull, the thayer's tends to have the deeper yellow color. The bill on a Thayer's gull is more smooth and slender overall.