A Sibley 1% Herring Gull

This bird was on Silver Beach last week.  It stood out to the naked eye with an apparently slightly darker mantle, and darkish eye.

 As it flew away it showed some narrow strips of black on the primaries.  Boom. Thayers. Done. Right?

 Well, not so fast.  If you look carefully at the spreadwing you'll notice that the outer 2 primaries are pretty solidly black.  (Note the darkish marks in the tail aren't actually pigmentation, just areas where the feathers are worn and the background is showing through).

And if you look a little closer you see that the dark eye is actually light (which some Thayer's can actually have), but this bird has a Herring Gull's head shape and facial expression.

 A look at the underwing, a pretty close match for what Sibley shows that 1% or so of Herrings will have.

Here's a real Thayer's from a few years ago.

The smaller bill, gentler face, and true Venetian blind primaries stand out as pretty different from the bird on Silver.