Many people (myself included) find Gull identification to be challenging. Even some species of pure adults can be difficult, so adding in the various hybrids that occur as well as up to 4 years of changing plumages as the birds mature makes things even harder. I have recently decided to make an effort to learn the gulls a little bit better.
The only time I have seen a Bonaparte’s Gull, it was easy to distinguish with its black head. I did not have too much trouble with Glaucous-winged Gulls when I realized that they are pretty much the only ones with gray wing tips. (An occasional Glaucous Gull makes it down to Sitka, but that’s fairly exceptional.) Mew Gulls were the next species. Although I am still not entirely comfortable with making the identification of them, they seem to look different enough in overall appearance, that they’re not too hard to distinguish. Most recently, I have started to make progress on the other two gulls commonly seen around Sitka, Thayer’s Gulls and Herring Gulls.
I took the picture of the this gull at Swan Lake in February. (Incidentally, winter seems to be a good time to get photos of the gulls, as they are commonly fed by people around town, so they are less wary than at other times of the year away from town.) Just this week, I finally got around to trying to figure out what species it was. With a little luck and lots of help from others, I learned that the bird is a Thayer’s Gull.
The lucky part of this was that I happened to get a picture that showed the undersides of the bird’s primary feathers. Without seeing those, the Herring Gull can be difficult to reliably distinguish from the Thayer’s Gull. However, Thayer’s Gulls have light undersides of the primaries while Herring Gulls have dark undersides (one helpful person described the Herring Gull’s wingtips as having been dipped in black paint).
The birds do differ in other respects, including eye color, beak size, shape of the head, and some other subtleties. However, it seems there is enough variation that it is difficult to make a confident identification based only on those characteristics.