Tips for Identifying Gulls
Here is an excellent guide for helping to identify gulls. This guide (composed in response to a question about how to identify a Thayer's gull and not meant to be comprehensive) was posted as a message on the Internet mailing list known as "Tweeters" by Gene Hunn, one of the state's most expert birders and a University of Washington anthtropology professor. If you are a beginner, this guide is valuable not so much because you will immediately be able to identify these gulls (you won't), but because it gives you an idea of the features you should note when you look at a gull.
By Gene Hunn
My id routine is roughly as follows, assuming, of course, an adult*:
[* Note by Jane Hadley: Juveniles are the dirty, brownish gulls.]
Pink legs? Yes. That rules out California, Ring-billed, Mew, etc., not to
mention Lesser Black-backed.
Mantle [middle and upper back] as pale or paler than all the hulking Glaucous-wings, Westerns, and
their hybrid spawn? Yes. Also rules out Slaty-backed, etc. Go on.
Wing with sharply contrasting black tips? Yes. Rules out Glaucous-wings and
Glaucous-winged X Western hybrids (the latter mostly have dark gray wing
tips, and not so sharply contrasting) again plus Iceland including Kumlien's
Iceland Gull, with wing tips of some shade of gray.
Eye not glaring pale yellow? Yes (that is, no, it's not glaring pale
yellow). Rules out Herring. The iris may vary from pale brown to rich yellow.
Head more round and bill less heavy than the other pink-footed gulls? Yes.
Rules out Herring, etc. again. Now you're beginning to feel confident about it.
Finally, check to see if the black on the wing tip is limited, that is, less
extensive and more interrupted by white "windows" than is normally the case
with Herring and Ring-billed Gulls (which have mantles as pale or slightly
paler than Thayer's) and that from the underside the primaries look pale
gray, not blackish (as in Herring Gulls). You can see this best in flight
but it is also noticeable when the bird is standing or sitting as you can
see the top of the wing and front and the underside of the tip of the wing
in back, which should appear pale.