I've hammered at this problem ever since Monterey Birds (1985).
It used to be routine for Monterey Peninsula Christmas counts to report
many Ring-bills (e.g., 801 in 1972, 564 in 1975, 58 in 1981) until I took
over as compiler in 1982 (a more realistic 4 birds, all in Seaside/Ft.
Ord area). In truth, all those "Ring-billed Gulls" were like this photo
-- a mid-sized gull with a ring around its bill -- and I would hazard a
guess that the vast majority were the species shown in this photo: California
Gull. A "ring-around-the-bill" effect is a common feature in second-summer
(this photo) through third-winter plumage of California Gull, but is also
shown by Mew, Thayer's, Herring, Western and Glaucous-winged gulls, all
of which are more common on the Monterey Peninsula than Ring-billed. While
an occasional Ring-billed is seen along the outer coast in migration (usually
spring), there is only one acceptable record of Ring-billed Gull on
the rocky shore of the Monterey Peninsula in winter: an adult at Lovers
Pt., Pacific Grove, 9 Dec 1989-27 Jan 1990, returning briefly (presumably
same bird) 5 Jan 1992.
A real Ring-billed Gull (adult in winter plumage) is shown at right. As discussed above, Ring-bills absolutely shun the rocky shoreline. They are birds of sandy shores and open fields, and are common at Elkhorn Slough, just 20 minutes north of Monterey. The easiest feature separating an adult Ring-billed Gull from California Gull (or Mew Gull) is the bright yellow eye (note the dark eye on the first photo). Ring-bills also have paler gray mantles than California or Mew, about the tone of Herring, but are easily separated from larger Herring & Thayer's gulls with "rings" on the bill by the Ring-billed's yellow legs & feet (all the larger gulls in question have pink legs at all ages). There are also differences in bill shape (compare the photos) and leg color (yellow in Ring-billed, typically greenish-gray in California), and quite a few differences in details of primary patterns on flying birds.