ID Challenge: Mew Gull NOT!

I photographed this small gull in Oceanside a few weeks ago. The tiny bill and round head made me seriously consider this as a possible rare Mew Gull, rather than the common Ring-billed Gull. I did observe Mew Gulls here last winter, and one was reported nearby recently, so I was on the look-out.

Small gull. Oceanside, California. November 8, 2015. Greg Gillson.

I remember an identification article that appeared in American Birds March 1980: "A method for separating juvenal and first-winter Ring-billed Gulls (Larus delewarensis) and Common Gulls (Larus canus)" by Anthony J. Lauro and Barbara J. Spencer. (pdf here)

This article was the first to show that the wing covert tip shape and color is the key to separating these very similar-looking birds in immature plumage. The wing coverts of immature Mew Gulls have medium dark solid rounded centers with pale edge. The wing coverts of immature Ring-billed Gulls have darker pointed centers with wider whiter tips.

Though the wing covert feathers are worn and faded, the photo below shows the pointed dark tips on the individual wing covert centers consistent with Ring-billed Gull, and not Mew Gull.