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