I took this picture last February at Swan Lake. Currently, my best guess is that this is a hybrid gull, possibly Glaucous-winged x American Herring Gull, or a somewhat odd (to my eyes) Thayer’s Gull.
I eliminate pure Glaucous-winged Gull because of the very dark on the primaries. I eliminate pure Herring Gull due to the dark eyes and not-black on the primaries.
I’m not sure that I can really eliminate Thayer’s Gull, though neither am I convinced it is a Thayer’s Gull. This may be due primarily to my inadequate understanding of the range of variation in Thayer’s Gulls, however. The orbital ring of this bird seems to be almost a mix of yellow and pink, suggesting the possibility of a hybrid, as Thayer’s Gulls should have a pink to pinkish-purple orbital ring. This bird also seems to be a little bit bulky for a Thayer’s Gull, though unfortunately I neglected to take a picture that included other, more easily identified, gulls for comparison. There is a skirt, which Thayer’s Gulls can show, but it’s more typical of Glaucous-winged Gulls. Also, the dark on the wing-tips seems a bit light for most Thayer’s Gulls. If there are other characters I am neglecting to consider that would help with the identification of this gull, I would be interested in learning about them.
I saw this gull the same day as the one considered above. It also seems to suggest the possibility of a hybrid Glaucous-winged x American Herring Gull. Though it looks more or less like a third-cycle Glaucous-winged Gull, the eye is unusually pale, and the wingtips are a bit dark for a pure Glaucous-winged Gull.