observers and I originally thought this bird, which I photographed near
the mouth of the Salinas River, was a Glaucous Gull. This species is
rare in Northern California, though a good number of first-year
Glaucous are seen here every winter. First-winter Glaucous are marked
by virtually all-white plumage, and a bicolored bill; the yellow eye is
characteristic of a second-winter bird.
However other observers pointed out that the bill shape, narrow in the middle and widening to a prominent bulb at the tip, with a prononced gonydeal angle, is inconsistent with Glaucous Gull. Perhaps the bird is a hybrid of Glaucous with one of the Gulls characterized by that bill structure, namely Western or Glaucous-winged.
Oddly, I photographed this bird at the same time and in the same flock as another "white gull" -- an albino Western or Glaucous-winged, which you can compare here .