This bird is one in which the close relationship between the Common Gull and Ring-billed Gull is particularly obvious.
Here, below, is a Ring-billed Gull, photographed in Santa Monica, CA, for comparison. This bird was photographed in April so is showing much more wear in the wing coverts.
Note that the Ring-billed Gull has a more parallel bill, stronger chevron-shaped markings on the neck and flanks and a paler silvery grey mantle. In addition,first winter Ring-billed Gull shows diamond shaped dark subterminal markings on P1-4, which are clearly absent in this Common Gull (below) - instead the markings are lance-shaped and restricted.