Age: A number of features on this individual point towards a 2nd cycle gull. This includes the rounded primary tips, the mixture of gray-brown feathers on the upperparts, the marbled greater coverts and white-tipped tertials - all these combined assure us this isn't a 1st cycle gull.
So which North American species look like this in their 2nd plumage cycle? Herring Gull immediately comes to mind, but the bill is awfully thin (and straight) for that species. The gray on the upperparts also looks suspiciously dark for Herring. The dark iris paired with an obvious post-ocular line, noticeable downward gape, thick brown neck-boa and bluish tibia are all diagnostic of no other than a California Gull.
This month's quiz bird was photographed in September, San Mateo County, California.