I love photos of gulls in flight! It would be far better if I could tell you exactly what species of gull we're looking at here, but I'm not entirely sure! In the first place, there are many species of gulls in California that we don't have around here (and vice-versa), so I didn't even try to ID all of them.
In the second,place this is not a fully mature gull, and I haven't memorized three or four years worth of different plumages even for the local gulls I know and love. I like this photo because it caught the gull in mid-landing. As I've said before, a beginning bird photographer can't choose a better subject than a gull!
This Mew Gull would be considered a rarity here in Massachusetts. Personally, I think he's a particularly appealing bird. The Mew Gull can easily be identified by it's small bill, roundish head, and small size. The fact that he is often described as a a 'cute' gull is another example of humans instinctual reaction to animals with rounder heads and smaller features. (See this post for another example).
Another gull we don't see at all in New England is the Heermann's Gull, but the Heermann's Gull is also very easy to ID. This is a juvenile bird, and it's the only uniformly dark colored gull with black legs and feet that you'll ever see.
Here he is actually chasing down another gull in order to steal it's supper - typical gull behavior despite it's atypical appearance. The adult Heermann's is even more startling-looking; it has a bright white head and a bright red bill!