we're lucky in NZ -- we've just got red-billed gull, black-billed gull and black-backed gull. All very easy. I don't know how you Northern Hemisphereans cope! They're worse than waders!
not with gulls! If I ever end up in America I'm going to just pretend the gulls aren't even there
However, looking at the photo [and bearing in mind I've never had to identify American gulls before!!], I'd say California gull is the one in the middle closest to the foreground (note the two-coloured bill spot), a ring-billed gull to its immediate left, a herring gull up the back (looking at the camera)...and two others (younger ones, I wouldn't know). But don't take any of that as definite!
Front center and the one behind that are both adult ring-billed gulls. Back center is also California as you suggest. However, yellow legs on front right individual point away from herring. The tail pattern makes me think it is California as well. Not sure about the one behind that though.
so if the front-most one is a ring-billed gull why does it have a two-coloured spot on the bill? (I'm not disputing its not a ring-billed, because as I said I have zero experience with American gulls, but shouldn't the bill spot make it a Californian? Or is that a variable or non-definite characteristic?)
The bill marking does somewhat resemble a California, but I've seen bill marking variation (though rare). California have a much darker gray than what is seen here, and fits in better with ring-billed or Herring. However, as Mr. Ituri points out, the legs may be pink, the more I look at them. This could make it Herring, if the iris and orbital ring did not look so dark, thus bringing me back to your original conclusion of it being a California gull.
