This gull was photographed in Monterey Bay, CA two weeks ago. It looks like a 3rd winter Herring Gull to me based on what I see in Sibley. I don't see any white patches on the tail coverts as in an adult. The time frame is off however (Aug-Apr according to Sibley) for this plumage .
The time frame is just a general guideline, and May is not far of of April. Could be Herring as you suspect. 2nd year Western Gull could be another option. Hopefully gull experts will chime in!
Hi Joe. I think this is a second cycle Western Gull. The large, two tone bill, pink legs and dark grey coming in on the mantle are the points that I am looking at. Also, on the west coast, this is a very common species.
Wow, those are too close for me to call. Another clue I see in Sibley is the head peaks in front of the eye on the Western, and behind the eye on the Herring. The bottom photo gives some indication of a peak in front of the eye.
I agree that this a second year Western Gull. The gray mantle color, which is definitely darker than Herring and a little darker than California, matches Western Gull well. Some mistakenly think Western Gull should appear black, but even the darker southern wymani subspecies isn't that dark. The bill shape looks wrong to me for Herring, being too long and I think the gonydeal angle (the drop on the lower mandible out near the tip) is too far forward. Overall shape also looks very heavy and blocky to me.
The brown retained wing coverts and tertials match a 2nd cycle well. In addition to color, you can tell they are old because they look very worn. By 3rd cycle, those feathers should be replaced.
Take a look at this gull. This looks like an adult Western Gull to me with even darker gray mantle than the ones above, but it lacks the white patches on the tail coverts ala the 2nd and 3rd cycle birds.