If you did not identify this bird in under 3 seconds, you clearly need to study up for gull season. Lake Merced, San Francisco, CA.
Well it's officially gull season here at BB&B...while California birders make blood sacrifices to that Big Gull In The Sky in attempts to get an early fall Little Gull (like most prayers, they usually go unanswered), gull season hasn't started in earnest until now. California's first Lesser Black-backed Gull of the winter has been reported from Alameda County. I did search for the (well-documented) Lesser Black-backed today, but as with most of my recent chase attempts, it ended in failure...but it turned up at a great gull-watching spot (not surprisingly, next to a dump) so I will be back soon. Birders up and down the state can now start their search for Black-tailed, Slaty-backed, Iceland, Black-headed and other species in earnest, and those looking for the state's first Greater Black-backed Gull have another crack at claiming Victory.
Much has been written about gulls...mostly how unpleasant it is to identify them, how people generally aren't very good at it, and even how some birds just can't conclusively be identified. Indeed, whinging about gulls is an atavistic endeavor at best, so I won't go rehashing the same old shit...but that doesn't mean what they say isn't true.
Well...it's a hybrid. The small, almost delicate bill points to Glaucous-winged X Herring in my opinion, but Glaucous-winged X Western is a possibility, especially considering the mottled breast and belly. Note the small bill with reddish base, checkered back and light brown primaries...these birds are deceptively similar to Thayer's Gulls, but don't look as neat and usually have paler primaries.
Here's a first-cycle Thayer's Gull for comparison...note the round head, big-eyed look, meager bill, and dark (not black) primaries with pale tips. This was photographed in late winter, young birds usually don't have so much pink on the bill right now. Lake Merritt, Oakland, CA.
Of course, real gull addicts don't want to be seeing straightforward looking birds unless they are extremely rare, so here is another genetic monstrosity...I would tentatively go with Glaucous X Herring Gull hybrid, or perhaps Glaucous X Glaucous-winged. Large, heavy-billed, extremely pale, with a dull (but extensive) pink base to the bill and a lot of white in the primaries. Lake Merritt.
Probably a very pale Glaucous-winged Gull...note the difference in bill shape from the above bird. Some may be tempted to call this a Glaucous, due to whiteness and pink in the bill, but of course they would be Wrong. Lake Merritt.
Ah, a Glaucous Gull. So white. So easy to identify. So rare but regular. What a relief. Golden Gate Park, San Francisco, CA.
Birders don't hold a grudge against Mew Gulls, since they aren't completely slutty and don't closely resemble other gulls. Lake Merced.