This bird requires all serious ABA birders to come to the Salton Sea at least once in their lives.
Lets face it...the best state/province to find gulls on the continent has to be California. There, I said it. Sorry to all you other landfill lurkers out there, but we are the best. Out of the 30 species of gulls recorded in the ABA Area, only Gray-hooded, Great Black-backed, Kelp and Yellow-legged have not been found in CA...and we all know Great Black-backed is just a matter of time. That means we have enjoyed species like Black-tailed Gull, Ivory Gull, Ross' Gull, Red-legged Kittiwake, Swallow-tailed Gull (!!!) and Belcher's Gull...the latter being the only one (and fortunately, one of the rarest) of these to grace my California list.
Being a relatively short distance from Mexico's Gulf of California (as the frigatebird flys), California boasts an endemic gull...not biologically speaking, but if you think in terms of being an ABA lister, Yellow-footed Gull is essentially only found in California. Yellow-footed Gulls are regularly found at the Salton Sea, most abundantly in summer and early fall. Despite many gull populations increasing, the Salton Sea remains the one and only place north of Mexico where you can reliably find this bird. If you dip on this bird at the sea (a dismal possibility for much of the year) and aren't obsessed with seeing one in the U.S., they are easy to get a few hours south of the border in places like San Felipe and Puerto Penasco (the latter town is pretty good birding in general), but most birders have gotten their lifer Yellow-foots at the Salton Sea.
This is a pretty typical looking YFGU. Similar to a Western Gull until you notice its feet (unless you are really colorblind, sorry).
Some of the best spots to find these coveted birds are at the end of Poe Road, near Red Hill, Obsidian Butte, and anywhere along the shore of the Salton Sea between the butte south to Young Road.
Another angle. Look at that massive bill. It's just a mean-looking bird, especially these pale-eyed individuals. Some YFGUs appear to have an especially orange cast to their bill and leg color, making them really glow.
Second-cycle (left) and first-cycle (right) Yellow-footed Gulls. Note the change in leg color as they age. Second-cycle YFGUs can retain pinkish legs though, which probably accounts for the number of Western Gull reports in the spring and summer at the Salton Sea. The majority of "real" WEGUs at the sea are typically found later in fall and winter.
Second-cycle Yellow-footed. This bird isn't very big-billed compared to some YFGUs, but note the yellowing legs and feet. The only Western Gulls that get yellowish feet (which is pretty uncommon) are spring adults, not young birds like this. The majority of WEGUs also have dark eyes. Howell and Dunn variously describe the standard Yellow-footed Gull bill as being "stout and bulbous-tipped" and the Western Gull bill as "stout, bulbous-tipped", so that should make things easy for you.
As shameful as it is to say, I'm sure more than a few larophobic birders have looked at a California Gull (abundant at the sea) and walked away feeling pretty happy that they saw the much-vaunted Yellow-footed Gull. Simply remembering that YFGUs are massive birds and that they don't form enormous flocks at the sea (well, not anymore) will help keep you in check. Here is a YFGU with a Ring-billed Gull for size comparison.
Not sure if this is a 3rd-cycle or adult bird...Howell and Dunn suggest that adults are always tattered and crappy looking. Sounds kind of racist to me, considering this is a Mexican endemic. Hey-oh!!! All photos in this post were taken along the southeastern shore of the Salton Sea.