Kelp Gulls at Costa del Este

Darién Montañez, Ken Wysocki and Steve Huggins saw five Kelp Gulls, four adults and a younger (three-year?) individual, at the water’s edge in Costa del Este. We were standing on the vacant lot right next to the Corredor Sur, by the Costa del Este exit (if you’re coming from the city), and they were standing next to a few Laughing Gulls, about 25 meters away from us. These gulls were huge, and their bold black-and-white plumage made them all the more conspicuous. Our first guess was Great Black-backed Gull, but the legs of our birds were dull grayish-green, not pink. Also, they did not have bold white mirrors on the primaries like Great Black-backeds would. Their bills were big and heavy, bright yellow with a red gonys-spot. The heads were snow-white with no visible streaking, as was the neck, rump, tail and underparts of the bird. Their backs and upperwings were dark slaty-gray, darker on the primaries, with broad white trailing edges. The underwings were white, dark gray on the primaries. The young bird was similar to the adults, but had dark dusky feathers on its wings.
We did not have the seabirds book on us at the time, but based on our memory we decided they might be Kelp Gulls. Now that I’ve checked the books I’m pretty sure they were Kelp Gulls after all. In fact, they looked exactly like the illustration on the National Geographic Guide (p. 201). Because, yes, there are a bunch of Kelp Gull reports from North America, and they are “casual to (the) Gulf coast”. And what do you think the shortest route from their South American range (“southern Brazil and Peru […] to Tierra del Fuego”) to the Gulf coast is?
But even if they turn out not to be Kelp Gulls, they will certainly be something new for the Panama List, as they were surely not Lesser Black-backeds or Band-taileds. My guess is that some of these birds will stick around, so I would strongly recommend you go check them out, the sooner the better. And in case a new gull is not enough to coax you into some good Costa del Este birding, you should know that a Long-billed Curlew was there too, mixed in with the rest of the shorebirds.