Kelp Gull at Costa del Este

Yesterday afternoon's Audubon Panama fieldtrip to Costa del Este had, as expected at this date, most of the common migratory shorebirds and gulls, some already in alternate plumage. Highlights included over 50 Black Skimmers, most of which showed the gray underwings and all-black tails of the South American subspecies cinerascens, a handful of Franklin's Gulls still in basic plumage, an alternate-plumage Ring-billed Gull, a few young Herring Gulls, a first-year Lesser Black-backed Gull and what we originally called a third-year Lesser Black-backed but are now calling a Kelp Gull. The gull in question was pretty big, with a large yellow bill that had a large black tip. The head was mostly white with dusky specks all over, merging into a dark smudge around and behind the eyes. The back was dark slaty gray with some older brown feathers in the coverts. Legs were putty-colored. No white tips on the primaries, black band across the tail. There is a size difference between Kelp and Lesser Black-backed, but that can be hard to assess in the field; I'd say our big gull was close in size to the young Herring Gulls we saw. The bill was definitely too large and blunt for Lesser Black-backed. Leg color on a third-year Lesser Black-backed (which is the age where they get their gray backs) would be noticeably yellow, and not non-descript like the ones on our bird. Also, a third-year Lesser Black-backed would not show the amount of black on the tail seen in our bird and have white mirrors on the tips of the primaries, which our bird lacked.
Leslie Lieurance digiscopefilmed it, and hereby present it for your examination. Extra points if you can spot the adult Ring-billed Gull.