This was my closest view of an adult sporting those black underwing flight feathers.
Dorsally there's not much evidence of that unique underwing. Note also the much smaller bill (and gray outer primaries) that are very different from a Bonaparte's.
One more view of the bird dashing through the melee of photo-bombing rat gulls.
Its flight was much more irregular than the other gulls', accelerating and side-slipping through the fray.
Finally in the Because Charmanders Won't Walk Themselves category, a neat little Golden-crowned...
Looks like it's keeping its feet dry.