We first saw the Mangrove Warbler off Scarlett Colley's boat in a offshore Mangrove thicket six or seven years ago. The Mangrove Warbler is a subspecies of the Yellow Warbler, which may become eventually be designated as a standalone species. Unlike the Yellow Warbler it has a rusty red head, has a slightly different song, does not migrate, frequenting only mangrove swamps of the Caribbean (and recently South Padre Island). Different appearance, different habitat, different song - to my mind, and almost all birders, it is simply a different bird.
Over time we have probably spent three or four hours stalking them on South Padre Island, listening to the loud clear song, getting fleeting glimpses of shadow shapes deep in the mangroves. Yesterday we finally actually got rather poor photos of it. Because of the patchy head markings this bird in the photo is likely a first year male.