This gull was photographed in the Coastal Plain near Houston and Galveston, Texas on December 28, 2005; I saw it briefly standing on the trash pile prior to getting these flight pics:- it somewhat had the jizz of a Caspian Gull, with fairly uniform-looking darkish first-generation wing coverts, and pale grayish tone to the upperparts (the scaps had thin anchor-shaped dark markings), and lightly-marked underwing coverts - clearly it is too darkly marked for a typical western Caspian Gull, plus the uppertail coverts and tail are too heavily-marked - might it be a taimyrensis? It is probable that it is the same individual as this bird seen earlier in the day at the same site:
The underwing coverts are quite Caspian-like (also some heuglini have underwing coverts like this), as is the "Venetian-blind" effect on the inner primaries created by the dark outer web and paler inner web of each feather. However the bulk of features look wrong for Caspian - at least for those better-known forms from the central/western part of their range. Some presumed Atlantic YLGUs (by range) have been photographed that have a Caspian-like structure, but I do not recall if such birds also have lighter underwing coverts - normally Atlantic YLGU has quite dark underwing coverts, rather like LBBG but with stronger pale barring on many of the feathers.