These pics were taken on December 20, 2006 in the Coastal Plain near Houston and Galveston, Texas; it is very suggestive of Slaty-backed Gull:
Note below the pale inner webs of the central primaries, the pale wing coverts (including greaters) contrasting with the dark secondaries, and the dark tail with basal pale speckling - not barring - on the outer two retrices that is limited to the inner webs of these feathers:
