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Update March 01, 2012: A while back I discussed this bird with an experienced guller who spent years in the northeast of the U.S.: Birds like this are seen occasionally, and the theory is that these are unusually old male birds, and as such they are highly-experienced in surviving winters in the north so that they can stay close to the breeding grounds and gain a territorial advantage over the males that must winter further south and will arrive on the breeding grounds later. A consequence is that a Texas guller like me will never see such a bird in my region.
NOTE: I saw a presentation at the IGM in Scotland in 2009 by Kalev Rattiste of Estonia about dark markings in the upperwing coverts of very old Common Gulls. His research (spanning 30 years) also established that COGUs gradually acquire more and more white (= less and less black) in the mirrors and subterminal area on the outer primaries up until about 25 years of age, after which the amount of white starts to diminish each year. My personal conclusion is that this points to a strong hormonal component in this process. It seems reasonable to assume a similar process happens in other medium/large white-headed gulls - including RBGU - thus I feel that this bird is not any kind of hybrid but a very old male RBGU:

This apparent adult Ring-Billed Gull (Larus delawarensis ) was photographed at Plymouth Harbor, Massachusetts on January 24, 2008; the primary pattern is beyond anything I've seen previously, and fits comfortably within the typical range for Short-billed/Mew Gull L. brachyrhynchus ; is it an extreme or aberrant RBGU, or a hybrid with some form of Mew Gull L. canus/brachyrhynchus/kamchatchensis?: