I found an odd dark mantled gull at Rock Cut SP the other day.
I took some photos which I attach here. I sent them to Amar Ayyash, who has a lot of expertise in gulls. He replied that he believes that this bird is a hybrid Great Black-backed Gull X Herring Gull. Notice the pink legs, the larger bill, paler mantle, and the white window in only the P10 primary, with small apical spots on the tips of other flight feathers. The subject compares reasonably well with photos of this type of hybrid that appear on page 293 of the Howell and Dunn Gull book, with the exception that their photo shows white windows on both P10 and P9.
Larry Balch also has commented (see his comment below) and believes that this bird is a 3rd winter Lesser Black-backed Gull. Larry meant that the mirror was in P10, not P9 as is in his comment. The subject has a mirror only in P10 (as I noted). Lesser Black-backs can have pink legs in immature individuals. I beleive that it is more likely to find a LBB Gull here than a hybrid which is unusual in the Canadian Maritime provinces and New England only in the winter (as stated by Howell and Dunn).
Dan Kassebaum believes that this bird is a subadult Lesser Black-backed too. He reports that he has observed a similar bird at Lake Carlyle. It had pinkish legs but with an undertone of yellow, indicating a subadult. Dan would like a better view of the flight feathers.