Leucophaeus atricilla

Bill: Relatively long, dark reddish bill (breeding), dark bill in other seasons and for juveniles. Bill looks drooped at the tip.
Legs: Dark reddish-black for adults, black in juveniles.
Wings: Relatively long wings. Gray above with extensive black on wing-tips, thin white trailing band.
Head: Black in breeding plumage is more extensive than Franklin's and Bonaparte's.
Thinner white eye-arcs than Franklin's, but still quite noticeable.
Non-breeding and juveniles mostly white, with gray streaking on back of head.