Slaty-backed Gull with raised wings towards the rear left of the image. This shot also contains at least one first-cycle Kumlien's Gull (lower left) but can you see the adult Kumlien's/Thayer's?
Image capture showing blackish leading edge to P10, and a clear black subterminal mark across the outer of an otherwise white wing tip. Other images (eg.below) show black outer edge to P10.
On the whole, another most interesting gull from Niles Pond.
NB. I'd be really interested to know if anyone's discovered a method of reading the wing tip patterns of Thayer's/Kumlien's on the closed wing?