This rather bulky, large gull is endemic to the Northwest Pacific Ocean. It is a rarity in North America but difficulty in field identification makes its status problematic. These photo convey their rather dumpy, thick-necked, heavy-chested look. Digiscoped with Panasonic DMC-LX5 | Nikon FS 3 | 30XWA | hand-held, no adapter (top). Canon PowerShot SX50 HS (bottom three).
References:
Burger, J., Gochfeld, M., Garcia, E.F.J. & Kirwan, G.M. (2017). Slaty-backed Gull (Larus schistisagus). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from http://www.hbw.com/node/53985 on 15 June 2017).
Howell, S.N.G. & Dunn, J.L. (2007) Gulls of the Americas. Houghton Mifflin, Boston.
Howell, S.N.G., Lewington, I. & Russell, W. (2014) Rare Birds of North America. Princeton Univ. Press
Malling Olsen, K. & Larsson, H. (2003) Gulls of Europe, Asia and North America. Christopher Helm, London.