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 photos convey their rather dumpy, thick-necked, heavy-chested look. Immatures come in a bewildering variety of plumages. One consistent feature seems to be their brownish rather than black wing tips. Many of the first cycle birds we saw would likely not be identified as Slaty-backed if seen in North America. In particular some individuals may be confused with Glaucous-winged Gull and others not safely distinguished from various Glaucous-winged Gull hybrids including Glaucous-winged X Herring and Glaucous-winged X Western. Canon PowerShot SX60.
References:
Burger, J., Gochfeld, M., Garcia, E.F.J. & Kirwan, G.M. (2019). 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 https://www.hbw.com/node/53985 on 15 March 2019).
Howell, S.N.G. & Dunn, J.L. (2007) Gulls of the Americas. Houghton Mifflin, Boston.
Malling Olsen, K. & Larsson, H. (2003) Gulls of Europe, Asia and North America. Christopher Helm, London.