This is an adult in breeding plumage. The population breeding in the Russian Far East and Bering Sea region is L. a. vegae characterized by darker back, and orange-red orbital ring compared with typical smithsonianus of North America. Note also smoky gray underside to primaries and white tongue tips on P6-P7.
Taxonomy is complex and unsettled. AOU and Clements/eBird retain a traditional arrangement of one species of Herring Gull. H&M4 and HBW split North American populations as a separate species, American or Arctic Herring Gull (L. smithsonianus). IOC further splits Vega/Siberian Gull (L. vegae) including Mongolian Gull citing Collinson et al. 2008, Yésou (Dutch Birding 24:271-298, 2002). Brazil (2009) further splits Mongolian Gull from Vega.
References:
Brazil, M. (2009). Birds of East Asia. Princeton Univ. Press.
Collinson, J.M., Parkin, D.T., Knox, A.G., Sangster, G. & Svensson, L. (2008) Species boundaries in the Herring and Lesser Black-backed Gull complex. Brit. Birds 101(7): 340–363.
del Hoyo, J., Collar, N. & Kirwan, G.M. (2017). Arctic Herring Gull (Larus smithsonianus). 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/467313 on 2 July 2017).
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.
Nisbet, Ian C., D. V. Weseloh, Craig E. Hebert, Mark L. Mallory, Alan F. Poole, Julie C. Ellis, Peter Pyle and Michael A. Patten.(2017).Herring Gull (Larus argentatus), The Birds of North America (P. G. Rodewald, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America: https://birdsna.org/Species-Account/bna/species/hergul