This poorly known taxon breeds in Central Asia and winters along the coast from Vietnam to southern Japan. It is variously considered a race of Herring Gull or of Vega Gull or a separate species depending on which authority one prefers. It is an uncommon to rare winter visitor in northern Japan. These are first cycle birds which may be identified by their pale coloration, unique mantle/scapular pattern, and distinctive shape with tapered neck when stretched and full-chested look when relaxed. Note also the bicolored bill which first cycle Vega Gulls usually lack. Canon PowerShot SX60.
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