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.
Regardless of taxonomy, this is an adult American Herring Gull in non-breeding plumage showing gray head streaking. It is a rarity in Japan and we were fortunate to see it and document it with photos. Note its long bill and pale eye with orange-yellow orbital ring. Its mantle color averages noticeably paler than other races, especially the local and abundant L. a. vegae which has an orange-red orbital ring. Eye color varies in vegae but usually dark, always pale in smithsonianus. Canon PowerShot SX60.
Identification of first cycle (bottom) is tentative.
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