Sometimes called Chinese Spot-billed Duck, this is one of the most common duck in Japan. Sexes are alike. This species was formerly considered a race of the Spot-billed Duck (Anas poecilorhyncha) now split under the name Indian Spot-billed Duck. The Indian bird has conspicuous red spots at the base of its bill which give this species its name. However, these spots are absent in the Eastern Spot-billed Duck seen here. Furthermore the two breed sympatrically in parts of China confirming that they are separate species. Canon PowerShot SX50 HS.
References:
del Hoyo, J., Collar, N. & Kirwan, G.M. (2017). Chinese Spot-billed Duck (Anas zonorhyncha). 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/467118 on 5 June 2017).
Kulikova, I.V., Zhuravlev, Y.N. & McCracken, K.G. (2004) Asymmetric hybridization and sex-biased gene flow between Eastern Spot-billed Ducks (Anas zonorhyncha) and Mallards (A. platyrhynchos) in the Russian Far East. Auk 121(3): 930–949.
Leader, P.J. (2006) Sympatric breeding of two Spot-billed Duck Anas poecilorhyncha taxa in southern China. Bull. Brit. Orn. Club 126(4): 248–252.