These active ducks were diving almost constantly making photography difficult. These small, unmistakable mergansers are winter visitors to Japan. Their breeding range largely coincides with that of the Black Woodpecker whose old tree cavities provide nesting sites. These ducks are the sole member of the genus Mergellus. They are genetically closer to Goldeneyes than to other mergansers and they have frequently hybridized with Common Goldeneyes in the wild. The name "Smew" dates to the 17th century but is of unknown origin. Canon PowerShot SX60.
References:
Carboneras, C. & Kirwan, G.M. (2019). Smew (Mergellus albellus). 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/52927 on 4 April 2019).
Gonzalez, J., Düttmann, H., & Wink, M. (2009). Phylogenetic relationships based on two mitochondrial genes and hybridization patterns in Anatidae. ''Journal of Zoology'' 279: 310–318.
Livezey, B. C. (1986). A phylogenetic analysis of Recent anseriform genera using morphological characters. ''The Auk'' 103: 737-754.
Todd, F. (1979). Waterfowl. Ducks, Geese & Swans of the World. Sea World, San Diego.