Although still common, this species seems to have declined as a winter visitor to Japan since the early part of the 20th century when it was considered to be the commonest waterfowl species. This widespread Eurasian race often goes by the name "Common Teal" and considered a separate species from the American Green-winged Teal (A. c. carolinensis) because of its highly divergent mitochondrial DNA. However, this split has not been widely adopted by American authors because of hybridization between the two as well as their physical similarity. Canon PowerShot SX60.
References:
Brazil, M.A. (1991) The Birds of Japan. Smithsonian Inst. Press.
Carboneras, C., Christie, D.A. & Kirwan, G.M. (2019). Common Teal (Anas crecca). 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/52868 on 24 April 2019).
Johnson, K. (1995). Green-winged Teal (Anas crecca), version 2.0. In The Birds of North America (A. F. Poole and F. B. Gill, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bna.193
King, J.R. (1999) OrnithoNews: Taxonomy of Anas ducks: Green-winged Teal is a separate species. Birding World 12(6):344.