Geographic variation is complex with about 21-23 subspecies currently recognized. A review by Red'kin and Konovalova (2006) confirmed the validity of several East Asian subspecies as well as the species status of the Siberian Nuthatch (S. [e.] arctica). The new subspecies, but not the species split have been endorsed by Clements in their most recent checklist. Brazil (2018) recognizes three subspecies in Japan, varying from the extremely pale, white-bellied S. e. clara (formerly included in S. e. asiatica) in Hokkaido to the darker chestnut-bellied S. e. roseilia in Kyushu. Clements and IOC both now recognize S. e. hondoensis ranging from southern Honshu to northern Kyushu but formerly included in S. a. amurensis. Regardless of taxonomy, this individual is the brightly colored, chestnut-bellied S. e. roseilia. Singing from the top of a snag. Canon PowerShot SX60
References:
Brazil, M. (1991) The Birds of Japan, Smithsonian Inst. Press.
Brazil, M. (2018) Birds of Japan: Helm Field Guide. Helm
Clements, J. F., T. S. Schulenberg, M. J. Iliff, D. Roberson, T. A. Fredericks, B. L. Sullivan, and C. L. Wood. 2018. The eBird/Clements checklist of birds of the world: v2018. Downloaded from http://www.birds.cornell.edu/clementschecklist/download/
Harrap, S., Christie, D.A. & Kirwan, G.M. (2019). Eurasian Nuthatch (Sitta europaea). 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/59914 on 13 May 2019).
Harrap, S. & Quinn. D. (1995) Chickadees, Tits, Nuthatches & Treecreepers. Princeton Univ. Press.
Red’kin, Y. and Konovalova, M. 2006. Systemic Notes on Asian Birds. 63. The eastern Asiatic races of Sitta europaea. Zool. Med. Leiden 80