This is a winter male characterized by its black mottled back and nape. Females are similar but have planer gray, less contrasting upperparts. Formerly treated as a race of Naumann's Thrush (Turdus naumanni), Dusky is more contrasting, especially on the head. It also lacks rufous tones on its underparts and the rich buff supercilium typical of Naumann's. However confusing hybrids between the two are known and have intermediate characters. Although Clements/eBird, IOC, H&M4, HBW and OBC split these species based on Russian studies, AOU and OSJ continue to treat them as subspecies. The new Birds of Japan field guide by Brazil (2018) misidentifies this species as a first winter female Fieldfare (plate 152). Canon PowerShot SX60
References:
Brazil, M. (2018) Birds of Japan. Helm Field Guides
Clement, P. & Hathway, R. (2000) Thrushes. Princeton Univ. Press, Princeton.
Collar, N. & de Juana, E. (2019). Dusky Thrush (Turdus eunomus). 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/58257 on 27 March 2019).