I found an interesting paper in English, on Nuthatches that I thought I would share. The paper concerns the Nuthatch in Russia and Asia, but my focus here will be on the sections concern Nuthatches seen in Hokkaido.
Credit is due to:
Yaroslav A. Red’kin, Zoological Museum, Moscow State University, Bolshaia Nickitskaia str. 6, 125009, Russia (e-mail: firstname.lastname@example.org).
Maria V. Konovalova, Zoological Museum, Moscow State University, Bolshaia Nickitskaia str. 6, 125009, Russia (e-mail: email@example.com).
In Hokkaido there are 2 different subspecies of the Eurasian Nuthatch that can be seen. The subspecies Asiatica and Clara. S.e.clara, previously being listed as a part of asiatica in the S.europaea group), with the addition of Arctica* existing. The Clara subspecies being the prevalent one here, with the occasional Asiatica appearing. The Siberian Nuthatch (previously S. europaea arctica), very rarely. Below is the differentiation, and drawings of each Hokkaido subspecies, followed by the bill comparisons of all the subspecies of eastern Asiatic races of Sitta europaea. The bill comparisons are especially valuable.
*Since writing this, it has been noted as of late that the “ARCTICA” subspecies as been classified as its own species; the “Siberian Nuthatch” (Sitta arctica), as well the range is much farther into Russia, and not occurring in Hokkaido.
SO, simple basic differentiation is based off the species group (europaea); which is defined as: The nominate subspecies. Upper parts blue-grey, black eye-stripe. Throat and underparts off-white or creamy, rusty red flanks and rear.
1. S. e. asiatica (Eurasian Nuthatch) – As nominate, but smaller. Shorter and more pointed bill, white forehead and supercilium.
2. S. e. clara (Eurasian Nuthatch) – Larger than S. e. asiatica, relatively pale upperparts, paler more obvious white forehead and supercilium.
3. Sitta arctica (Siberian Nuthatch) – Large with a narrower and shorter black eye-stripe than any other form, little white on the forehead or supercilium. Breast and center of underparts white, flanks extensively chestnut.Via Wikipedia https://en.wikipedia.org/wiki/List_of_Eurasian_nuthatch_subspecies
The Bill length and shape, and as well where (what position) the black eye mask touches or connects to the bill ; can aid in ID-ing the subspecies.
The link to the original paper is below ,where detailed information on distinguishing the subspecies is discussed.
(Below) Bill Comparisons: