canadensis group, which he defined as six species, which are also sometimes treated as the subgenus Sitta (micrositta) (Buturlin, 1916). The Yunnan nuthatch (S. yunnanensis) was not included in the study. Pasquet concludes that the Krüper's nuthatch is phylogenetically related to the Algerian nuthatch, with both species forming the sister group of a clade that includes the Chinese nuthatch, the Corsican nuthatch, and the red-breasted nuthatch. In 2014, Éric Pasquet and colleagues published a phylogeny based on nuclear and mitochondrial DNA from 21 species of nuthatches and confirmed the 1998 study's relationships within the canadensis group, adding the Yunnan nuthatch, which was found to be the most basal of the species. The conclusions of the study are in agreement with the morphology of the species, the red-breasted, Corsican, and Chinese nuthatches sharing, in particular as a derivative characteristic, the entirely black only present in males, a unique trait in Sittidae and related families. The second clade, grouping together Krüper's and Algerian nuthatches are thought to have the front of the black crown in males, this sexual dimorphism being absent in young individuals. Phylogeography A study on the phylogeography of the species was published in 2012, involving five sample sites in Turkey. The authors conclude that there are significant genetic diversities between the different localities, showing that the species occurred in at least three refuges during the last glaciation maximum. The southern populations are significantly different from the northern populations of the country, but the different northern populations have intensively mixed their genes after the glacial retreat, and have quite similar nuclear genetic material, although a marked geographical structure is observed by studying their mitochondrial genomes. Description The Krüper's nuthatch is a small to medium-sized nuthatch, measuring in length. In males and females, the folded wing measures and respectively, the tail and . The measures and the . The adult male weighs . The study of 41 different measurements of many individuals across different localities in Turkey showed small variations in size along the range. The are blue-gray with a black front crown in adults of both sexes, and the primary and secondary flight feathers are gray-brown. The supercilium is white and underlined by a black that is a little less well defined behind the eye. The throat is white and the rest of the are pale gray with a large reddish patch on the crescent-shaped breast. The underwing-coverts are off-white