The Nuthatches are all included in the genus Sitta. The occur across the Northern Hemisphere, from Eurasia to North America, but the greatest diversity is found in Southern Asia. Two species are known to interbreed Eurasian Nuthatch (S. europaea) and Chestnut-vented Nuthatch (S. nagaensis).
Genetic analysis of the Eurasian Nuthatch uncovered three lineages (Caucasus, Europe and Asia) with limited gene flow between them (Hung, Drovetski & Zink, 2012). Similarly, the White-breasted Nuthatch (S. carolinensis) is comprised of four monophyletic clades and there is close to zero (if not zero) gene flow among these clades (Walstrom, Klicka & Spellman, 2012).
Hung, C. M., Drovetski, S. V. & Zink, R. M. (2012). MULTILOCUS COALESCENCE ANALYSES SUPPORT A mtDNA-BASED PHYLOGEOGRAPHIC HISTORY FOR A WIDESPREAD PALEARCTIC PASSERINE BIRD, SITTA EUROPAEA. Evolution 66, 2850-2864.
Walstrom, V. W., Klicka, J. & Spellman, G. M. (2012). Speciation in the White-breasted Nuthatch (Sitta carolinensis): a multilocus perspective. Molecular Ecology 21, 907-920.