A revised phylogeny of nuthatches (Aves, Passeriformes, Sitta) reveals insight in intra- and interspecific diversification patterns in the Palearctic

Nuthatches of the Holarctic and partly Indo-Malayan genus Sitta have been subject to a number of phylogenetic analyses; however, the most complete phylogenetic hypothesis to date missed several Asian species-level taxa, was based on a limited sampling, and included only one sample per species. Other recent studies were mainly focused on phylogeographic patterns of single Asian species but failed to unambiguously resolve their phylogenetic relationships. In this study, we provide a time-calibrated multi-locus phylogeny of nuthatches (Sitta) including 27 out of 28 currently recognized species. To account for intraspecific variation we included a number of subspecific taxa in our sampling, e.g. for the phenotypically diverse Eurasian nuthatch, S. europaea . Within the latter species (our clade X) three phenotypically distinct phylogroups started diversifying from the early Pleistocene on: (i) brown-bellied forms of the sinensis group from China and Taiwan with smallest body size dimensions, (ii) white-bellied forms of the Central and Eastern Palearctic europaea group with average body size dimensions, and (iii) brown-bellied forms of the Western Palearctic and Caucasian caesia group with largest body size dimensions. The three phylogroups are connected by chains of phenotypically intermediate populations in Eastern Europe (e.g. ssp. homeyeri) and in Far East Russia and north-eastern China (ssp. amurensis). The Eurasian nuthatch was sister to a monophyletic clade IX comprising five Sino-Himalayan species: S. nagaensis, S. cashmirensis, S. castanea, S. neglecta, and S. cinnamoventris. In the Sino-Himalayas, ecological segregation along the elevational gradient was established from the mid-Miocene onset of nuthatch diversification until the recent ecological segregation among chestnut-bellied forms of the S. castanea complex during the Pleistocene.