Taxonomy
The nuthatch family, Sittidae, was described by René-Primevère Lesson in 1828. Sometimes the Wallcreeper (Tichodroma muraria), which is restricted to the mountains of southern Eurasia, is placed in the same family as the nuthatches, but in a separate subfamily "Tichodromadinae", in which case the nuthatches are classified in the subfamily "Sittinae". However, it is more often placed in a separate family, the Tichodromadidae. The Wallcreeper is intermediate in its morphology between the nuthatches and the treecreepers, but its appearance, the texture of its plumage, and the shape and pattern of its tail suggest that it is closer to the former taxon. The Nuthatch Vanga of Madagascar (formerly known as the Coral-billed Nuthatch) and the sitellas from Australia and New Guinea were once placed in the nuthatch family because of similarities in appearance and lifestyle, but they are not closely related. The resemblances arose via convergent evolution to fill an ecological niche.
The nuthatches' closest relatives, other than the Wallcreeper, are the treecreepers, and the two (or three) families are sometimes placed in a larger grouping with the wrens and gnatcatchers. This superfamily, the Certhioidea, is proposed on phylogenetic studies using mitochondrial and nuclear DNA, and was created to cover a clade of (four or) five families removed from a larger grouping of passerine birds, the Sylvioidea.
The nuthatches are all in the genus Sitta Linnaeus, 1758, a name derived from sittē, Ancient Greek for this bird. Nuthatch refers to the propensity of some species to wedge a large insect or seed in a crack and hack at it with their strong bills. Species boundaries in the nuthatches are difficult to define. The Red-breasted Nuthatch, Corsican Nuthatch and Chinese Nuthatch have breeding ranges separated by thousands of kilometres, but are similar in habitat preference, appearance and song. They were formerly considered to be one species, but are now normally split into three and comprise a superspecies along with the Krüper's and Algerian Nuthatch. Unusually for nuthatches, all five species excavate their own nests.
The Eurasian, Chestnut-vented, Kashmir and Chestnut-bellied Nuthatches form another superspecies and replace each other geographically across Asia. They are currently considered to be four separate species, but the south-Asian forms were once believed to be a subspecies of the Eurasian Nuthatch. A recent proposed change in this taxonomy is a split of the Chestnut-bellied Nuthatch into two species, namely the Indian Nuthatch, Sitta castanea, found south of the Ganges, and the Chestnut-bellied Nuthatch sensu strictu, S. cinnamoventris, which occurs in the Himalayas. Mitochondrial DNA studies have demonstrated that the white-breasted northern subspecies of Eurasian Nuthatch, S. (europea) arctica, is distinctive, and also a possible candidate for full species status. This split has been accepted by the British Ornithologists' Union.
A 2006 review of Asian nuthatches suggested that there are still unresolved problems in nuthatch taxonomy and proposed splitting the genus Sitta. This suggestion would move the red- and yellow-billed south Asian species (Velvet-fronted, Yellow-billed and Sulphur-billed Nuthatches) to a new genus, create a third genus for the Blue Nuthatch, and possibly a fourth for the Beautiful Nuthatch.
The fossil record for this group appears to be restricted to a foot bone of an early Miocene bird from Bavaria which has been identified as an extinct representative of the climbing Certhioidea, a clade comprising the treecreepers, Wallcreeper and nuthatches. It has been described as Certhiops rummeli.
Read more about this topic: Nuthatch