Nuthatches are small, short-tailed, large-headed birds in the family Sittidae in the order Passeriformes, the perching birds. There are 25 species of nuthatches, occurring on all continents except South America, Africa, or Antarctica.
Most species of nuthatches are forest birds which clamber over the bark of trees seeking insects, spiders, and arthropod eggs. Nuthatches can climb in any direction-including head-first down tree trunks, and even clamber upside-down beneath large limbs. The preferred food is arthropods, but when these are not abundant (during autumn, winter, and spring), nuthatches eat fruits and seeds, including the relatively large nuts of trees such as beech, oak, hazel, and chestnut. The edible matter of hard fruits such as acorns and hazelnuts can be rather difficult to extricate from their protective tissues. Some species of nuthatches accomplish this task by wedging the nut into a woody crevice and then hammering it open using their relatively stout beak. Hence the origin of their common name, nuthatch.
Two species of nuthatch, the brown-headed nuthatch of North America (Sitta pusilla) and the orange-winged sittella (Neositta chrysoptera) of Australia, are known to manipulate small twigs with their beaks for use in drawing insects within reach from deep in bark crevices or rotted wood. These are rare examples of the use of tools by birds.
Nuthatches are not gregarious, although during winter they will sometimes flock with other small forest birds, such as chickadees, tits, and kinglets. Presumably, this is done for reasons of safety, because flocks of small birds have a better chance of detecting predators early.
Nuthatches defend territories, and are non-colonial breeders. The typical nuthatches (genus Sitta) nest in deep crevices or cavities in trees. The cavities may be natural, or the nuthatch may excavate it in soft, rotted wood. Most nuthatch species will also use an abandoned cavity previously excavated by another species, such as a woodpecker, and they may also use nestboxes. If the entrance to a cavity is too large, Eurasian species of nuthatches will make the hole smaller, and safer, by plastering its edges with mud. North American nuthatches do not do this.
Nuthatches have their greatest species diversity in central Asia, where there are 13 species, of which 12 are in the genus Sitta. There are four species of nuthatches in North America. The red-breasted nuthatch (Sitta canadensis) breeds widely in northern coniferous and mixed wood forests. This species is non-migratory, generally remaining in the same locale during winter. The red-breasted nuthatch usually excavates its own nest cavity in rotted wood of dead trees or stumps, and it smears the edge of the entrance hole with conifer pitch, although the reason for this behavior is not known. The white-breasted nuthatch (S. carolinensis) is a widespread resident of broadleaf and mixed wood forests. The brown-headed nuthatch occurs in southeastern pine forests, and the pygmy nuthatch (S. pygmaeus) inhabits pine forests of the west.