Nuthatches belong to the bird family Sittidae which is a Greek word and used by Aristotle for a bird that pecks at the bark of trees. It is also a corruption of the older English name "nuthack" which the British nuthatches were given because of their habit of wedging nuts, insects and other food in tree crevices and then hacking them open with its long, sturdy bill.
Nuthatches are a thickset, stub-tailed, nervous little bird that scurry down tree trunks headfirst, hops, jerks, takes short jumps and moves along underside of branches in the same way with their back to the ground. They fly up and down like woodpeckers. When climbing down tree trunks, nuthatches depend entirely upon their claws. They stretch one foot forward under the breast and the other back under the tail, and hitch nimbly down the trees, digging in with their strong hind toes. The reason why these birds climb downward is that they may find food in bark crevices overlooked by birds that climb upward like tree creepers and small woodpeckers.
Of the 22 species of nuthatches in the world, there are only four members of the family that reside in North America, the white-breasted, red-breasted, brown-headed and pygmy nuthatches. North American nuthatches measure from 3 3/4" to 6" in length and the sexes resemble each other in plumage. They are gray-blue above, white below, bills thin, straight and sharp-pointed, nostrils are somewhat covered with stiff feathers, the wings are long and pointed, legs short and strong, and long toes have sharp claws for clinging to tree bark.
These birds may breed in bird boxes, but most often choose a tree hole. All nuthatches are cavity nesters. The nest is constructed by stuffing the cavity with shredded bark, small twigs, grasses and rootlets, and then lined with fur/hair and feathers for cushioning of the eggs and chicks. Both sexes help with the nest construction; but the male's efforts are not as industrious as the females. Incubation lasts 12 to 14 days and is mainly the female's chore.
Most North American nuthatches come to feeding stations. It is always a pleasure to watch these aggressive birds extend their wings to dissuade finches and other birds. They will most likely snatch a chunk of suet, sunflower seed or peanut and fly off to a tree to wedge it into a crevice or do what they are known to to -- hacking the food into pieces for immediate consumption.