A cockatoo is a parrot that is any of the 21 species belonging to the bird family Cacatuidae, the only family in the superfamily Cacatuoidea. Along with the Psittacoidea (true parrots) and the Strigopoidea (large New Zealand parrots), they make up the order Psittaciformes (parrots). The family has a mainly Australasian distribution, ranging from the Philippines and the eastern Indonesian islands of Wallacea to New Guinea, the Solomon Islands and Australia. Cockatoos are recognisable by the showy crests and curved bills. Their plumage is generally less colourful than that of other parrots, being mainly white, grey or black and often with coloured features in the crest, cheeks or tail. On average they are larger than other parrots; however, the cockatiel, the smallest cockatoo species, is a small bird. The phylogenetic position of the cockatiel remains unresolved, other than that it is one of the earliest offshoots of the cockatoo lineage. The remaining species are in two main clades. The five large black coloured cockatoos of the genus Calyptorhynchus form one branch. The second and larger branch is formed by the genus Cacatua, comprising 11 species of white-plumaged cockatoos and four monotypic genera that branched off earlier; namely the pink and white Major Mitchell's cockatoo, the pink and grey galah, the mainly grey gang-gang cockatoo and the large black-plumaged palm cockatoo. Cockatoos prefer to eat seeds, tubers, corms, fruit, flowers and insects. They often feed in large flocks, particularly when ground-feeding. Cockatoos are monogamous and nest in tree hollows. Some cockatoo species have been adversely affected by habitat loss, particularly from a shortage of suitable nesting hollows after large mature trees are cleared; conversely, some species have adapted well to human changes and are considered agricultural pests. Cockatoos are popular birds in aviculture, but their needs are difficult to meet. The cockatiel is the easiest cockatoo species to maintain and is by far the most frequently kept in captivity. White cockatoos are more commonly found in captivity than black cockatoos. Illegal trade in wild-caught birds contributes to the decline of some cockatoo species in the wild.
Cockatoos are monogamous breeders, with pair bonds that can last many years. Many birds pair up in flocks before they reach sexual maturity and delay breeding for a year at least. Females breed for the first time anywhere from three to seven years of age and males are often older. Sexual maturity is delayed so birds can develop the skills for raising and parenting young, which is prolonged compared with other birds; the young of some species remain with their parents for up to a year. Cockatoos may also display site fidelity, returning to the same nesting sites in consecutive years. Courtship is generally simple, particularly for established pairs, with the black cockatoos alone engaging in courtship feeding. Established pairs do engage in preening each other, but all forms of courtship drop off after incubation begins, possibly due to the strength of the pair-bond. Like most parrots, the cockatoos are cavity nesters, nesting in holes in trees, which they are unable to excavate themselves. These hollows are formed from decay or destruction of wood by branches breaking off, fungi or insects such as termites or even woodpeckers where their ranges overlap. In many places these holes are scarce and the source of competition, both with other members of the same species and with other species and types of animal. In general, cockatoos choose hollows only a little larger than themselves, hence different-sized species nest in holes of corresponding (and different) sizes. If given the opportunity, cockatoos prefer nesting over 7 or 8 metres (23 or 26 ft) above the ground and close to water and food. The nesting hollows are lined with sticks, wood chips and branches with leaves. The eggs of cockatoos are oval and initially white, as their location makes camouflage unnecessary. However, they do become discoloured over the course of incubation. They range in size from 55 mm × 37 mm (2.2 in × 1.5 in) in the palm and red-tailed black cockatoos, to 26 mm × 19 mm (1.02 in × 0.75 in) in the cockatiel. Clutch size varies within the family, with the palm cockatoo and some other larger cockatoos laying only a single egg and the smaller species laying anywhere between two and eight eggs. Food supply also plays a role in clutch size. Some species can lay a second clutch if the first fails. Around 20% of eggs laid are infertile. The cockatoos' incubation and brooding responsibilities may either be undertaken by the female alone in the case of the black cockatoos or shared amongst the sexes as happens in the other species. In the case of the black cockatoos, the female is provisioned by the male several times a day. The young of all species are born covered in yellowish down, bar the palm cockatoo, whose young are born naked. Cockatoo incubation times are dependent on species size, with the smaller cockatiels having a period of around 20 days and the larger Carnaby's black cockatoo incubating its eggs for up to 29 days. The nestling period also varies by species size, with larger species having longer nestling periods. It is also affected by season and environmental factors and by competition with siblings in species with clutch sizes greater than one. Much of what is known about the nestling period of some species is dependent on aviary studies – aviary cockatiels can fledge after 5 weeks and the large palm cockatoos after 11 weeks. During this period, the young become covered in juvenile plumage while remaining in the hollow. Wings and tail feathers are slow to grow initially but more rapid as the primary feathers appear. Nestlings quickly reach about 80–90% of adult weight about two thirds of the time through this period, plateauing before they leave the hollow; they fledge at this weight with wing and tail feathers still to grow a little before reaching adult dimensions. Growth rate of the young, as well as numbers fledged, are adversely impacted by reduced food supply and poor weather conditions.