Cockatoo - Morphology

Morphology

The cockatoos are generally medium to large parrots of stocky build, which range from 30–60 cm (12–24 in) in length and 300–1,200 g (0.66–2.6 lb) in weight; however, one species, the Cockatiel, is considerably smaller and slimmer than the other species, being 32 cm (13 in) long (including its long pointed tail feathers) and 80–100 g (2.8–3.5 oz) in weight. The movable headcrest, which is present in all cockatoos, is spectacular in many species; it is raised when the bird lands from flying or when it is aroused. Cockatoos share many features with other parrots, including the characteristic curved beak shape and a zygodactyl foot, with the two middle toes forward and the two outer toes backward. They differ however in a number of characteristics, including the presence of a gall bladder, some other anatomical details and their lack of the Dyck texture feather composition which causes the bright blues and greens seen in true parrots.

Like other parrots, cockatoos have short legs, strong claws, a waddling gait and often use their strong bill as a third limb when climbing through branches. They generally have long broad wings used in rapid flight, with speeds up to 70 km/h (43 mph) being recorded for galahs. The members of the genus Calyptorhynchus and larger white cockatoos, such as the Sulphur-crested Cockatoo and Major Mitchell's Cockatoo, have shorter, rounder wings and a more leisurely flight.

Cockatoos have a large bill, which is kept sharp by rasping the two mandibles together when resting. The bill is complemented by a large muscular tongue which helps manipulate seeds inside the bill so that they can be de-husked before eating. During the de-husking the lower mandible applies the pressure, the tongue holds the seed in place and the upper mandible acts as an anvil. The eye region of the skull is reinforced to support muscles which move the mandibles sideways. The bills of male cockatoos are generally slightly larger than those of their female counterparts but this size difference is quite marked in the Palm Cockatoo.

The plumage of the cockatoos is less brightly coloured than that of the other parrots, with species generally being either black, grey or white. Many species have smaller areas of colour on their plumage, often yellow, pink and red, usually on the crest or tail. The Galah and Major Mitchell's Cockatoo are more broadly coloured in pink tones. Several species have a brightly coloured bare area around the eye and face known as a periophthalmic ring; the large red patch of bare skin of the Palm Cockatoo is the most extensive and covers some of the face, while it is more restricted in some other species of white cockatoo, notably the corellas and Blue-eyed Cockatoo. The plumage of males and females is similar in most species. The plumage of the female Cockatiel is duller than the male but the most marked sexual dimorphism occurs in the Gang-gang Cockatoo and the two species of black cockatoos in the subgenus Calyptorhynchus, namely the Red-tailed and Glossy Black Cockatoos. The iris colour differs in a few species, being pink or red in the female Galah and Major Mitchell's Cockatoo and red-brown in some other female white cockatoo species. The males all have dark brown irises.

Cockatoos maintain their plumage with frequent preening throughout the day. They remove dirt and oil and realign feather barbs by nibbling their feathers. They also preen other birds' feathers that are otherwise hard to get at. Cockatoos produce preen-oil from a gland on their lower back and apply it by wiping their plumage with their heads or already oiled feathers. Powder-down is produced by specialised feathers in the lumbar region and distributed by the preening cockatoo all over the plumage.

Moulting is very slow and complex. Black cockatoos appear to replace their flight feathers one at a time, their moult taking two years to complete. This process is much shorter in other species, such as the Galah and Long-billed Corella, which each take around six months to replace all their flight feathers.