ern faces. The western ridge divides the northwest and south fac

atoos have a large bill, which is kept sharp by rasping the two jaws 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 jaw applies th

e pressure, the tongue holds the seed in place and the upper jaw acts as an anvil. The eye region of the skull is reinforced to support muscles which move the jaws 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 the pink cockatoo and red-brown in some other female white cock