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The family Casuariidae includes three living species, all of the genus Casuarius. They live on island of New Guinea, and its nearby islands, and the northeastern part of Australia. All three cassowary species prefer rainforest habitat, as the birds require a large volume and diversity of fruit for their diet. The three species are generally segregated by altitude, ranging from lowland swamp forests to higher altitude montane forests.
Cassowaries are large ratites, and are among the largest birds in the world. Their drooping plumage is black and coarse, and they have brightly colored skin red and blue) on their necks. All three species possess a casque, or helmet, on the top of the head, which grows slowly throughout the bird's first few years. The function of the casque is poorly understood; it was originally thought that the birds use it to push aside underbrush as they travel through the dense forests. The casque may also be important in social dynamics, as it signals age of the individual. Two species of cassowary (the northern and southern cassowaries) have distinctive wattles, which are long folds of unfeathered skin that hang from the neck.
In general, the sexes are fairly similar, though females are slightly larger and more brightly colored, and have larger casques. Cassowaries can run at speeds up to 50 km/hr, and can jump 1.5m from a standing position. Another of their well-known features is the dagger-like claw on their innermost toe; this claw can be a deadly weapon and cassowaries may use it in defense by jumping and kicking with both feet. Humans have been killed by such attacks, as males can be extremely aggressive when protecting nests or young. Other distinctive features include an extremely long aftershaft, nearly as long as the main feathers, and remiges that are reduced to bare quills and curve under the body. Their wings are stunted, with a smaller body-to-wing proportion than in some other ratites, and, like most other ratites, cassowaries have no tail feathers. They have small, rudimentary clavicles, a small procoracoid process, no syrinx, and reduced caeca. They have three toes like most ratites, and short middle phalanges.
They are generally shy birds, difficult to spot in the wild, though they travel regular paths in the forest and establish regular crossing points at rivers. They tend to remain solitary for much of the year, though pairs do form during the breeding season. Males will claim territories and pair with a female for a period of several weeks, during which time she will lay 3-5 eggs. Females may then mate with another male. Males will tend the eggs and young, remaining with the chicks for 9 months. Young cassowaries are striped brown, and do not fully gain adult plumage for 3 years.