Hornbills (family Bucerotidae) are a group of birds characterized by a long, down-curved bill, sometimes with a casque on the upper mandible. Frequently, the bill is brightly coloured. Many of the birds are now endangered. Some species of hornbill are protected in Malaysia. Both the common English and the scientific name of the family refer to the shape of the bill, "buceros" being "cow horn" in Greek. In addition, they possess a two-lobed kidney. Hornbills are the only birds in which the first two neck vertebrae (the axis and atlas) are fused together; this probably provides a more stable platform for carrying the bill.
There are two subfamilies: the Bucorvinae contain the 2 ground-hornbills in a single genus, whereas the Bucerotinae contain all other taxa. In the Sibley-Ahlquist taxonomy, hornbills are separated from the Coraciiformes as a separate order Bucerotiformes, with the subfamilies elevated to family level. Given that they are almost as distant from the rollers, kingfishers and allies as are the trogons (Johansson & Ericson 2003), the arrangement chosen is more a matter of personal taste than any well-established taxonomic practice. All that can be said with reasonable certainty is that placing the hornbills outside the Coraciiformes and the trogons inside would be incorrect.
The most distinctive feature of the hornbills is the heavy bill, supported by powerful neck muscles as well as by the fused vertebrae. The large bill assists in fighting, preening, and constructing the nest, as well as catching prey. Where present, the casque on the upper mandible appears to serve no function beyond reinforcing the bill in most species. In the Black-casqued Hornbill, however, it is hollow and serves as a resonator.
The plumage of hornbills is typically black, grey, white, or brown, although typically offset by bright colours on the bill, or patches of bare coloured skin on the face or wattles. Some species exhibit sexual dichromatism; in the Abyssinian Ground-hornbill, for example, pure blue skin on the face and throat denotes an adult female, and red and blue skin denotes an adult male. The calls of hornbills are loud, and vary distinctly between different species.
Hornbills are omnivorous birds, eating fruit, insects and small animals. They cannot swallow food caught at the tip of the beak as their tongues are too short to manipulate it, so they toss it back to the throat with a jerk of the head. They range in size from the Black Dwarf Hornbill (Tockus hartlaubi), at 102 grams (3.6 oz) and 30 cm (1 foot), to the Southern Ground-hornbill (Bucorvus leadbeateri), at up to 6.2 kg (13.6 lbs) and 1.2 m (4 feet). Males are about 18% larger than females on average.
Hornbills generally form monogamous pairs, defending a fixed territory. The female lays up to six white eggs in existing holes or crevices, either in trees or rocks. Before incubation, the females of all Bucorvinae—sometimes assisted by the male—begin to close the entrance to the nest cavity with a wall made of mud, droppings and fruit pulp. When the female is ready to lay her eggs, the entrance is just large enough for it to enter the nest, and after she has done so, the remaining opening is also all but sealed shut. There is only one narrow aperture, big enough for the male to transfer food to the mother and the chicks. During the incubation period the female undergoes a complete moult. When the chicks and the female are too big to fit in the nest, the mother breaks out, then both parents feed the chicks. In some species the mother rebuilds the wall, whereas in others the chicks themselves rebuild the wall unaided. The ground-hornbills are conventional cavity-nesters instead.
Most species' casques are very light, containing a good deal of airspace. However, the Helmeted Hornbill has a solid casque made of a material called hornbill ivory, which is greatly valued as a carving material in China and Japan. It is often used as a medium for the art of netsuke.