14, see text Hornbills are birds found in tropical and subtropical Africa, Asia and Melanesia of the family Bucerotidae.

They are characterized by a long, down-curved bill which is frequently brightly coloured and sometimes has a horny casque on the upper mandible.

They are the only birds in which the first and second neck vertebrae (the atlas and axis respectively) are fused together; this probably provides a more stable platform for carrying the bill.

A number of mainly insular species of hornbill with small ranges are threatened with extinction, mainly in Southeast Asia.

The smallest species is the black dwarf hornbill (Tockus hartlaubi), at 99.1 g (3.50 oz) and 32 cm (1 ft 1 in) in length.

[3] The most distinctive feature of the hornbills is the heavy bill, supported by powerful neck muscles as well as by the fused vertebrae.

In other species it is quite large, is reinforced with bone, and has openings between the hollow centre, allowing it to serve as a resonator for calls.

[14] The oldest known hornbill is from the Early Miocene of Uganda, around 19 million years ago, which is similar to modern Tockus.

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.

[1] Territoriality is related to diet; fruit sources are often patchily distributed and require long-distance travel to find.

Before incubation, the females of all Bucerotinae—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 her to enter the nest, and after she has done so, the remaining opening is also all but sealed shut.

For example, some species of hornbills in Africa have a mutualistic relationship with dwarf mongooses, foraging together and warning each other of nearby birds of prey and other predators.

[24] The family Bucerotidae was introduced (as Buceronia) by the French polymath Constantine Samuel Rafinesque in 1815;[25][26] it comes from the genus name Buceros given by Carl Linnaeus in 1758 from the Greek word βούκερως bōukeros which means "cow horn".

In the Sibley-Ahlquist taxonomy, however, hornbills are separated from the Coraciiformes into an order of their own, 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,[28] the arrangement chosen is more a matter of personal taste than any well-established taxonomic practice.

If these two species are classified in congeneric, Tropicranus becomes a junior synonym of Horizocerus, as that was one of the old names used for the Black dwarf hornbill.

[30][31] The following cladogram showing the relationships between the genera is based on a molecular phylogenetic study by Juan-Carlos Gonzalez and collaborators that was published in 2013.

[30] The number of species in each genus is taken from the list of world birds maintained by Frank Gill, Pamela C. Rasmussen and David Donsker on behalf of the International Ornithologists' Union.

The Iban people of Borneo regards the Rhinoceros hornbill (known as Kenyalang) as the king of the worldly birds, who acts as the intermediary between the man and the God.

The Wreathed hornbill (Undan) is believed by the Iban people to be the guide of dead souls to the lower world.

The latter species is one of the world's rarest birds, with only 20 breeding pairs or 40 mature individuals, and faces imminent extinction.