Hornbills arebirds found in tropical andsubtropical Africa, Asia andMelanesia of thefamilyBucerotidae. They are characterized by a long, down-curved bill which is frequently brightly coloured and sometimes has a hornycasque on the upper mandible. Hornbills have a two-lobedkidney. They are the only birds in which the first and secondneck vertebrae (theatlas andaxis respectively) are fused together; this probably provides a more stable platform for carrying the bill.[1] The family is omnivorous, feeding on fruit and small animals. They aremonogamous breeders nesting in natural cavities in trees and sometimes cliffs. A number of mainlyinsular species of hornbill with small ranges arethreatened withextinction, mainly in Southeast Asia.
The brightest colours on most hornbills, like this pair ofknobbed hornbills, are found on the beaks and bare skin of the face and throat.
Hornbills show considerable variation in size and colors. The smallest species is theblack dwarf hornbill (Tockus hartlaubi), at 99.1 g (3.50 oz) and 32 cm (1 ft 1 in) in length.[3][4] The largest and most massive species appears to be thesouthern ground hornbill which has an average weight of 3.77 kg (8.3 lb), and can weigh up to 6.3 kg (14 lb) and span about 180 cm (5 ft 11 in) across the wings.[3][4][5] Other species rival the southern ground species in length, at up to about 130 cm (4 ft 3 in), including theAbyssinian ground hornbill (Bucorvus abyssinicus), thegreat hornbill (Buceros bicornis) and, probably the longest of all (perhaps exceeding 150 cm (4 ft 11 in)) thanks in part to its extended tail feathers, thehelmeted hornbill (Rhinoplax vigil).[3][6] Males are always bigger than the females, though the extent to which this is true varies according to species. The extent ofsexual dimorphism also varies with body parts. For example, the difference in body mass between males and females is 1–17%, but the variation is 8–30% for bill length and 1–21% in wing 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.[3] The large bill assists in fighting, preening, constructing the nest, and catching prey. A feature unique to the hornbills is thecasque, a hollow structure that runs along the upper mandible. In some species it is barely perceptible and appears to serve no function beyond reinforcing the bill. 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 forcalls.[1] In thehelmeted hornbill the casque is not hollow but is filled withhornbill ivory and is used as a battering ram in dramatic aerial jousts.[7] Aerial casque-butting has also been reported in thegreat hornbill.[8][9]
Theplumage of hornbills is typically black, grey, white, or brown, and is frequently offset by bright colours on the bill, or by patches of bare coloured skin on the face or wattles. Some species exhibitsexual dichromatism, where the colouration of soft parts varies by sex.Hornbills possess binocularvision, although unlike most birds with this type of vision, the bill intrudes on their visual field.[10] This allows them to see their own bill tip and aids in precision handling of food objects with their bill. The eyes are also protected by large eyelashes which act as a sunshade.
The Bucerotidae include about 55 livingspecies, though a number ofcryptic species may yet be split, as has been suggested for thered-billed hornbill. Their distribution includesSub-Saharan Africa and theIndian subcontinent to the Philippines and theSolomon Islands, but no genus is found inboth Africa and Asia. Most are arboreal birds, but the large ground hornbills (Bucorvus), as their name implies, are terrestrial birds of opensavanna. Of the 24 species found in Africa, 13 are birds of the more open woodlands and savanna, and some occur even in highly arid environments; the remaining species are found in dense forests. This contrasts with Asia, where a single species occurs in open savanna and the remainder are forest species.[3] TheIndian subcontinent has 10 species of hornbills, of which 9 are found in India and adjoining countries, while theSri Lanka grey hornbill is restricted to the island. The most common widespread species in the Indian subcontinent is the Indian grey hornbill.According to theInternational Union for Conservation of Nature (IUCN),Indonesia has 13 hornbill species: 9 of them exist inSumatra, and the rest exist inSumba,Sulawesi,Papua andKalimantan. Kalimantan has the same hornbill species as Sumatra, except that the great hornbill is not found there.[11] Meanwhile, the neighboring archipelago of thePhilippines has 11 hornbill species, all of which are endemic to certain small islands of the country, making them one of the most endangered hornbills in the world.[12] In theNeogene (at least in the lateMiocene), hornbills inhabited North Africa and South Europe. Their remains have been found inMorocco[13] andBulgaria.[14] The oldest known hornbill is from the Early Miocene of Uganda, around 19 million years ago, which is similar to modernTockus.[15]
Hornbills arediurnal, generally travelling in pairs or small family groups. Larger flocks sometimes form outside the breeding season. The largest assemblies of hornbills form at some roosting sites, where as many as 2400 individual birds may be found.[16]
Femalegreat hornbill feeding on figs. Fruit forms a large part of the diet of forest hornbills.
Hornbills areomnivorous 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. While both open country and forest species are omnivorous, species that specialise in feeding on fruit are generally found in forests, while the more carnivorous species are found in open country.[3] Forest-dwelling species of hornbills are considered to be important seed dispersers.[17] Some hornbill species (e.g.,Malabar pied-hornbill) even have a great preference for the fruits of thestrychnine tree (Strychnos nux-vomica), which contain the potent poisonstrychnine.[18]
Some hornbills defend a fixedterritory.[1] Territoriality is related to diet; fruit sources are often patchily distributed and require long-distance travel to find. Thus, species that specialise in fruit are less territorial.
Hornbills generally formmonogamous pairs, although some species engage incooperative breeding. The female lays up to six white eggs in existing holes or crevices, either in trees or rocks. The cavities are usually natural, but some species may nest in the abandoned nests ofwoodpeckers andbarbets. Nesting sites may be used in consecutive breeding seasons by the same pair. 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. There is only one narrow aperture, big enough for the male to transfer food to the mother and eventually the chicks. The function of this behaviour is apparently related to protecting the nesting site from rival hornbills.[20] The sealing can be done in just a few hours; at most it takes a few days. After the nest is sealed, the hornbill takes another five days to lay the first egg.Clutch size varies from one or two eggs in the larger species to up to eight eggs for the smaller species. During the incubation period the female undergoes a complete and simultaneousmoult. It has been suggested that the darkness of the cavity triggers a hormone involved in moulting.[21] Non-breeding females and males go through a sequential moult.[22] When the chicks and the female are too big to fit in the nest, the mother breaks out the nest and both parents feed the chicks.[1] In some species the mother rebuilds the wall, whereas in others the chicks rebuild the wall unaided. The ground hornbills do not adopt this behaviour, but are conventional cavity-nesters.[1]
A number of hornbills have associations with other animal species. For example, some species of hornbills in Africa have amutualistic relationship withdwarf mongooses, foraging together and warning each other of nearbybirds of prey and other predators.[23] Other relationships arecommensal, for example following monkeys or other animals and eating the insects flushed up by them.[24]
There are twosubfamilies: theBucorvinae contain the two ground hornbills in a singlegenus, and the Bucerotinae contain all othertaxa. Traditionally they are included in the orderCoraciiformes (which includes alsokingfishers,rollers,hoopoes andbee-eaters). In theSibley-Ahlquist taxonomy, however, hornbills are separated from theCoraciiformes into anorder 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 thetrogons,[28] 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.
Genetic data suggests thatground hornbills andBycanistes form a clade outside the rest of the hornbill lineage.[29] They are thought to represent an early African lineage, while the rest of Bucerotiformes evolved in Asia. However, another study claims that the ground hornbills diverged first, followed byTockus. WithinTockus, two clades have been identified based on genetics and vocal types—'whistlers' and 'cluckers'. The 'cluckers' have been placed in a separate genus,Lophoceros.[30][31]
Bycanistes belongs to a clade of mostly African species that also includesCeratogymna andTropicranus. Another member of this clade is theBlack dwarf hornbill. The Black dwarf hornbill is typically classified in the genusTockus but in this study, is a sister species to the White-crested hornbill. If these two species are classified in congeneric,Tropicranus becomes a junior synonym ofHorizocerus, as that was one of the old names used for the Black dwarf hornbill. This clade also includes one Southeast Asian species, thewhite-crowned hornbill.
The following cladogram showing the relationships between the genera is based on amolecular 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 byFrank Gill,Pamela C. Rasmussen and David Donsker on behalf of theInternational Ornithologists' Union.[31]
Most species' casques are very light, containing much airspace. However, thehelmeted hornbill has a solid casque made of a material calledhornbill ivory, which is greatly valued as a carving material in China and Japan. It was used as a medium for the art ofnetsuke. Also used for hunting purposes in places like India. TheIban 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.
^abDunning, John B. Jr., ed. (2008).CRC Handbook of Avian Body Masses (2nd ed.). CRC Press.ISBN978-1-4200-6444-5.
^Kemp, A. C., & Kemp, M. I. (1980).The biology of the southern ground hornbill Bucorvus leadbeateri (Vigors)(Aves: Bucerotidae). Annals of the Transvaal Museum, 32(4), 65-100.
^Robson, C. (2015).Birds of South-East Asia: Concise Edition. Bloomsbury Publishing.
^Kinnaird, Margaret F.; Hadiprakarsa, Yok-Yok; Thiensongrusamee, Preeda (2003). "Aerial jousting by Helmeted HornbillsRhinoplax vigil: observations from Indonesia and Thailand".Ibis.145 (3):506–508.doi:10.1046/j.1474-919X.2003.00188.x.
^Cranbrook, Earl of; Kemp, A. (1995). "Aerial casque-butting by hornbills (Bucerotidae): a correction and an expansion".Ibis.137 (4):588–589.doi:10.1111/j.1474-919X.1995.tb03271.x.
^Martin, Graham; Coetzee, Hendri C. (2003). "Visual fields in hornbills: precision-grasping and sunshades".Ibis.146 (1):18–26.doi:10.1111/j.1474-919X.2004.00211.x.
^abBrunet, J. 1971. Oiseaux miocènes de Beni Mellal (Maroc); un complément à leur étude. Notes Mem. Serv. geol. Maroc, 31 (237): 109–111.
^abBoev, Z., D. Kovachev 2007. Euroceros bulgaricus gen. nov., sp. nov. from Hadzhidimovo (SW Bulgaria) (Late Miocene) – the first European record of Hornbills (Aves: Coraciiformes). – Geobios, 40: 39–49.
^Kalina, Jan (1988). "Nest intruders, nest defence and foraging behaviour in the Black-and-white Casqued HornbillBycanistes subcylindricus".Ibis.131 (4):567–571.doi:10.1111/j.1474-919X.1989.tb04791.x.
^Stonor, C. R. (1937). "On the attempted breeding of a pair of Trumpeter Hornbills (Bycanistes buccinator) in the gardens in 1936; together with some remarks on the physiology of the moult in the female".Proceedings of the Zoological Society of London, Series A.107 (Part 3):89–94.doi:10.1111/j.1469-7998.1937.tb08502.x.
^Gaietti, Mauro; McConkey, Kim (1998). "Black HornbillAbthracoceros malayanus following Gibbons in central Borneo".Ibis.140 (4):686–687.doi:10.1111/j.1474-919X.1998.tb04716.x.
^Woodruff, D. S. & Srikwan, S. 2011. "Molecular genetics and the conservation of hornbills in fragmented landscapes." In Poonswad, P. (ed)The Asian Hornbills: Ecology and Conservation. National Center for Genetic Engineering and Biotechnology, Bangkok, pp. 257–264.
Zimmerman, Dale A.,Turner, Donald A., &Pearson, David J. (1999):Birds of Kenya and Northern Tanzania (Field Guide Edition). Princeton University Press.ISBN0-691-01022-6
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