Bucerotidae – Hornbills

The Bucerotidae or Hornbills are a family of bird found in tropical and subtropical Africa, Asia and Melanesia. They are characterized by a long, down-curved bill which is frequently brightly coloured and sometimes has a casque on the upper mandible. Both the common English and the scientific name of the family refer to the shape of the bill, ‘buceros’ being ‘cow horn’ in Greek. Hornbills have a two-lobed kidney. 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. The family is omnivorous, feeding on fruit and small animals. They are monogamous breeders nesting in natural cavities in trees and sometimes cliffs. A number of species of hornbill are threatened with extinction, mostly insular species with small ranges.

Hornbills show considerable variation in size, ranging from the Black Dwarf Hornbill Tockus hartlaubi, at 102 grams and 30cm. Males are always bigger than the females, though the extent to which this is true varies according to species. The extent of sexual dimorphism also varies with body parts. For example, the difference in body mass between males and females is 1% to 17%, but the variation is 8% to 30% for bill length and 1% to 21% in wing length.

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, constructing the nest, and catching prey. A feature unique to the hornbills is the casque, 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 for calls. In the Helmeted Hornbill the casque is not hollow but is filled with hornbill ivory and is used as a battering ram in dramatic aerial jousts. Aerial casque-butting has also been reported in the Great Hornbill.

Their plumage 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 exhibit sexual dichromatism. The calls of hornbills are loud and vary distinctly between different species. They possess binocular vision, although unlike most birds with this type of vision, the bill intrudes on their visual field. 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. They are diurnal, 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.