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Description

Hornbills show considerable variation in size and colors. 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. The largest and most massive species appears to be the southern 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. Other species rival the southern ground species in length, at up to about 130 cm (4 ft 3 in), including the Abyssinian ground hornbill (Bucorvus abyssinicus), the great 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, the helmeted hornbill (Rhinoplax vigil). 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 1–17%, but the variation is 8–30% for bill length and 1–21% 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.

The plumage 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 exhibit sexual dichromatism, where the colouration of soft parts varies by sex.Hornbills 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.