Hornbill

About the hornbill

Hornbills are a family of birds, the Bucerotidae, found across tropical and subtropical Africa, Asia, and Melanesia. About 62 living species make up the family, distributed from sub-Saharan Africa and the Indian subcontinent to the Philippines and the Solomon Islands — though no single genus is found in both Africa and Asia. The family's defining feature is a long, down-curved bill, often brightly colored and sometimes topped with a horny casque on the upper mandible. Hornbills also have a two-lobed kidney, and they are the only birds in which the first two neck vertebrae, the atlas and axis, are fused together, likely to give the bill a more stable base.

Size varies enormously across the family. The smallest species, the black dwarf hornbill, weighs just 99.1 g (3.50 oz) and measures 32 cm (1 ft 1 in) long. At the other extreme, the southern ground hornbill averages 3.77 kg (8.3 lb), can reach 6.3 kg (14 lb), and spans about 180 cm (5 ft 11 in) across the wings, making it the largest and most massive species in the family. Several other species rival it in length, including the Abyssinian ground hornbill, the great hornbill, and the helmeted hornbill, whose extended tail feathers may push its total length past 150 cm (4 ft 11 in). Males are always larger than females, though by how much varies by species and by body part — the difference in body mass runs 1 to 17 percent, while bill length can differ by 8 to 30 percent and wing length by 1 to 21 percent.

The bill itself, powered by strong neck muscles and braced by those fused vertebrae, is used for fighting, preening, nest building, and catching prey. A structure unique to hornbills, the casque, runs along the top of the bill; in some species it barely shows and seems to do little beyond reinforcing the bill, while in others it is large, bone-reinforced, and hollow enough to work as a resonating chamber for calls. The helmeted hornbill is the exception — its casque is solid, filled with a dense material called hornbill ivory, and used as a battering ram in aerial jousts with rivals, a behavior also reported in the great hornbill. Plumage across the family runs to black, gray, white, or brown, often set off by bright bill colors or bare patches of colored skin on the face. Hornbills also have binocular vision unusual among birds because the bill itself intrudes into the visual field, letting them see the tip of their own bill and handle food with precision; large eyelashes shade the eyes like a built-in sunshade.

Behavior and diet

Hornbills are diurnal birds that generally travel in pairs or small family groups, though larger flocks can form outside the breeding season — at some roosting sites, as many as 2,400 individual birds have been counted together. They are omnivorous, eating fruit, insects, and small animals, but their tongues are too short to manipulate food caught at the tip of the bill, so they toss each item back toward the throat with a jerk of the head. Open-country species tend to be more carnivorous, while forest species specialize more heavily in fruit, and forest hornbills are considered important seed dispersers for the trees they feed on. Some, such as the Malabar pied-hornbill, favor the fruit of the strychnine tree despite its potent poison. Species that rely on patchily distributed fruit generally travel farther and defend territory less than species with steadier food sources.

Breeding hornbills typically form monogamous pairs, though some species practice cooperative breeding. A female lays up to six white eggs in a natural cavity in a tree or rock, sometimes reusing the abandoned nest of a woodpecker or barbet, and pairs may return to the same site in consecutive seasons. Before laying, the female — sometimes helped by the male — begins sealing the nest entrance with a wall of mud, droppings, and fruit pulp, leaving only a narrow slit just wide enough for her to pass through, and later small enough for the male to feed her and the chicks. This sealing typically takes only a few hours, occasionally a few days, and about five days pass between the sealing and the first egg. Clutch size ranges from one or two eggs in the largest species up to eight in the smallest. During incubation the female molts completely and all at once, which researchers suspect is triggered by the darkness of the sealed cavity; non-breeding females and males molt gradually instead. Once the chicks and female outgrow the nest, the wall is broken open and both parents feed the brood — in some species the female reseals the nest afterward, and in others the chicks rebuild the wall themselves. Ground hornbills are the exception to all of this: they nest conventionally in cavities without sealing the entrance.

Range and habitat

Hornbills range across sub-Saharan Africa and the Indian subcontinent through to the Philippines and the Solomon Islands, though never with the same genus present on both the African and Asian sides of that range. Of the 24 African species, 13 live in more open woodland and savanna, some in quite arid conditions, while the rest occupy dense forest; in Asia the pattern flips, with only a single species in open savanna and the remainder confined to forest. The Indian subcontinent holds 10 hornbill species, nine of them in India and neighboring countries, while the Sri Lanka gray hornbill exists only on that island. Indonesia has 13 species, concentrated in Sumatra, Sumba, Sulawesi, Papua, and Kalimantan, and the Philippines has 11 species, every one of them found only on specific small islands within the country.

Most hornbills are arboreal, but the large ground hornbills are terrestrial birds of open savanna. Fossil evidence shows hornbills once lived in North Africa and southern Europe during the late Miocene, with remains found in Morocco and Bulgaria, and the oldest known hornbill fossil comes from Early Miocene Uganda, around 19 million years old.

Conservation status

Hornbills have no single conservation status, because the IUCN Red List assesses each species separately, and the picture varies widely: African hornbills are not seriously threatened, but many Asian hornbills face pressure from hunting and habitat loss tied to their dependence on primary forest. In the Philippines, where all 11 species are found only on specific islands, the Palawan hornbill is vulnerable and the Mindoro hornbill is endangered; the Visayan hornbill is listed as endangered by the IUCN, and a subspecies, the Ticao hornbill, was declared extinct in 2013. The rufous-headed hornbill and the Sulu hornbill are both critically endangered, with the Sulu hornbill among the rarest birds in the world, down to an estimated 20 breeding pairs. The helmeted hornbill, also critically endangered, is threatened by uncontrolled hunting tied to the trade in its solid hornbill-ivory casque.