Birds
Hornbill
Bucerotidae · also called hornbills
A hornbill is a big, noisy, heavy-billed bird of Old World forests and grasslands, and it does two things almost no other bird does. It carries a hollow horn — the casque — on top of its bill, braced by the only fused neck vertebrae in any bird. And when it breeds, the female climbs into a tree hole and walls herself in with mud, staying imprisoned for weeks while the male feeds her through a slit.
Overview
Hornbills are the family Bucerotidae, around 60 species across sub-Saharan Africa and southern and Southeast Asia. Forest species eat mostly fruit, especially figs, plus insects and small animals; the African ground hornbills are nearly all-carnivorous, striding through grassland after snakes, frogs, and small mammals. They have eyelashes — stiff modified feathers shading the eye — which few birds do.
The sealed nest
In most hornbills the female seals herself into a tree cavity with mud and droppings, leaving only a slit, and stays inside for six weeks to four months, moulting all her flight feathers at once while the male brings food. It protects the eggs and chicks from snakes and monkeys.
The casque, and hornbill ivory
The casque is usually a hollow keratin-over-bone chamber that amplifies calls and signals age; a few species use it to joust in flight. The helmeted hornbill is unique in having a solid casque of dense red keratin — “hornbill ivory” — and poaching for it has driven the species to Critically Endangered.
Status
The ~60 species span the range: many African savanna hornbills are Least Concern, but the overall trend is decreasing, many Asian forest species are threatened, and both ground hornbills are declining under a very slow breeding rate. Large hornbills are keystone seed dispersers, so their loss slows forest regeneration.
Hornbill: common questions
Why does the female hornbill seal herself inside the nest?
It is a defence against nest predators, especially snakes, monkeys, and other birds, which are a major threat to eggs and chicks in a tree hole. The female chooses a cavity, climbs in, and then she and the male wall up the entrance using mud, their own droppings, and regurgitated food, leaving only a slit wide enough for a beak. She is then sealed in for the whole nesting period, which can be six weeks in a small species and up to four months in a large one. During this time she moults all her wing and tail feathers at once, so she could not fly even if she left, and she depends entirely on the male bringing food. In some species she breaks out partway through and both parents then feed the chicks, which reseal the hole themselves.
What is the casque on a hornbill's bill for?
It varies by species and is not fully settled. In many hornbills the casque is a hollow, lightweight chamber of thin bone under a keratin skin, and it appears to act mainly as a resonating chamber that makes the bird's honking and cackling calls louder and carry further through dense forest. It also grows with age and differs between the sexes, so it signals maturity and identity. In the great hornbill and a few others, males use the reinforced casque to butt each other in mid-air jousts over territory and mates. The helmeted hornbill is the exception, with a nearly solid casque used as a battering ram in head-to-head clashes.
How is a hornbill different from a toucan?
They look alike, both being big-billed fruit-eating forest birds, but they are unrelated and live on different continents, a classic case of convergent evolution. Hornbills are an Old World family, Bucerotidae, of Africa and tropical Asia, and their nearest relatives are thought to be the hoopoes and wood hoopoes. Toucans are a New World family, Ramphastidae, confined to the Americas and related to woodpeckers and barbets. The clearest behavioural difference is the nest. Hornbills wall the female into a tree hole with mud, which no toucan does. Structurally, many hornbills have a casque and fused neck vertebrae, and hornbills have eyelashes, while toucans have a serrated bill edge and a very long feather-like tongue.
Why is the helmeted hornbill so endangered?
Because of its casque. Unlike other hornbills, the helmeted hornbill has a solid front to its casque made of dense keratin, coloured deep red and yellow, which is prized as a carving material known as hornbill ivory and has been worked in China and Japan for centuries. A surge in demand from about 2011 drove intense poaching across Borneo and Sumatra, with tens of thousands of birds killed in some years for the single block of material in each skull. The species was moved to Critically Endangered in 2015. It is also a slow breeder that needs large old rainforest trees for nesting, so habitat loss compounds the hunting, and it is now the focus of an international recovery effort.