Hornbills are a family (Bucerotidae) of more than fifty species of bird found in Africa and Asia. Hornbills originated in Africa, and are initially divided into ground-dwelling species (two species found only in Africa), and arboreal (tree-dwelling, the remainder), and subsequently into Asian and African species. 

The primary theory for the origins of hornbills, is that they evolved initially in Africa spreading into Asia when the Indian continental plate split from Africa around 55 million years ago, colliding around 10 million years later with Asia, forming the Himalayas, and carrying hornbills into Asia. One Asian species, Berenicornis comatus, the white-crowned hornbill, is more closely related to African hornbills than to the rest of the Asian hornbills, having therefore split earlier, whereas other Asian species share subsequent Asian common ancestors.

The hornbills have long bills, which they use for feeding on fruit and insects (different species vary in diet between exclusively insect-eating, fruit-eating, or both). They swallow food by tilting their head back, as their tongues are too short for this job.

They are not related at all to toucans, which live in South America. Many hornbill species, unlike toucans, possess casques, which are the large growths on top of the bird's bill. Hornbills are typically monogamous, and those found in the jungles of Sumatra form an important road in dispersing fruit seeds in the jungle.

The female hornbill makes her nest in a hole in a jungle tree, which she and her mate seal, first using chewed fruit, and then clay. After she has been sealed in the nest, her mate will feed her through a tiny slit, and she will remain sealed in the nest for around five months, until her chicks have hatched and are ready to leave the nest. This sealing is thought to protect the eggs and chicks from predators.(This behaviour does not apply to the two species of ground-dwelling hornbills).

The nine hornbill species found in Sumatra (including around Bukit Lawang) (none of which are endemic) are:

Berenicornis comatus - white-crowned hornbill - this widely-distributed hornbill is the closest relative of the African hornbills.

Buceros bicornis - great hornbill - the three Buceros species and the closely related Rhinoplax genus are an early evolution of Asian hornbills
Buceros rhinoceros - rhinoceros hornbill
Rhinoplax vigil - helmeted hornbill

Anorrhinus galeritus - bushy-crested hornbill 
Anthracoceros albirostris - oriental pied hornbill
Anthracoceros malayanus - black hornbill

Cranobrontes corrugatus - wrinkled hornbill
Rhyticeros undulatus - wreathed hornbill

Helmeted hornbill

The helmeted hornbill is unique among hornbills, because its casque is solid, forming a material known as 'hornbill ivory'. The killing of helmeted hornbills has increased hugely since 2012, with traders sending thousands of their casques to China each year, where the ivory is carved into trinkets. Whereas it was recently locally common in many areas, it now faces imminent extinction, as poachers exterminate the birds en masse.