Birds
Rhinoceros Hornbill
Buceros rhinoceros · also called rhinoceros hornbill
The rhinoceros hornbill is a big black-and-white rainforest bird crowned with a huge upswept orange horn — a hollow structure it colours itself, with oil from a gland near its tail. Its nesting habit is stranger still: the female bricks herself inside a tree.
Overview
Rhinoceros hornbills live in lowland and hill rainforest of the Malay Peninsula, Borneo, Sumatra, and Java, dependent on tall, mature forest. They eat mostly figs and other fruit, plus insects, small reptiles, and the nestlings of other birds. At 80–90 cm plus a long tail, they are among the largest Asian hornbills.
The casque
The upturned casque on top of the bill is hollow keratin over an air-filled honeycomb — light for its size. Naturally pale, it is stained orange and red by oil the bird wipes on from its preen gland. It probably works as a resonating chamber for the loud honking calls and as a status signal; males have the largest, most curved casques.
The sealed nest
The female enters a tree cavity and walls the entrance almost shut with mud and droppings, leaving a narrow slit. She stays inside for three months or more, moulting all her flight feathers, while the male brings every scrap of food — dozens of fruits at a time, regurgitated through the slit. If the male dies, the family usually dies with him.
Farmers of the forest
By swallowing fruit whole and flying long distances, hornbills scatter the large seeds of many rainforest trees.
Status
The rhinoceros hornbill is Vulnerable and declining, hit by logging and oil-palm clearance and by hunting for meat, tail feathers, and casques. It needs big old cavity trees to breed.
Rhinoceros Hornbill: common questions
What is the casque on a hornbill's bill for?
The casque is the hollow, horn-like structure sitting on top of the rhinoceros hornbill's already large bill. It is built of keratin over a bony, air-filled honeycomb, so despite its size it is fairly light. Its exact function is still debated, but the leading ideas are that it acts as a resonating chamber that amplifies and deepens the bird's loud honking calls, that it is a signal of age, sex, and status (males have larger, more upturned casques), and that it may be used in the aerial "casque-butting" clashes that males of some hornbill species perform in flight. In the rhinoceros hornbill the casque curves dramatically upward at the front, which is where the "rhinoceros" name comes from.
Why is the casque orange when it is made of white keratin?
The colour is cosmetic and applied by the bird. Like many hornbills, the rhinoceros hornbill has a preen gland (the uropygial gland) at the base of its tail that produces an oily secretion tinted orange-red by carotenoid pigments. The bird transfers this oil onto its bill and casque during preening, gradually staining the naturally pale keratin a rich yellow, orange, and red. Because the colour has to be maintained by grooming, a bright, evenly coloured casque may signal that a bird is healthy and well-fed enough to keep up its appearance.
Is it true the female hornbill seals herself inside a tree?
Yes, and it is one of the most extreme nesting strategies of any bird. The female chooses a natural cavity in a large tree, climbs inside, and — with help from the male passing her material — plasters the entrance almost completely closed using mud, her own droppings, and fruit pulp, leaving only a narrow slit. She then stays imprisoned for around three months — she lays her eggs, incubates them, and broods the chicks, while moulting all her wing and tail feathers at once so she temporarily cannot fly anyway. The male passes food to her and later the chicks through the slit. When the chicks are partly grown the female breaks out, the nest is resealed by the chicks, and both parents provision them until they fledge. The wall protects the family from snakes, monitor lizards, civets, and other hornbills.
Why is the rhinoceros hornbill endangered?
It is classed as Vulnerable and declining, mainly because it depends on exactly the forest that is disappearing fastest. Rhinoceros hornbills need large tracts of tall, old lowland rainforest, both for the fruiting trees they eat from and for the big trees with natural cavities they must have to breed — and Sundaland's lowland forests have been extensively logged and converted to oil palm plantations. On top of habitat loss, the birds are hunted for food, for their tail feathers (used in the ceremonial dress of several Bornean peoples, for whom the bird is also a sacred symbol), and for their casques. Their slow reproduction — one brood a year, dependent on a single cavity and a single male — makes recovery difficult.