An introduction to hornbills

More from the bird book. For the back-story, see the previous owls article.

Hornbills are among the most distinctive and spectacular of Old World tropical birds. Often flaunting bright colours and sometimes reaching huge sizes (the largest species have wingspans of 1.8 m), they're well known for their enormous, curved bills and large bony crests. [Image above shows Great Indian hornbill skeleton Buceros bicornis (l) and male Wreathed hornbill Rhyticeros undulatus (r) (by Blijdorp, from wikipedia). Hornbills like the Great Indian hornbill are forest-dwelling omnivores that eat fruit as well as small animals. In the Great Indian hornbill, the casque has a flattened upper surface and is sometimes used in head-butting. This is a long-lived bird, reaching its sixth decade in cases. Image below shows two Rhinoceros hornbills B. rhinoceros - by JP Bennett, from wikipedia - at top, and Knobbed hornbill or Sulawesi wreathed hornbill R. cassidix - by Tobias, from wikipedia - below. The Rhinoceros hornbill is a giant, forest-dwelling species of south-eastern Asia. Its horn-like casque is larger and more prominent in males. The red colour on the casque is derived from the oil gland under the tail.]

Life in tropical forests

Spectacular colours, gigantic bills and head crests, and remarkable pieces of behaviour make hornbills among the most obvious birds of the African and Asian tropics. People have long revered these birds and, in some cultures, their images are incorporated into art, heraldry and architecture. What appear to be the most archaic of hornbills - the ground hornbills and the grassland-dwelling Tockus hornbills - are African. Early in their evolution (apparently during the Eocene*), hornbills invaded Asia, and it is here that many kinds of tropical, forest-dwelling hornbill evolved (Viseshakul et al. 2011). One Asian lineage then appears to have re-invaded Africa (Kemp 1995, Kinnaird & O'Brien 2007). [Helmeted hornbill Rhinoplax vigil shown below; by Doug Janson, from wikipedia.]

* Molecular clocks and fossil data from related bird groups show that hornbills had certainly appeared, and apparently undergone much of their initial diversification, during the Eocene. Yet their fossil record hardly reflects this. The alleged Eocene hornbill Geiseloceros robustus is not a hornbill at all, and undoubted hornbills are rare fossils in Miocene and post-Miocene strata.

The bill is straight in some species, but is more typically curved along its length. Its superficial resemblance to a cow's horn explains the group's name. This massive bill is a versatile tool, allowing different species to exploit tropical forest canopies, dry woodlands and even grasslands and semi-deserts. The majority of species use it to pick fruit and to grab small animals while foraging in trees, but some use it as a hammer to excavate bark or soil when searching for insects. Ground hornbills are able to subdue such large prey as snakes and rabbits. Their size, large appetites and ability to range far and wide in search of fruit make hornbills important seed dispersers.

In many species, the tips of the upper and lower jaws are the only parts of the jaws that are in full contact. The birds use these to perform precise grasping actions, and they also manipulate objects before swallowing. Hornbills have a particularly short tongue that doesn't play a role in the manipulation of food items: once an object is positioned as desired at the jaw tips, it is thrown backwards into the throat. This technique has been referred to as 'ballistic transport' (Baussart & Bels 2010) (the same feeding technique has been convergently evolved in toucans: this is all rather different from the 'catch and throw' technique practised by ratites and some other birds) [figure below, from Baussart & Bels (2010), shows food transport as used by three different hornbill species. The object follows a ballistic curve as it's thrown from the jaw tips to the throat]. It seems logical to assume that hornbills can see their own bill tip, and studies of their visual fields show that sophisticated binocular vision allows this (Martin & Coetzee 2004). This is unusual among birds, as the bill tip is normally outside the bird's field of vision. The hornbill palate is roofed with bone and is hence reinforced and strong compared to that of most other birds (Burton 1984). After grabbing a prey animal, a hornbill will often beat it to death against a perch. The long, thick eyelashes of some species help shield the eyes from sunlight.

Like most forest-dwelling birds, hornbills have broad, rounded wings. They are very noisy in flight, and the whooshing sound of their wing feathers means that they are sometimes heard before they are seen.

Hornbill breeding biology is remarkable. Most species are monogamous and defend territories, but co-operative breeding is present in others. When the time comes to nest, the pair selects a cavity in a tree or rock face. Using mud and droppings, the female walls herself into the chamber. A slit remains the only point of contact with the outside world, and the female and chicks then rely on the male to collect and deliver food. In some species, the female remains in the chamber for as long as five months.

Hard heads and hollow crests

Running along the top of the hornbill beak and often extending on to the skull roof is a hollow bony ridge or crest, sheathed in protective keratin. This is properly termed the casque. In the most primitive hornbills, the casque is merely a low ridge, and it is thought that its initial development was related to the addition of strength to the bill. More advanced hornbills have enlarged the casque and possess a mass of supporting bony struts inside it.

One of the primary roles of the casque is as a social signaller. It only develops once the bird reaches sexual maturity, and is typically larger in males. In the Black-casqued hornbill Ceratogymna atrata the casque works as a resonating chamber (Alexander et al. 1994). Perhaps the most remarkable casque is the one possessed by the Helmeted hornbill. Composed of a solid block of bone, it accounts for about 11% of the bird's weight [world-famous photo of sectioned R. vigil casque shown here by Matt Wedel]. Perhaps, by adding weight to the bill, it helps the bird use its bill as a hammer. More remarkable is the fact that Helmeted hornbills use their crests in aerial jousting: males engage in prolonged, noisy head-butting matches while in flight (Kinnaird et al. 2003).

The ground hornbills

By far the least typical of hornbills are the two ground hornbills of southern and eastern Africa. These large, predominantly black hornbills have bright red or blue skin on their faces and necks. They can reach 4 kg and may have a wingspan of about 2 m. One species - the Southern ground hornbill Bucorvus leadbeateri virtually lacks a casque while the other - the Northern or Abyssinian ground hornbill B. abyssinicus - has a tall, short casque. Today, ground hornbills are restricted to sub-Saharan Africa, but fossils show that they once inhabited northern Africa and eastern Europe (Boev & Kovachev 2007). They appear to be the sister-group to all the other hornbills, and are so distinct from all the others that some experts regard them as belonging to their own group termed Bucorvidae.

Hornbills typically have short legs and toes, and when moving on the ground they hop. Ground hornbills are terrestrial predators, and their anatomy is somewhat different. Their legs are long, their short-toed feet are heavily padded on their undersides, and they walk with a striding gait. It's said that they can run at speeds approaching 30 km/h. They are also capable fliers, however, and fly up into trees to roost.

Ground hornbills also differ from other hornbills in that the female does not become walled into the nest chamber. Between one and three eggs are laid, but it seems that they never succeed in raising more than a single chick.

The great classic modern source on hornbills is Alan Kemp's excellent 1995 book The Hornbills, part of the 'Bird Families of the World' series published by Oxford University Press. As is true of so many of those big, monographic books (and echoing statements I've made before), it's essentially out of reach to a huge sector of the community because of its price: it normally sells for about £150 (about $US243 or EUR170) these days. Surely by now someone has thought to turn these books into pdfs? Ahem.

If you're a regular reader you'll know that this is hardly the first time hornbills have been featured on Tet Zoo. Please also see...

• You're not a proto-phorusrhacid, but you're still a cariamaen, and that's alright with me (ode to the Ameghinornithidae) (discusses alleged fossil hornbill Geiseloceros robustus)
• The other ground hornbill
• Ground hornbills: savannah-dwelling, avian pseudo-hominids

Refs - -

Alexander, G. D., Houston, D. C. & Campbell, M. 1994. A possible acoustic function for the casque structure in hornbills (Bucerotidae). Journal of Zoology 233, 57-67.

Baussart, S. & Bels, V. 2010. Tropical hornbills (Aceros cassidix, Aceros undulatus, and Buceros hydrocorax) use ballistic transport to feed with their large beaks. Journal of Experimental Zoology Part A: Ecological Genetics and Physiology 313A, 72-83.

Boev, Z. & Kovachev, D. 2007. Euroceros bulgaricus gen. nov., sp. nov. from Hadzhidimovo (SW Bulgaria) (Late Miocene) - the first European record of hornbills (Aves: Coraciiformes). Geobios 40, 39-49.

Burton, P. J. K. 1984. Anatomy and evolution of the feeding apparatus in the avian orders Coraciiformes and Piciformes. Bulletin of the British Museum of Natural History (Zoology) 47, 331-443.

Kemp, A. C. 1995. The Hornbills. Oxford University Press, Oxford.

Kinnaird, M. F., Hadiprakarsa, Y.-Y. & Thiensongrusamee, P. 2003. Aerial jousting by Helmeted hornbills Rhinoplax vigil: observations from Indonesia and Thailand. Ibis 145, 506-508.

- . & O'Brien, T. G. 2007. The Ecology and Conservation of Asian Hornbills: Farmers of the Forest. University of Chicago Press, Chicago.

Martin, G. R. & Coetzee, H. C. 2004. Visual fields in hornbills: precision-grasping and sunshades. Ibis 146, 18-26.

Viseshakul N, Charoennitikul W, Kitamura S, Kemp A, Thong-Aree S, Surapunpitak Y, Poonswad P, & Ponglikitmongkol M (2011). A phylogeny of frugivorous hornbills linked to the evolution of Indian plants within Asian rainforests. Journal of evolutionary biology, 24 (7), 1533-1545 PMID: 21545425