Tetrapod Zoology

An introduction to hornbills

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

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ResearchBlogging.org

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.]

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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.

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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.

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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.

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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

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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.

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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…

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

Comments

  1. #1 Bill
    June 22, 2011

    Not only a great book about great birds, but Alan Kemp is great gentleman as well.

  2. #2 Gunnar
    June 22, 2011

    Which species does the “this species” in the first paragraph refer to?

  3. #3 Ranjit Suresh
    June 22, 2011

    When I see the skeleton of the Great Indian Hornbill with it’s enormous head supported on a long thin neck, I think of Pterodactyloids like Tapejara, Pteranodon, Nyctosaurus, and Quetzalcoatlus. In particular, the ground hornbills are slightly reminscent of Azhdarchids.

    But, when you see the other photos of living hornbills, the long thin cervical column doesn’t look so thin at all. I wonder if life restorations of pterosaurs may make them look too thin-necked sometimes. Which also made me think of this:

    http://svpow.wordpress.com/2009/05/

  4. #4 heteromeles
    June 22, 2011

    In reference to the older idea of ground hornbills as avian pseudo-hominids, according to what you wrote above, it appears that walking ground hornbills are ancestral, and hopping arboreal hornbills are derived. Does that shoot down the “pseudo-hominid” hypothesis?

  5. #5 mo
    June 22, 2011

    They are like avian gibbons :P

  6. #6 Allen Hazen
    June 22, 2011

    About the paucity of the Hornbill fossil record… My impression from miscellaneous reading is that there is something about the soil chemistry of forests (more acidic?) that is inimical to the preservation of skeletons. (Somebody please set me right if I’m off-base here!) So shouldn’t one just EXPECT the fossil record of a primarily (except for the Ground Hornbills, which apparently do have a useful fossil record) forest-dwelling taxon to be disappointing?

    (Or: why paleoprimatologists should be extra-grateful for the scraps that HAVE been found of paleogene primates!)

  7. #7 John Harshman
    June 22, 2011

    heteromeles:

    Why do you assume that ground hornbills show the primitive condition? They’re the sister group of other hornbills, so if we look at hornbills only, the either condition would be equally parsimonious as primitive. If, however, we look at their closest relatives (hoopoes first, then coraciiforms and piciforms, then trogons), all those groups are chiefly arboreal, and thus arboreality is most parsimonious as the primitive condition.

    I love ground hornbills, by the way. Very cool birds. Both species.

  8. #8 J. S. Lopes
    June 22, 2011

    An African Paleogene endemic origin for hoopoes-hornbills seems likely.

  9. #9 Riggi
    June 22, 2011

    This is really impressive! I know you don’t do requests, but can you upload the Psittacoformes pages, if you did it? I am really curious if that article you incoporated that songbirds are related to parrots and that there are three families. Consider the fact I am sick and tired of people (even in latest of books like The Bird) suggest parrots are still a single group, while the rest have been accurately grouped. Oh I wish I can have it to my bird/dinosaur bookshelf. My other dream books will be of fossil bird lineages and coelurosaurs that are covered to this extent.

  10. #10 Vladimir Dinets
    June 22, 2011

    I often wonder if hoopoes are actually tiny hornbills. The similarity in color pattern and flight style is rather striking.

    heteromeles: even if ground-hornbills were a relatively primitive group, that would have nothing to do with them being, or not being, pseudo-humans. Are you sure we are the end product of biological evolution?

  11. #11 Hai~Ren
    June 22, 2011

    Great article. I happen to live in a region where hornbills are diverse. Singapore herself lost all her hornbill species many years back due to deforestation, but 1 species has recolonised Singapore (and is spreading), with a few other species represented by escapees.

  12. #12 heteromeles
    June 22, 2011

    Sigh. For everyone who didn’t read Dr. Naish’s wonderful piece on ground hornbills as hominid analogues and is wondering what I’m asking about…what can I say? It’s referenced right up top there.

    I didn’t know that the outgroups on the hornbill group are arboreal. That does change things.

    The idea of hornbills as helmeted hoopoes has some alliterative appeal also.

  13. #13 Dartian
    June 23, 2011

    Darren:

    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.

    Are you suggesting that from now on, in popular and semi-popular contexts, we should just replace genus/family/order/class/phylum/etc. with the catch-all term ‘group’? Frankly, from a science communication point of view, I’m not entirely convinced that that’s a genuine improvement.

    Riggi:

    parrots are still a single group

    Well, they are. They do form a monophyletic clade (or ‘group’, if you prefer). Whether psittaciforms should be divided into different ‘families’ is a different question.

    Vladimir:

    The similarity in color pattern

    Dunno about that; IMO, the hoopoe’s colour pattern is pretty unique in the avian world. Which particular hornbill species’ colour pattern would you say the hoopoe’s is similar to?

  14. #14 Brian
    June 23, 2011

    Dartian; There are more hoopoes than just *Upupa*. Wood hoopoes and scimitarbills are more hornbill-like in colour. They also lack the unusual crest of *Upupa*.

  15. #15 David Marjanović
    June 23, 2011

    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.

    Fossils of other bird clades of course can’t tell anything about the diversification of hornbills, so that must come from molecular divergence dating.

    How were those analyses calibrated, then?

    If there’s no fossil record of tree hornbills, where did the internal calibration points come from? If there are no internal calibration points, all dates are too old (unless another source of error is even stronger).

  16. #16 Dartian
    June 23, 2011

    Brian:

    There are more hoopoes than just *Upupa*.

    Vladimir spoke of “hoopoes”, not “wood hoopoes”. And do you know for sure if Upupa and wood hoopoes really even are sister taxa (their vernacular names notwithstanding)?

    Wood hoopoes and scimitarbills are more hornbill-like in colour.

    More like which hornbills, exactly? Tockus? Do you know for sure what the ancestral hornbill colouration was like?

  17. #17 Vladimir Dinets
    June 23, 2011

    Dartian: the greater hornbill, for example. It also has that target-like pattern on wings and tail.

  18. #18 John Harshman
    June 23, 2011

    Dartian:

    And do you know for sure if Upupa and wood hoopoes really even are sister taxa (their vernacular names notwithstanding)?

    Yes. Even Sibley & Ahlquist 1990 can show you that. And so will Hackett et al. 2008.

    Sibley, C. G., and J. A. Ahlquist. 1990. Phylogeny and classification of birds. Yale U. Press, New Haven.

    Hackett, S. J., R. T. Kimball, S. Reddy, R. C. K. Bowie, E. L. Braun, M. J. Braun, J. L. Chojnowski, W. A. Cox, K.-L. Han, J. Harshman, C. J. Huddleston, B. D. Marks, K. J. Miglia, W. A. Moore, F. H. Sheldon, D. W. Steadman, C. C. Witt, and T. Yuri. 2008. A phylogenomic study of birds reveals their evolutionary history. Science 320:1763-1768.

  19. #19 Surroundx
    June 25, 2011

    The Ticao Tarictic Hornbill (Penelopides panini ticaensis Hachisuka) was last seen in 1971, and is sadly now extinct as its former habitat has been reduced to “(< 10 ha) of degraded
    lowland rainforest”.

  20. #20 Surroundx
    June 25, 2011

    *reduced to less than 10 hectares of “degraded lowland rainforest”

  21. #21 Schenck
    June 27, 2011

    I’m reading ‘The Inner Bird’, and he has a sketch of the Helmeted Hornbill photo section, nice to see the photo. He cites it as Pycraft History of Birds, I presume that Wedel is given credit there.

    “it’s essentially out of reach to a huge sector of the community because of its price: it normally sells for about £150 ”
    Hmm, not unlike “Dinosaurs and other extinct Saurians”……(but I love it anyway!)

  22. #22 Dartian
    June 28, 2011

    Vladimir:

    the greater hornbill, for example. It also has that target-like pattern on wings and tail.

    Hmm, OK. I don’t really see any obvious similarity there, but it’s in the eyes of the beholder, I suppose.

    John:

    Even Sibley & Ahlquist 1990 can show you that.

    I won’t dispute your expertise on the subject of upupiform phylogeny. But I’m a bit surprised to see anyone still today, in 2011, favourably referring to Sibley & Ahlquist as a source. I’m not an ornithologist, so my views on their methods and their results are admittedly mostly based on their hominoid phylogeny papers from the eighties (which, for various reasons, are rarely cited these days). I am aware of your 1994 paper in Auk, but that was a while ago; how would you say that Sibley & Ahlquist, generally speaking, are regarded in the ornithological community today?

  23. #23 John Harshman
    June 28, 2011

    Dartian,

    S&A’s regard among ornithologists is highly variable, as one might expect. However, their regard among avian systematists specifically is quite low, and has been so ever since the 1980’s. Part of that rep is richly deserved. But that doesn’t mean we can reject all their results because of the source. Much of what they did still stands up, and if you look at the data you can tell what is supported and what isn’t. It happens that the hoopoe conclusions are robustly supported by the data.

  24. #24 Dartian
    June 30, 2011

    John: thanks for the reply. Sibley & Ahlquist’s publications have pretty much been off my radar for the last ten years or so. Should have guessed, though, that they would eventually make an appearance here on Tet Zoo. ;)

  25. #25 Darren Naish
    June 30, 2011

    “Eventually”? Maybe you’ve forgotten this Tet Zoo article?

  26. #26 Dartian
    June 30, 2011

    Maybe you’ve forgotten this Tet Zoo article?

    Oh crap! I had.

    Hmph. You win this time, Mr. Naish.

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