Hornbills are among the most charismatic, fascinating and awesome of birds, yet surprisingly little is known about them, dedicated studies are few, and they are incredibly elusive and hard to study. Approximately 60 hornbill species occur across tropical Africa and Asia, and also in the Middle East and Australasia. These are birds of superlatives. The largest species have wingspans of over 1.5 m and weigh as much as 6 kg (in the case of the Southern ground hornbill Bucorvus leadbeateri); some (the Great hornbill Buceros bicornis and Southern ground hornbill) can reportedly live for more than 60 or even 70 years. Sexual dimorphism is extreme in some species, with the males of some being as much as 66% larger than the females. Hornbills are slow to mature, Buceros not breeding until four or five years of age.

Hornbills are also birds of splendid and remarkable anatomy. Great casques that form huge cylinders or curved, rhino-like horns decorate the heads of some species. Furrows, grooves and serrations sometimes mark their great, curved bills; giant, lavish eyelashes, enormous tail feathers and brightly coloured neck and facial skin are present in some species; and fused cervical vertebrae and bilobular kidneys are peculiar to the group. The chicks of some hornbills possess paired air sacs located on either side of the dorsal midline, the function of which (if they have one) is completely unknown.

They are also birds of incredible habits. In most, the female becomes walled up – self-incarcerated – within the nest chamber; the Helmeted hornbill Rhinoplax vigil engages in aerial jousting contents [adjacent photo of live Rhinoplax by Doug Janson]; some species ‘paint’ their feathers and rhamphotheca with the oily secretions of their under-tail glands; and co-operative breeding is the norm in some groups of species (like the Anorrhinus brown hornbills). As large, slow-breeding animals that typically rely on large tracts of forest and reliable access to fruits and cavities in trees, hornbills are seriously endangered by habitat loss and degradation, and potentially by climate change and selective hunting.

Margaret Kinnaird and Timothy O’Brien’s 2007 The Ecology and Conservation of Asian Hornbills [previously mentioned here on Tet Zoo] is not a popular retelling of other people’s research on these fascinating birds, but a major piece of primary literature that presents, analyses and discusses a huge amount of new data (Kinnaird & O’Brien 2007). It is thus more like a monograph on the diversity, distribution, evolution, behaviour and conservation biology of Asian hornbills. Numerous subtitled sections, sidebars of text, graphs and tables of data feature throughout. The volume is very much required reading for anyone seriously interested in the biology, evolution, ecology and conservation of hornbills, but it’s comprehensive enough and well-illustrated enough to be of broader appeal as well. A colour plate section features a selection of beautiful, spectacular photos by Tim Laman. Small drawings by Jonathan Kingdon also feature throughout the book.

Great questions about hornbills

There are several great questions about hornbills, and I hoped that some or all might be discussed or even answered here. How is the hornbill casque used? Does hornbill behaviour or ecology give us an insight as to which pressures contributed to casque evolution? How is the casque formed in anatomical terms? What role do hornbills play in the distribution of plant seeds and are they ‘keystone’ frugivores? How dependent are they on pristine areas of forest, and can they maintain viable populations in secondary or disrupted forest? How and why did their remarkable breeding strategy evolve? Do they find fruit (a notoriously ephemeral and sometimes unpredictable resource in the tropics) thanks to a well-developed memory, or are they merely opportunistic? How has their evolution and distribution been shaped by that of other fruit-eating groups, like primates? And so on. We’re not in the position to answer or even test many of these questions adequately for the simple reason that the relevant data hasn’t been collected (though note that some have been studied since Kinnaird & O’Brien (2007) was published: see Viseshakul et al. (2011) and Gonzalez et al. (2013a)). Nevertheless, Kinnaird and O’Brien provide copious data relevant to these issues, and discuss them within context. [Adjacent photos by JP Bennett and Tobias.]

Because the hornbill casque only develops at sexual maturity and often exhibits sexual dimorphism (it’s usually larger in males than in females), it seems plausible that it functions as an indicator of maturity, and evolved within the context of sexual selection pressure (as is probably the case for other cranial casques and crests in other archosaurs: Hone et al. 2012, Hone & Naish 2013). The concept that the casques are ‘species identification badges’ lacks compelling support, and might be contradicted by hybridisation events recorded between anatomically distinct species (Chamutpong et al. 2013). Adding support to a sexually selected role is the fact that the casques of some hornbill species seemingly function as acoustic resonating chambers used to broadcast their territorial vocalisations (Alexander et al. 1994) and that those of others are heavily reinforced internally and used in aerial head-butting contents (Kinnaird et al. 2003). However, suggestions have also been made that the heavy casques of some hornbills (the Helmeted hornbill in particular) help the bill work as a hammer.

The degree and distribution of sexual dimorphism in hornbills is unusual: some species are monomorphic, others have males that are slightly larger than females, and others have males that are more than twice as large as females (Kinnaird & O’Brien 2007). Quite why this range of dimorphism occurs, and why it’s distributed in the way that it is, remains mysterious, since there are few consistencies within particular hornbill lineages, or within species that share given regions or islands. More studies that link ecology and behaviour with phylogeny and distribution are much needed. [Image below by Magalhães.]

Hornbills: the historical perspective

While mostly based on the conservation biology, current threats, and future of hornbills, a substantial section of the book reviews the evolutionary history of these birds. Based on the fossil record of their close relatives (hoopoes and wood-hoopoes and their fossil kin), hornbills must have originated in the Eocene. However, essentially nothing is known in the way of their fossil record until the Miocene, and even then remains are scant and not especially informative with respect to patterns and trends in the evolution of the group (Naish 2012).

Of the several different phylogenetic hypotheses that have been published, most agree that ground hornbills (Bucorvus) are the sister-group of most remaining hornbills, and that early hornbill evolution occurred in Africa. Kinnaird & O’Brien (2007) favour the view that a hypothetical ancestral hornbill – they call it a ‘proto-Buceros’ and imagine it as a large, territorial, carnivorous hornbill – descended from Bucorvus-like ancestors and gave rise both to (assumed) endemic African taxa like Tockus and Tropicranus, and to a more forest-adapted, frugivorous lineage that moved into Asia. Here, the group radiated extensively, later reinvading Africa to give rise to Ceratogymna and Bycanistes. This model is more or less consistent with the topologies recovered in recent phylogenetic analyses (Gonzalez et al. 2013b).

Kinnaird & O’Brien (2007) provide an extensive discussion of hornbill phylogeny and biogeography in southeast Asia and northern Australasia, discussing the key features, distribution and biology of each hornbill genus in turn. Hornbill biogeography across the so-called Asian hornbill realm is an area made highly complex by the fact that different parts of the Sunda Shelf region were exposed and submerged at different points in the geological past, and that forests, woodlands and savannahs waxed and waned across this region in step with climatic cycles. Hornbills must have employed overwater dispersal at times: even during those parts of the Pleistocene when sea levels were about 180 m lower than present, the Philippines, for example, were separated from Borneo by long stretches of water, and yet members of the group ended up here in the form of several tarictic hornbills, and certain Buceros, Anthracoceros and Aceros species.

From a historical perspective, the most interesting case of all concerns Rhyticeros, since these occur on New Guinea and some of the surrounding islands, but are otherwise birds of peninsular south-east Asia, Borneo, Sumatra and Java. The Rhyticeros species are “known as the great hornbill dispersers [being able to] fly 10-15 km in a day” (Kinnaird & O’Brien 2007, p. 30), and their absence from Sulawesi, Flores, Timor and even Australia is puzzling, since the birds either passed across these areas in getting to where they are today, or are close enough for them to be within easy flying distance. We might hope for fossils, archaeological specimens or even ethnic tales that demonstrate or hint at the presence of Rhyticeros hornbills in these regions, but nothing like this has been reported yet (Meijer 2014).

“Farmers of the forest”

Hornbills are specialised frugivores, able both to ingest huge quantities of fruit in a short period of time and (almost certainly) able to successfully capture and metabolise the very low protein concentrations present in many of the fruits they eat. Fruits are so crucial to hornbills that Kinnaird & O’Brien (2007) discuss fruit diversity and biology, and the importance to hornbills of the species concerned, at length. In keeping with other studies on tropical ecology, figs are emphasised as a keystone resource, searched for and utilised by frugivores even when other fruits are available (it should be noted that figs contain over 750 species, over 500 of which occur within the Asian hornbill realm). Chapter 4 – ‘Feeding ecology; how to survive on fruits’ – includes a huge amount of data and discussion as goes the figs and the other fruits utilised by hornbills. [Adjacent photo by Lip Kee Yap.]

Despite their efficiency as processors and digesters of fruit, hornbills still need to consume 60-600 g of them per day, quantities equivalent to 20-33% of their body weight (Kinnaird & O’Brien 2007). Presumably as a consequence of their fruit-rich diet, they hardly ever drink, and seem especially efficient at processing water. Kinnaird & O’Brien (2007) note that this may be linked to the unusual, bilobed form of hornbill kidneys. This efficient water extraction also almost certainly explains why hornbill faeces are drier than tends to be the case for birds, and this is turn might help explain how dung came to be co-opted as a nest-building material in the group.

A future for hornbills?

The last section of The Ecology and Conservation of Asian Hornbills is devoted to the threats that face hornbills and their environments. Logging, the spread of plantations and loss of connected forest tracts, the ecology of fire, human population expansion, poor management, government corruption, hunting, fuelwood collection, local poverty, the development of infrastructure and other factors paint a highly complex picture of interaction, the links between them being confusing, sometimes counter-intuitive, complicated and under-researched. It is a deeply topical subject given the current pace of habitat change in the Asian hornbill realm and the near-unstoppable, unregulated monster that is the palm oil industry. [Adjacent photo by Llimchiu.]

Exactly what this grand, evolving mess means for the distribution and health of hornbill populations, and for sympatric plants and animals, is neither simple nor clear. Urbanisation, for example, means that people may have less impact on forests, and hence on hornbills, and also that regulations and rules concerning land-use will increasingly come into play... theoretically, that is. Indeed, as Kinnaird & O’Brien (2007) discuss, collusive corruption, bribery and unregulated logging have been serious problems across the hornbill realm, especially in Indonesia, the Solomon Islands and Papua New Guinea.

Anyway, as goes the future, hornbill species are under threat as forest blocks are broken up, degraded and made ever more accessible to hunters, loggers and others who exploit hornbill habitats. Birds like hornbills may persist across such fragmented landscapes, but at lower population densities. However, some studies indicate that big, frugivorous birds and other animals are relatively resilient to the activities of the logging industry or, counter-intuitively, may even benefit from them (Plumptre & Greiser-Johns 2001). Caveats that need to be kept in mind are that the term ‘frugivore’ is slightly ambiguous and not used consistently across all studies, and that it may be dangerous or misleading to assume that what goes for one hornbill species may go for another (Datta (1998) found that Great hornbills, Oriental pied hornbills Anthracoceros albirostris and Wreathed hornbills Rhyticeros undulatus differed in how they responded to logging disturbance). Kinnaird & O’Brien (2007) discuss simulations and projections that pertain to forest fragmentation and what it means for hornbills – there is, again, a huge quantity of data and discussion here.

The Ecology & Conservation of Asian Hornbills: Farmers of the Forest is excellent and data-packed and will be used regularly by those interested academically in hornbills, in the ecology or biology of tropical forest birds, or in avian conservation in the tropics. It is definitely not a general guide to hornbills or to Asian hornbills as a whole, however, and should be considered primarily focused on seed dispersal and ecology, and on conservation. As usual, the price is problematic and means that it is out of reach to interested amateurs and those without grants or institutional support. In summary, it is a highly impressive and important tour-de-force that provides a wealth of information on the past, present and future of Asian hornbill biology.

Margaret F. Kinnaird and Timothy G. O’Brien 2007. The Ecology and Conservation of Asian Hornbills: Farmers of the Forest. University of Chicago Press, Chicago. ISBN 13:978-0-226-43712-5, pp. 315. £47.50. Buy it here.

Hornbills have been discussed at Tet Zoo on several previous occasions. See...

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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.

Chamutpong, S., Ponglikitmongkol, M., Charoennitikul, W., Mudsri, S. & Poonswad. P. 2013. Hybridisation in the wild between the Great hornbill (Buceros bicornis) and the Rhinoceros hornbill (Buceros rhinoceros) in Thailand and its genetic assessment. Raffles Bulletin of Zoology 61, 349-358.

Datta, A. 1998. Hornbill abundance in unlogged forest, selectively logged forest, and a forest plantation in Arunachal Pradesh, India. Oryx 32, 285-294.

Gonzalez, J.-C. T., Sheldon, B. C., Collar, N. J. & Tobias, J. A. 2013b. A comprehensive molecular phylogeny for the hornbills (Aves: Bucerotidae). Molecular Phylogenetics and Evolution 67, 468-483.

- ., Sheldon, B. C. & Tobias, J. A. 2013a. Environmental stability and the evolution of cooperative breeding in hornbills. Proceedings of the Royal Society B 280, 20131297.

Hone, D. W. E., Naish, D. & Cuthill, I. C. 2012. Does mutual sexual selection explain the evolution of head crests in pterosaurs and dinosaurs? Lethaia 45, 139-156.

- . & Naish, D. 2013. The ‘species recognition hypothesis’ does not explain the presence and evolution of exaggerated structures in non-avialan dinosaurs. Journal of Zoology 290, 172-180.

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.

Meijer, H. J. 2014. The avian fossil record in Insular Southeast Asia and its implications for avian biogeography and palaeoecology. PeerJ 2:e295 http://dx.doi.org/10.7717/peerj.295

Naish, D. 2012. Birds. In Brett-Surman, M. K., Holtz, T. R. & Farlow, J. O. (eds) The Complete Dinosaur (Second Edition). Indiana University Press (Bloomington & Indianapolis), pp. 379-423.

Plumptre, A. & Greiser-Johns, A. 2001. Changes in primate communities following logging disturbance. In Fimbel, R. A., Grajal, A. & Robinson, J. G. (eds) The Cutting Edge: Conserving Wildlife in Logged Tropical Forests. Columbia University Press, New York, pp. 71-92.

Viseshakul, N., Charoennitikul, W., Kitamura, S., Kemp, A.C., 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, 1533-1545.