Evolution of hornbills

Our understanding of the relationships between different hornbill species has evolved from early classifications in 1945 and 1960, followed by further studies based on morphological and behavioural characteristics by Kemp & Crowe in 1985 and later by Kemp in 1988 and 1995. Sibley and Ahlquist were the first ones to use the DNA-DNA hybridization technique to understand the genetic relationships between the different hornbill species in 1990. There were also attempts to classify hornbills based on mitochondrial DNA data and allied taxonomic data from the feather louse of a limited subset of hornbill species. A phylogeny for frugivorous Asian hornbills linked to tree species in Asian forests was also recently published by Visheshakul et al. 2011.

But it was only in 2013 that the complete phylogeny that included DNA samples from all the extant hornbill species was published by a group of biologists from University of Oxford and Birdlife International (see below). This has helped us understand the evolution of the different hornbill species that have distinct morphology, behaviour and geographic distribution across the continents of Asia and Africa. This study has provided objective support for some of the earlier classifications. It has also objectively resolved some of the prevailing confusion in relationships between different hornbill species.

Current evidence suggests that hornbills first evolved in Africa and they entered Asia and Europe where they went extinct. They shared a common ancestor with the present-day hoopoes more than 75 million years ago (mya). After the evolution of the Bucerotiformes (Hornbill) clade (branch), they first diverged into the Ground (Bucorvus genera) and other hornbills approximately 50 mya. The divergence in Bucorvus genus is thought to be associated with the spread of grasslands in North and East Africa. The earliest known hornbill fossils are of Ground Hornbills Bucorvus that have been found as north as Morocco and Bulgaria. The diversification of hornbills and their invasion of the Asian continent had important implications for the spread of tree species belonging to the Laurel group (Lauraceae family).

Current hornbill species are typically classified into six clades that include Bucorvus, Tockus, Berenicornis, Buceros, Anorrhinus and Aceros. The Tockus clade that is found in Africa, comprises of two genera Tockus and Rhynchaceros which can be distinguished by their calls ('clucks' in Tockus and whistles in Rhynchaceros).

The Berenicornis clade is a very interesting clade that includes the Asian genus Berenicornis and the African genera Bycanistes, Ceratogymna and Tropicranus. Genus Berenicornis is represented by a single species (White-crowned Hornbill Berenicornis comatus) that is found in Thailand, Indonesia and Malaysia. The presence of this species in the clade otherwise comprising of African species is possibly a consequence of either double invasion of Hornbills from Africa to Asia or a re-colonization of Africa from Asia. Unlike the Ceratogymna and Bycanistes which are predominantly frugivorous, the closely related Tropicranus and Berenicornis have high representation of animal matter in their diet.

The Buceros clade comprises of two genera: Rhinoplax that is represented by a single species (Helmeted Hornbill Rhinoplax vigil) and the genus Buceros which is represented by three species found in forests of India, Thailand, Indonesia, Malaysia and Phillipines. These four species are known to use their uropygeal gland (preen gland) secretions for cosmetic coloration.

The Anorrhinus clade is comprised of three genera that include Ocyceros represented by three species found in dry and wet forests of the Indian subcontinent, Anorrhinus is represented by three species found in north-east India, Myanmar and Thailand and Anthracoceros is represented by five species.

Aceros clade is represented by four genera that are found mostly in south-east Asia. Rufous-necked Hornbill represents the only species belonging to the genus Aceros. The clade comprising of the genera Rhyticeros (that includes the Wreathed and Narcondam Hornbills), Penelopides (tarictic hornbills) and Cranobrontes (that includes Walden's Hornbill Cranobrontes waldeni) is closest (sister) to the Rufous-necked Hornbill clade. The closest current relative of the Narcondam Hornbill is the Papuan Wreathed Hornbill Rhyticeros plicatus. Interestingly, while Narcondam Hornbill is found only on the Narcondam Island in Bay of Bengal, the Papuan Wreathed Hornbills is found in Papua New Guinea archipelago, 4000 km east of Narcondam Island.

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

Hornbill diet

Hornbills are the largest fruit-eating birds (also called frugivores) in Asian rainforests. The availability of fruits in the landscape is mainly responsible for various decisions made by an individual and/or species with respect to its habitat selection and may directly or indirectly affect its behaviour, physiology, abundance and ultimately its persistence in a landscape.

Hornbills mostly eat fruits (75-100% of diet) and occasionally supplement it with animal matter in lean seasons/during nesting season when there are chicks inside the nest (with the exception of ground hornbills in Africa that are predominantly meat-eaters). Among the Asian Hornbills, some genera like the Ocyceros, Anthracoceros and Anorrhinus also have more animal matter in their diet. Studies have shown a relationship between the availability of fruits in the landscape and increasing number of hornbills in different seasons.

A single species of hornbill may consume fruits of as many as 30-40 tree species in a year depending on its availability and the season. Studies in Asia have found that there is an increase in ripe fruit availability during the middle of the breeding season or towards the end of the breeding season when the chicks are ready to fledge and leave the nest.

Hornbills prefer fruits that are black or red in color. They rarely consume fruits that are less than 10 mm in size. Fruits of tree species belonging to the families Lauraceae, Meliaceae, Myristicaceae dominate the diet of hornbills in Asian forests. Hornbills consume many types of fruits that include drupes like Beilschmiedia sp., Phoebe sp., Polyalthia sp., arillate capsular fruits like Dysoxylum sp., Aglaia sp., Chisocheton sp., and berries like Zizyphus sp. Some species like the Great Hornbill feed mainly on figs with as much as 80% of their diet comprising of figs.

Each fruit type also varies in its nutritional content. Drupes are known to be high in fat levels while moderate in their sugar and fiber levels, while figs are high in fiber content but lower in sugar, lipids and protein content, while arillate capsular fruits are high in fat content. So, the choice of a fruit is based not only on its availability but also on its nutritional content, thereby compelling the Hornbills to include several species of fruits to make up for its total nutritional requirements. During the breeding season after chick hatching, animals like frogs, snakes, lizards, crabs, beetles and other insects are brought to the nest by the male to feed the female and chick inside the nest. In the breeding season, hornbill diets include more of lipids (from oily fruits) and protein (from animal matter), since the female is stationary inside the nest, and the chick is small.

Hornbills travel large distances and seeds are often regurgitated away from the parent plant, thereby making their feeding behaviour extremely important for the regeneration of seedlings in the forest. Hornbills are like gardeners who take care of their orchards and ensure continuity of fruits in the future.

Breeding

Hornbills use tree hollows to nest in. Hornbills cannot make a hole themselves, so they have to search for and choose suitable tree-holes in the forest that may have been made by woodpeckers, or a natural hole that is formed where a branch breaks off.

They look for hollows that are large enough for the female and her chicks to stay inside in till they are big enough to fly. When the female is ready to lay eggs, she goes into the hollow, after a lot of coaxing from the male. Then she locks herself inside by sealing the opening with her droppings. In some species, the male may bring sealing material such as mud and assist the female to seal the nest cavity. This sealing activity can often take up to 2 weeks and is time and energy consuming, so hornbills choose an optimum cavity size, neither too large nor too small. She only leaves a slit open through which the male passes food items. This nest-sealing behaviour in hornbills is unique among birds and is thought to have evolved to avoid predation.

The female usually lays two eggs, although in most of the large hornbills, only one chick survives. In some of the smaller hornbills like the Brown Hornbill up to 5-6 chicks can fledge. When the chicks hatch, one can hear them calling hungrily from inside. Their calls become frantic when the male arrives on one of his feeding visits.

The male hornbill has to work very hard. If he does not supply enough food to his family, then the female may break out of the nest. However, on her own, she is not able to take care of her chicks, which will die if something happens to the male.

While being locked up in the nest, the female does not need to fly. So she sometimes sheds her wings and tail feathers, and re-grows a new set before it is time to leave the nest again. During this time, the female and chick or chicks are completely dependent on the male hornbill to feed them. Sometimes, the mother Hornbill only sheds her tail feathers, so that she is able to fly, if anything happens to the male hornbill.

The male hornbill can carry lots of fruit in his throat pouch. Up to 200 small fig fruits can be brought by the male and passed on in a single visit. Animal food items (crab, lizard or a beetle) are usually brought and fed singly. The father hornbill makes several visits throughout the day, usually once every 1-2 hours. After the chicks hatch, his visits become more frequent.

It can get hot and stuffy inside the dark cavity, so, the female sits inside facing the slit with her mouth open, perspiring - usually, the tips of the beak are visible when watching with a pair of binoculars.

In Arunachal Pradesh, most hornbill species start nesting in mid to end-March and the mother and chick come out only in mid-July to early August. The female is cooped up inside for almost 3-4 months (this varies from species to species). The breeding cycle ranges from around 90 days for the Oriental Pied Hornbill to about 120-130 days for the Wreathed Hornbill. In Arunachal, in the low-elevation forest, the breeding season of hornbills overlaps with the peak availability of ripe fruit of many tree species of the Meliaceae, Lauraceae and Myristicaceae families (April to July).

The female does not dirty the nest with droppings, so she turns around with her back towards the cavity, aims and shoots the droppings out of the slit. Even her chicks soon learn to do this. So, if you happen to be walking in the forest below a hornbill nest tree, watch out or else you may get splattered with the hornbill's fruity droppings! Droppings usually contain undigested pulp, fig seeds and insect chitin. Through the slit, she also spits out (regurgitates) the seeds of the larger non-fig fruits she eats. Many hundreds of seeds fall on the forest floor below the nest.

When the chicks are big enough, the female hornbill breaks the seal and comes out first, and the chick usually comes out a few days later. In some species, like the Great Hornbill, the mother hornbill comes out much earlier and helps the male feed the chick.

The chick/chicks stays with the parents for some months and family groups of 3 are often seen during August-September. It is believed that for some hornbill species, the chicks/juveniles may stay with the parents for 9 months to a year, while juveniles of some species like the Great Hornbill and Wreathed Hornbill can also be seen together in larger flocks in the non-breeding season.

Seed Dispersers

Hornbills are unique birds: they get their name from the horn-like projection called a casque on top of their beak. They are larger than other forest birds. Hornbills are flashy with their over-sized beaks, bright skin around their eyes and long eyelashes. Most have a brilliantly colored pouch of loose skin at their throat. In this, they carry lots of fruit, which is their favorite food.

Hornbills love figs. When a fig tree in the forest is in fruit, you are sure to find hornbills gorging noisily on them. They also eat many other kinds of fruits. Hornbills are very picky; they eat only the ripest juicy fruits. They test the softness of the fruit with their beaks before deciding to eat it.

Hornbills swallow fruits whole and do not damage the seeds. Most fruits they eat have large seeds (2 to 4 cm). The fleshy part of the fruit is removed in the hornbill's stomach and digested. The cleaned seeds travel up from the stomach, come out in the mouth and are spat (regurgitated) out. Most seeds are regurgitated out from the mouth, while only the tiny seeds of figs are passed out in its droppings.

Freshly dropped seeds are warm and have a pinkish colour. The colour maybe due to acids produced in the hornbill's stomach when the fruit is being processed. Hornbills do not spend a long time on the tree from which they have eaten fruits; usually they spend less than half an hour on fruiting trees before flying off to perch on some tree or to another fruiting tree. Most fruits are processed for more than an hour in the gut. Therefore, the seeds of fruits they have eaten will be spat out several hours later somewhere else.

If you ever go roaming in a forest with hornbills, be sure to look under trees where hornbills have perched, or under their nest and roost trees and you will find seeds dropped below the trees. But under nest trees, many hundreds of seeds fall on the ground below, accumulating through the long breeding season. You will find a garden of seedlings below the nest tree. However, of the thousands of seeds that are thrown out, only some grow into seedlings and even fewer into trees.

Hornbills fly over long distances looking for fruits. As they digest the fruits and spit out countless seeds, some of these seeds find a place to grow and become new trees. That's why many trees in tropical forests produce big juicy coloured fruits: so that hornbills can 'plant' the seeds elsewhere in the forest.

Without hornbills, seeds could not travel so far in the forest, and find new places to grow. More than a quarter of the tree species in some tropical forests bear fruits adapted for dispersal by hornbills.

Hornbills symbolize a healthy forest. They are important in keeping the forest alive and growing. If we lose hornbills, many forest trees that depend on them to spread their seeds may eventually disappear from the forest too.

Threats to hornbills in India

The Rufous-necked Hornbill is listed as 'Vulnerable' by the IUCN, while the Great Hornbill, Brown Hornbill and the Malabar Pied Hornbill are listed under the Lower risk/Near threatened category. Other Hornbill species in India are listed as 'Least Concern'. Collar et al. (1994) lists ten globally threatened Hornbill species, of which two species occur in India, the Rufous-Necked Hornbill and the Narcondam Hornbill, while three species (Malabar Grey Hornbill, Malabar Pied Hornbill and the Brown Hornbill are listed as 'Near threatened'. Six species of hornbills are listed in Schedule I of the Wildlife Protection Act of India (1972): the Great Hornbill, Rufous-necked Hornbill, Wreathed Hornbill, Narcondam Hornbill, Oriental Pied Hornbill and the Brown Hornbill. Surprisingly, the endemic Malabar Grey Hornbill and the generally rare Malabar Pied Hornbill are not listed in any Schedule.

The most threatened hornbills in India are the Rufous-necked Hornbill, Narcondam Hornbill, Malabar Pied Hornbill, and the Brown Hornbill. All four have restricted distributions. The Narcondam Hornbill is considered vulnerable because the global population of the species is confined to a single small island and any future habitat loss, catastrophe or disease could wipe out the population. The Malabar Pied Hornbill has a patchy distribution and is often confined to riverine forest patches within deciduous forests. The reasons for its rarity are not known, but hunting and habitat loss are major contributing factors in some areas, such as in Bihar and Orissa. The Brown Hornbill and Rufous-necked Hornbill are threatened because of habitat loss and hunting in north-east India. The other five hornbill species have wider distributions, but are locally rare or even extinct in some parts of their range. Among these, the species of immediate concern is the Great Hornbill, mainly because of hunting, habitat loss, modification and fragmentation. The species is naturally less abundant in the Western Ghats and northern India, than in north-east India, probably because these areas are in the western limits of its overall range. Though they do persist in fragmented rainforest patches to some extent, natural rarity is also aggravated by habitat fragmentation and loss. Some degree of poaching, especially of chicks from nests also occurs in some areas in the Western Ghats. In northern India, it is not so common. In north-east India, especially in Arunachal Pradesh, it is more common, but the species faces severe hunting pressure and is locally extinct in some areas. The Wreathed Hornbill, though restricted to the north-east within India, is locally abundant in many areas, but is also affected by hunting and habitat loss. Within north-east India, the status of hornbills is better in Arunachal Pradesh, where they are more commonly sighted in many areas than in other states like Mizoram, Nagaland, Tripura and Meghalaya where hunting and habitat loss to cash crop plantations, agricultural expansion, logging and shifting cultivation has been greater. The Malabar Grey Hornbill though endemic to the Western Ghats is relatively common in modified and fragmented habitats and is able to breed in habitats modified by man. The Indian Grey Hornbill has the widest distributional range of all hornbills in India and is a common bird even in city gardens, parks, woodlands and agricultural tracts. The Oriental Pied Hornbill faces some hunting pressure but is relatively widely distributed in India. It is a generalist in habitat choice and is often able to breed and survive in degraded habitats. It is less common in northern India, and is hunted in northeast India, though less than the larger Hornbill species.

Historically, hornbills have also been subjected to hunting over most of their range, adding to their vulnerability. The fact that they still persist some areas of northeast India and the Western Ghats is largely due to the existence of tracts of forest that may soon become small islands in a landscape mosaic created by different land-use patterns such as cash crop plantations, agriculture and logging. Large forest areas exist outside sanctuaries and national parks in some states like Arunachal Pradesh, and therefore contiguous patches of forest still remain. Even though habitat modification and fragmentation due to shifting cultivation and logging are serious threats, hornbills are able to persist in logged and secondary forest patches in part due to their high mobility and as long as there is proximity to primary forest areas. Monocultures created by large-scale cash crop plantations like tea, rubber, oil palm, orange orchards and other horticulture.

Hunting and the use of hornbill body parts in Arunachal Pradesh

Hornbills are vulnerable in north-east India due to the traditional value of these birds for their feathers, beaks, casques, flesh and supposed medicinal value of their fat, among many tribal groups. A major conservation issue is the existence of hunting for several species (particularly of the Great Hornbill). Many areas, especially in eastern and central Arunachal Pradesh have had such high hunting pressures that the Great Hornbill has become extremely rare or locally extinct. The Great Hornbill is also the state bird of Arunachal Pradesh and several tribes have myths and stories about hornbills that form an important part of their folklore. Most tribes have their own names for the different hornbill species. The Great Hornbill is the most valued and hunted by most tribal groups, followed by the Rufous-necked Hornbill. The men of the Nyishi tribe wear the upper beak or both the casque and beak as part of traditional ceremonial headgear, and up to Rs. 2000 ($ 45) would be paid for hornbill beaks and casques in the nineties. Currently, prices range from Rs. 4000-6000 for the real beaks, although in some places people have started using the artificial beaks (but these are priced lower). The Wancho tribe adorn themselves with the feathers, particularly the tail feathers of the Great Hornbill. Among the Wanchos, feathers are most highly prized, worn by the raja (king) and other important people in the community. In earlier times, Great Hornbill feathers were traded for pigs, mithun horns and wild boar tushes by the Wancho, but now, are also sold. Great hornbills are now not seen in most of Tirap district, and are believed to be locally extinct. In the nineties, two body feathers were being bought for Rs. 260, while a single tail feather cost between Rs. 600 to Rs. 1600. Wanchos consider the Great Hornbill and Rufous-necked Hornbill to be more beautiful and the tail feathers of these are considered showier than those of the Wreathed Hornbill. The head and neck feathers of the Rufous-necked hornbills are also used to adorn Wancho headdresses. Among the Wanchos, women wear the feathers of the Oriental Pied Hornbill during traditional dances. Wreathed Hornbill feathers are not used. The Nyishi in Lower Subansiri district also use hornbill feathers during dances, though paper substitutes are also used. Apart from the meat that is consumed, the fat of all hrnbill species is used for medicinal purposes. Mishmi women in Dibang valley are apparently only allowed to eat rat and hornbill meat. In Dibang valley, the Wreathed Hornbill is hunted more; while the Great Hornbill is hunted very rarely, mainly because it is anyway rare. The two other smaller species, Brown Hornbill and the Oriental Pied Hornbill, are hunted much less (mainly for food) being smaller in size and less spectacular, though evidence of hunting of Brown hornbills was seen in Tangsa and Wancho villages in eastern Arunachal Pradesh.

Conservation Initiatives

Hornbill species are accorded high legal protection (on paper) and the strongholds for populations of most species are in Protected Areas. However like most other wildlife hornbills face a variety of threats, sometimes even within existing Protected Areas.

Listed below are a few of the targeted specific conservation initiatives for hornbills in India and a few other Asian countries. Most of these initiatives involve partnerships and collaborations between local communities, scientists and conservationists, government officials and urban citizens.

In the Western Ghats, a conservation program is underway for the last decade in the Vazhachal forests in Kerala to monitor and protect Great Hornbill and Malabar Pied Hornbill nests with the Kadars, a local tribal community. In this area, earlier, the chicks used to be taken from the nests to sell. Read more about the work of the Western Ghats Hornbill Foundation.

A pioneering and long-running hornbill research and conservation program by the Hornbill Research Foundation in Thailand (protects nests of several Hornbill species through an innovative hornbill family adoption program (Hornbill Research Foundation, Thailand) and diverse research, conservation and education activities at several forest sites in Thailand.

A similar effort in north-east India, the Hornbill Nest Adoption Program to protect Hornbill nests and habitat in reserve forest areas outside the Pakke Tiger Reserve, Arunachal Pradesh has been initiated from 2011 in a partnership between the Ghora-Aabhe society (council of village heads), Arunachal Pradesh Forest Department and the Nature Conservation Foundation (www.ncf-india.org). Monitoring and protection is undertaken by Nyishi community members/youth, while urban citizens who adopt hornbills and become foster parents mainly support the program. In 2013, 26 nests were found outside the reserve, out of which 12 were active (5 Great, 6 Oriental Pied, and 1 Wreathed) and 11 were successful. Nesting success in the Papum RF was 91%, while inside Pakke; there were 33 nests out of which 17 were active and 13 successful (76.5%).

Apart from this, NCF through its Rainforest Restoration Program in the Valparai landscape in the Western Ghats is restoring rainforest habitat for hornbills and other wildlife.

Several other institutions such as the BNHS, SACON and local organizations/Forest Departments in various cities and states in India undertake research and conservation activities on hornbills. There has been some recent successful attempt to breed Indian grey hornbills using artificial nest boxes in some cities.

In Arunachal Pradesh, a program to distribute artificial Great Hornbill beaks (fiberglass substitutes) in 2004 has been successful in making many Nyishi people use these artificial beaks on their headgear and give up wearing real beaks. This has created awareness among many community members in some areas in Arunachal Pradesh. The program was an initiative of the Wildlife Trust of India and the Arunachal Pradesh Forest Department

Various scientists/conservationists and governments are also active in other Asian countries like Sri Lanka, Bangladesh, Nepal, Bhutan, Malaysia, Indonesia, Singapore, Philippines, Cambodia, Vietnam and Laos.

Apart from the better documented conservation initiatives, there are many local efforts to protect hornbills by banning hunting of hornbills along with other wildlife and the imposition of fines by local village councils and community members.

Several zoos are involved in supporting Hornbill research and conservation in the wild in different ways apart from education activities. For example, the Woodland Park Zoo in Seattle works in partnership with the Hornbill Research Foundation, Thailand. The Greater Vancouver Zoo in Canada has partnered with the Hornbill Nest Adoption Program in Arunachal Pradesh, India to provide support through their 'Quarters for Conservation' Program.

Asian Hornbill bibliography

Abundance, distribution, threats and conservation

• Anggraini, K., Kinnaird, M.F. and O'Brien, T.G. 2000. The effects of fruit availability and habitat disturbance on an assemblage of Sumatran hornbills.Bird Conservation International 10:189-202. Journal article
• Bennett, E. L., Nyaoi, A.J. and Sompud, J. 1997. Hornbills Buceros spp. and culture in northern Borneo: can they continue to co-exist.Biological Conservation 82:41-46.Journal article
• Chantarat, S., Barrett, C.B., Janvilisri, T., Sittichai M., Niratisayakule, C. and Poonswad, P. 2011. Index insurance for pro-poor conservation of hornbills in Thailand. Proceedings of the National Academy of Sciences 108 (34): 13951-13956.Journal article
• Dasgupta, S. and Hilaluddin. 2012. Differential effects of hunting on populations of hornbills and imperial pigeons in the rainforests of the Eastern Indian Himalaya. Indian Forester 138:902-909.Journal article
• Datta, A. 1998. Hornbill abundance in unlogged forest, selectively logged forest and a forest plantation in Arunachal Pradesh, India. Oryx 32:285-294. Journal article
• Datta, A. 2004. An overview of hornbills: biology, distribution and conservation in Arunachal Pradesh. Arunachal Forest News 20:40-64. Journal article.
• Datta, A. 2009. Observations on Rufous-necked Aceros nipalensis and Austen's Brown Anorrhinus austeni hornbills in Arunachal Pradesh: natural history, conservation status and threats. Indian Birds 5:108-117. Journal article
• Gale, G. A. and Thongaree, S. 2006. Density estimates of nine hornbill species in a lowland forest site in Northern Thailand. Bird Conservation International 16:57-69. Journal article
• Gonzalez, J.-C. T. 2011. Enumerating the ethno-ornithological importance of Philippine hornbills. The Raffles Bulletin of Zoology Supplement No. 24:149-161. Journal article
• Ho, H. C. and Sutari, S. 1997. Spectacular movements of hornbills, possibly plain-pouched hornbills Aceros subruficollis, in Peninsular Malaysia. Oriental Bird Club Bulletin 25:59-61. Journal article
• Ho, H. C. and Sutari, S. 2000. Observations of Plain-pouched Hornbills Aceros subruficollis in TasekTemengor, Peninsular Malaysia. Forktail 16:65-67. Journal article
• Hussain S. A. 1984. Some aspects of the biology and ecology of Narcondam Hornbill (Rhyticeros narcondami).Journal of the Bombay Natural History Society 81(1): 1-18. Journal article
• Johns, A. D. 1985. Selective logging and wildlife conservation in tropical rainforest: problems and recommendations. Biological Conservation 31:355-375. Journal article
• Johns, A. D. 1989. Recovery of a Peninsular Malaysian rainforest avifauna following selective timber logging: the first twelve years. Forktail 4:89-106.Journal article
• Jornburom, P.,. Chimchome, V., Pattnavibool, A. and Poonswad, P. 2010. Density estimation of hornbills in Huai Kha Khaeng Wildlife Sanctuary, Uthai Thani Province. Thailand Journal of Forestry 29:1-11.Journal article
• Juhaeni, D. 1993. Little known bird. Sumba Hornbill Aceros everetti.Oriental Bird Club Bulletin 18:19-20.Journal article
• Kannan, R. and James, D.A. 1999. Fruiting phenology and the conservation of the Great Pied Hornbill (Buceros bicornis) in the Western Ghats of Southern India.Biotropica 31:167-177.Journal article
• Kannan, R. and James, D.A. 2008. Fig trees, captive elephants and conservation of hornbills and other frugivores in an Indian wildlife sanctuary. Journal of the Bombay Natural History Society 105(2):238-242.Journal article
• Katti, M. V., Singh, P., Manjrekar, N., Mukherjee, S. and Sharma, D. 1992. An ornithological survey in eastern Arunachal Pradesh, India. Forktail 7:75-89.Journal article
• Kemp, A. C. 2001. Family Bucerotidae (Hornbills). Pages 436-523 in J. d. Hoyo, A. Elliot, J. Sargatal, and D. A. Christie, editors. Handbook of the Birds of the World. Lynx Edicions, Barcelona, Spain. Book chapter
• Kemp, A., Kemp, M.E.G and Thong-Aree, S. 2011. Use of lookout watches over forest to estimate detection, dispersion and density of hornbills, Great Argus and diurnal raptors at Bala forest, Thailand, compared with results from in-forest line transects and spot maps.Bird Conservation International 21:394-410.Journal article
• Khan, T., Lenin, J., Mistry, U., Mudappa, D., Raman, T. R. S., Varma, K. & Whitaker, R. 2010.Goats and the Narcondam Hornbill Aceros narcondami.Indian Birds 6:28.Journal article
• Kinnaird, M. F., O'Brien, T.G. and Suryadi, S. 1996. Population fluctuation in Sulawesi Red-knobbed Hornbills: tracking figs in space and time. Auk 113:431-440.Journal article
• Krishna, C. M., Sarma, K. and Kumar, A. 2012. Rapid assessment of Wreathed Hornbill Aceros undulatus (Aves: Bucerotidae) populations and conservation issues in fragmented lowland tropical forests of Arunachal Pradesh, India. Journal of Threatened Taxa 4:3342-3348.Journal article
• Marsden, S.J. 1998. Changes in bird abundance following selective logging on Seram, Indonesia.Conservation Biology 12(3):605-611.Journal article
• Marsden, S. J. 1999. Estimation of parrot and hornbill densities using a point count distance sampling method.Ibis 141:377-390.Journal article
• Marsden, S. J. and Pilgrim, J. D. 2003. Factors influencing the abundance of parrots and hornbills in pristine and disturbed forests on New Britain, PNG. Ibis 145: 45-53.Journal article
• McConkey, K. R. and Chivers, D.J. 2004. Low mammal and hornbill abundance in the forests of Barito Ulu, Central Kalimantan, Indonesia.Oryx 38:439-447.Journal article
• Mudappa, D. and Raman, T. R. S. 2007.Hornbill populations in important conservation units along the Western Ghats, India.Page 76 in The Active Management of Hornbills and their Habitats For Conservation. Edited by A. C. Kemp & M. I. Kemp. CD-ROM Proceedings of the 4th International Hornbill Conference, MabulaGame Lodge, Bela-Bela, South Africa. Naturalists & Nomads, Pretoria.Conference proceedings
• Mudappa, D. and Raman, T.R.S. 2009. A conservation status survey of hornbills (Bucerotidae) in the Western Ghats, India.Indian Birds 5:90-102.Journal article
• Naniwadekar, R. and A. Datta. 2013. Spatial and temporal variation in hornbill densities in Namdapha Tiger Reserve, Arunachal Pradesh, north-east India. Tropical Conservation Science 6:734-748.Journal article
• Naniwadekar, R., Mishra, C., Isvaran, K., Madhusudan, M.D., and Datta, A. 2014. Looking beyond parks: conservation value of 'unprotected areas' for hornbills in Arunachal Pradesh, Eastern Himalaya. Oryx.doi:10.1017/S0030605313000781.Journal article
• O'Brien, T. G., Kinnaird, M. F., Jepson, P. and Setiwan, I. 1998. Effect of forest size and structure on the distribution of Sumba Wreathed Hornbills Aceros everetti. In P. Poonswad (Ed.), The Asian Hornbills: ecology and conservation, pp. 209-218. Thai studies in Biodiversity No. 2, Biodiversity Research and Training Program, Bangkok.Book chapter
• Pasuwan, C., Pattanakiat, S., Navanugraha, C., Chimchome, V., Madsri, S., Rattanarungsikul, P., Thiensongrusamee, P., Boonsriroj, T. and Poonswad, P. 2011. An assessment on artificial nest construction for hornbills in Budo Sungai National Park, Thailand.The Raffles Bulletin of Zoology Supplement No. 24: 85-93.Journal article
• Poonswad, P., Tsuji, A. and Ngarmpongsai, C. 1986. A comparative ecological study of four sympatric hornbills (Family Bucerotidae) in Thailand. In H. Ouellet (Ed.), Acta XIX Congressus Internationalis Ornithologici, pp. 2781-2791. Ottawa, Canada, 22-29 June, 1986. University of Ottawa Press.Conference proceedings
• Poonswad, P., Sukkasem, C., Phataramata, S., Hayeemuida, S., Plongmai, K., Phitaya, C., Thiensongrusame, P., and Jirawatkavi, N. 2005.Comparison of cavity modification and community involvement as strategies for hornbill conservation in Thailand. Biological Conservation 122:385-393.Journal article
• Poonswad, P., Chimchome, V., Mahannop, N. and Mudsri, S. 2013. Conservation of hornbills in Thailand. Chapter 19, Pages 157-166 in Conservation Biology: Voices from the Tropics, 1st Edition, Navjot S. Sodhi, L. Gibson, P. Raven, eds. John Wiley & Sons, Ltd. Book chapter
• Raman, T. R. S., Rawat, G.S. and Johnsingh, A.J.T. 1998. Recovery of tropical rainforest avifauna in relation to vegetation succession following shifting cultivation in Mizoram, north-east India.Journal of Applied Ecology 35:214-231.Journal article
• Raman, T. R. S. 2001. Effect of Slash-and-Burn Shifting cultivation on rainforest birds in Mizoram, Northeast India.Conservation Biology 15(3):685-698.Journal article
• Raman, T.R.S. 2003. Assessment of census techniques for interspecific comparisons of tropical rainforest bird densities: field evaluation in the Western Ghats, India.Ibis 145:9-21.Journal article
• Raman, T.R.S. 2006. Effects of habitat structure and adjacent habitats in tropical rainforest fragments and shaded plantations in the Western Ghats, India.Biodiversity & Conservation 15: 1577-1607.Journal article
• Raman, T. R. S. and Mudappa, D. 2003. Correlates of hornbill distribution and abundance in rainforest fragments in the southern Western Ghats.Bird Conservation International 13:199-212.Journal article
• Raman, T. R. S. 2007.Effects of habitat alteration on hornbills and frugivorous birds in tropical rainforests of India. Pages 383-394 in The Active Management of Hornbills and their Habitats for Conservation. Edited by A. C. Kemp & M. I. Kemp. CD-ROM Proceedings of the 4th International Hornbill Conference, Mabula Game Lodge, Bela-Bela, South Africa. Naturalists & Nomads, Pretoria.Book chapter
• Raman, S. T. R., Mudappa, D., Khan, T., Mistry, U., Saxena, A., Varma, K., Ekka, N., Lenin, J. and Whitaker, R. 2013. An expedition to Narcondam: observations of marine and terrestrial fauna including the island-endemic hornbill.Current Science 105:346-360.Journal article
• Rane, A. and Datta, A. in review. Protecting a hornbill haven: A community-based conservation initiative in Arunachal Pradesh, north-east India. Hornbill Conference Proceedings, Manila Philippines, April 2013. Conference proceedings
• Reddy, M. S., Muralidhar, K.S., Gandhi, M. and Basalingappa, S. 1990. Distribution and variation in number of Malabar Pied Hornbills Anthracoceros coronatus(Boddaert) in selected areas of North Kanara forest of Western Ghats in Karnataka, India. The Indian Zoologist 14:63-73. Journal article
• Saikia, P. K. and Saikia, M.K. 2011. Present distribution, status and ecology of White-winged Wood Duck and hornbills in Nameri National Park, considering the tropical forest disturbances of AssamZoo's Print Journal 26:1-11. Journal article
• Setha, T. 2004. The status and conservation of hornbills in Cambodia.Bird Conservation International 14:S5-S11.Journal article
• Singh, P. 1995. Recent bird records from Arunachal Pradesh, India. Forktail 10:65-104. Journal article
• Sitompul, A. F., Kinnaird, M.F., and O'Brien, T.G. 2004. Size matters: the effects of forest fragmentation and resource availability on the endemic Sumba Hornbill Aceros everetti. Bird Conservation International 14:S23-S37.Journal article
• Trisurat, Y. 2005. Modeling of hornbills and other wildlife distributions in a forested landscape at Khao Yai National Park, Thailand. The Ecology of Hornbills: Reproduction and Populations. Pimdee Karnpim Co. Ltd., Bangkok.Book chapter
• Trisurat, Y., V. Chimchome, A. Pattanavibool, S. Jinamoy, S. Thongaree, B. Kanchanasakha, S. Simcharoen, K. Sribuarod, N. Mahannop, and P. Poonswad. 2013. An assessment of the distribution and conservation status of hornbill species in Thailand. Oryx47(3):441-450.Journal article
• Velho, N., Ratnam, J., Srinivasan, U., and Sankaran, M. 2012.Shifts in community structure of tropical trees and avian frugivores in forests recovering from past loggi>ng. Biological Conservation 153:32-40.Journal article
• Winarni, N. L. and Jones, M. 2012. Effect of anthropogenic disturbance on the abundance and habitat occupancy of two endemic Hornbill species in Buton Island, Sulawesi. Bird Conservation International 22:222-233.Journal article
• Ved, N. 2011. Hornbills in southern Mizoram: History, beliefs and recent sightings. Indian Birds 7:117-119. Journal article

Evolution

• Baussart, S. and Bels, V. 2007. Evolutionary analysis of feeding mechanism of hornbills. Comparative Biochemistry and Physiology 146:1\20. Jounal article
• Delport, W., Ferguson, J.W.H. and Bloomer, P. 2002. Characterization and evolution of the mitochondrial DNA control region in hornbills (Bucerotiformes).Journal of Molecular Evolution 54:794-806.Journal article
• Gonzalez, J.-C. T., Sheldon, B.C., Collar, N.J., and Tobias, J.A. 2013a. A comprehensive molecular phylogeny for the hornbills (Aves: Bucerotidae). Molecular Phylogenetics and Evolution 67:468-483.Journal article
• Gonzalez, J.-C. T., Sheldon, B.C. and Tobias, J.A. 2013b. Environmental stability and the evolution of cooperative breeding in hornbills. Proceedings of the Royal Society B 280:1-9. Journal article
• Kemp, A.C. 1988b.The systematics and zoogeography of Oriental and Australasian Hornbills (Aves: Bucerotidae). Bonner Zoologische Beitraege: 39: 315-345.Journal article
• Leighton, M. 1986. Hornbill social dispersion: variations on a monogamous theme. Pages 108-130 in D. I. Rubenstein and R. W. Wrangham, editors. Ecological Aspects of Social Evolution.Princeton University Press. 551 pp.Book chapter
• Viseshakul, N., Charoennitikul, W., Kitamura, S., Kemp, A.C., Thongaree, S., Surapunitak, Y., Poonswad, P. and 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.Journal article

Diet and ranging

• Balasubramanian, P., Saravanan, R. and Maheswaran, B. 2004. Fruit preferences of Malabar Pied Hornbill Anthraoceros coronatus in Western Ghats, India. Bird Conservation International 14:S69-S79.Journal article
• Chaisuriyanum, S., G. A. Gale, S. Madsri, and P. Poonswad. 2011. Food consumed by Great Hornbill and Rhinoceros Hornbill in tropical rainforest, Budo Su-NgaiPadi National Park, Thailand.The Raffles Bulletin of Zoology 24:123-135.Journal article
• Datta, A. and Rawat, G.S. 2003. Foraging patterns of sympatric hornbills during the nonbreeding season in Arunachal Pradesh, Northeast India.Biotropica 35:208-218.Journal article
• Foeken, S. G., de Vries, M., Hudson, E., Sheppard, C.D. and Dierenfeld, E.S. 2008. Determining nitrogen requirements of Aceros and Buceros hornbills. Zoo Biology 27 (4): 282-293.Journal article
• Ganesh, T. and Davidar, P. 1999. Fruit biomass and relative abundance of frugivores in a rain forest of southern Western Ghats, India.Journal of Tropical Ecology 15:399-413. Journal article
• Hadiprakarsa, Y.-Y. and Kinnaird, M.F. 2004. Foraging characteristics of an assemblage of four Sumatran hornbill species.Bird Conservation International 14:S53-S62.Journal article
• Keartumsom, Y., Chimchome, V., Poonswad, P., Pattanavibool, A. and Pongpattananurak, N. 2011. Home range of Great Hornbill (Buceros bicornis Linnaeus, 1758) and Wreathed Hornbill (Rhyticeros undulatus (Shaw) 1881) in non-breeding season at KhaoYai National Park, NakhonRatchasima Province. Journal of Wildlife in Thailand 18:47-55. Journal article
• Leighton, M. and Leighton, D.R. 1983. Vertebrate responses to fruiting seasonality within a Bornean rain forest. In S. L. Sutton, T. C. Whitmore, and A. C. Chadwick (Eds.), Tropical Rain Forest: Ecology and Management, pp. 181-196.Blackwell Science Publications, Oxford. 498 pp.Book chapter
• McNab, B. K. and Bosque, C. 2001. Energetics of toucans, a barbet, and a hornbill: implications for avian frugivory.The Auk 118:916-933.Journal article
• O'Brien, T. G., M. F. Kinnaird, E. S. Dierendfeld, N. L. Conklin, R. W. Wrangham, and S. C. Silver. 1998b. What's so special about figs? Nature 392:668. Journal article
• Ouithavon K., Poonswad P., Bhumbhakpan N., and Laohajinda V. A comparative study of the feeding ecology of two sympatric hornbill species (Aves: Bucerotidae) during their breeding season in Huai Kha Khaeng Wildlife Sanctuary 2005. In: Lum S., Poonswad P. (Eds). The ecology of hornbills: reproduction and populations. Bangkok: Pimdee Karnpim Co. Ltd. p 59-74.Book chapter
• Poonswad, P. and Tsuji, A. 1994. Ranges of males of the Great Hornbill Buceros bicornis, Brown Hornbill Ptilolaemus tickelli and Wreathed Hornbill Rhyticeros undulatus in KhaoYai National Park, Thailand.Ibis 136:79-86. Journal article
• Poonswad, P., Tsuji, A., Jirawatkavi, N. and Chimchome, V. 1998. Some aspects of the food and feeding ecology of sympatric hornbills in Khao Yai National Park, Thailand. In P. Poonswad (Ed.), The Asian Hornbills: Ecology and conservation, pp. 137-157. Thai Studies in Biodiversity No. 2., Biodiversity Research and Training Program, Bangkok, 325 pp.Book chapter
• Poonswad, P., Tsuji, A. and Jirawatkavi, N. 2004. Estimation of nutrients delivered to nest inmates by four sympatric species of hornbills.Ornithological Science 3:99-112.Journal article
• Reddy, M. S. and Basalingappa, S. 1995. The food of the Malabar Pied Hornbill.Journal of Ecological Society 8:23-28.Journal article
• Suryadi, S., Kinnaird, M.F., O'Brien, T.G., Supriatna, J. and Somadikarta, S. 1994. Food preference of the Sulawesi Red-knobbed Hornbill during the non-breeding season.Tropical Biodiversity 2:377-384.Journal article
• Suryadi, S., Kinnaird, M.F., O'Brien, T.G., and Supriatna, J. 1996. Time budget of the Sulawesi Red-knobbed Hornbill,Rhyticeros cassidix, during the non-breeding season at Tangkoko-Duasaudara Nature Reserve, North Sulawesi. In D. J. Kitchener and A. Suyanto (Eds.), Proceedings of the First International Conference on eastern Indonesian-Australian vertebrate fauna, pp. 123-126. Manado, Indonesia, November 22-26, 1994.Conference proceedings
• Suryadi, S., Kinnaird, M.F. and O'Brien, T.G. 1998. Home ranges and daily movements of the Sulawesi Red-knobbed HornbillAceros cassidix during the non-breeding season. Pages 159-170 in P. Poonswad, editor. The Asian Hornbills: ecology and conservation. Thai studies in Biodiversity 2, Bangkok. 325 pp.Book chapter
• Tifong, J., Chimchome, V., Poonswad, P. and Pattnavibool, A. 2007. Home range and habitat use of Rufous-necked Hornbill (Aceros nipalensis) by radio tracking in Huai Kha Khaeng Wildlife Sanctuary, Uthai Thani Province.Thailand Journal of Forestry 26:28-39.Journal article
• Tsuji, A., Poonswad, P. and Jirawatkavi, N. 1987. Application of radio-tracking to study ranging patterns of hornnbills (Bucerotidae) in Thailand. Proc. Jean Delacour/IFCB Symp. Breeding birds in Captivity: 316-351. North Hollywood, Calif.: International Foundation for the Conservation of Birds.Conference proceedings
• Walker, J. S. 2006. Resource use and rarity among frugivorous birds in a tropical rain forest on Sulawesi.Biological Conservation 130:60-69.Journal article
• Walker, J. S. 2007. Dietary specialization and fruit availability among frugivorous birds on Sulawesi.Ibis 149:345-356.Journal article

Breeding

• Charde, P., Kasambe, R. and Tarar, J.L. 2011. Breeding behavior of Indian Grey Hornbill in Central India.The Raffles Bulletin of Zoology 24:59-64.Journal article
• Chimchome, V., A. Vidhidharm, S. Simchareon, S. Bumrungsri, and P. Poonswad. 1998. Comparative study of the breeding biology and ecology of two endangered hornbill species in Huai Kha Khaeng Wildlife Sanctuary, Thailand. In P. Poonswad (Ed.), The Asian Hornbills: Ecology and Conservation, pp. 111-136. Thai Studies in Biodiversity No.2: 1-336, Biodiversity Research and Training Program, Bangkok, 325 pp.Book chapter
• Choy, P. K. 1980. Breeding the Great Indian Hornbill (Buceros bicornis) at Jurong Bird Park.International Zoo Year Book 20: 204-206.Journal article
• Chuailua, P., Plongmai, K. and Poonswad, P. 1998. Status of nest cavities of hornbills in KhaoYai National Park, Thailand. In P. Poonswad (Ed.) The Asian Hornbills: Ecology and Conservation, pp. 219-226. Thai Studies in Biodiversity No.2: 1-336, Biodiversity Research and Training Program, Bangkok, 325 pp.Book chapter
• Datta, A. and Rawat, G.S. 2004. Nest-site selection and nesting success of three hornbill species in Arunachal Pradesh, north-east India:Buceros bicornis, Aceros undulatus and Anthracoceros albirostris Bird Conservation International 14: S249-S262.Journal article
• Golding, R. R. and Williams, M.G. 1986. Breeding the Great Indian Hornbill Buceros bicornis at the Cotswold Wild Life Park.International Zoo Year Book 24/25:248-252. Journal article
• Hutchins, M. 1976. Breeding biology and behaviour of the Indian Pied Hornbill Anthracoceros m. malabaricus.International Zoo Year Book 16: 99-104.Journal article
• James, D.A. and Kannan, R. 2007. Wild great Hornbills (Buceros bicornis) do not use mud to seal nest cavities.The Wilson Journal of Ornithology 119(1):118-121.Journal article
• Kannan, R. and James, D.A. 1997. Breeding biology of the Great Pied Hornbill (Buceros bicornis) in the Anaimalai Hills of southern India.Journal of the Bombay Natural History Society 86:448-449.Journal article
• Kinnaird, M. F. and O'Brien, T.G. 1993. Preliminary observation on the breeding biology of the endemic Sulawesi Red-knobbed Hornbill (Rhyticeros cassidix).Tropical Biodiversity 1(2):107-112.Journal article
• Kinnaird, M. F. and O'Brien, T.G. 1999. Breeding ecology of the Sulawesi Red-knobbed Hornbill Aceros cassidix.Ibis 141:60-69.Journal article
• Madge, S. G. 1969.Notes on the breeding of the bushy-crested hornbill.Malay Nature Journal 23:1-6.Journal article
• Maheswaran, B. and Balasubramanian, P. 2003. Nest tree utilization by the Malabar Grey Hornbill Ocyceros griseus in the semi-evergreen forest of MudumalaiWildlife Sanctuary (S India). Acta Ornithologica 38:33-37. Journal article
• Marsden, S. J. and Jones, M.J. 1997. The nesting requirements of the parrots and hornbill of Sumba, Indonesia. Biological Conservation 82:279-287. Journal article
• Mudappa, D. and Kannan, R. 1997. Nest-site characteristics and nesting success of the Malabar Grey Hornbill in the southern Western Ghats, India. Wilson Bulletin 109:102-111. Journal article
• Mudappa, D. 1998.Nesting habitat, breeding biology, and conservation of the Malabar Grey Hornbill (Ocycerosgriseus) in southern Western Ghats, India. In PilaiPoonswad (ed.). Proceedings of the 2nd International Asian Hornbill Conservation workshop, Bangkok, Thailand 13-18 April 1996. Conference proceedings
• Mudappa, D. 2000. Breeding biology of the Malabar Gray Hornbill (Ocyceros birostris) in southern Western Ghats. Journal of Bombay Natural History Society 97:15-24. Journal article
• Mudappa, D. 2005. Eight years monitoring of Malabar Grey Hornbill Ocyceros griseus nest cavity use and dynamics in the Anamalai rainforest, India. Pages 3-10 in The ecology of hornbills: reproduction and population. Edited by S. Lum and P. Poonswad, Pimdee Karnpim Co., Ltd, Bangkok. Book chapter
• O'Brien, T.G. 1997. Behavioural ecology of the North Sulawesi Tarictic Hornbill Penelopides exarhatus exarhatus during the breeding season. Ibis 139:97-101. Journal article
• Plongmai, K., Savini, C., Poonswad, P., Phitaya, C., Kitamura, S. and Wohandee, P. 2007. Nesting and flocking behavior in relation to the phenology of non-fig hornbill food plants at KhaoYai National Park, Thailand In: Kemp AC, Kemp MI (Eds.) The active management of hornbills and their habitats for conservation: CD-ROM proceedings of the 4th International Hornbill conference, Mabula Game Lodge, BelaBela, pp 371-380. Conference proceedings
• Poonswad, P., Tsuji, A. and Ngarmponsai, C. 1983. A study of the breeding biology of hornbills (Bucerotidae) in Thailand. Proceedings of the Jean Delacour/ICFB Symposium on Breeding Birds in Captivity.February 24-27, 1983, Los Angeles, California, USA. Conference proceedings
• Poonswad, P., Tsuji, A. and Ngampongsai, C. 1987. A comparative study on breeding biology of four sympatric hornbill species (Family Bucerotidae) in Thailand with implications for breeding in captivity. Proceedings of the Jean Delacour/IFCB Symposium on Breeding Birds in Captivity: 250-315. February 11-15, 1987, North Hollywood, California, USA: International Foundation for the Conservation of Birds. Conference proceedings
• Poonswad, P., Tsuji, A., Liewviriyakit, R. and Jirawatkavi, N. 1988. Effects of external factors on hornbill breeding and population. Proceedings of the 5th World Conference on Breeding Endangered Species in Captivity.October 9-12, 1988, Cincinnati, Ohio. Cincinnati Zoo and Botanical Garden. Conference proceedings
• Poonswad, P. 1995. Nest site characteristics of four sympatric species of hornbills in KhaoYai National Park, Thailand. Ibis 137:183-191. Journal article
• Witmer, M. 1993.Co-operative breeding by Rufous Hornbills on Mindanao Island, Philippines. Auk 110(4): 933-936. Journal article

Frugivory and Seed Dispersal

• Becker, P. and Wong, M. 1985. Seed dispersal, seed predation and juvenile mortality of Aglaia sp. (Meliaceae) in lowland dipterocarp forest. Biotropica1 7:230-237. Journal article
• Corlett, R.T. 1996. Characteristics of vertebrate-dispersed fruits in Hong Kong. Journal of Tropical Ecology 12: 819-833. Journal article
• Corlett, R. T. 1998.Frugivory and seed dispersal by vertebrates in the Oriental (Indomalayan) region. Biological Review 73:413-448. Journal article
• Datta, A. and Rawat, G.S. 2008. Dispersal modes and spatial patterns of tree species in a tropical forest in Arunachal Pradesh, north-east India. Tropical Conservation Science 1(3):163-183. Journal article
• Datta, A. and Rane, A. 2013. Phenology, seed dispersal and regeneration patterns of Horsfieldia kingii, a rare wild nutmeg. Tropical Conservation Science 6(5):674-689. Journal article
• Ganesh, T. and Davidar, P. 2001. Dispersal modes of tree species in the wet forests of southern Western Ghats. Current Science 80(3):394-399. Journal article
• Kinnaird, M. F. 1998. Evidence for effective seed dispersal by the Sulawesi Red-knobbed Hornbill Aceros cassidix. Biotropica 30:50-55. Journal article
• Kitamura, S., Suzuki, S., Yumoto, T., Poonswad, P., Chuailua, P., Plongmai, K., Noma, N., Maruhashi, T., and Suckasam, C. 2004a. Dispersal of Aglaia spectabilis, a large-seeded tree species in a moist evergreen forest in Thailand. Journal of Tropical Ecology 20:421-427. Journal article
• Kitamura, S., Yumoto, T., Poonswad, P., Chuailua, P., and Plongmai, K. 2004b. Characteristics of hornbill-dispersed fruits in a tropical seasonal forest in Thailand. Bird Conservation International 14:S81-S88. Journal article
• Kitamura, S., T. Yumoto, P. Poonswad, N. Noma, P. Chuailua, K. Plongmai, T. Maruhashi, and C. Suckasam. 2004c. Pattern and impact of hornbill seed dispersal at nest trees in a moist evergreen forest in Thailand. Journal of Tropical Ecology 20:545-553. Journal article
• Kitamura, S., Suzuki, S., Yumoto, T., Poonswad, P., Chuailua, P., Plongmai, K., Maruhashi, T., Noma, N., and Suckasam, C. 2006. Dispersal of Canarium euphyllum(Burseraceae), a large-seeded tree species, in a moist evergreen forest in Thailand. Journal of Tropical Ecology 22:137-146. Journal article
• Kitamura, S., Yumoto, T., Noma, N., Chuailua, P., Maruhashi, T., Wohandee, P. and Poonswad, P. 2008. Aggregated seed dispersal by wreathed hornbills at a roost site in a moist evergreen forest of Thailand. Ecological Research 23:943-952. Journal article
• Kitamura, S. 2011. Frugivory and seed dispersal by hornbills (Bucerotidae) in tropical forests. Acta Oecologica 37:531-541. Journal article
• Kitamura, S. and Poonswad, P. 2013. Nutmeg-vertebrate interactions in the Asia-Pacific region: importance of frugivores for seed dispersal in Myristicaceae. Tropical Conservation Science 6(5):608-636. Journal article
• Lambert, F. and Marshall, A.G. 1991. Keystone characteristics of bird-dispersed Ficus in a Malaysian lowland rain forest. Journal of Ecology 79:793-809. Journal article
• Sethi, P. and Howe, H.F. 2009. Recruitment of hornbill-dispersed trees in hunted and logged forests of the Indian Eastern Himalaya. Conservation Biology 23:710-718. Journal article
• Sethi, P. and Howe, H.F. 2012. Fruit removal by hornbills in a semi-evergreen forest of the Indian Eastern Himalaya. Journal of Tropical Ecology 28:531-541. Journal article
• Velho, N., Datta, A. and Isvaran, K. 2009. Effects of rodents on seed fates of hornbill-dispersed tree species in a tropical forest in north-east India. Journal of Tropical Ecology 25:507-514. Journal article
• Velho, N., Isvaran, K. and Datta, A. 2012. Rodent seed predation: effects on seed survival, recruitment, abundance and dispersion of bird-dispersed tropical trees. Oecologia 169:955-1004.DOI10.1007/s00442-012-2252-9. Journal article

Behavior and Physiology

• Raman, T. R. S. 1998. Aerial casque-butting in the Great Hornbill Buceros bicornis. Forktail 13:123-124. Journal article
• Gamble, K. C. 2007. Internal anatomy of the hornbill casque described by radiography, contrast radiography, and computed tomography. Journal of Avian Medicine and Surgery 21:38-49. Journal article
• Martin, G. R. and Coetzee, H.C. 2004. Visual fields in hornbills: precision-grasping and sunshades. Ibis 146:18-26. Journal article

Educational material and general awareness

• Datta, A. 2003. Pakke Tiger Reserve (IBA site): Hornbill haven in Arunachal Pradesh. Mistnet Vol. 4, No 3 & 4, Jul-Dec 2003. Newsletter article
• Datta, A., Singh, P., Athreya, R.M., and Karthikeyan, S. 1998. Birds of Pakhui Wildlife Sanctuary in western Arunachal Pradesh. Newsletter for Birdwatchers 38(6):91-96. Newsletter article
• Datta, A. and Manjrekar, N. 2004. Walk the rainforest with Niwupha. Katha, New Delhi, October 2004, 32 pp. Children’s book
• Datta, A. and Manjrekar, N. 2005. Let's discover our hornbills. Colouring and activity book on Indian Hornbills. Hornbill Conservation Program. A Nature Conservation Foundation Publication. 42 pp. Children’s book
• Datta, A. 2005. High on hornbills. Wildlife Conservation magazine. June 2005. Popular article
• Datta, A. 2012. Hornbills: farmers of our forests. The Hindu in School, April 4, 2012. http://www.thehindu.com/todays-paper/tp-in-school/article3278676.ece Popular article
• Datta, A. 2012.A tree hole for a home. The Hindu in School, April 11, 2012. http://www.thehindu.com/todays-paper/tp-in-school/article3301581.ece Popular article
• Datta, A., Rane, A. and Tapi, T. 2012. Shared parenting: Hornbill Nest Adoption Program in Arunachal Pradesh. The Hindu Survey of the Environment July 2012. pp. 88-97. Popular article
• Kapoor, V. and Datta, A. 2008. Secrets of the Rainforest - Nature Activity Book for children in Arunachal Pradesh.A Nature Conservation Foundation Publication.196 pp. Children’s book
• Mudappa, D. and Raman, T. R. S. 2012.The feathered foresters. Saevus 1(4, Sep/Oct): 28-33. Popular article
• Rane, A. 2014. Nitya in the rainforest. The Hindu in School, July 16, 2014. http://www.thehindu.com/todays-paper/tp-in-school/nitya-in-the-rainforest/article6214586.ece Popular article
• Rane, A. 2014. From hunters to protectors. The Hindu in School, July 23, 2014. http://www.thehindu.com/todays-paper/tp-in-school/hunters-to-protectors/article6239006.ece Popular article
• Raman, T. R. S. and Mudappa, D. 1998. Hornbills: giants among forest birds. Resonance-Journal of Science Education 8: 56-65. Popular article
• Raman, T. R. S. and Mudappa, D. 2012. Islands in peril: Conservation caveats. The Hindu Magazine, 25 February 2012, page 4. Popular article
• Sidhu, S. The incredible adventures of a SEED. The Hindu in School, 28 November, 2012. http://www.thehindu.com/todays-paper/tp-in-school/the-incredible-adventures-of-a-seed/article4141700.ece Popular article
• Sidhu, S. The talking tree. The Hindu in School, 5 December, 2012. http://www.thehindu.com/todays-paper/tp-in-school/the-talking-tree/article4165259.ece Popular article
• A Hunter's Tale (12 minute Audio-visual CD for children in English & Hindi). Edited by KalyanVarma.
• Poster "Hornbills: Farmers of the Forest" 2004 on Hornbills of north-east India.
• Poster "Hornbills: feathered foresters" 2005 on Hornbills of Western Ghats.

  Books and theses

• Ali, S. and Ripley, S.D. 1987. Compact Handbook of the Birds of India and Pakistan. 2nd edition. Oxford University Press, 737 pp. Book
• Collar, N., Crosby, M.J. and Stattersfield, A.J. 1994. Birds to watch 2: the world list of threatened birds. Birdlife International, Cambridge, United Kingdom.408 pp. Book
• Datta, A., 2001. An ecological study of sympatric hornbills and fruiting patterns in a tropical forest in Arunachal Pradesh. Ph.D. thesis, SaurashtraUniversity, Rajkot, Gujarat, India. Thesis
• Datta, A. 2002. Status of hornbills and hunting among tribal communities in eastern Arunachal Pradesh. Unpublished Report. Submitted to the Wildlife Conservation Society, New York and WCS-India Program, Bangalore. Report
• Datta, A. and Rane, A. 2011. Long-term hornbill nest and roost monitoring in Pakke Wildlife Sanctuary and Tiger Reserve (2003-2010). Unpublished Report. Submitted to Arunachal Pradesh Forest Department, Itanagar, Arunachal Pradesh, November 2011. Report
• Kemp, A. C. and Kemp, M.I. 1974. Report on a study of hornbills in Sarawak, with comments on their conservation. World Wildlife Fund Project Report 2/74.Unpublished manuscript. Report
• Kemp, A.C. 1995. The Hornbills. Oxford University Press.Oxford, U.K. 302 pp.
• Kinnaird, M.F. and O'Brien, T.G. 2007. The Ecology and Conservation of Asian Hornbills: Farmers of the Forest. The University of Chicago Press. Book
• Kitamura, S. 2000. Seed dispersal by hornbills in a tropical rain forest in KhaoYai National Park, Thailand. M.Sc. thesis, Kyoto University, Japan.185 pp. Thesis
• Leighton, M. 1982. Fruit resources and patterns of feeding, spacing and grouping among sympatric Bornean hornbills (Bucerotidae). Unpublished Ph.D. thesis. University of California, Davis. Thesis
• Mudappa, D. 1994. Nesting habitat of the Malabar Grey Hornbill (Ocycerosgriseus) in Anamalais, southern Western Ghats, India. M. S. Dissertation, Pondicherry University, Pondicherry. Thesis
• Mudappa, D. 1998. Long-term monitoring of nest cavity use and dynamics of the Malabar Grey Hornbill in the Anamalai rainforests: The first six years. Report submitted to the Centre for Ecological Research and Conservation, Mysore. Report
• Mudappa, D. and Raman, T. R. S. 2008. Hornbills and endemic birds: a conservation status survey across the Western Ghats, India. NCF Technical Report No. 17, Nature Conservation Foundation, Mysore. Report
• Naniwadekar, R. 2014. Seed dispersal by hornbills, conservation status and consequences of their decline in tropical forests of Arunachal Pradesh. PhD Thesis, Manipal University, 216 pp. Thesis
• Pawar, S. and Birand, A. 2001. A survey of amphibians, reptiles and birds in Northeast India. CERC Technical Report #6, Centre for Ecological Research and Conservation, Mysore. Report.
• Poonswad, P. and Kemp, A.C. 1993. Manual to the Conservation of Asian Hornbills. Hornbill Project, Thailand, Faculty of Science, Mahidol University, Bangkok. 511pp. Book
• Poonswad, P. 1998. (Editor). The Asian Hornbills: Ecology and Conservation. Biodiversity Research and Training Program, Bangkok, Thailand. Book
• Poonswad P., Kemp A.C., and Strange M. 2013. Hornbills of the world: A photographic guide: Draco Publishing and Distributions Pte. Ltd., Singapore and Hornbill Research Foundation, Thailand; 2013. Book
• Reddy, M. S. 1988. Some aspects of the ecology and behaviour of hornbills with special reference to Anthracoceros coronatus (Boddaert) from North Kanara District of Western Ghats. Unpublished Ph.D. Thesis, Karanatak University, Dharwad. Thesis
• Rane, A. and Datta, A. 2011. 2011 Hornbill Nest Adoption Program Report.
• Rane, A. and Datta, A. 2012. 2012 Hornbill Nest Adoption Program Report.
• Rane, A. and Datta, A. 2013. 2013 Hornbill Nest Adoption Program Report.
• Singh, P. 1999. Bird survey in selected localities of Arunachal Pradesh, India. Unpublished report, Wildlife Institute of India, Dehradun. Report
• Tsuji, A. 1996.Hornbills - Masters of Tropical Forests.Sarakdee Press, Bangkok, Thailand. 93 pp. Book