Hornbills (Bucerotidae)

(vertebrate zoology) The hornbills, a family of Old World tropical birds in the order Coraciiformes.

(Bucerotidae)

Class: Aves

Order: Coraciiformes

Suborder: Bucerotes

Family: Hornbills (Bucerotidae)

Thumbnail description
Medium to large-sized, stocky, highly vocal birds with long, slightly decurved bills topped by casques of various shapes, sizes, and colors

Size
11.8–47.3 in (30–120 cm); 0.22–13.2 lb (100g–6 kg)

Number of genera, species
14 genera; 54 species

Habitat
Forest, woodlands, and savanna

Conservation status
Endangered: 2 species; Critically Endangered: 2 species; Vulnerable: 5 species; Near Threatened: 12 species

Distribution
Sub-Saharan Africa, south and Southeast Asia, New Guinea, and the Solomon Islands

Evolution and systematics

In his classic work The Hornbills published in 1995, Alan Kemp wrote "Trying to decide what other groups of birds are most closely related to hornbills is not quite so easy." Time has not made that decision any easier; the classification of these bizarre, large-billed birds is still debated. Most modern taxonomic treatments place hornbills within the order Coraciiformes together with their closest relatives: the hoopoe (Upupidae) and the woodhoopoes and scimitarbills (Phoeniculidae). This classification is based primarily on similarity in foot and jaw morphology, and a prolonged retention of quills by nestlings, which gives them a prickly "pin-cushion" look. All birds of these families nest in tree holes, but only hornbills seal the entrance to their cavities.

As of 2001, science recognizes 54 species of hornbills grouped within 14 genera and two subfamilies. All but two species are classified within the subfamily Bucerotinae. The exceptions are the terrestrial ground-hornbills, which fall within the subfamily Bucorvinae. The distinction between Bucerotinae and Bucorvinae is based on unique feather lice and anatomical and behavioral differences such as a greater number of neck vertebrae and the lack of nest-sealing behavior in the Bucorvinae. In 2001, S. Huebner and colleagues conducted detailed molecular studies of the two groups. They found that ground-hornbills were probably the earliest form.

All 54 hornbill species display unique anatomical features that clearly identify their affinities. These include being blessed with long, sweeping eyelashes on their upper lids and a fusion of the first two cervical vertebrae to provide support for large bills. All hornbills lack carotid arteries as well as the short feathers under the wings that cover the primary and secondary flight feathers of other birds. Finally, hornbills have unusual kidneys in that they are two-lobed instead of three, and the Z chromosome, one of a pair of sex chromosomes, is oversized.

Physical characteristics

Hornbills are among the most flamboyant birds of their habitat. The oversized, slightly decurved bills topped by sometimes outlandish casques shaped as bumps, ridges, or horns make hornbills an unforgettable component of any landscape. Hornbills vary tremendously in size and shape, starting with the large, long-legged southern ground-hornbill (Bucorvus leadbeateri) weighing up to 13.2 lb (6 kg), and going down to the 0.26 lb (120 g) red-billed dwarf hornbill (Tockus camurus). Males are always larger and stouter than females but the greatest dimorphism often occurs in bill length with males having up to 30% longer bills. Horn-bill plumage is described as "drab," lacking the brilliant colors of relatives such as the kingfishers (Alcedinidae) and rollers (Coraciidae). However, the bold black-and-white patterns of many forest hornbills and the delicate gray pied patterns of many Tockus species are far from dull. Add in bills and casques of brilliant orange, yellow-gold, deep crimson, or shiny black, and patches of bare skin around the eyes and throat in a kaleidoscope of garish hues, and you have a colorful group of birds.

Plumage color and size and shape of the casque identify the age and sex of an individual. Newly fledged hornbills have underdeveloped casques and small bills, but after the first year of life, appearances converge on that of their adult counterparts. In species where sexes differ in color as adults, determining the gender of the young can be difficult. For example, in almost all Aceros, Rhyticeros, Penelopides, and Tockus species, the young, regardless of their sex, resemble their fathers for the first year of life. The opposite is true for the Bycanistes and Ceratogymna who resemble the adult female. Young of the northern ground-hornbill (Bucorvus abyssinicus) and a few Tockus species show plumage true to their sex while chicks of the rufous hornbill (Buceros hydrocorax) are radically different from both parents.

Numerous authors have described the noise produced by flying hornbills as that of an approaching train. This incredible "whooshing," produced in different pitches depending on the species' size, is a result of wing structure. Because horn-bills lack the small feathers that normally cover the shafts of the primary and second flight feathers, each powerful stroke of the wing allows air to pass through and vibrate the large feathers.

The most outstanding feature, and the one from which hornbills acquire their common name, is the casque on the top of the bill. Casques vary from the mere ridge of the redbilled hornbill (Tockus erythrorhynchus) to the wash-board bumps of the wreathed hornbill (Rhyticeros undulatus) and the elaborate banana of the rhinoceros hornbill (Buceros rhinoceros). The function of casques, which may take up to six years to develop, is the topic of many debates. It is possible that casques provide structural support for a long bill. Casques may also serve an acoustic function by helping amplify a horn-bill's call. Additionally, casques may be attractive to the opposite sex. The helmeted hornbill uses its casque in bizarre, aerial displays where individuals of either sex collide in midair, casque-to-casque. The head-butting competitions always occur near fruiting fig trees (Ficus spp.). Although Gustav Schneider once reported that helmeted hornbills perform this comical ritual when they are intoxicated on fermented figs, observations from Sumatra indicate that this acrobatic act may be in defense of clumped food resources.

Distribution

Hornbills occur across sub-Saharan Africa, through India and southern Asia, across the Indonesian and Philippine archipelagos, and east to the Solomon Islands. There are no horn-bills in the New World. Within the hornbill family, 23 species inhabit Africa, while the remaining 31 are found in Asia. The largest and most widespread genus in Africa, Tockus, is represented by 13 species. The most ubiquitous of the Tockus species is the African gray hornbill (Tockus nasutus), a medium-sized gray and white bird with populations occurring from the shores of Mauritania in the west, east to the Red Sea, and as far south as Namibia and South Africa. In Asia, Aceros and Rhyticeros hornbills dominate with five genera each, occurring from Bhutan and northern India in the west to the Solomon Islands in the east. Two species of the genus Ocyceros, the Malabar gray hornbill (Ocyceros griseus) and Indian gray horn-bill (Ocyceros birostris), occur exclusively in India while the third species, the Sri Lankan gray hornbill (Ocyceros gingalensis), is restricted, as its name implies, to the island of Sri Lanka. Several hornbill species, especially those occupying oceanic islands, have restricted distributions. The tarictic hornbills are a prime example. The Luzon (Penelopides manillae), Visayan (Penelopides panini), and Mindanao (Penelopides affinis) tarictic hornbills occur only on a few neighboring islands within the Philippine archipelago. A fourth Philippine species, the Mindoro tarictic hornbill (Penelopides mindorensis) is endemic to the small island of Mindoro while the fifth species of the genus, the Sulawesi tarictic hornbill (Penelopides exarhatus) is found only on the Indonesian island of Sulawesi. Several other species are endemic to single islands, including the Sulawesi red-knobbed hornbill (Aceros cassidix), the Sumba hornbill (Rhyticeros everetti), and the Narcondam hornbill (Rhyticeros narcondami).

Habitat

Arid deserts, scrubby woodlands, cool mountains, and steamy rainforests all constitute hornbill habitat. In general, however, hornbills are birds of the forest. Of the 30 species found in India and Southeast Asia, only the Indian gray horn-bill lives in open savanna. In Africa, where forests are less extensive, the proportion of savanna-dwelling species increases accordingly; 13 of 23 species reside in savannas and woodlands while the remaining 10 inhabit forests. Species occupying savannas tend to have more extensive ranges but, like the red-billed hornbill, may be separated into many distinct populations by imposing bands of woodland. Endemic species are, by default, limited to habitats available within their restricted range. This is particularly true of insular species like the Sumba hornbill, which occupies all forest types on its native island.

There are key features that must be present in all hornbill habitats—an ample number of large trees for nesting, an adequate year-round supply of food, and enough habitat area to support a viable population. Each species has a particular set of requirements, which may help explain why several species can simultaneously occupy the same habitat. In the forests of Thailand, where nine hornbill species may occur together, a small Tickell's brown hornbill (Anorrhinus tickelli) is able to use nest holes of smaller dimensions than the larger great hornbill (Buceros bicornis). On Sumatra, where a similar number of species coexist, they generally forage on different diet items; when diet overlap occurs, as with rhinoceros and helmeted hornbills, they partition their habitat by feeding at different heights in the canopy. Habitat quality will influence the number of hornbills an area can support. Habitat size also limits hornbill populations. On the island of Sumba, hornbills are rare or absent from forest patches less than 3.6 mi² (10 km²) in size.

Behavior

Hornbills generally wake at dawn, preen their feathers, then begin their search for food. Normally, hornbills move about in pairs, but some species are found in family groups of three to 20 individuals. Some hornbills gather in large flocks around clumped food resources. The Sulawesi redknobbed hornbill is occasionally seen in groups of more than 100 individuals at large fruiting figs. In Thailand, wreathed hornbills roost in flocks of over 1,000 individuals. The plain-pouched hornbill (Aceros subruficollis) takes the record for the largest aggregations; over 2,400 individuals were counted in Malaysia in 1998 traveling to roost. Roosts may serve as "information centers" where individual birds can reduce foraging time by following a knowledgeable, long-term resident. As Alan Kemp summarizes, these massive gatherings are "wonderfully noisy and visually stunning, and must surely rate among the foremost spectacles of the bird world."

Hornbills are believed to be monogamous. The only research on the faithfulness of hornbills failed to find evidence of extra-pair paternity in Monteiro's hornbill (Tockus monteiri), boosting confidence in their monogamous behavior. Monogamy may have many variations on the theme. Among cooperative social groups, there is generally one monogamous breeding pair and a number of offspring who become "helpers" during the nesting season, delivering morsels to their mother and siblings and defending a mutual territory. Cooperative breeding occurs more often in hornbills than any other bird family, and may characterize up to one-third of all hornbill species.

Many hornbills range widely but none of these movements is considered migratory. Most hornbills are sedentary and many are territorial. The majority of Tockus and small-bodied forest hornbills are territorial throughout the year. Larger hornbills such as the Aceros and Rhyticeros that rely on scattered fruit resources, may range over 21 mi² (58 km²) and only defend temporary territories around nest sites.

Hornbills communicate through a wide range of spectacular calls and each species can be identified by its vocalizations. Loud calls announce territories, or in the non-territorial species, aid in maintaining contact. Territorial ground-hornbills "boom" when their boundaries are invaded and non-territorial wreathed hornbills bark like dogs while coordinating flocks. While calls are important in dense forest habitats, visual displays are more prevalent in open grasslands. For example, the Hemprich's hornbill (Tockus hemprichii) has an elaborate territorial display that resembles the mechanical movements of a wind-up toy; the bill is pointed skyward, while the bird whistles, and lifts and fans its tail over its back.

Feeding ecology and diet

Hornbill diets span the spectrum from animals to fruits and seeds but most are omnivorous, mixing meat and fruit in their meals. Among Tockus, diets tend more toward insects, scorpions, lizards, snakes, and small mammals, while Ocyceros

and tarictic diets include more fruit. Omnivory is the rule among the territorial, group-living hornbills. Because animal prey often occurs at low density and is available year-round, hornbills may develop defendable territories in which dietary needs for the pair or group are satisfied. Additionally, these species maximize exploitation of their territories by using abundant but ephemeral fruit resources as they become available. The availability of fruit resources within a habitat may determine the degree of omnivory observed.

Heavy reliance on fruits requires that hornbills have large home ranges, and may affect reproductive rates. Fruit diets combined with large home ranges have important consequences for forest ecology. As hornbills travel, they disperse seeds of the fruits they relish, playing a role in regenerating the forests in which they live.

Reproductive biology

Hornbill reproduction tends to coincide with rainfall and increased food supply. In seasonal African savannas, Tockus species begin courtship and reproduction with the rains, when invertebrates and fruits are plentiful. The opposite occurs on Sulawesi where lack of rainfall stimulates reproduction in the Sulawesi red-knobbed hornbill, so the burst in fruit supply occurs immediately after fledging. In aseasonal Bornean rain-forests, reproduction appears to be supra annual, tied to highly cyclical peaks in food supply. Breeding in these populations may be controlled by the rate at which pairs regain condition between reproductive cycles. In fig-rich forests of North Sulawesi, hornbills breed every year, usually returning to the same nest tree.

The hornbill's unique nesting behavior is the feature that has most fascinated students of nature. All hornbills are hole-nesters, preferring natural cavities in trees or rock crevices. Unlike any other group of birds, the female hornbill seals the entrance to her nest cavity, leaving only a narrow slit through which she, and later her chicks, receive food from her mate. In most species, the male ferries mud to the female who then works for several days to seal the cavity entrance. Where mud is a rare commodity, the female uses her own feces as building material.

Nest sealing is believed to have evolved as a form of predator defense, for protection against other intruding hornbills, and to enforce male fidelity. Nest sealing has been described as an example of male chauvinism in which the male cloisters his female, forcing her to depend on him for survival. In reality, the female incarcerates herself and later frees herself, forcing the male to provide for her and their offspring. Because the male is busy provisioning his family, he is incapable of maintaining two nests, and the female can be sure of his complete attention.

The onset of breeding begins with courtship. When in flight, courting pairs act as though they are attached by an invisible rubber band, reacting swiftly to each other's movements. They perch in cozy proximity, engage in mutual preening, and exchange food gifts as a demonstration of their ardor. Other clues of the onset of breeding include the intensification in color of the exposed fleshy areas around the face and throat, reflecting hormonal changes. Nest inspection increases in frequency until copulation occurs and the female enters the nest cavity.

The number of eggs, their size, and the length of incubation are all correlated with body size. Clutch size ranges from two to three eggs in large hornbills and up to eight for smaller hornbills. Incubation runs from 23-49 days in small and large species, respectively. Eggs hatch in intervals and the emerging chicks are naked and translucent pink with closed eyes. Feather growth begins within a few days and as chicks develop, the skin blackens and begging calls change from feeble cheeps to loud, insistent calls.

The timing of female emergence varies tremendously; some females accompany their chicks from the nests and others leave well before chicks fledge. Research on Monteiro's hornbill suggests that females emerge to ensure survival when their body condition reaches its lowest point.

Male hornbills can be impressive providers. Although many Tockus species carry items to the nest one-by-one, most hornbills collect multiple food items, stuffed into a bulging gullet before delivering a load to the nest. A Sulawesi redknobbed hornbill once delivered 162 fruits in one trip, a load equivalent to nearly 20% of his body weight.

Nesting success is high for those species studied. In southern Africa, chicks fledged from 90–92% of the nests of four Tockus species and in Thailand, 80% of great hornbill nests monitored fledged young. Sulawesi red-knobbed hornbills averaged 80% nesting success over three years, but this figure plummeted to 62% during the 1997 El Niño/ENSO fires. Smaller hornbills fledge up to four chicks, but large hornbills rarely fledge more than one chick per year.

Conservation status

Only 16% of all hornbill species are classified as being under some level of threat, ranging in increasing degree from Vulnerable to Critical and Endangered, according to the IUCN. An additional 12 species, however, are considered Near Threatened and will probably experience a decline in status within the twenty-first century. Africa presently has no hornbills in danger of extinction; only two West African forest inhabitants, the yellow-casqued hornbill (Ceratogymna elata) and the brown-cheeked hornbill (Bycanistes cylindricus), are classified as Near Threatened. All nine species suffering endangerment reside in Asia, and most (77%) occur on small oceanic islands. The Sumba and Narcondam hornbills, both single-island endemics, are classified as Vulnerable, with total populations hovering around 4,000 and 300, respectively. The situation in the Philippines is especially urgent. Rapidly dwindling forests contain two species ranked as the most endangered hornbills in the world, the Visayan and Mindoro tarictic hornbills, as well as two species classified as Critical, the Sulu (Anthracoceros montani) and rufous-headed (Aceros waldeni) hornbills, and one Vulnerable species, the Palawan hornbill (Anthracoceros marchei). There are no rigorous population estimates for these species, but we assume populations are extremely small and may vanish within decades unless conservation measures are adopted.

The underlying threat to hornbill populations is habitat alteration resulting in forest loss and fragmentation. As forests become smaller and more isolated, hornbill populations decline, resulting in increased vulnerability to extinction from natural disasters such as disease. Protection of hornbill populations and their habitats within conservation areas of adequate size offer some hope for their long-term persistence. In the late 1990s, two parks were established on Sumba to aid in the conservation of the Sumba hornbill, and the Philippines have proposed to establish the Central Panay Mountains National Park (NP) for the Visayan tarictic hornbill. In India and Africa, vast tracts of savanna and forest have been protected as parks for decades. The long-term success of hornbill conservation in these parks, however, depends on active management to ensure that they are more than "parks on paper."

Unsustainable hunting for food, pets, and body parts is also a problem. Although illegal, trade in helmeted hornbill ivory continues. Great and oriental pied hornbill casques are common souvenirs in Thai and Laotian markets. Traditions that require feathers or skulls take a toll on living birds. Female Kenyalang dancers of Malaysia carry up to 10 hornbill tail feathers in each hand, thus supplying a full complement of 20 dancers can cost up to 80 hornbills.

The 1990s have seen a dramatic increase in awareness of hornbill ecology and conservation needs. The number of horn-bill studies, especially those by range-country biologists, escalated during this time and continues to increase. Developments in hornbill research and conservation are quickly communicated to the global community through the IUCN Species Survival Commission's Hornbill Specialist Group and facilitated by Internet communication. Only with such global attention are we able to finance local initiatives and put pressure on a range of state governments to conserve these unusual birds.

Significance to humans

Like many other groups of birds, hornbills are hunted for food and consumed for medicine. In Africa, parts of the ground-hornbill are eaten to improve health and sagacity, whereas in India, the great hornbill, the Indian pied hornbill, and the Indian gray hornbill are rendered into oils that supposedly aid in childbirth and relieve gout and joint pains. In Indonesia, the meat of the Sumba hornbill is roasted and eaten to relieve rheumatism and asthma. Because they are easily tamed, horn-bills are captured and traded for pets or exhibition. Unlike any other group of birds, however, hornbills play special roles in the folklore and ceremonies of the countries where they occur. Long, elegant tail feathers are the most sought-after hornbill part, but heads and casques are also coveted. The Nishis people of Arunachal Pradesh, India, attach the upper beak of the great hornbill to rattan bopiah caps as traditional male headgear. Neighboring Wanchos of eastern Arunachal use the warm, chestnut-colored neck feathers of rufous-necked hornbills to cover caps. On Borneo, the helmeted and rhinoceros hornbills reach mythical proportions in the eyes of the local inhabitants. The helmeted hornbill, in particular, is strongly associated with headhunting. C. Hose, an early twentieth century naturalist and explorer, reported that only someone who has taken a human head is allowed to wear the intricately carved earrings created from the "ivory" of the helmeted hornbill casque, or to adorn themselves with the bird's long, central tail feathers. Helmeted hornbills are also believed to judge souls leaving their mortal existence.

Today, hornbills are increasingly highlighted as local mascots or state birds. This is especially true in Asia. The great hornbill is the state bird of Arunachal Pradesh, northern India. The rhinoceros hornbill has been adopted as the state bird of Sarawak, Malaysia, where it appears on tourism advertisements, T-shirts, and even the state coat-of-arms. In Indonesia, the helmeted hornbill, the Sulawesi red-knobbed hornbill, and the Sumba hornbill proudly serve as official mascots for three provinces.

Species accounts

Resources

BirdLife International. Threatened Birds of Asia: BirdLife International Red Data Book. Cambridge, United Kingdom: BirdLife International, 2001. del Hoyo, J., A. Elliott, and J. Sargatal, eds. Handbook of the Birds of the World. Vol. 6, Mousebirds to Hornbills. Barcelona: Lynx Edicions, 2001.

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

Poonswad, P., ed. The Asian Hornbills: Ecology and Conservation. Bangkok: Thai Studies in Biodiversity, 1998.

Anggraini, K., M. Kinnaird, and T. O'Brien. "The Effects of Fruit Availability and Habitat Disturbance on an Assemblage of Sumatran Hornbills." Bird Conservation International 10(2000): 189–202.

Hadiprakarsa, Y., and M. Kinnaird. "Foraging Characteristics of an Assemblage of Four Sumatran Hornbill Species." The Third International Hornbill Workshop Abstracts. (2001). Hornbill Research Foundation, Bangkok, Thailand.

Huebner, S., R. Prinzinger, and M. Wink. "Phylogenetic Relationships in Hornbills (Aves, Bucerotiformes): Inferences from Nucleotide Sequences from the Mitochondrial Cytochrome b gene." Journal für Ornitologie (in press).

Kemp, A. "The Role of Species Limits and Biology in the Conservation of African Hornbills." Ostrich (in press)

Klop, E., T. Hahn, M. Kauth, S. Engel, L. Lastimoza, and E. Curio. "Diet Composition and Food Provisioning of the Visayan Tarictic Hornbill (Penelopides panini panini) During the Breeding Season." Ecololgy of Birds 22 (2000).

Hornbill Research Foundation. c/o Department of Microbiology, Faculty of Science, Mahidol University, Rama 6 Rd, Bangkok, 10400 Thailand. Phone: +66 22 460 063, ext. 4006. E-mail: scpps@mucc.mahidol.ac.th

Coraciiformes Taxon Advisory Group. "Hornbills." (1 June 2001).

Threatened Birds of Asia. The BirdLife International Red Data Book. "Threatened Bird Species Account." (10 June 2001).

[Article by: Margaret Field Kinnaird, PhD]