are a family of bird found in tropical and sub-tropical Africa and Asia.
They are characterized by a long, down-curved bill which is frequently
brightly-colored and sometimes has a casque on the upper mandible. Both the
common English and the scientific name of the family refer to the shape of the
bill, "buceros" being "cow horn" in Greek. In addition, they possess a two-lobed
kidney. Hornbills are the only birds in which the first two neck vertebrae (the
axis and atlas) are fused together; this probably provides a more stable
platform for carrying the bill.
The family is omnivorous, feeding on
fruit and small animals.
They are monogamous breeders nesting in natural
cavities in trees and sometimes cliffs.
A number of species of hornbill
are threatened with extinction, mostly insular species with small ranges.
There are two subfamilies: the Bucorvinae contain the 2 ground-hornbills in a
single genus, whereas the Bucerotinae contain all other taxa.
In the Sibley-Ahlquist taxonomy, hornbills are separated from the Coraciiformes
as a separate order Bucerotiformes, with the subfamilies
elevated to family level. Given that they are almost as distant from the
rollers, kingfishers and allies as are the trogons, the arrangement chosen is more a matter of personal
taste than any well-established taxonomic practice. All that can be said with
reasonable certainty is that placing the hornbills outside the Coraciiformes and
the trogons inside would be incorrect.
Distribution and habitats
Bucerotidae include some 57 living species,
though a number of cryptic species may yet be split in some insular forms.
Their distribution ranges from Africa south of the Sahara through tropical
Asia to the Philippines and Solomon Islands. Most are arboreal birds, but the
large ground-hornbills (Bucorvus), as their name implies, are
terrestrial birds of open savanna. Of the 23 species found in Africa 13 of these
species are birds of the more open woodlands and savanna, and some species even
occur in highly arid environments. The remaining species are found in dense
forests. This contrasts with Asia, where a single species occurs in open savanna
and the remainder are forest
considerable variation in size as a family, ranging in size from the Black Dwarf
Hornbill (Tockus hartlaubi), at 102 grams (3.6 oz) and
30 cm (1 foot), to the Southern Ground-hornbill (Bucorvus
leadbeateri), at up to 6.2 kg (13.6 lbs) and 1.2 m (4 feet).
Males are always bigger than the females, though the extent to which this is
true varies dependent upon species. The extent of sexual dimorphism also varies
with body parts, for example the difference in body mass between males and
females is between 1-17%, but the variation is 8-30% for bill length and 1-21%
in wing length.
The most distinctive feature of the hornbills is the
heavy bill, supported by powerful neck muscles as well as by the fused
vertebrae. The large bill assists in fighting, preening, and constructing the
nest, as well as catching prey. A feature unique to the hornbills is the casque,
a hollow structure that runs along the upper mandible . In some species it is
barely perceptible and appears to serve no function beyond reinforcing the bill.
In other species it is quite large, is reinforced with bone, and has openings
between the hollow centre allowing serve as a resonator for calls. In the Helmeted Hornbill
the casque is not hollow but is filled with ivory and is used as a battering ram
used in dramatic aerial jousts. Aerial casque-butting has also been reported in
the Great Hornbill.
The plumage of hornbills is typically black, grey, white, or brown, although
typically offset by bright colours on the bill, or patches of bare colored skin
on the face or wattles. Some species exhibit sexual dichromatism; in the
Abyssinian Ground-hornbill, for example, pure blue skin on the face and throat
denotes an adult female, and red and blue skin denotes an adult male. The calls
of hornbills are loud, and vary distinctly between different
Hornbills possess binocular
vision, although unlike most birds with this type of vision the bill intrudes on
their visual field. This allows them to see their own bill tip and aids in
precision handling of food objects with their bill. The eyes are also protected
by large eyelashes which act as a sunshade.
Hornbills are diurnal, generally
travelling in pairs or small family groups. Larger flocks sometimes form in the
non-breeding season. The largest assemblages of hornbills form at some roosting
sites, where as many as 2400 individual birds may be
Hornbills are omnivorous
birds, eating fruit, insects and small animals. They cannot swallow food caught
at the tip of the beak as their tongues are too short to manipulate it, so they
toss it back to the throat with a jerk of the head. While both open
country and forest species are omnivorous, species that
specialise in feeding on fruit are generally found in forests while the more
carnivorous species are found in open country. Forest living species of
hornbills are considered to be important seed dispersers.
instances hornbills defend a fixed territory. Territoriality is related to diet;
fruit sources are often patchily distributed and require long distance travel in
order to find, thus species that specialise in fruit are less
generally form monogamous pairs, although some species engage in cooperative
The female lays up to six white eggs in existing holes or
crevices, either in trees or rocks. The cavities are usually natural, but some
species may nest in the abandoned nests of woodpeckers and barbets. Nesting sites may be used in consecutive breeding
seasons by the same pair.
Before incubation, the females of all
Bucerotinae—sometimes assisted by the male—begin to close the
entrance to the nest cavity with a wall made of mud, droppings and fruit pulp.
When the female is ready to lay her eggs, the entrance is just large enough for
it to enter the nest, and after she has done so, the remaining opening is also
all but sealed shut. There is only one narrow aperture, big enough for the male
to transfer food to the mother and eventually the chicks. The function of this
behaviour is apparently related to protecting the nesting site from rival
hornbills. The sealing can be done in just a few hours, at most it takes a few
Having sealed the nest it takes a further five days for the first
egg to be laid. Clutch size varies from one or two eggs in the larger species to
up to eight eggs for the smaller species. During the incubation period the
female undergoes a complete and simultaneous moult. It has been suggested that
the darkness of the cavity triggers a hormone involved in moulting.
Non-breeding females and males go through a sequential moult.
chicks and the female are too big to fit in the nest, the mother breaks out,
then both parents feed the chicks. In some species the mother rebuilds the wall,
whereas in others the chicks themselves rebuild the wall unaided. The
ground-hornbills are conventional cavity-nesters instead.
Associations with other
A number of hornbills have associations with other
animal species. For example hornbills in Africa have a mutualistic relationship
with Dwarf Mongooses, in which they forage together and warn each other of
nearby birds of prey and other predators. Other relationships are commensal, for
example following monkeys or other animals and eating the insects flushed up by
Most species' casques are very light, containing
a good deal of airspace. However, the Helmeted Hornbill has a solid casque made of a
material called hornbill ivory, which is greatly valued as a carving material in
China and Japan. It is often used as a medium for the art of
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