2. Hornbills (Bucerotidae) 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-coloured 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.
3. There are two subfamilies:
the Bucorvinae contain the 2 ground-
hornbills in a single genus, whereas
theBucerotinae contain all other taxa.
In the Sibley-Ahlquist taxonomy,
hornbills are separated from
theCoraciiformes 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.
4. Hornbills show 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 dimorphismalso 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.
5. The plumage of hornbills is typically black, grey, white, or brown, although
typically offset by bright colours on the bill, or patches of bare coloured 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 species.
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.
6. 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
found.
7.
8. 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.
In some 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 territorial
9.
10. Hornbills generally form monogamous pairs, although some species engage in
cooperative breeding. 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 days. 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. When the 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.
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23. Come & Check Out The
Hornbills
Experience it from July until October
every year!
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