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The southern yellow-billed hornbill

18 January 2016

 

www.leopard.tvThe southern yellow-billed hornbill Tockus leucomelas is one of ten types of hornbill in southern Africa of which the southern ground hornbill Bucorvus leadbeateri is by far the largest, while Monteiro’s hornbill Tockus monteiri and the Damara hornbill Tockus damarensis only occur in central, north-western Namibia and south-western Angola. Bradfield’s hornbill Tockus bradfieldi occurs from extreme north-central Namibia into south-eastern Angola, north-western Zimbabwe and south-western Zambia. The rarest hornbill in southern Africa is the silvery-cheeked hornbill Bycanistes brevis of north-eastern southern Africa, but its range extends north along mountain forests into the Ethiopian Highlands.

The southern yellow-billed hornbill was first described scientifically in 1842 as Buceros leucomelas by Lichtenstein based on a specimen that was collected between Bloemhof and Commando Drift along the Vaal River. The generic name Tockus refers to its typical clucking call and the specific name leucomelas is Latin for black and white. It occurs from southern Angola, southern Zambia and southern Malawi south through Namibia, Botswana and Zimbabwe into the north-central and north-eastern bushveld parts of South Africa.

The adult, southern yellow-billed hornbill is 40 to 60 cm tall and the male weighs some 210 g as opposed to 170 g in the female. The sexes are alike and, as its scientific name indicates, the plumage is a combination of grey, white and black patches and stripes. The head and neck is grey with a broad white eyebrow that extends to the nape, while the underside is white. The large, curved, horny bill is yellow with dark brown cutting edges. In an adult male the bill has a ridge along its entire length but the female only has a ridge close to the body. The eyes are yellow, the bare facial skin parts are pink and the legs and feet are grey. Young birds look like the adults but have grey eyes that first become brown before becoming yellow. Geographical variations mainly involve differences in the intensity of the colour of the plumage. The southern yellow-billed hornbill can be confused with te red-billed and Damara hornbills but the latter two basically have red bills with a yellow base. The southern yellow-billed hornbill flies with a direct flap-and-glide action, basks and preens in the morning sun and roosts against tree trunks.

Yellow-billed hornbills are territorial residents and sedentary over much of their range. However, they rarely may form small flocks in the dry season and during a drought when these flocks may move around locally to areas with a high quality of food. In South Africa there may be more regular local movements into the Limpopo River valley and to the base of the escarpment in the Lowveld. The preferred habitat consists of dry, open savannas with Acacia and broad-leaved trees, but southern yellow-billed hornbills occur in many wooded vegetation types with a low ground cover. Unlike the red-billed hornbill, the yellow-billed hornbill is not common in mopane (Colophospermum mopane) and miombo (Brachystegia) woodlands.

Foraging is done mainly on the ground where the hornbill will run around to pick up small animals and fruit. It will also search for food in bushes and trees and may swoop down on prey from a perch. It may infrequently look for food under dung or bark, but mainly does so in the dry season. The food consists of a wide range of invertebrate and smaller vertebrate animals. It includes the nestlings of the red-billed quelea Quelea quelea, rodents, smaller snakes, chameleons, frogs, bird’s eggs, ants, beetles, termites, crickets, emperor moths, insect larvae, spiders, scorpions and centipedes, but wild fruits and young leaves are eaten often.

The southern yellow-billed hornbill is monogamous and a solitary nester. Courtship usually occurs in the morning and involves displays, including rocking back and forth. The courting male will feed his female for up to a month before laying of the eggs start and he carries the food to her in his bill. Copulation occurs away from the nest site which is usually a hole facing north-east, in a tree. The nests are usually at least 75 cm above the ground and are lined with dry foliage or small flakes of bark that are brought to the nest mainly by the male. Nest boxes can be used in the wild and in captivity. A nest consists of a natural chamber that is 200 to 300 mm wide and there is  an escape hole above the chamber. The floor of the nest is 100 to 300 mm above the lip of the entrance that is 70 mm high and 40 mm wide. After the first good rains, the female seals herself in the nest with her own faeces, but she leaves a narrow slit of 5 to 15 mm wide to be fed by the male. Laying of the eggs starts then and two broods may be produced in a year. The first of the two to six, oval, pitted, white eggs are laid four to six days after the female has sealed herself in the nest and the clutch takes at least eight days to completed. Incubation starts with the first egg that is laid, lasts 24 days and the eggs hatch in the sequence of having been laid. Consequently a brood of nestlings will vary in size.

The hatchlings are pink and naked with open eyes and weigh 12 g when hatched. The eyes only open at an age of five days when feathers also first appear. The chicks squirt their faeces through the nest entrance slit from an age of ten to15 days. They reach the adult size at an age of 35 days and the female emerges from the nest when the oldest chick is 19 to 27 days old. The chicks that leave the nest reseal the nest entrance and perch in its vicinity in a tree to be fed by both adults for a few day before starting to follow the adults to forage. The main causes of nest failure are addled eggs, starvation of the younger chicks and predation by adult hornbills, baboons and rock monitors. Moulting occurs in the rainy season.

 

Reference:

Hockey, P A R, W R J Dean and P G Ryan (Eds) 2005. Roberts – Birds of southern Africa, seventh edition. Cape Town: Trustees of the John Voelcker Bird Book Fund, pp 152 - 153.

article by Prof J du P Bothma

 

 

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