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The Black stork

22 July 2015

 

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The black stork Ciconia nigra was first described scientifically as Ardea nigra by Linnaeus in 1758 based on a specimen from Sweden which shows its wide distribution. As does its common name, the name nigra indicates its black upper colour because it is Latin for black.

It is an uncommon but fairly large stork with a height of 95 to 110 cm when standing, weighs around 2.8 kg and has a wingspan of up to 1.55 m. The sexes look alike but the male is larger than the female and has a longer, heavier and at times recurved bill. When in breeding plumage the head, neck, back and wings are black with a glossy green sheen, while the wing coverts have a purplish sheen. The feathers of the foreneck are long and the belly and undertail coverts are white. The bill, the bare skin of the lore and around the eyes are bright red, but the base of the upper part of the bill is yellowish. The eyes are dark brown and the legs and feet are red. When not breeding, the red skin and bill become darker red and the overall plumage becomes more dull. Young birds are browner than the adults and the bill, facial skin legs and feet are dull grey-green. The bill and legs turn red at an age of about 10 months. The black stork can sometimes be confused with Abdim’s stork Ciconia abdimii but the latter’s lower back, rump and upper tail coverts are white, the shorter bill is greyish, the facial skin is blue, the legs are whitish green and the toes are reddish.

The black stork breeds from western Europe eastward to north-eastern China and southward to Japan, with an islolated breeding population from eastern Zambia to South Africa. It occurs widely in southern Africa but is generally absent from the Caprivi Strip, Namib Desert, central Kalahari, Okavango Basin and the Transkei region of the Eastern Cape province. It is suspected to have complex, seasonal movements and is locally nomadic in arid areas such as the Karoo where it will move around to find better foraging areas following localized rainfall. It inhabits dams, pans, wetlands, shallow rivers and pools in otherwise dry riverbeds, and sometimes will move into estuaries and marshes.

It usually occurs alone but will form pairs or small flocks, usually of less than 20 birds. It roosts alone or with a mate on cliffs, trees or pylons. In the breeding season it roosts on cliffs. It occasionally roosts communally with the woolly-necked stork Ciconia episcopus. It takes flight from the ground easily, soars well, agilely manoeuvres between branches in forests and flies with 159 strong wing beats per minute in level flight.

www.leopard.tvForaging is done alone but occasionally in pairs or small groups in aquatic areas. When foraging, a black stork walks around slowly in shallow water while stabbing at prey with the sharply pointed bill. It finds prey visually or probes into water and grabs the prey with a rapid forward lunge. It may shade the water with its wings at regular intervals to be able to see better under the water, before darting forward and seizing its prey. The diet mainly consists of fish that are usually from 120 to 180 mm, but occasionally up to 300 mm, long but it also eats frogs and toads, tadpoles, small mammals, nestling birds, reptiles such as small tortoises, large insects, caterpillars and freshwater snails.

Black storks start to breed when they are three years old, are monogamous and may pair for life. They nest alone but occasionally share a cliff nest site with the Cape Vulture Gyps coprotheres, Southern Bald Ibis Geronticus calvus, Verreauxs’ Eagle Aquila verreauxii, Peregrine Falcon Falco peregrinus or Lanner Falcon Falco biarmicus. They are sometimes joined at a nest by other black storks that may be juveniles from the previous year. A small territory is defended around the nest. Courtship and pair-bonding displays start two weeks before laying the eggs and may continue while brooding.

A new nest is built in a cave, a mine or on top of a cliff some 10 to 100 m above the ground and often over water, or an old one is refurbished by both sexes as the same nest may be used repeatedly. In one instance a nest in Botswana is known to have been used for at least 27 years. Sometimes a nest is built on top of the nest of the Hamerkop Scopus umbretta, Verreaux’s Eagle or African Harrier Hawk Polyboroides typus. The nest consists of a flattish platform of dry vegetation such as sticks, reeds or other vegetation. It has a shallow, central cup that is lined with dry grass and soft material. The nest has an outer diameter of around 1 m and is 200 mm high, but nests in caves are smaller.

Breeding is done in the winter and is believed to be an adaptation to exploit food resources when water levels receed. Two to five oval, dull, chalky-white eggs are laid at intervals of two days and they soon become nest-stained. Incubation by both sexes, but with mainly the female at night, starts after laying the first or second egg and lasts for 35 to 36 days. The eggs do not hatch synchronously and the hatchlings are helpless and covered with white down. They have yellow bills, pale pink or flesh-coloured legs and feet. The bill later becomes orange-yellow at the base but grades to pale olive-green towards the tip. The feathers appear at an age of 30 to 35 days and the chicks leave the nest when they are 63 to 71 days old, but return to the nest for another two weeks to be fed.

Overhead cables and powerlines are responsible for occasional deaths in adults, while nestlings are prone to predation by the Martial Eagle Polemaetus bellicosus, Verreaux’s Eagle and the Baboon Papio hamadryas.

 

Reference:

Hockey, P A R, W R J Dean and P G Ryan (eds) 2005. Roberts – birds of southern Africa, seventh edition. Cape Town: The John Voelcker Bird Book Fund, pp 620 - 621.

article by Prof J du P Bothma

 

 

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