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The yellow-billed egret

8 September 2015

 

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Egretta intermedia occurs from sub-Saharan Africa eastwards through India and south-eastern Asia to Japan and eastern Australia. It is widespread in southern Africa but it is generally absent or sparsely distributed in the drier western parts such as much of Namibia, southern and western Botswana, the Northern Cape province and the central and western Karoo of South Africa. It was first described scientifically as Andea intermedia by Wagler in 1829 based on a specimen from Java. The scientific name intermedia is Latin for intermediate and refers to the size of this bird, being intermediate between that of the greater and little egret.

An adult yellow-billed egret is some 69 cm tall, and a male weighs around 460 g as opposed to 370 g in a female. The plumage of the sexes is alike and is wholly white, with a moderate crest and dense plumes on the breast. Despite its common name the bill is red, but it is tipped in orange-yellow. The eyes are bright red with a green eye ring. The upper legs are crimson to yellow and the lower ones and feet are black. The yellow-billed egret is sometimes confused with the great egret but the latter is considerably larger and has a proportionally slimmer neck and longer bill, while the legs are uniformly black. The yellow-billed egret croaks and clatters with its bill.

In South Africa, the yellow-billed egret is especially common from the north-eastern parts south to the Free State and at the Cape Peninsula of the Western Cape. In southern Africa its main concentrations are in the Okavango Basin, northern KwaZulu-Natal, the central South African Highveld and the south-western Cape. It is present year-round but it does move around in response to changing water conditions. A nestling that was ringed in the Western Cape province was recovered as an adult bird in Zambia. It is known to be preyed upon by the tawny eagle Aquila rapax and has a life expectancy of up to ten years.

The habitat is seasonally flooded wetlands and grasslands, the shores of lakes, estuaries, saltpans and rivers where it prefers shallow water. However, it also forages in dry grasslands close to water and on occasion along the seashore in the Western Cape. It is usually solitary but may form loose flocks of up to 20 birds. It roosts communally with other water birds in reedbeds or trees over water. It flies with slow wings beats.

The yellow-billed egret hunts for food by day in shallow water or grass and from floating vegetation in deeper water. When hunting, it wades or walks slowly while stabbing with its bill at any possible prey. It will also stand stock still and wait for prey with its head and neck tilted to one side, forages in the wake of moving hippopotamuses and will follow other egrets and herons as they forage. It flushes prey from the gras by flapping its wings and hovers over water to stab at prey with its bill after plunging head first into the water. The diet consists of small fishes, various types of frog, aquatic insects and their larvae, grasshoppers, and occasionally terrestrial insects and spiders. It is known to eat the chicks of the yellow-crowned bishop bird Euplectes afer and on sea shores it will forage among stranded kelp for food. In Zimbabwe it has been estimated that a population of 23 to 35 yellow-billed egrets will eat up to 1,81 tonnes of fish per year.

It is monogamous and usually nests colonially in scattered nests with other types of water bird, but sometimes only with others of its kind. There can be up to 70 pairs in a breeding colony and the males defend small territories around their nests from others of its kind. A breeding pair will spend much time by preening each other’s feathers while being perched on a nest. The nest is built by the female with material that is brought to her by the male. The nest consists of a platform of sticks and reeds that is 200 to 800 mm in diameter and 150 to 400 mm high. The platform is lined with grass and is placed 1,5 to 6 m above water in a tree or a reedbed.

Two to three sub-elliptical, pale greenish-blue, smooth but slightly pitted eggs are laid from July to March, but with regional laying peaks. Both sexes incubate the eggs for a period of 24 to 27 days but they only change over infrequently. The newly-hatched chicks are covered in white down, the eyes are yellowish-brown, the bill is bright yellow, the legs are greyish-green and the skin of the wings is bright green. The young leave the nest when they are about 21 days old and the feathers appear when they are 35 days old. They are brooded and fed by both the parent birds. Yellow-billed egrets are sensitive to human disturbance when they are nesting.


Reference:

Hockey, P A R, W R J Dean & P G Ryan (Eds) 2005. Roberts – birds of southern Africa, seventh edition. Cape Town: The John Voelcker Bird Book Fund, pp 584 - 585.

 

article by Prof J du P Bothma

 

 

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