|Range of the dwarf bittern. Yellow: breeding, green: year-round|
This is a member of the genus Ixobrychus, which contains many of the smallest herons in the world. Going on reported length, from 25 to 30 cm (9.8 to 11.8 in), this is perhaps the smallest species of heron. The weight is similar to other Ixobrychus species, reportedly from 60 to 150 g (2.1 to 5.3 oz), averaging 75–110 g (2.6–3.9 oz). The wingspan reportedly averages 45 to 50 cm (18 to 20 in). The male has a dark slate grey head and neck, with elongated head and neck feathers. The bill is dark overall, being black to dark green on top and yellow on the lower bill. The lores and orbital skin are blue to yellow green and the irises are red brown to dark red. The back, upper wings and other upper parts are dark slate grey. Its throat and upper breast are pale buff, darkening to tawny on the abdomen, heavily streaked black. The legs and feet are green yellow in front and yellow in back. In courtship legs and feet turn bright orange. The female is paler, with a more rufous tinge on its belly and yellow irises. The immature bird is a more buff and pale version than the adult. It is distinguished from the black bittern of Asia by its striped belly, dark neck tufts, shorter, darker bill and much smaller size. It is distinguished from the green and striated herons by its smaller size and slow flight, its pale and heavily streaked underparts, and its uniform (not barred) upper parts.
The dwarf bittern occurs year-round in much of tropical and sub-tropical Africa, with the most southern habitats only visited during the breeding season. It has been reported from Angola, Benin, Botswana, Burkina Faso, Burundi, Cameroon, Central African Republic, Chad, Republic of the Congo, Democratic Republic of the Congo, Ivory Coast, Equatorial Guinea, Ethiopia, Gabon, Gambia, Ghana, Guinea, Guinea-Bissau, Kenya, Liberia, Malawi, Mali, Mauritania, Mozambique, Namibia, Niger, Nigeria, Rwanda, Senegal, Sierra Leone, Somalia, South Africa, Spain, Sudan, Swaziland, Tanzania, Togo, Uganda, Zambia, and Zimbabwe.
This is a migratory species within Africa, particularly in the north and south. Movements of equatorial birds are less clear. Birds occur in South Africa in the November to April wet season. In the dry season, birds from both north and south the migrate toward the equator (Brown et al. 1982). The species does wander outside its usual range, including further into South Africa (Ingram 1998), the Canary Islands, and France.
Dwarf bitterns feed solitarily or in pairs, hunting by night and, especially if it is cloudy, by day. This it does silently and unobtrusively by standing and by walking slowly. This daily schedule overlaps availability of frogs (Tarboton 1980, Hustler and Williamson 1985). Dwarf bitterns sometimes prefer to sneak through vegetation and reeds rather than climb or perch on them. When disturbed, they stay put at the water's edge in an exaggerated version of the bittern posture or it often fly up into trees moving only short distances. It is likely that individuals defend feeding territory, and its use of a forward display[clarification needed] has been described (Riddell 1987). Insects, such as grasshoppers and water bugs, spiders, small fish, crabs, snails and frogs form the main items of the diet. Local populations specialize in grasshoppers and frogs, and can take advantage of seasonal prey availability such as frogs in fishless seasonal pans.
The breeding season is highly variable geographically. It nests primarily in the rains, but in some places in the early dry season. Nesting begins in September in Senegal, July in Sierra Leone and Ghana, August–October in Mali, December–January in the Democratic Republic of the Congo, May–June in Kenya, January–March and September–October in Zimbabwe, February–March in Namibia, and January–April and September–December in South Africa.
It nests along the margins of flood plains, usually when the water inundates the forests. It appears to prefer thicket vegetation in which it nests solitarily or in small groups. Although not usual, they have been seen to nest near squacco heron and rufous-bellied heron. The nest is a platform of twigs lined with coarse grass, 23 cm × 38 cm (9.1 in × 15.0 in) across and 7 cm (2.8 in) deep. The nest is placed near the central trunk of the tree, on a branch or in bushes overhanging the water, at a height of over 0.5–4 m (20–157 in) above the water. They often use thorn bushes for nesting. The dwarf bittern builds its nest quickly (Tarboton 1980).
The courtship behaviour of this species is not known. The eggs are white. Some fresh eggs are faint blue, but this fades within days. Eggs are about 34 mm × 28 mm (1.3 in × 1.1 in). Clutches consist of three to four eggs, although the range is two to five. Incubation lasts for 18 days or longer. Nests have not been reported with asynchronously hatched young, so it is possible that incubation begins with the last eggs.
The young are fed by regurgitation, typically in the evening. They develop quickly and at seven days they leave the nest if disturbed, but return. When fully feathered they wander widely. When sitting on a branch, they adopt a typical bittern posture, making them hard to see. In one study only 69% of eggs hatched.
- BirdLife International (2012). "Ixobrychus sturmii". IUCN Red List of Threatened Species. Version 2013.2. International Union for Conservation of Nature. Retrieved 26 November 2013.
- Newman, Kenneth (2002). Newman's Birds of Southern Africa. Struik. ISBN 1868727351.
- IUCN-SSC Heron Specialist Group. "Dwarf Bittern Ixobrychus sturmii (Wagler)". Heron Conservation.
- Elphick, Jonathan (2007). The Atlas of Bird Migration: Tracing the Great Journeys of the World's Birds. Struik.
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