Black-backed Bittern

Identificazione

The species measures from 25 to 36 cm (9.8 to 14.2 in), has a total weight of 60–120 g (2.1–4.2 oz), averaging 84 g (3.0 oz). This is a very small bittern and one of the smallest herons in the world. The adult male has largely black upperparts, including a black cap, while the underparts, as well as the neck, breast and the sides of the head, are rich chestnut. There are large buff patches on the shoulders, conspicuous in flight. The female is duller, brown and streaked on back and crown; immature birds are similar. The irides are yellow, the bill is yellow with a black culmen, and the feet and legs greenish-yellow.

Tassonomia

The bittern has sometimes been regarded as a subspecies of the little bittern (Ixobrychus minutus), or of the New Zealand little bittern (I. novaezelandiae). However, molecular evidence has shown it to be more closely related to the yellow bittern (I. sinensis) than to the African and Palaearctic forms of the little bittern, and it is now recognised as a full species.

Nome scientifico

Botaurus dubius

Autorità

Mathews, 1912

Ordine

Pelecaniformi

Famiglia

Ardeidae

Genere

Botaurus

Areale e habitat

The birds are mainly found in freshwater wetlands, where they inhabit dense emergent vegetation of reeds and sedges, and inundated shrub thickets. They are also occasionally found in brackish and saline wetlands such as mangrove swamps, Juncus-dominated salt marsh and the wooded margins of coastal lagoons. In Australia the bittern is found in the south-east of the continent, with most records deriving from the Murray-Darling Basin, as well as patchily along the east coast, and in south-west Western Australia where it is locally common on the Swan Coastal Plain. Some scattered records are given from elsewhere, including coastal locations in the Kimberley region, the Top End, and the Torres Strait islands, with vagrants occasionally reaching Lord Howe Island and New Zealand.<p>Apart from records of vagrants, circumstantial evidence that at least part of the population makes long-distance seasonal migrations is that most sightings, and the highest reporting rates, of the bitterns in southern Australia occur in spring and summer, with the birds largely absent in autumn and winter. There are few breeding records from the Australian tropics.<p>In New Guinea the bittern is found seasonally in the reed beds of the Waigani Swamp near Port Moresby from November to April, only being reliably recorded elsewhere on the island from the lowlands of the Trans-Fly region. No firm breeding records are from New Guinea, though a specimen taken in the middle Fly River lagoons was of a bird ready to lay, indicating breeding in the Fly River marshes in September. The lack of good breeding records suggests that at least some birds in New Guinea are seasonal migrants from Australia.<p>The bittern was sighted in New Caledonia in 2001, where it bred.

Migrazione

Migratori completi

Comportamento

Comportamento migratorio: Migratori completi.

Conservazione

Tendenza della popolazione: in diminuzione.

Tendenza della popolazione

in diminuzione