The Australasian Bittern is a large, stocky, thick-necked heron-like bird with camouflage-like plumage. Individuals grow to a length of 66–76 cm, with a wingspan of 1050–1180 cm. Males weigh up to 1400 g and females weigh up to 900 g (Marchant & Higgins 1990).
The upperparts of the body are brown, dark brown to black and mottled buff in complex patterns that aid the bird's concealment in swamp vegetation. The underparts of the body are streaked and scalloped brown and buff. The bird has a prominent black-brown stripe running down the side of the neck, the eyebrow is pale, and the chin and upper throat are white. The species’ bill is straight and pointed, straw yellow to buff in colour, with a dark grey culmen (dorsal ridge of the bill). The legs and feet are pale green to olive; and the iris orange-brown or yellow (Jaensch 2005 pers. comm.; Marchant & Higgins 1990; Pizzey & Knight 1997).
Dark and pale variants of the plumage have been observed in adults, but the variations are not well understood. The sexes are similar in appearance, but females are smaller (Marchant & Higgins 1990). Juveniles are generally paler than adults (Marchant & Higgins 1990; Pizzey & Knight 1997), with heavier buff flecking on the back; adults and juveniles are probably not separable in the field (Marchant & Higgins 1990).
The Australasian Bittern is generally solitary, but sometimes occurs in pairs or dispersed aggregations of up to 12 birds. The species apparently breeds in solitary, territorial pairs but little more is known about its breeding ecology. It is probably sedentary in permanent habitats, but some individuals possibly make regular short-distance movements during winter, and occasional movements to inland areas have been recorded during extensive flooding (Marchant & Higgins 1990; Jaensch 2005 pers. comm.).
The Australasian Bittern occurs in terrestrial freshwater wetlands and, rarely, estuarine habitats. It favours wetlands with tall, dense vegetation, where it forages in still, shallow water up to 0.3 m deep, often at the edges of pools or waterways, or from platforms or mats of vegetation over deep water. The species favours permanent and seasonal freshwater habitats, particularly those dominated by sedges, rushes and/or reeds (e.g. Phragmites, Cyperus, Eleocharis, Juncus, Typha, Baumea, Bolboschoenus) or cutting grass (Gahnia) growing over muddy or peaty substrate (Marchant & Higgins 1990).
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