The Blue-billed Duck - Oxyura australis - is so named because breeding males have a blue bill; females have grey-brown bill all year. Breeding males have black head and neck, sometimes chestnut tinged. The back is rich chestnut, wings are brown, upper chest and flanks chestnut, underside brown flecked with black or dark brown. Under tail black. Eye dark brown, feet and legs brown. After the breeding season males have black and grey speckled heads with grey chin and some black speckles; breast is dark grey, feathers edged with dark brown, bill slate grey, feet grey. Males are 35 to 44 cm long.
Females all year are black-brown above, each feather barred with narrow bands of light brown. Upper tail black. Chin and throat brown speckled with black; breast and belly mottled light brown and black; each feather black with a light tip. Eye is brown, bill grey-brown, feet grey-brown. Females are 36 to 44 cm long.
Immature dusk of both sexes are coloured similarly to females but are paler; the bill is grey-brown.
Male tails are mainly black with stiff, pointed feathers; the tail is often held horizontal above or below the water surface. Sometimes held erect and vertical.
The Blue-billed Duck is a wary bird living in deep, permanent bodies of water with seasonal variation. In spring and summer pairs disperse widely to sheltered pools and swamp backwaters to breed. In autumn and winter many birds gather in rafts of sometimes a thousand or more on open sheets of water. Great concentrations have been observed along the central Murray River. The bird is completely aquatic and rarely seen on land. Can fly well but does not take-off easily needing several metres of running across the surface to become airborne. If disturbed will dive and reappear well away from the disturbance.
Dives and swims underwater to get most of its food from the bottom of swamps and pools; remains submerged for 30 seconds at a time. Also swims beneath overhanging vegetation stripping seeds and feeding from the water surface. Eats seeds, buds and leaves of submerged and surface plants as well as larvae.
Breeding takes place regularly in spring and early summer. The female builds a nest in dense vegetation over water, occasionally using empty nests of other species. She tramps plants into a platform and makes a deep, cup-shaped nest 250 mm across and 80 mm deep, often covering it with a roof of reeds bent over. Other birds are charged at and driven off.
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