The Blue-billed Duck (Oxyura australis) is a small Australian stiff-tailed duck, with both the male and female growing to a length of 40 cm (16 in).[2][3] The male has a slate-blue bill which changes to bright-blue during the breeding season, hence the duck’s common name (see photo). The male has deep chestnut plumage during breeding season, reverting to a dark grey. The female retains black plumage with brown tips all year round. The duck is endemic to Australia's temperate regions, inhabiting natural inland wetlands and also artificial wetlands such as sewage ponds in large numbers. It can be difficult to observe due to its cryptic nature during its breeding season through autumn and winter. The male duck exhibits a complex mating ritual. The blue-billed duck is omnivorous, with a preference for small aquatic invertebrates. BirdLife International[1] has classified this species as Near Threatened. Major threats include drainage of deep permanent wetlands, or their degradation as a result of introduced fish, peripheral cattle grazing, salinisation and lowering of ground water.[4]

The Blue-billed Duck is almost entirely aquatic. While they have been observed on land, they have difficulty walking,[2] exhibiting a penguin-like gait.[3] During non-breeding season, many ducks gather in flocks totalling several hundred,[8] especially juveniles and younger adults, in open lakes or dams in autumn and winter, far from the shore. For the rest of the year, during breeding season, the Blue-billed duck prefers deep, freshwater swamps, with dense vegetation including cumbungi Typha orientalis (broadleaf cumbungi) and Typha domingensis (narrow-leaved cumbungi); although it has appeared in lignum swamps in more coastal areas,[2][3][9] especially in drier seasons.[7] They have also occasionally been found in large rivers and saline water bodies such as billabongs.[2][3]