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. The male has a slate-blue bill which changes to bright-blue during the breeding season, hence the duck's common name. 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 has classified this species as Least concern. Major threats include drainage of deep permanent wetlands, or their degradation as a result of introduced fish, peripheral cattle grazing, salinization, and lowering of ground water.