Introduction
The Lake Duck (Oxyura vittata) is a South American stiff-tailed duck of the family Anatidae. It is associated mainly with freshwater lakes, lagoons, marshes and reed-fringed wetlands. The species is also known as the Argentine Ruddy Duck in some older sources, although Lake Duck is the accepted English name used by major modern bird checklists.
Physical Description
The Lake Duck is a compact, medium-sized diving duck, measuring about 30 cm in length in Argentine field accounts. Like other stiff-tailed ducks, it has a long, narrow, rigid tail that may be held upright when the bird is resting or alert.
Adult males are strongly patterned. They have rich chestnut-brown body plumage, a black head and neck, and a conspicuous bright blue bill during the breeding season. The lower belly and undertail area are paler. Females and juveniles are much less colourful, with brown, finely barred plumage, a pale throat and a dark bill. The female’s plumage provides effective camouflage among reeds and other wetland vegetation.
The species is notable for its unusual reproductive anatomy. Research has documented an exceptionally long, spiny male phallus, measuring approximately 20 cm when fully extended. This feature is discussed in the scientific literature in relation to sexual selection and reproductive anatomy in waterfowl, but it is not normally useful for identifying birds in the field.
Range and Habitat
The Lake Duck occurs widely in southern and central South America. Its range includes much of Argentina and Chile, with additional populations in Uruguay, Paraguay and southern Brazil. In Chile, it is distributed mainly through lowland and temperate areas rather than the high Andes. Some populations move seasonally, while others appear to remain resident throughout the year.
It favours freshwater lakes, lagoons, shallow marshes and ponds with aquatic vegetation, reed beds or other dense marginal cover. Argentine field accounts report the species from steppe lagoons and record it at elevations of up to approximately 900 m. Reed beds provide shelter, feeding areas and suitable nesting habitat.
The Lake Duck is generally found in open water close to vegetation. It may use larger lakes and lagoons outside the breeding season and smaller wetlands during warmer months where suitable cover and food are available.
Behaviour and Diet
The Lake Duck is an accomplished diver. It usually swims in shallow or moderately deep water close to cover and dives to feed, often disappearing quickly when disturbed. It is commonly observed alone or in small groups, sometimes alongside other stiff-tailed ducks.
Available accounts indicate a varied diet consisting of aquatic plant material, seeds and small aquatic invertebrates. The exact proportions may vary with habitat and season. Feeding usually takes place underwater or among submerged vegetation.
Courtship includes postures and movements typical of stiff-tailed ducks, such as head and tail movements, splashing and display calls. Males may produce popping or mechanical sounds during display. Breeding information is limited in comparison with that available for better-studied waterfowl, but the species nests among reeds and other wetland vegetation. In Argentina, breeding has been reported mainly from October to January.
The male’s unusual reproductive anatomy is one of the best-known biological features of the species. Studies of Lake Ducks have contributed to broader research into sexual selection, genital morphology and the evolution of reproductive traits in waterfowl.
Identification Tips
- Look for a small, compact diving duck with a long, stiff tail, often held at an angle or upright.
- A breeding male shows a black head and neck, rich chestnut body plumage and a vivid blue bill.
- Females and juveniles are brown and finely barred, with a pale throat and dark bill.
- The species is usually seen close to reeds or other aquatic vegetation and may dive repeatedly when approached.
- The Lake Duck can resemble the larger Andean Duck (Oxyura ferruginea), especially where their ranges meet. The Lake Duck is generally smaller, and the male has a flatter-looking head, a bright blue bill and paler undertail areas.
- It can also be confused with other brown female stiff-tailed ducks. Bill colour, body size, head pattern, habitat and geographical location are important identification clues.
Conservation Status
The Lake Duck is currently assessed as Least Concern on the IUCN Red List. It has a broad distribution and is not regarded as globally threatened. A reliable current global population estimate should not be inferred from local counts, and the available assessment does not justify presenting a precise population figure here.
Wetland loss, alteration of aquatic vegetation, pollution and changes in water availability may affect local populations, particularly where shallow lakes and marshes are heavily modified. Protecting freshwater wetlands, maintaining reed beds and managing pollution are appropriate conservation measures. Continued monitoring is also important because information on population trends and breeding ecology remains uneven across the species’ range.
Frequently Asked Questions
Where does the Lake Duck live?
The Lake Duck occurs in Argentina, Chile, Uruguay, Paraguay and southern Brazil. It occupies freshwater lakes, lagoons, marshes and ponds, usually where reeds or other aquatic vegetation provide cover.
What does the Lake Duck look like?
The male has a chestnut-brown body, a black head and neck, and a bright blue bill. Females and juveniles are brown and barred, with a pale throat and a dark bill. Both sexes have the stiff, pointed tail typical of the genus Oxyura.
What does the Lake Duck eat?
It feeds mainly on aquatic vegetation, seeds and small aquatic invertebrates. The bird usually obtains food by diving in shallow or moderately deep water.
Is the Lake Duck migratory?
The species is partly migratory. Some populations undertake seasonal movements, while others are resident. The extent and timing of movement vary across South America.
Where does the Lake Duck nest?
It nests in wetland vegetation, especially reed beds and other dense marginal cover. Breeding has been recorded in Argentina mainly from October to January, although timing varies geographically.
Is the Lake Duck endangered?
No. The Lake Duck is currently listed as Least Concern by the IUCN. Local populations may nevertheless be affected by wetland degradation, pollution and changes in water levels or aquatic vegetation.
References
- BirdLife International. 2021. Oxyura vittata. The IUCN Red List of Threatened Species 2021: e.T22679824A189960778. IUCN, Cambridge, UK. https://www.iucnredlist.org/species/22679824/189960778. Accessed 8 September 2026.
- Gill, F., Donsker, D. and Rasmussen, P. (eds). 2026. IOC World Bird List, version 15.2. International Ornithologists’ Union. https://www.worldbirdnames.org/. Accessed 8 September 2026.
- McCracken, K. G. 2000. The 20-cm spiny penis of the Argentine Lake Duck (Oxyura vittata). The Auk 117(3): 820-825. https://doi.org/10.1642/0004-8038(2000)117[0820:TCSPOT]2.0.CO;2.
- Brennan, P. L. R., Prum, R. O., McCracken, K. G., Sorenson, M. D., Wilson, R. E. and Birkhead, T. R. 2007. Coevolution of male and female genital morphology in waterfowl. PLoS ONE 2(5): e418. https://doi.org/10.1371/journal.pone.0000418.
- Sistema de Información de Biodiversidad, Administración de Parques Nacionales, Argentina. n.d. Oxyura vittata, Pato Zambullidor Chico, Lake Duck. SIB Argentina. https://sib.gob.ar/especies/oxyura-vittata. Accessed 8 September 2026.
- Mataloni, G. and Quintana, R. D. 2022. Freshwaters and wetlands of Patagonia. In: Mataloni, G. and Quintana, R. D. (eds), Patagonia: Natural History, Ecology and Conservation. Springer. Conservation-status summary for waterbirds including Oxyura vittata.
- Consejo Nacional de Investigaciones Científicas y Técnicas. n.d. Atlas ornitogeográfico de la República Argentina. CONICET Digital. Distribution account for Oxyura vittata. https://ri.conicet.gov.ar/. Accessed 8 September 2026.
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