A duck seen on rivers and ponds across Japan. Its black bill has a yellow tip, and males and females have similar body colours.
A dark line runs through the eye across the pale face. Both sexes are subdued in colour, unlike ducks with conspicuous male breeding plumage.
Eastern spot-billed ducks can eat the small underground tubers of Hydrilla waterweed. In a feeding experiment, one duck caught near Lake Biwa and kept in captivity ate more tubers than stems and leaves, showing a preference between parts of the same plant.
In November 2007, after approximately 12 hours without food but with drinking water available, the duck received stems and leaves and tubers simultaneously in separate trays. Three grams of each were offered for five minutes. In both trials, almost no tubers remained, whereas roughly half the stems and leaves were left.
A field experiment the following year compared open plots and bird-exclusion plots in the Echi River. At sites initially about 60 centimetres deep, fewer tubers remained in open plots. Eastern spot-billed ducks, Eurasian wigeons and other waterbirds gathered there and fed at the surface.
An extinction-risk assessment has not been added yet.
Around Lake Biwa, rising water may put aquatic-plant food beyond the reach of eastern spot-billed ducks and other waterbirds. At the Echi River study site in 2008–2009, water depth increased sharply from January and exceeded one metre in the deeper plots by late February. The authors suggested that artificial water-level regulation could restrict access to food. Regional wintering-habitat conservation therefore involves considering accessible feeding depths alongside aquatic vegetation.