We think of ducks, here in the American Southwest, as Floating Goats. They eat durned near anything!
They eat small fish and amphibians, reptiles, too! They eat grain & algae & fecal matter deposited by cattle. They do, of course, eat bread and virtually any grain product, such as commercially made breakfast cereals. They eat worms and insects.......Their provender is a major factor in their taste, too! Roast a wild duck or a bred duck but one of them what feeds in the city Park is likely to taste a tad fowl!