Swans eat aquatic vegetation, which their long necks equip them to take from the riverbed. They take the molluscs that cling to the vegetation, and also eat small fish, frogs and worms.
They will graze big grassy fields, and can survive quite successfully in a field of short-cropped grass. Flocks of Bewick's and whooper swans can be seen in potato fields during the winter, feeding on the potatoes left after harvesting.
Swans normally find enough food in the wild without supplementary feeding. It is only in freezing weather that extra food can be helpful. Many people like feeding bread to swans and while this is unlikely to do them any real harm in the long term, it is no substitute for the proper diet that the birds themselves will seek out.
Grain, such as wheat, and vegetable matter, especially lettuce and potatoes, can be fed to swans. Food should be thrown into the water to avoid encouraging the birds onto the bank.