Swans eat aquatic vegetation, which their long necks equip them to take from the riverbed. They take the molluscs which 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, which is fine as long as it's not mouldt, but when it's fed in large quantities it can cause dietary problems.
Grain, such as wheat, and vegetable matter, especially lettuce and potatoes, can be fed to swans, together with corn, chopped cabbage, spinach, frozen peas and floating specialised swan and duck food pellets are the most suitable foods for swans and larger cygnets.Food should be thrown into the water to avoid encouraging the birds onto the bank.
In common with advice from the RSPB and The Canals & Rivers Trust, Swan Lifeline does not recommend feeding human food to swans and other wildfowl. This includes cooked meats, bread, pizza bases, cakes, sandwiches which contain butter and meat products as these contain high levels of salt, sugar, fats and other additives. They will also deteriorate in the river if they're not eaten, or go mouldy, which can cause severe intestinal problems.
Human foods are not a substitute for a swan’s natural diet.