Types
The six types of swans include tundra swan, black swan, trumpeter swan, mute swan, black-necked swan and whooper swan. Black and black-necked swans live in the south hemisphere, such as Australia and South America, while the others are found in the north. It should be noted that swans, with their long necks, have a distinct advantage in reaching food over both ducks and geese.
Diet in the Wild
Although the specifics of the diets differ by location, with certain plants and insects more abundant in certain locations than others, swans worldwide consume a fairly common diet. Young swans, called cygnets, often begin by eating aquatic insects for protein as they grow. Adult swans tend toward exclusively eating vegetation. Common feeding plants include duck potato and waterweed.
Diet from Humans
It's common enough for park visitors to want to feed the birds they see, including swans, that the birds may soon become accustomed to and at times dependent upon human feeding. Swans often benefit from more vegetation, especially during the colder months when they may not be able to find much on their own. Alfalfa, broccoli and spinach are all good winter foods for swans, while in the summer, when vegetation is plentiful, they may prefer legumes and grains such as rice or light popcorn. Regardless of the type of food you feed a swan, it should always be broken into small pieces to prevent choking.
Foods to Avoid
Although most park goers toss bits of bread to the swans, this is not an especially good food for them to eat as it may lead to stomach pains. As well, swans should never be fed any processed foods, and avoid baked goods. Generally people who enjoy feeding swans should stay away from offering them anything too far from their natural diet, as anything too different may cause problems with their digestion, or even illness.