You should feed the birds lettuce, small amounts of bread if any and poultry layer pellets mixed with cracked corn. You might also want to check to see what your local and state ordinances state about feeding wildlife. In many states, it is illegal to feed wildlife.

If the birds are Trumpeter or Tundras, they may be federally protected and you cannot approach, touch, feed or harrass. If they are Mute swans, your state may have a statute regarding feral populations. So, please check with your state officials before feeding any wildlife. The Regal Swan