If the behavior was seen only after eating the bread, it may have been trying to dislodge a piece in its throat or nose. Food should always be given in water, otherwise swans and other waterfowl can choke.

If the bird keeps shaking it's head, even when not eating, it may have ear mites which are easily treatable. If the earshaking is not due to food, the swan needs to be seen by a licensed avian (bird)/waterfowl veterinarian for examination and treatment. There are exotic and bird veterinarians as well as wildlife sanctuaries and zoos in Hawaii, finding a veterinarian should not be a problem. The Regal Swan