Bread, especially white bread, is not the best food, but it is certainly better than corn flakes. Too much white bread can cause a change in the swan's blood chemistry, but bread's major problem is that it serves no nutritional value for the bird. In effect, the bird is getting no energy(no calories).

The best food source for supplemental feeding is cracked corn because it acts as a filler while supplying the swan with much needed Vitamin A.
The cracked corn should be mixed 50%-50% with poultry layer pellets as the pellets provide much needed minerals and micronutrients such as calcium, phosphorous, magnesium, etc.

If you are looking more for a treat, use lettuce as this will provide the swan with additional vegetation, especially during the winter months. Swans are grazers and will eat grass and aquatic vegetation. So, when speaking about treats, lettuce is the best treat of all. The Regal Swan