Either someone is extremely un-informed or is trying to scare people to not feed the birds bread, thus providing healthier food such as lettuce and a mixture of cracked corn and poultry layer pellets. If swans died from the yeast in bread, every duck, goose, swan and bird for that matter around the world would be dead because this just seems to be the preferred food for people to feed birds. So, no it does not kill them.

In essence, bread is not a good nutrition source for swans, i.e., it does not contain necessary minerals and vitamins and only serves as a mild filler. The sugar in the bread can change the blood sugar level in the swan which may not be an optimal occurance. THe Regal Swan