Have you tried to have the swans necropsied? They may be eating something poisonous in the field that is causing them to waste. Lead poisoning or other heavy metals may cause the wasting.

The type of vegetation they are eating may not be of nutrient value and there normal vegetation may not be available at certain times of the year. If you are in the U.K., you might try to contact Wendy Hermon at Swan Lifeline in the U.K. If a necropsy is completed and you find the cause, please let us know. The Regal Swan