This could be any number of conditions from fungal, bacterial or viral infection to a nutritional deficit or poisoning. All can cause neurological problems. A leg or foot injury could also cause the problem. The only way to properly diagnose the problem is to get the swan to a licensed waterfowl veterinarian. If you do not have one in the area, see if a veterinary medical school is located in your area as they may have an avian or exotic species veterinarian that can help. In any case, if it is some thing that can affect the other cygnets, you need to have it diagnosed and treated immediately if possible. The Regal Swan