The cygnet may be able to survive a broken leg depending on how severe and the location of the break. It will not survive without veterinary care. You need to take the cygnet to a licensed avian/waterfowl veterinarian to have it examined and determine the course of treatment. With a break, the cygnet will probably need to undergo a somewhat prolonged treatment (2-3 weeks at least) meaning that it will not be able to be returned to its parents. You will need to find a new home so that it can be raised either alone or in the best case, with other young waterfowl such as a wildlife rehabilitation center, etc. The Regal Swan