You need to get the small swan to an avian veterinarian immediately. Whether the injury was caused by a trap, predator or something else, the infection can kill the swan. The swan's leg will need to be examined, any dead tissue debrided, and the swan will need to go on antibiotics. The debriding may require minor surgery under anesthesia. But, the best advice is to get the swan to the vet ASAP. The Regal Swan