If the cygnet was not returned to the parents in a timely fashion, within a few hours of separation (which would not be possible if it had to be rehabilitated), then the parents do not recognize the young bird and will try to kill it or removed it from the rest of the family. The bird may have been too slow in keeping up with the rest of the brood which invites attack by predator and ultimately, the chances of the predator finding the other cygnets. So, the parents will kill the weak cygnets to protect the stronger ones. Unfortunately, nature is not kind and this allows for the survival of the strongest. The Regal Swan