Unfortunately, swan parents, especially males will kill cygnets that are sick or that they perceive are different from the rest of the brood. Differences or illnesses can make the rest of the brood susceptible to predation, etc.

The dad is an old male and is probably not wanting to take on the responsibility of having little babies around him. Also, most males will not re-mate if the original female is killed. So, he may have mated with the new swan but now has reservations about her. You will have to wait and see if he will try to take her back, but if he continues to show aggression towards her, you will either need to relocate him or the new swan and her cygnets. Since the male is truly the owner of this pond after 23 years of age, it would probably be better to keep him and let him live alone for his remaining years on the pond he is accustomed. The Regal Swan