If the cygnets were not exhibiting illness until a later time, that would be the reason that the parents may have killed them at a later date. But, that would mean someone would have had to actually observe the parents non stop 24 hours a day to determine that in fact, the parents were responsible for the cygnets' death.

The more probable cause of the cygnets' death was due to predation. Most any wading or bird of prey will eat the young swans, egrets, herons, owls, hawks, other wildlife, mink, otters, turtles, large mouth fish, raccoons, coyote, snakes, foxes, bobcats, panthers, bears, etc.

Whatever predator is in your area is capable of attacking and eating young birds. Once a predator kills and eats one member of a flock/family/herd, the greater the probability that the predator will return to finish the rest of the members. Young birds and animals are easy prey for a predator, even with the best of care from the parents. This situation is no different than seeing 10 ducklings behind a mother duck and by the end of the week, all or most of the ducklings are gone because of predation. We hope this explanation helps to understand that there are many other creatures that can kill the cygnets than just the parents. The Regal Swan