There could be any number of reasons for the cygnets' deaths. Even though you may not see predators, many exist including snapping and soft-shelled turtles, egrets, herons, owls, hawks, raccoons, and foxes are just a few along with domestic animals such as dogs, cats and unfortunately humans.

If predation did not cause their deaths, they may have had some type of infection that caused their demise. Without a necropsy (autopsy for animals) or having seen the moment of death, there is no real way to determine what killed them. Hopefully, the parents will pay attention to the remaining cygnet and protect it as long as an infection is not the cause. The Regal Swan