There could be snapping or soft shelled turtles, bass, heron, raccoon, otter, owl, hawk and many other predators that could kill the young cygnets. The geese may be nesting or taking their goslings to different areas of the pond at night away from these predators. Unfortunately, there is another predator, humans. In many states wildlife officials or their representatives will sneak into swan habitats killing the parents and cygnets deeming them an invasive species which is not true. They may kill the cygnets to control their population and in some instances, it might be representatives of a homeowners association or private pond owner that is the culprit. So unless you witness the demise of the cygnets, it is difficult to determine the specific cause for the cygnets deaths. The Regal Swan