Are you sure that the cygnets are not under mom’s wings? A female swan will carry her young under the wings, especially if they are smaller than the rest of the brood or can’t keep up.

If the cygnets were truly predated, there are several predators that come to mind. They include, Egrets, Herons, Owls, Hawks, Turtles, Minks and Foxes. The Regal Swan