Not sure why someone would do this except maybe he thought that once the cygnets hatched, the swans do not need the nest. If the neighbor was tired of the nest, in some area that he needed to access and was tired of an overly aggressive parent, this might also explain why he would get rid of the nest. The parents will find a place for the cygnets, but you can facilitate this assistance by placing hay or straw in an area near the pond that the cygnets can readily access the nesting material, i.e., low banks and a site where they are protected from predators. Good luck! The Regal Swan