No, this does not sound normal. The parents may eventally return, but if they do not, the cygnets may not learn the migratory path or be able to protect themselves in severe weather. If you are located in the U.K., you might alert the RSPCA or call Swan Lifeline in Eton. They may be able to asess the situation and help rescue and relocate the young birds to a safer area if needed. The Regal Swan