More than likely, the cygnets were walked until they tired. Then, the female swan could have transported them along a small stream that you may not know about. The mother swan can also carry the cygnets on her back if they tire, so she could have certainly walked a short distance with them on her back or at least floated along a small stream to cross the field. As far as the fencing, many animals and birds will find a small opening in the fence line, especially if there is erosion along the bottom rail line or any other opening. The Regal Swan