This is a juvenile mute swan. The young swan's feathers have not turned completely white and it probably got off the beaten migratory path and landed on your pond Your swans (being territorial),do not want any part of this swan becoming a new resident so they are letting him know he is not welcome. You need to watch to ensure that your swans do not seriously injure or kill the young bird. You might be able to capture him and take him to a local wildlife rehabilitation center or let them know that he is in distress and they may be able to help. Please let us know how this situation is resolved. The Regal Swan