How long as the swan lived on the pond? If it has been several years, then apparently it is doing quite well. If it has just recently been placed, then there might be a problem. Also, how large of an area of weeds and how high are the weeds? The weeds can also serve as an adequate shelter.

Swans were built for cold winters in mind as the Northern Hemisphere swans have many feathers which include down to keep them warm. The only concern would be the diminished food sources during the winter and the lake freezing over so that the swan becomes trapped. As long as these two issues are addressed so that the swan can be fed and move around the pond, everything should be fine. If you do have concerns, you might want to contact the owner of the swans to see what accommodations he/she is making for the swan during the winter months. The Regal Swan