There may be several reasons for this short "migration". The new pond may not freeze and/or has more aquatic vegetation and other resources during winter's inclement weather.

The new pond may be a gathering place for all swans in the area to meet before leaving en masse with the entire flock to their wintering migration grounds.

There may be predators in the immediate area during the winter months and the new body of water may be safer for the swans now and in the future during the cold winter months. The Regal Swan