It could be a juvenile or injured swan that was chased from the flock. If the bird does have eggs, then it means that there is a possibility that something happened to its mate. If a predator attacked the other bird or caused the mate to leave, then the chances for the remaining swan to successfully hatch and raise possible cygnets is not going to be of high probability.

We would suggest that you contact the local RSPCA or Wendy Hermon at Swan Lifeline, Eton to see if they can check on the swan to determine if it might be injured or if there are eggs in the area. The Regal Swan