Usually, if the parents want the cygnets to leave, all cygnets will hang out together, but the parents will still be in the area in case something happens. If the first cygnet was lost to a predator or something scared the parents, they will abandon the young to save their own life. If you can catch or at least watch the remaining cygnet, we would suggest that you contact the nearest swan rescue facility to advise them of the situation. They may be able to intervene and catch the young bird and hold it until it is ready to go out on its own. Five months is a relatively young age for the bird to be on its own.

We are not sure your exact location, but you might try the Swan Lifeline Rescue Facility in Eton and contact Wendy Hermon. She may be able to give you some options. The Regal Swan