Hi Caroline
There are two possibilities:
The swan could be a former cygnet of the parents and hasn’t gotten the message to leave. Whether the new swan is an offspring or not, it may be a female. Even though the male parent may act defensively against the new swan, he will usually not hurt a female. So, our guess is that this new swan has known the parents and is a female. Depending on whether the bird flies, it may or may not leave if there are no other swans or suitable habitats in the area. The Regal Swan