I have discovered why this Whooper Swan is here out of season. She, yes, it's a female, was placed here by the RSPCA. She must have been injured in some way when it was time to migrate, and they must have placed her here to recover with a lot of her wild cousins, in relative safety. Her wings have been clipped so she can't fly at the moment. Hopefully they will regrow and come the winter, she can rejoin the returning members of her own species.