Did the parents have cygnets from last year? Did the juveniles leave the nesting/mating area prior to this year’s nesting? If so, this could explain the behavior. The young female is the daughter from last year, the female may raise her wings but not enough to exhibit overly defensive behavior because the female is too young to mate and therefore, not a threat. So, the young bird may have found its dad and returned to its old habitat. Even though it is possible, the dad found a new mate, it probably is unlikely this late in the season and the daughter scenario seems more plausible. The Regal Swan