Yes, they will probably leave her. Nature is cruel, but if it means one death vs the family, the one will certainly be left to die. It is just about time for them to fly and if she does not get her flight feathers within the next few weeks, she may be left behind.

State wildlife officials are useless as most states view mute swans as feral and would just as soon see them dead. See if you can get a wildlife rehabber to intervene if she does not fly away with the flock. Maybe, they can hold the female until next year when the swan family returns. If by then, she has flight feathers, she will be able to fly away during the migratory season.

Again, if migration occurs next month or so, she may have her feathers and will migrate with the family. This is going to be a wait and see proposition, but the rehabber will be able to tell if the flight feathers are going to come in on time and if they are growing normally.

Lastly, just because everyone seems to be a great family right now, this does not necessarily remain so in nature. Once the parents take the cygnets on the migration pattern and they get to the stopovers for the winter, the cygnets (actually juveniles) will basically be dumped off to find a mate and the parents will return, possibly to your lake or region during the next nesting season. Sometimes, cygnets and their new mates will join the parents in the same area for nesting season, but they will eventually be run from the immediate area once nesting occurs. So, don't feel like this family will remain forever, because they won't.

Let us know how this transpires. The Regal Swan