Swans, continued

I am getting concerned about the two young Trumpeter Swans on my pond (above). Mid-summer, the parents and two of the Cygnets were gone one day, and have not been seen since. It is very unusual for the parents to abandon their young - what could have happened? I was worried from the beginning that our pond was too small to sustain them all, maybe they tried to walk on to another larger body of water and for some reason the two remaining were unable to follow.... My fear is that these two have never learned how to fly?! They need quite a bit of surface water to run on and to take off from, and our pond is really small, especially at this time of year. On top of that, it is surrounded by 6 ft. tall native prairie, and if they took to land thinking they could run there to take off, they would have a hard time with the grass... I check on them daily; they are always there, just poking around for something to eat, slowly swimming away when I get too close. I have never seen them fly or attempt to. I finally contacted a person with the swan program at the Park District. She thought maybe they could be slightly malnourished, so I have started throwing out corn. If they are still around when the ice starts to freeze, we will have to catch them and release them in the Swan Refuge at Lake Rebecca Park, in Wright County. I need a happy ending.