Cygnet all alone.

We have had a pair of adult swan in our quay for a long time, this year they had one cygnet survive then suddenly another young cygnet arrived with them so the four of them were always frequenting the quay. Now that the young ones are going a little white, suddenly the adult swans and one of cygnet have gone leaving just one cygnet alone? It still goes off out the quay but is in the quay a lot more as residents are feeding it as concerned it has been abandoned? It looks so lonely... is this normal behaviour of a swan to desert there cygnets?

The residents did say that one of the cygnets had a funny foot and often had it up on its back and sometimes you see it swimming with just one foot... but then it will put it down and move as normal.. so not sure if there is a problem with the foot or not. Would this affect its ability to take off or can they take off on the water?

The adult swans have tags on there legs.