There are a pair of mute swans that successfully hatched and fairly fledged three cygnets. However, now only two of the cygnet, still mostly grey but flying, remain with the parents. I despaired that the third cygnet, after surviving to practically adult size, had died. Then the other day I saw a lone cygnet that appeared to be the appropriate age of the missing cygnet; neither parent was in sight. Would the parents have ousted the "third" cygnet? She - based on how the back feathers sat, it appears to be female (not angled correctly for display?) appeared heathly and not in any distress. I'm rather distressed. Advice?