Our first thought would be that the cygnet ingested something poisonous while feeding. If you have been experiencing any drought or heavy rains in the area, the cygnet could have been exposed to run-off or in the case of drought, allowed to access an area which would normally not be accessible to a cygnet.

There is also the possibility that it was suffering from an infection, i.e., bacterial, fungal or viral which would not necessarily been observable. If the infection was not visible, then a blood test would be the only way to determine if the cygnet's antibodies were working in an infection fighting mode.

As far as the flocks in Europe, the swans do fight and chase the cygnets from the area. Once the migratory season occurs, the families fly to migration stations that act as a rest-stop on the bird's way to their wintering grounds. Once winter is over, the birds again stop-over to rest before continuing their flight onto their nesting sites.

During this migratory flight (to and from) and the stop-overs, the cygnets are encouraged by the parents to meet other juveniles and try to find an eventual future mate. If the juveniles continue back to the nesting site with the parents, then they are chased and nipped until they leave the area so that the parent's can go about preparing a nest and starting a new family.

If a juvenile is forced from the parent's area and finds itself in another swan's territory, it is also chased and nipped. If the juvenile is a male and the testosterone level is so high that it begins to fight a male swan in an existing area, then yes, the swans could seriously injure each other which could lead to eventual death. Normally, this does not happen. Once a male swan is engaged in a fight, the victor maintains dominance in the area and the loser could still remain in the area, but alludes to the wishes of the other swan.

All of this is normal behavior and is found throughout the animal kingdom. Dominance rules, submission follows. This normal pecking order allows for strong breeders and protectors of the flock or herd.

This behavior is more readily seen in a captive setting because the birds are pinioned (in most cases by state law so that they do not start feral populations), and cannot escape from the parents or look for another habitat. So, fights continue. The Regal Swan