As traumatic as this might be, it is nature. All wildlife species will kill their young in order to put them out of their misery if there is something wrong with the offspring. Better to have the parents kill the young than a predator follow a sick or injured member of the family and kill the other members of the family or flock/herd. Additionally, if the young has a disease, the last thing the parent wants is the other young to die from a disease that also might wipe out the family/herd/flock.

Although this killing may seem very cruel, just think that these parents know something is wrong, but they don't have a gun or other device to kill the birds, instantly, so unfortunately, as effective as the method of drowning, it may take some time for the young to die. However, many times what you think is an inordinate amount of time may not be as long as you think and the young may be barely lingering with the parent just ensuring that it does not suffer and is in fact dead. The Regal Swan