Even though this seem senseless to humans, there is always a reason in nature. Species do not regularly kill their young without a good reason, otherwise, the species would die out. There was probably something wrong with the baby swan and the parents knew it. Unfortunately, this is how diseases and injuries are dealth with in nature. There are no hospitals, rehabilitation centers, etc. Nature inbreeds into species the need for healthy hardy offspring, otherwise the genetic pool would be greatly endangered and the overall flock, herd, etc., would suffer. More than likely, the young swan drowned was probably a runt and count not keep up. This would signficantly endanger the rest of the family and allow predators to be attracted and follow the rest of the family.

Another possibility is of a new parent. If the parents do not understand what a cygnet is, why it is constantly following them around and screaming at the top of their lungs, then a new parent could accidently attack the cygnet without understanding that is not something that can or should be done. First time parents have been seen to exhibit this behavior, but by the second brood, become much more experienced and understand that the cygnets are something to be protected. The Regal Swan