Many years ago (approximately 11 years)when we first began researching swans, we saw this behavior in a family of swans. The male chased one of the cygnets and tried to kill it. Intervention was successful and the cygnet was removed from the area due to many complaints of people watching the spectacle.

Regal Swan researchers contacted various swan specialists from around the world to see if they had ever observed infanticide. Many stated never. However, in the ensuing years of our research, we observed this behavior many times.

As cygnets were chased from the habitats and nearly killed, the cygnets were rescued and given extensive veterinary medical exams. Some of the interventions were successful as diagnosis of an underlying cause was found and treated, and some were unsuccessful as treatment was too late and the cygnets died.

What we found is that the parents knew something about the cygnets health or behavior well before we ever observed an illness or peculiarity in behavior. Many were ill with parasites, had internal infections such as egg sac peritonitis or had been attacked by a predator and ensuing infection or trauma was too great for the young bird to survive without human intervention.

The young cygnets may have been attacked by a predator and the parents either knew that they were going to die from the attack or that they were too ill to keep up and the predator would return to finish the task. In these instances, nature will assume that it is better to put one of their own out of its misery than to let it suffer or risk extermination of all the family. So, to answer your question, yes the killing of the swans' own young can be fairly common if there is an underlying health or behavior issue. The Regal Swan