Infanticide in swans has been observed by our research team many times. We have even stepped in and rescued some of the cygnets to ascertain why the parents may have turned on the cygnet. In most cases, the cygnet had something wrong with it. In several of our interventions, we found cygnets dying from aspergilliosis (fungal infection which leads to neurological compromise and then death), egg sac peritonitis (in which the egg sac did not disolve and the cygnet developed peritonitis of the abdominal cavity), and various other infections. In all of the illnesses, the cygnets eventually died.

We did have a couple of predator related injuries in which we were able to rescue and save the cygnets. With surgery and antibiotics. These cygnets lived and are still living as adult swans.

So, the answer to this parental infanticide is that the parents know the young bird is sick or injured and will not make it in the wild. If they do not kill it, a predator will find it and possibly follow it to the rest of the family and more lives will be lost, including possibly the parents. Nature may seem cruel, but in the interest of the species, death is a means to rid the genetic pool of any illness, genetic anamoly or any other issue that could jeopardize the entire flock. The Regal Swan