Unfortunately, I too just witnessed this past weekend a pair of swans parents who drowned one on their cygnets. There were 6 cygnets and all were swimming and feeding with the parents when out of the blue, the parents forced one of the cygnets under the water and drowned it.

I read your previous response that it perhaps got tangled in weeds or the parents were protecting it. That was not the case here.

I was really hoping to gain some insight from you as to why this behavior occurs because it was so upsetting to me to witness.