This does occur in nature because both swans and geese are extremely territorial when cygnets or goslings are around. Geese are notorious for taking on swans and vice versa. Unfortunately, sometimes, a severe injury or death occurs to the perpetrator.

In the case of ducks, they are usually not aggressive towards something larger than themselves. So, once the swan or goose chases a duck, the duck will leave and not try to antagonize the goose or swan any further. Also, some ducks are normally the same color as the young cygnets which as long as they are not white in color, the ducks are not seen as a threat. This is why when the cygnets get older and become white in color, they know their parents, but the adult swans then begin to chase the juvenile swans from the area and act as if they no longer know them. The parents see another white swan and think that it is an intruder in their habitat. This is nature and as disturbing as it may seem, it is the reason that various species and individual animals and birds survive. They protect their young from perceived or real predators. The Regal Swan