Those aren't ducks though. They're
http://en.wikipedia.org/wiki/Coot's.
At least some coots have difficulty feeding a large family of hatchlings on the tiny shrimp and insects that they collect. So after about three days they start attacking their own chicks when they beg for food. After a short while, these attacks concentrate on the weaker chicks, who eventually give up begging and die. The coot may eventually raise only two or three out of nine hatchlings. In this attacking behaviour, the parents are said to "tousle" their young. This can result in the death of the chick.
(Edited 1 minute later.)
@previous (Meta)
Such is nature :( If they didn't there would probably be a large chance that all of their young will die, due to being underfed.
@previous (B)
Then the swan...was the good guy all along?
@previous (Green)
Not really. Depending on if it's one or two coot families in the picture, they either have 3 or 6 youngs to feed. Two-three seems to be amount they can support, so the "great purging" has already happened. If not, it may happen irregardless of the swan, depending on how much food they can find. The swan was probably just being a dick, unless it was starved in which case it may have been trying to eat the young (I think they prefer a herbivorous diet).
In any case, it's nature. There are no good or bad guys, and applying human moralities to it doesn't make much sense. There's just the will to survive.
(Edited 1 minute later.)