Actually, American Coots will attack chicks for two reasons.
One reason is, as you said, due to heavy nest parasitism. Coots are highly territorial, and if a female coot isn’t able to snag a decent territory (or wants to lay more eggs than her territory can support), she might lay her eggs in another coot’s nest. If a pair of coot parents can tell that they have eggs or chicks that aren’t theirs (for example, eggs that look significantly different, or chicks that hatched later than their other chicks), they’ll attack and kill the foreign eggs and/or chicks. However, parents can’t always differentiate between their own chicks and foreign chicks, in which case they aren’t culled.
The second reason is resource management. In any given environment, there are good years and bad years. Coots will lay the maximum number of eggs that their territory can support in a good year (plenty of food for their chicks, etc.). Because not all years are excellent ones, oftentimes there are going to be more chicks than a coot’s territory can support. Once the weakest chicks die of starvation, coot parents will ensure the survival of the rest of their brood by bullying and attacking the strongest chicks, to make sure that the weakest of the surviving chicks get enough food.
Coots are weird birds.
Thank you for adding! I don’t think I had heard the resource management part before, it does make sense though.