Update: The duckling has been separated from the rest of the flock, but its condition hasn't improved. It is lively enough (not listless at all), and is eating and drinking some, but is very unstable on its feet. It will easily flip over onto its back and not be able to get up. This weekend, I've had to right it several times. I am wondering now if it is sick, and that was the reason the young ducks were "attacking" it.