Was it medicated chick starter? I just read two days ago that ducks are not supposed to eat medicated chick starter because it can poison them. (that freaked me out since I have raised one duck and 3 geese on that prior, but I have finally found some duck starter)
I am so sorry about your babies. I hope your are able to find the cause. When my goose Silly suddenly dropped dead on me, I ran around screaming and crying just holding her. I could not bear to bury her without knowing what killed her. I took her to the vet for a necropsy and in her intestines was a metal washer that had gotten stuck and blocked and killed her digestive track. She was broody so I did not see any abnormal behavior, just just acted broody.