It's a little sad in real life too, because ducklings think you are their real mother, and they become anxious when you get away or eventually leave them alone at farm (with other ducklings). The sound I used is probably a call of a duckling for their mother; when they are relaxed they chirp differently. But it's a growing process for both ducklings and children they are attached to. Don't treat animals like playthings, that is what my whole game could also be about.