To the OP I am sorry for your loss I have 6 dogs 5 I can trust with the chickens of those 2 are full chi, and 2 are mixed chi, the other is a beagle mix. Now my 6th dog there is no way I can trust her, she is locked up when chicks/chickens are out the chicks/chickens are locked up when she is out. I would think that the puppy is being a puppy and it does happen. Some dogs do have a higher prey drive then others. Maybe some submissive training with both dogs would help. Also a good fence. Good luck and sorry you lost your duck.
