Our chickens hunted down shrews and frogs . . . I think ducks just eat what is available. They don't know the chickies are special. We had bantam chicken hens that would attack, mutilate and kill chicks from other hens. The chickies were probably too little to get away from the duck . I'm so sorry -- that must have been so upsetting!
my grandparents have the same problem but with their male muscovy. if any of the babies try to snuggle with him he will kill. so when the babies start hatching, they have to seperate him from mom