About 4 weeks ago I hatched some Snowy Mallard ducklings. I gave 2 to a Friend's Daughter so She and her Husband could raise them to have as pets. One of them died so I replaced it. She just called and said that the first one drowned the replacement and they think that's how the other one died now. They were letting them swim in the bath tub and the first one stood on the other one and drowned it. Has any one had this happen before? If so why is this happening and how can we stop it. I don't know what to tell her as they are very attached to the ducklings. Help!