Hi there, I just want to share with your our success story of rehabilitating a chicken-killing dog.
When we first got our chickens, we had a pit bull dog and a lab cross in a fenced back yard. The hens were in a pen about 200 feet from the back yard. One day we went to town, 30 miles away, and left the chickens out roaming, thinking we'd be home well before dark and would lock the chickens when we got home. We took longer in town than we expected, and when we got home we found that our pit bull had chewed/dug his way through the fence and chased and killed chickens for sheer pleasure. He had killed 6, just leaving them where he caught them. He was still chasing them when we pulled into the driveway, so we were just minutes too late getting home. We rounded up terrified, hiding chickens, and got them closed in for the night.
I was very angry at DH's dog, and insisted that he kill it, or find it a new home, or something, but the dog couldn't live at my house anymore. DH half-heartedly tried to find his dog a new home, but ultimately never did. He worked with his dog quite a bit, and now the dog is a good protector of the birds when he's loose in the yard with them. We, unfortunately, do not live far from neighbors, even though we're out in the boonies. Our dogs spend most of their day in the house, but when they're not right with us they are tethered. We have moved to a new home with no yard, and have yet to make a run for the dogs. A chicken coop came first...
We had a lone little duck who was not accepted by the geese, so she hung out with my hubby's dog. They'd nap together in the shade, and spend most of the day together. The geese and chickens eat out of the dogs' dishes, and he's very mellow with them. He is excited at the brooder full of new chicks, and loves to sit with his big, fat nose in the box watching the babies. He mothers orphan kittens, and is an excellent all-around protector dog.
Best of luck to you and Lucy! I hope she will take the new ducks under her "wing" and take care of them like Rex does.