I was given a Great Pyrenees because she was left unsupervised on 3 acres with lots of free range chickens. Yup, they made great toys and she "played" with them to death. I don't know what those people did to her, but she absolutely HATES chickens now. Mine are always cooped and the few times one or more has got out the door, my GP is right on them. I have tried various things, but I got her after the damage was done and so far, nothing has worked. Otherwise she is a wonderful dog. You have your work cut out for you. Maybe you can kennel your dog while the chickens are out and vise-versa, until you can impress on her that the chickens belong to you and she is not to "mouth" them. There are lots of threads on here about training a dog to not chase chickens by some very knowledgeable people, do a search and study. I wish you the best with your dog.