The thing i'd recommend for retraining a dog is to learn about pack behavior. If you can make your dog believe you are the alpha and those chickens belong to you, he'll view them as off limits. Making hom view you as the alpha is yhe easy part; every day, take him on a leash walk and make him...
This has been my experience as well.
We were fortunate to get great pyr puppy from a working mom, which I would recommend if possible.
The only thing on, with great pyr, you need really good fences: they are bred to roam big territories.