what i did to break my animals from messing with the chickens was to put them in a cage..wrap chicken wire around it as a sort of shield..then i went back inside by a window and waited. as soon as they would approach the cage, i would run outside, banging on pots and pans and yelling no at them until they realized that i was going to do that every time they even thought about going near them. im sure my neighbors thought i was nuts but it worked great!
I'm lucky that my 2 dobermans are very good with the birds....but that was not always the case. It took one of the chickens to teach the problem dobe a lesson. It was a silky hen actually, that had a chick with her. When the dog showed too much interest in her, she turned and flew right up in the dog's face and gave her the scare of her life! Now she won't even look at them
LOL! DObermans by nature want to protect what is theirs or their family, and as long as they view the chickens as something to be protected and not eaten, we all get along. I know that most agree that positive reinforcement is best for training, but maybe your dog just needs a little negative experience with the birds to learn a lesson.