Well, I don't know very much about chickens, but I DO know a lot about dogs. German Shepards are very intelligent, for the most part, so you have that on your side. I see that a few people have recommended shock collars, this is your choice. However, here is a warning. The dog will most likely learn to "understand" the workings of it. He will know when he isn't wearing it, and eventually, he will be able to sense when the battery is low. I know this sounds unbelievable, but I have seen it happen. Also, I think that you would be teaching the dog that approaching the chickens = pain, startle, etc, and that may not be a good idea either.
If you caught your dog chewing on the couch, you would correct it, letting the dog know that the couch is YOURS, not his to chew on. Same thing with the chickens! If I were you, I would spend a lot of time in your yard, with the chickens out and at a safe distance. Dog on leash. When the dog gets used to them, (no staring, drooling, facing them, etc), reward the dog. Every session, get a little closer to the birds. Even with this, though I would not leave dogs alone in the yard while the birds are out, because like I said, you have a smart dog, and he might try something!
Just remember, it is easier to train a dog when he is tired. A long walk will realx the both of you, and make for an easier training session. I trained both my dogs to stay away from stray cats the same way, and it worked wonders!!! Good luck and be patient!!
YOU are the pack leader! The dogs need to know and respect that!