At some point she/he'll discover that they look like fun to play with. When that happens, any time the puppy goes for a bird grab it by the nape and sharply pin it to the ground while saying "no". Also teach it to be gentle and not to bite. My big girl comes to the coop with me at lock-up to say "good night". I let her "handle" the chickens but I stop her when she tries to grab one and pull it off the roost. I give her "babies" once in a while, too. She likes to take them to the couch and give them a thorough bathing. These exercises I use for teaching "gentle" and "no bite". Of course, trying to tell the difference between a bite and a gentle mouth grab is a bit tricky. They try to escape so, naturally, she has to pick them up and put them back in position.
Guarding should come naturally. Will a shepherd fight to the death to save one of its chickens? Probably not. For that level of dedication you would need to look at one of the true LGD breeds. If all you're after is keeping the smaller predators away and an alarm when a visitor comes calling, I'm sure the shepherd will perform fantastically.