I had a Newfoundland with zero hunting skills who did this exact thing, he'd "pin them," I found him laying down with his monster arms on the spread out chicken wings, both of them frozen as if he was defying the chicken to move. It has to be that prey drive in most dogs just got the best of him with chicken movement. Since then I found that with all of my dogs it's a movement thing. My Leonberger (similar to shepherd,) isn't interested in chickens until they run. Something about them running totally triggers her to go after them. Anyhow.... That "pinned" chicken died overnight from shock. So I would not allow my Newfie access to the chickens anymore and I started "chicken training," with him and every dog after him. On leash, BE NICE TO CHICKENS, let him sniff, rub his back with side and back with chicken, done. Eventually, chicken is on ground, let him sniff, if he takes a step, STAY and LEAVE IT. Takes as long as it takes as every breed is different. Shepherds are so smart, you'll have him trained in no time! I love Sheppies!!!