I had a dilemma with my own three cats going after my chickens... I had the cats first, adult adept hunters. Because of this, I didn't even let the chicks out in a covered run until they were six weeks old. Despite the chicks size, the cats were way too interested in them for my comfort.
So I tried an old trick from childhood when my mom and I raised chickens. We would 'chickenize' the problem pet. This works well with young dogs and cats. I was unsure if it would be successful on an accomplished bird catcher.
Here's the method: one person holds the chick steady and safely at nose height to the dog or cat. Another person draws the pet up close enough to smell the chick while hanging on tightly to control the dog or cat. The INSTANT the pet smells the chick BELLOW, "BAD DOG-BAD DOG-BAD DOG!!" LOUDLY and continously and sharply rapp it on the nose or top of the head. Then be quiet and back off with the pet. Do not make any more noise. Wait a few moments for the encounter to settle into the pet's mind, then repeat. Twice is usually enough.
You are not trying to injure the pet, so the head whacks are just raps to startle them. Hopefully the pet concludes that little chick is a dangerous creature, indeed.
Test the pet after treatment to see if they got the message by approaching with the chick. A convinced pet will avert their face, duck, or back away. If they choose to be interested instead, reinforce the lesson by repeating, "BAD DOG!"
A few pets are more stubborn or stupid and require repeat 'treatments'. If your dog has killed a chicken, all bets are off. It might still work if the animal is young enough. This is why I was so worried about my cats. The biggest male has caught pigeons in the barn and grown rabbits from the fields. But the treatment worked!