Positive reinforcement
It has been proven to work and have a lasting effect over punishment.
First start off a good distance away from the chickens with the dog leashed. Each time he looks at the chickens calmly, treat him. It is probably good
to use a clicker with this so you can mark the exact behavior with reward. Look up "charging the clicker" to get started.
Work up to moving closer to the chickens and reward calm behavior. If he goes to move towards them or any unwanted behavior, take him away immediately.
What you want is for his mind to associate chickens with reward. Eventually you should be able to have him laying next to the chickens and looking up at you for
a treat. You can even incorporate a ball or special game he likes. Each time he is calm around the chickens, play the game or throw the ball for him to chase. This is redirect and it works well for your working type dogs. They quickly associate the chickens with fun!
It will take a lot of time and effort...but in the end it will be worth it.
Unfortunately, punishment based training really only teaches the dog not to warn you of the bad behavior, hence avoiding the bad feeling. For instance, if you physically punish a dog for chasing a cat say, the dog will usually not display that behavior when you are within eye sight because they associate the punishment with you. But, once they cannot see you, game on. There isn't any motivation there for them not to do the behavior....and this is a must especially when trying to redirect instinct behavior.
Take yourself for instance.....say you were on a diet and your body was craving chocolate. If every time you went to eat a piece someone smacked you, you would figure out that if you didn't eat it you wouldn't get smacked. But, your body still craves that chocolate and eventually you would sneak it when no one was looking. Now, same instance, you were on a diet and craved chocolate but this time when you picked it up someone said, "You are beautiful in every way! Here, take this special diet chocolate, no calories and tastes exactly the same!" Much better, huh! No need to sneak around but you learned an alternative method.