I'm by all means not an expert on this subject as I don't have chickens but here is what I would do.
I would have the chickens in their pen and take your dog as far as you have to away , where he is most calm but can still see the chickens. C/T for him staying calm in a sit. You could do some training right there so that he is focusing on you and not the chickens. Gradually progress closer but ONLY if he stays calm. C/T for him staying calm when he looks at the chickens. Work on him being able to look at the chickens but staying calm. Once you are close , 5 feet or so, you could start letting a few chickens out of the pen to roam and see if you can keep him calm? If he stays calm reward him and throw a ball or something for him to chase... instead of the chicken? This is a long shot guess but maybe it could work? Point is to take his focus off the chicken and reward him with something more fun.
Check out the book "Control Unleashed" It is a great book and can help you with your dogs reactive behavior towards the chickens. The auther also has a yahoo group you can join and get advise directly from her. Just go to yahoo groups and search CU dogs.
Good luck! Hope that helps?