This is basic instinctual canine behavior... And unless he's been taught from a young age to protect instead of eat chickens, there's no reason why he shouldn't be attacking them. In other words, at a basic, fundamental level, this dog is doing nothing wrong. I think most pet owners would be shocked and horrified at what their lovable pooch is capable of if let loose in a yard with chickens... But that having been said, of course this is not a desirable behavior, and you desire to control it is certainly understandable... I just want you to understand that this is not a "bad" dog, but a dog without proper training and restraint.
I know you've asked this question before, and I refrained from responding to it because there is no easy answer, and with so little information, it's hard to give really useful advice. For the sake of giving you something to work with, I'm going to assume that the dog in question is has not in the past been acting as guardian to the chickens and has suddenly turned on them (this would be a whole different problem).. But that he has recently been given the opportunity to go after chickens, and did so with exuberant glee! It is possible to reprogram any dog to not give in to it's base instincts... But this is one of the most difficult to overcome if not started in very early puppy hood. What it basically comes down to, is you being the absolute dominant presence in your dogs consciousness (top dog).. And learning how to use that position of authority to exert absolute control over your dog in these situations. Unfortunately, this is something that you probably need a mentor to help you through... It's learning how to read a dog, and in essence be a dog, so that you and your dog are speaking the same language. If you aren't familiar with the series of Dog Whisperer books by Cesar Millan, that's where you need to start.. And if you can get a hold of the Dog Whisperer t.v. Episode entitled, "Dog Whisperer: Bella & Big Boy and Gracie", you'll be able to see him address this situation directly.. Which could prove invaluable.