"Playing" with the chicken is a self rewarding behavior - ie. the dog received positive reinforcement just by doing it (it was FUN!). That makes your job of teaching the dog to leave the chickens alone more difficult, obviously. I am going through this same thing with one of my Mastiffs, although she just chased and pinned, but didn't end up hurting the bird, because I was close enough to get her off.
There are
many ways to go about it, but I recommend using the "leave it/get it" commands. It is easier to teach with 2 people at first - one person holding a treat (something really tasty) a couple of feet in front of the dog, and the other controlling the dog. When the dog moves toward the food, say "leave it" , tug on the leash and if necessary the quickly cover the treat so he can't get it. Repeat and praise when he starts to figure it out. To keep from frustrating the dog, either give the dog a treat when does the exercise correctly or teach the "get it" command at the same time, allowing him to get a treat. Repeat until the dog responds to your "leave it" command while off leash, with no one close enough to cover the food.
Then repeat the process with the chickens. Start with the chickens locked in their pen, and the dog on leash. When the dog has a "soft eye" looking around at the chickens that is ok, but if he starts to "fixate" on one chicken, not moving his eye away and starting to tense in his body, say" leave it", then tug on the leash. If he obeys and looks away from the chicken, praise and treat. If he doesn't respond, check him, and if necessary back him up from the chickens until he is far enough away that he will obey. Work him there and move closer as you progress.
Repeat until he will respond to you when he is off leash and not next to you. Then, when you feel ready, I would recommend getting a Gentile Leader head collar for him, get him used to it, and have that on him when you first let the chickens out of the coop. With a head collar I have absolute control over a dog that out weighs me by 40 lbs. It is wonderful for controlling the head/face which is what you need to prevent an accident. Repeat the training process with the chickens loose. Remember to strongly reward him for leaving the chickens.
For the next step, letting the dog off leash with the chickens loose, an e-collar is great as you have control even when not next to the dog. Teach the dog the command first, then use the collar to reinforce it if the dog disobeys. Using the collar without any training, as a negative reinforcement for going near the chicken, may work with some dogs/breeds, but for my Mastiffs, it may make them want to get the chickens, because the chickens zapped them. I want them to know that the discipline came from me - that I disapprove of their behavior.
If you want faster results, use clicker training along with the leave it command. You are competing with the memory of how much fun it was to chase the chicken, and clicker training gets the animal investing in doing the right thing.
Also, using a trainer is great, but be sure to check into the trainer you plan on using. I have met many that are very bad at what they do, and a bad trainer can cause more issues than he/she resolves.
I hope this helps.