Chickens are birds, and it's actually pretty common for retrievers to target, chase and worry live birds. So common in fact that specific methods have been developed to 'break' that behavior for field dogs. Those methods would work just as well for any retriever even if it is not going to be used in the field.
The training does make use of ecollars, check lines, etc., and a lot of corrections. The corrections can be "hard" and some positive-training-only people will be aghast at that, but that's the way it is. If you want to stop it, you've got to apply some pretty intense counter-conditioning.
You can find these methods in any book or source on retriever training - as I said, it's a common problem behavior. I'm sure you can figure ways to adopt the methods so as to be specific for your penned chickens if that is what you want.
What Poly said, except to add that these hardwired drives are not equally intense in all sporting dogs. Dogs from show/pet stock may very well have the same drive, but it will likely be correctable with methods the average pet owner would be comfortable with. Though you may find you are comfortable with a good deal more after the process is complete, than you were before you began.
On the other end of the spectrum is the bird-crazy, field bred dog. This dog's brain goes off like a high-score-super-jackpot pinball machine when he sees birds running and flapping on the ground. If you have one of those, it will take some doing just to get his attention in the presence of poultry. Acute situational deafness must be overcome before any progress can be made.
There was a Dog Whisperer episode once where he rehabbed a dog who ate the family chickens (I know, please don't kill me for mentioning Caesar Milan). Anyway, he used a pretty generic counter-conditioning technique if I recall correctly, where he had the dog sit or lie down in a relaxed state. Then he brought a chicken closer and closer until he could stand it. By the end I think he was putting it on the dog's head (which I think is somewhat ridiculous) but it's a pretty standard training method.
Bring your dog as close (or far enough away) to the pen as he can be while remaining relaxed and not fixating on the chickens. Treat him and praise him a lot. Eventually move a little closer and reward his calm behavior. The goal would be to get him able to be near the pen without worrying about the chickens. He'll associate good things with not bothering them.
I don't have much experience with dogs with high prey drives, so obviously there may be more effective methods to use instead or in combination with this one. But it's always helpful to show the dog what behavior you DO want, as opposed to just being angry when they do something you don't want.