My husband had that problem before when they had a German Shepherd who chases the chickens in their yard. What he did was to put the dog in a cage and get hold of a chicken. Placing the chicken near the cage and talking as if introducing it to the dog would give the dog some familiarization. Do that for 1 week and you can check if it is effective.

At first, the dog would react negatively but after some days, the dog's reaction should be milder until it ignores the chicken.

As Corzhens stated, chances are the dog goes after chickens is because it's not familiar with them or thinks of them as prey/a threat. If you show the dog that a chicken isn't food/something that needs to die, then in time it will come to see the chicken as something else. It's kind of like when you have to introduce cats and dogs that haven't seen each other before. There's going to be some hair-raising moments, but tolerance can be achieved. Sometimes there is an ingrained instinct though that may not be overridden so easily.

Another methods might be to set up a run that you could place near the chickens when they're out free and let your pooch get used to them that way. Interact with the chickens, but leave the dog alone. This worked for my dog when it came to the wild turkeys that would roam our backyard. Eventually he just got bored of trying to get free and attack them. Now the turkeys waddle up to the dog house and peck at it. He won't even react.

The fact remains that at bottom, dogs are carnivores and this orientation will always appeal to their base instincts. I have observed that dogs can be trained to protect other animals like some tribesmen in various parts of the world do to protect their animals. Dogs can also be deterred from attacking chicken through restraint by harnessing them in an area barely within reach of chicken and then gently reproaching it with a bristle to teach that its wrong to do that. One can also design positive reinforcement experiments based on those of Pavlov, the Russian scientist to train them on coexistence with other animals.

To avoid that, locals here use a fence to separate the chicken from the dogs and curb any of the curiosity of the dog to go near them. If they are not deterred form the act of killing chicken they will do it over and over again. If they are shown that chicken are not to be touched then eventually they lose interest.

Free range chicken and chicken that roam at a yard unchecked can be pray to even stray dogs that come near.

Your dogs probably see them as games and that why they hunt the chickens. Poor chickens, I feel sorry for them. You should have a fence made to separate and prevent your dogs from attacking the chickens, otherwise your owner is going to sue you one day he's had enough. 

Every dog can chase chickens wherever they see them. I have seen it as their own method of games but what I am against is the dog killing the chicken. It's better to keep the dogs away from the chicken to avoid such stories.

Some dogs breed can be aggressive when it comes attacking other smaller animals. Pitbulls hunt cats as games which can be worrying. My neighbour's pitbull killed his cats last week.