Dogs are programmed this way-- it is up to the owners/masters to ensure the dogs do not feed their "wild" side, but strongly reinforce the domesticated side. If your dog is a fear biter, you need to be between the dog and anything it may want to bite, and aggressively ensure that this negative behavior is stopped.
And for you guys that think its ok to let your dog’s run free it is not; there is nothing worse than having a strange pack of dogs running through your property feeding their "wild" side.
With respect to the chickens, SSS! If you feel really guilty go buy them some new chicks.
I’m a lifelong, large breed, dog owner. My 2 cents worth.