I've got a 4 yo anatolian mix shepard that's the same way, he won't even bark at people unless I tell him to, or a strangerr gets close to me. He loves people, especially children, but animals, forgettaboutit. As soon as he hit maturity all dogs and other animals must submit or it's a fight. He does "RELEASE" upon command and a tap on his haunches though. He's a good obedient boy.
I watched our dog around the chickens after many were killed, she would try and herd them back into the coop, (that is where they belonged in her mind). Once the rooster decided to stay outside and "defend" the hens, bad move. If he would have went into the coop I think our dog would have left him alone.
Our dog allows cats to sleep on her in her doghouse, she doesnn't randomly kill cats, just the ones that don't submit.
I've seen this in other dogs, my mother in law has a herd of buffalo that the St. Bernard doesn't want to see from the house. They were frustrated with the dog chasing the buffalo, I watched, the chasing wasn't random, it was only when the buffalo invaded his territory.