Well, if he isn't killing them...perhaps he's trying to care for them or hurd them.
Nevertheless, it sounds as though he's confused and thinks you or his pack leader desires this behaviour.
If you've ever watched Cesar Milan....what he does is take the dog to the chickens.
Have him sit or stand with you and only focus on you....not the chickens.
If he looks at the chickens, surprise him with a gentle poke under the ribs....or a "pssssst", whatever it takes to break his attention so that he looks back to you.
Keep doing this until he is no longer interested in looking at the chickens.
Cesar sometimes holds the chickens above the dog to make the dog submit to the chicken....lie down and look away.
Cesar can make these things happen pretty quick, I think the average everyday dog owner needs to ensure he's the boss first....not the dog.

You might want to consider, these chicks may be looking to get picked up............

Surfcrest