My two boxers aren't MEAN towards the chickens, but their preditor drive takes over from time to time (I can count on one hand) and they PLAY with the chickens... the problem is... the chicken doesn't know it's a game. They have never killed one but they did shove one INTO... yes, into... the pallet that is the base of the coop. That standard full grown hen fit in that pallet... 4 inches or so! Of course, I had to really work to get her out and she was plucked all down the center of her back, but the dog thought it was fun. We called her "bareback" for about a year until she finally got all her feathers back.
So I guess the answer to your question is... if it is a hunting or working breed... I wouldn't trust them ever to be alone. There are those that are so well trained that they can be around your chickens, but let's be real... those dogs are few and far between. Can't blame them for doing what they were bred to do for thousands of years, though.