Our foster German Shepherd initially thought he would like to chase/kill/eat the totally free range chickens. He also thought it was ok to snarl at me for getting to close to DH. We worked with him on both issues (plus lots of others -he's a piece of work, this one) and he can now be loose around the chickens. He is very interested in their doings but understands he is not to chase them or show aggression towards them.
One of my Speckled Sussex just hatched 3 chicks, and while she was broody if the dog stuck his nose in the big door of the coop to see what I was doing in there, she'd go nuts and attack him, wings aflappin', pecking at his head, and squawking away. He looked confused and retreated from the door. However, about 15 minutes later I heard some chicken hollering outside the coop, looked out the window to see chickens headed past with a German Shepherd in pursuit. He was ticked that Little Chicken had the audacity to warn him away, and was going to take it out on someone! He was careful, though, to run just slow enough to not catch them. We discussed it and they are all back to their truce again. It's pretty funny if I am in the coop to see a German Shepherd head stick through the pophole, and a chicken inside merely look up and then go back to eating.
The roo will act as though he is going to chase the dog away if the dog so much as heads in his direction, even if it's clear he isn't planning to chase. I don't think they are afraid of Cutter at all, I think that, just as Cutter has learned that we think they are part of the "pack" the chickens have assigned him some sort of addendum status in the "flock". I don't see this as a good thing as I sometimes care for other dogs, and the chickens will run right towards a strange dog if I am holding the leash. I think they assume that a.) I ALWAYS have treats for them and b.) I will keep them safe. Neither is always going to be true!