We used to have pair of Maremma dogs.
They were lovely, and although we didn't have chickens then, they took care of our goats, and also us.
I have no doubt they also would have been good with chickens, as where we got them from they were kept for that purpose.
One of them once killed a fox, and they also protected us a couple of times.
Now we have a shar pei x Australian Cattle dog, and mix that is anyone's guess. Both to be trusted with chickens, but they don't really protect them very much either. (although just having them keeps the foxes away)