I agree with the previous poster and think diligence is the key. We bought our first adult chickens when my very well behaved dog, Boone, was about a year old. Of course, he was curious so every single time the chickens were free-ranging and Boone went near them, we very firmly expressed our displeasure. When we bought chicks, we introduced him to them and once again made sure he understood that they were off limits. I love my dog and he's a good boy, but dogs have powerful instincts, and I while I let him meander with the adults, he is never allowed to be with the chicks unsupervised.
When our game rooster decided to get mean, he started attacking our dog and you would not believe the restraint that Boone showed. He very easily could of killed him but instead he ran away and came right over to my side and sat down. I could be wrong, but I think my dog understands the situation enough that he is waiting for permission to take action. Of course, my preferred course of action is for him to leave them be.
Sorry to hear about your momma hen.
Good luck with your puppy!
Oh, another thing I should mention, we feed our chickens table scraps and treats, so Boone has now figured out that chickens = food. I've caught him rooting around in their leftovers, so from now on chickens only get scraps in their coop to discourage any sort of confrontations. Hope this helps.