We just got a year old Boston, and she was really chasing the chickens. She actually ran by us and grabbed one. Luckily the bird was ok, and the dog we really yelled at and shook our finger at. The Boston's are so sensitive, they don't need a lot of redirection. She is so hyper we worried about her. She still tries to play too rough with the ferret, but the other day the chickens were in the kitchen (don't ask!) and she was standing next to me, not looking at them. I went in the other room, and had to walk past the chickens and heard her crying. She wouldn't walk past them! I called her and she ran as fast as she could right past them. Of course they were squawking at her and made her go even faster! LOL!
So, if yours if the typical Boston, I would think just letting him know that the chickens are ok and supposed to be there and that it makes you upset if he looks at them wrong, that he will probably ignore them. I wouldn't trust him completely, and I still don't trust mine all the way, but I am pretty sure that she won't attack them or snatch at them while I am standing there, which is the only time she is around them.
Good luck with your dog. The Boston's are such people pleasers you shouldn't have any trouble. I've had my dog about 6 wks and she is doing great. But, mine are older now, too. Not sure how she'd do with babies.
Shelly