I have been an avid hunter with my labs for years. I have hunted quail, pheasant, dove, ducks and geese.
My almost 5 year old Chocolate female, loves the chickens and will lay down and let them cuddle up with her or they will even go over and sit on her. I can let her outside with my free ranging birds and I never have a problem. If my ducks get in the pond and don't want to come back to the coop at night, I just send her into the water after them and she will retrieve them for me without even hurting a feather on their head. It's kinda funny to watch, and the ducks now just allow her to carry them, they have gotten so used to it.
Now, my yellow male was a different story. He had the same training as my chocolate and never hurt a bird while I was out hunting, however, one day I had both labs outside in their fenced in yard (my chickens couldn't get into the yard, 5ft fence and their wings had been clipped to avoid problems if any existed) and I went off to the local community college for my classes. Well, when I got home, it looked like a million feather pillows exploded on a murder scene.
My male lab had jumped the fence and slaughtered 22 of my chickens. Ripping them to shreds. There were feathers everywhere, pieces of chickens everywhere and the amount of blood was unreal. I came home to him mauling another chicken as I drove up the driveway. Needless to say, he got rehomed because I couldn't risk him doing this again. And now he couldn't be trusted not to tear apart a game bird and keep it for himself instead of bringing it to me. It was a very sad day
but I was also so mad
I guess my whole point to the story is that it will vary between dog to dog regardless of training. Your best bet is to take him out on a lead once the chicks are bigger or allow him near their pen to get used to them. You don't want to let him have free will. Be there with a lead to control him should anything go wrong. I also like the approach of putting them in a down position. It makes it less threatening to the chicks and also makes it harder for him to lunge at the babies. I hope this helps and good luck!