I mentioned my two Shelties in an earlier post. Gracie, now nearly 11, was about 6 months old when we got adult hens. I think she was a bit intimidated by them. She just never bothered them. She always loved their feed, however, and also the scratch we tossed out twice daily. She might think she is a chicken. When she was younger she played a game we called "Bowling for Chickens." While the birds were bunched up over their scratch, she would barrel through the middle of them, sending them scattering in every direction, squawking and flapping while she ran off laughing her fool head off. Then they would all settle down, Gracie included, to share the scratch. It was obviously a game - to her, anyway! Like when your little brother jumps out of a closet and yells "BOO!"
We just got Sammy a couple of years ago, at the age of 18 months. To acclimate him to the hens, I put him in the grow-out pen* next to the chickens for at least an hour, sometimes several hours, every day for about a month. Then one day after his session I put him on leash and walked him around in the very large run with the chickens. I tied him to a post and chased the chickens to see what he would do if they flapped. He looked at me like I'd lost my last marble.
I unhooked him and let him wander around with the chickens while dragging the 15-foot leash. If he had chased, I could have stopped him by stepping on it. He stayed close to me and showed no interest in the chickens. But this was not a training exercise, it was simply a test. Which he passed with flying colors!
The next day I turned the chickens out to free range and let him drag the leash again while watching from the window. He again showed no reaction to the chickens. So the third day I just turned him loose with the chickens and watched. Again, nothing. And, other than following them around to "clean up" after them, he still has no interest in them.
I recommend this "see-no-touch" method of intoducing your dog to your chickens if at all possible. Take yourself out of the equation. If you are always there to tell your dog NO or Leave It!, then he only learns to leave the chickens alone when you are there! If he spends hours unattended in a pen next to them but can't get to them, then, IN MY EXPERIENCE, he will get bored and learn to ignore them. He will lose interest and nap right next to them. That's the response you want.
If you are there with him, holding the chicks under his nose, focusing his attention on them while at the same time trying to teach him to IGNORE them, you are doing a self-defeating exercise.
Imagine staring at a bowl if ice cream right in front if you, while constantly telling yourself you can't have it. Now imagine that same ice cream in a freezer in another room while you play a video game or chat with a friend. Which is easier to ignore? For the dog, the chickens are present, but unavailable, like the ice cream in the freezer. His urge, his desire, to have them, fades away over time. They become just another part of the scenery.
Good luck!
*The "grow-out" pen is variously a bachelor pen, an infirmary, or whatever the need calls for at any given time.