We have a 4 1/2 yr old border collie who pays no attention at all to our parttime free range flock. I would echo what a lot of folks have said already, that you just need to take it slow and give the dog a long probabtion before trusting him alone. There are so many knowledgeable BYC dog people who give great detailed advice!!
Here is my experience... we got chicks for the first time last spring when our dog Maverick was 3 1/2 and weren't sure what to expect.
I was alarmed at how interested in the chicks he was. He would snap at them sometimes when we had them out of the brooder in a little "pet fence" designed for rabbits. When he honed in on them, I verbally discliplined him and re-directed him by throwing one of his toys. He has a very strong herd/prey drive that we had channeled in to ball fetching when he was a pup. His interest was less intense when they were outside but he would sit outside their run and watch them. Later I let them out around him supervised and he pretty much ignored them.
I think partly it was the fact that he is just more interested in balls, but also I kind of suspect that part of it was they got less interesting as they got bigger. Picture a hoppy litle chick that peeps-- a bit like a fuzzy squeaky ball, right?
So anyway, he now ignores them completely and we leave him outside with them all the time and have no qualms. In fact a couple weeks ago a hawk got one of my Buff Orps while he was outside with them! I think he was hanging out on the other side of the house, though, but I can't say for sure. I drove into the driveway, the hawk flew off from the chicken carcass, and the dog came around the corner wagging hello. I was like, what kind of operation are you running here, bud???