I have two rescue dogs, both mutts, and with varying degrees of training, both are now great with chickens and can be left alone with them without consequence.
My younger dog, a boxer x hound/pointer, was easy as pie. He's a submissive dog with a low prey drive and an incredibly soft mouth. We did get him as a pup, but he didn't meet any chickens until he was at least a year old. What I did with him was, I let him go into the room with me and sniff the brooder when I had chicks. I'd just ignore him (unless he looked a little
too interested) and do paperwork or something while he scoped out the chicks. He figured out pretty quick that chickens are part of the furniture. Once, when the birds were fully grown, he got out in the yard unsupervised (he learned how to open the back door), and the only sign that he showed any interest anyone was a particularly disgruntled buff orpington with some slime on her back. Since then, he's been thoroughly henpecked, and will let the hens chase him off the porch with his tail between his legs.
My older dog, a border collie x lab-chow, had more trouble because she has a higher prey drive, is a bit higher on the totem pole, and has big, sharp teeth. She is incredibly eager to please, though, which helped tremendously with her training. We got her as an adult, and she was probably six by the time the chickens came in. I knew she had a high prey drive, so I was very careful when introducing her to the birds. Unfortunately, she had a few 'accidents' and we lost a small handful of birds to her. It was heartbreaking, but the good news was she never ate one - she just played with them until they stopped squeaking. We told her to "leave it" whenever she saw a chicken and she learned to avoid them. She was eventually able to be left alone with the chickens and can stay with them for hours, unsupervised, without mishap. (We learned this on accident when our other dog opened the back door while we were at work. It took a couple times for us to realize that, not only could he open the door, he could also
unlock it.
)
I would recommend looking for a more submissive dog (or at least one that will let you earn its respect) with a lower prey drive. And make sure to take the time to train your dog - which is tricky when what he's supposed to ignore runs, squeaks and tastes like chicken. An all-around command like "Leave it" can be very helpful.
