No matter how much you trust him. Last Monday, I had to leave the Dog alone in the backyard with the chickens because I was going to be gone all day. I thought, "well, he never messes with them when I am around and the chickens are getting big enough." Nope. Bad IDEA! I came home to count 6 chickens, instead of 8. I looked and looked and finally found one cowering inside my fenced grapes a little ruffed up but fine. Then I found the last one trying desperately to hide in a corner. And the dog had scratched the skin off of her back. It was horrible. I couldn't think of how a chicken could live with out the skin on her back. I wrapped her into a towel, and then researched. Someone told me that their chicken lived with a hunk of meat taken out of it's back from an attack and eventually even grew back feathers. So I gave her a saline back rinse and covered her in triple antibiotic ointment. I then put her in a Rubbermaid container with a heat lamp and her own food and water. I actually fed her scrambled egg yokes and prayed. She ate and drank just fine that late evening. Then a few days ago, I noticed that she was cleaning herself and making light chicken noises. Then today, I found her perched on the top of the Rubbermaid container. She had a huge limp but I thought some sunshine would do her good. I took her out to the other girls outside.
Man, what a difference sunshine and friends make. Her limp was 90% better. She used her wing feathers to cover up her back so it was hardly noticeable. And she kept up with her fellow ladies all day long. AND they didn't peck at her at all. When it came time to come in for the evening, she went straight into the coop with the rest of them and hopped herself right up to the perched and settled. Here I thought for sure she would die and it seems she is making a full recovery. I couldn't be more giddy!
However, the dog is never allowed to be alone with them again. I am even going to pick up a chain soon for those times that we have to leave them both outside again.
So my advice, never trust the most trustable dog alone with chickens.