She is loose with them in your backyard? When a chicken runs it sets off a prey drive in the dog. They really can’t help it. Some can be trained not to but some will never be broken. I always do a temperament test and keep the the low prey drive dogs if possible. What about a crate?
No chickens in the back yard. Either the hen came under the fence, or Josie went under and got her and came back. She has a crate but she’s been in too much because we’ve been busy. Since we were home I was trying to leave her out. I didn’t trust DH to watch her well enough to stop an inside accident because it’s openening day of baseball. We had just finished a little training session and it had gone really well.

I know it happens, I just don’t really have any chickens to spare right now. This puts me down to 10 hens total, and 3 roosters. Then two brooders of littles, but I can’t count them yet.