Hi,
I have Rat Terriers and Chickens. You have to keep in mind that they are terriers so the possibility of them hurting or killing a chicken is there.
Having said that, I have 6 Rat Brats and they are in the barn and yard daily with the chickens. Once in a while one of them will scare a chicken into running then they want to chase. I am there with them and tell them to knock it off and they stop. The chase is the game and what they are after. Not so much to kill the bird. When they do chase a chicken and get it cornered they stand and bark at it...not attack it.
They do the same thing with the barn cats once in a while. Get one to run so they can chase it. My cats are onto them now and just ignore them. Once in a while I will catch them chasing one and yell at them.
The exception are Guineas. Maybe because they are more flighty. My dogs did kill a Guinea. I had 8 Guineas at the time. I caught them in the act and really read them the riot act. But I wouldn't trust them around Guineas.
Most of the time they will walk or sit or stand side by side with the chickens and as is well. But occasionally one will decide it would be a fun game to chase a chicken and the rest will follow suite. It always seems to be the same two dogs that get it started. Both females.
What I do with my pups is get them around an aggressive rooster and sooner or later they will get into the roo's space and he will go after them. The pups don't really get hurt by the roo, but it is a lesson they remember. After that they tend to stay a lot farther back from the chickens.
So, many many chickens free ranging for years with 6 Rat Terriers and occasional pups and one Guinea was killed. I was in the house on the phone, heard them barking and ignored it to finish my call. My fault.
You do need to be within ear shot so if you do hear the dog barking you can supervise. I think having 6 dogs out there at the same time also creates pack behavior and makes it more likley they they will get bored and start up a game of chase the chicken.
There favorite thing to hunt are field mice. And they are very good at it. It's interesting to watch as one will go into the center of a field area and start walking to scare up a mouse. The others wait on the outside perimeter and once a mouse is out and running the inside dog will chase it out towards the others. I never taught them how to do it. It's just how they hunt. I don't have rats here, so I have never seen them hunt them.
Hope this helps. Carrie