We have two dogs - although we use invisible fence and the chickens have learned exactly where that invisible line exists, there is still one large open common area. The chickens literally fly through that area. The scent of the dogs seems to keep all critters at bay.
We also have large Bald Eagles in our area, but they seem completely uninterested in the hens - maybe because of the dogs or the fact that it is easier going for small mammals and snakes than a mad 8lb hen.
Mother Henn