I would avoid purchasing dogs for dealing with a single issue (such as rats) when the flock size is typical of backyards. Dogs will prove more expensive than other options, especially once the rats are no longer an issue. Secondly, the activity of dogs going after rats may cause damage to grounds where ratting is taking place. As a youth I remember neighbors operating a feed lot with a couple hundred head beef where they used a feeding system using silage and grain-based feed. The cattle had access to a barn with water and feed mangers. The rats created a burrow system under barn, silo's and a couple acres serving as barnyard where implements and vehicles were stored. What the opted to do was purchase a couple young adult rat terriers that promptly went to work hunting and killing the rats. The dogs were not helped nor did they have an easy to dig location like the situations linked in videos earlier in thread. The dogs had to do a lot more digging through harder ground. They also caught a lot of rats scurrying above ground. The digging part got to the point that the ground became treacherous for the cattle to walk over in the barn. Some of the dog digging was hard to discern from that of the rats when it comes to the damage done to stones serving a the barns foundation. The barn was of the very large wooden variety where structure was made of locally acquired materials.
It may prove more feasible to make so rats cannot get to chickens. Some selective trapping of persistent rats trying to get into where chickens are housed can also be employed. I am not an advocate of poisons owing to risks / costs it has posed for our dogs and other animals in the past. Shooting rats with a BB gun or light rifle can be entertainment for some, but those days are behind me at least.
It may prove more feasible to make so rats cannot get to chickens. Some selective trapping of persistent rats trying to get into where chickens are housed can also be employed. I am not an advocate of poisons owing to risks / costs it has posed for our dogs and other animals in the past. Shooting rats with a BB gun or light rifle can be entertainment for some, but those days are behind me at least.