Great Pyreneese (GP) appear to be standard for guarding poultry although they were not bred for purpose. Most livestock breeding dogs developed to stay out 24/7 year round with four-legged livestock that are often mobile requiring livestock guarding dogs to be imprinted, in part so dogs will stay with flock.
Most poultry flocks not mobile, as they have ranges anchored to a central roost. Imprinting not as important for poultry guarder but dog ideally not a roamer. GP's and thier like can do this easily although so can other breeds that are suitable for guarding the home. My facility is relatively small and not by choice I have had to use dog breeds not developed for livestock guarding. My guarders were infact dual purpose, most hunting (black and tan hounds) but also had dalmations and border collie that guarded. Dogs had to be given outlet for bred in activities so when on guard duty they are relaxed. Smaller dogs are cheaper in terms of up keep and for us could go from point to point faster when fences present. Speed sometimes important when evicting predator. Any dog with proper mentality and 45 pounds or larger can repell even a large racoon or fox.
Larger dogs, often in pairs or greater, needed for repelling coyotes or domestic dogs. Coyotes can be pretty smart and as a group can still get birds if just one dog involved, especially if birds spread out.
I am setting up to use a pair of German shorthair pointers now (male just acquired as pup). Female will be brought in a pup 2 years later. It will be three years before I have good coyote protection but by spring racoons will not be a concern. Imprinting on chickens well underway, despite breed being used to hunt birds. Pointer was not my first choice but studying up on breed has indicated they are very mobile which is desirable when foxes will be doing snatch and grab. Being a pointer will also keep some sight oriented attention on birds of prey. Biggest concern I have with this breed is that it is prone to roam.