I have Anatolian Shepherds, and have had them for years. This is the breed of dog that works best for me. The dogs referred to as lgds or livestock guardian dog breeds have been selectively bred for hundreds of years to help shepherds guard the flocks. They laze around during the day, seemingly unaware, until a percieved threat appears. They will bark warnings, then get as aggressive as they need to. My dogs had to be introduced to new pets or animals, but accepted them into "their" flock. They guard their territory more than specific animals. When the dog barks an alarm, the goats all run toward the dog. The chickens are pretty oblivious. At night, the dogs are more on guard because that's when most of the predators are active. The dogs even bark at hawks or turkey vultures that happen to come too low. They are protective of what is "theirs". My dogs, as puppies, behave more like adult dogs of other breeds. They are very laid back. The adults come up to greet you then go back to what they were doing. They don't jump all over and constantly seek attention.
While many dogs can be trained to protect livestock and there is always exceptions to rules, it is hard to go against instinct. Labs have been selectively bred to retrieve birds and waterfowl. Border collies and blue heelers have been bred to herd livestock. Terriers and dachshunds have been bred to kill vermin. These dogs could possibly make good poultry guardians, but might not. I had labs and always lost poultry to them their first year. When they matured, they left them alone (they didn't protect them either).
If I wanted a poultry guard dog that I had to train, I would start with a breed that has natural instincts for the job. Anatolian Shepherd, Great Pyrenees, Akbash, Maremma, Kuvasz, Komondor are just a few. There are more and more of these dogs in shelters, so if you wanted to help rescue, you could still try for these breeds. I don't have much experience with the other breeds, but have contact with breeders and owners. Pyrs tend to roam a little more than others. While you should have good fencing for any animals, they like to add to their territory. Dogs bred for specific jobs like to work. If you get rid of your flock or herd, your lgd might go in search of another.