We have a Great Pyr/Anatolian/Akbash mix and he is wonderful. He is very laid back and is gentle with all of our critters. He patrols our 25 acres during the day and we bring him in at night. He is an excellent flock guardian and is serious about his job. In fact, last week we found out, sadly, just how good of a job our dog does. Someone had dumped a dog on our country road. We could not catch the dog and the dog would not leave. To make a long story short, we kept our dog in the house for four days so he would not follow the dumped dog out into the road and get hit. With Jeb, our dog, out of commission a coyote snatched our 6-year-old daughter's Bourbon Red hen, Juliet, as she nested in the horseradish in the middle of the day. It was devastating to all of us, but especially our daughter. She loved that turkey. Anyway, my point is a good flock guardian is invaluable and ours is back on the job.
My 2-cents here would be: training over breed. We have a rescue, mixed-breed dog and he was about 6 years old when we first brought chicks home. When the chicks started hopping out of the brooder box (coop was still under construction), he would do anything to get a mouthful of those interesting, little, squeaky toys. He did manage to get a chicken slobbery once, amazingly without any damage done. When the chickens finally moved outside for good, poor doggy had to give up his free run of the yard. If the chickens were out, he'd stay on leash until his "business" was done and he'd go back into the house. That was the routine for quite some time and I finally decided to see how the animals would manage if the dog were let loose in the yard under strict supervision. It was a little awkward, but overall a success. Now, while he's still a housedog, he gets to run around outside again and he is sooo good about leaving the birds alone. The birds tolerate him too and rarely move out of his way. I still keep an eye on things though, since it's been less than a year. So, young or old, All-American breed or purebred, if it's a dog that respects YOU, you should have an easy time teaching him/her that the chickens belong to YOU and YOU ALONE. He's got his own squeaky toys.
I rescue dogs for a hobby from the humane society so have the uh, opportunity....ahem....to see a lot of different new dogs interacting with poultry.
Overall they tend to fall into two major groups, those that are either more interested in something else or not interested and those that will never be good with them. While usually it is rather predictable by breed, I was a little surprised when our golden retriever (bird dog) who will chase down and kill a rat and return it like a chew toy became one of my best dogs with the poultry and the one I would depend on to defend them should he happen to be around when Mr. Coyote comes to town. However I agree a dog is not a match for a coyote or group of coyotes. They can trick dogs into thinking they are playful and then attack en masse. So I only let my dogs out at night in groups and of course the chickens are in their coop by then. I would generally not trust dogs in groups around any prey.
My border collie is also excellent with poultry but I'm not sure she would defend them and she certainly does not care enough about them to stay by them. I have not had the opportunity to foster dogs that had been trained as guardians or were that breed except one anatolian shepherd and he was indeed great with the livestock, but was already very people-oriented and not that interested in the birds. I think starting with a puppy of the correct breed would be the way to get one that has a real interest in staying with the animals.
I agree training helps but you have to start with a dog that hasn't already eaten one and doesn't have a high prey drive if you are working with one you already have. I do have one dog with a high prey drive (pit/boxer) who after one year of being on the other side of the fence from the poultry proved herself to be trustworthy. They were just boring I think after all this time and she got tired of me telling her not to follow them along the fence, and she is a good dog. So training took one year or more in her case, so I suggest you have some good fencing in place. I also have a karelian bear dog that guards the pen, but would kill and eat them herself if they fly out. She is actually the one that keeps coyotes away at my house. That dog is just plain less trainable and I would never ever trust her around livestock no matter how many years of training I might try to put in.
So what works at my house is a good fence. I don't let my untrustworthy dogs that are here "free range" at the same time as my chickens and have a yard for each.
I have a yellow Lab, Drake. My chickens run in a very large wood fenced backyard in the day, where Drake lives. He has been around chickens most of his life. Because it is so hot in Mississippi now, I placed a small fan under the covered patio where Drake sleeps. I've found "Betty" sitting in front of the fan with Drake. I think the nature of most dogs would be protective or atleast alarm you if something is not suppose to be with chickens.
I agree with Wolfwoman! We have 2 mixed breeds, one is a herder - the other retriever. Both keep predators at bay (whether they know it or not). We have 2 acres and our 4 chickens free range with the dogs outside during the day and we haven't lost any of the chickens. Our retriever mix has treed several raccoons and our herder just hangs out and watches them, barking every once-in-awhile when they get on the deck....
I will mention, we have 2 other species of fowl (pair of Lady Amherst Pheasants and a Japanese Coturnix Quail) that our retriever is much more inclined to want to chase, but is NOT allowed to be in contact with (they are protected in their pen) - otherwise all canine and fowl live in harmony.
Most of the breeds I have seen recommended in this thread are going to be good at the job you have for them. But I have also read other posts where many of those same breeds, after a period of time, snapped and ate a few birds. I think the best thing is to have chickens already grown and to raise the LGDs from just weaned puppies with the chickens/other poultry they are destined to guard. That way they will view the poultry as fellow pack members. We know for certain, from genetic research, that all dogs are bred down from wolves. 95% of their brain is still wolf--they still think in pack terms (no matter how much you anthropomorphize.)
When I was a dog trainer, I learned that it is important to teach them based on the way THEY think, NOT the way WE think. Treating them like furry four-legged people just confuses them.
Since everyone else has suggested a breed, I will throw my two cents worth in as well. I highly recommend the English Shepherd. Very intelligent (compared to many others,) and quite versatile. You can have one breed that is easily trained as guardian and herder simultaneously. As with all breeds, proper training is required.