Birds are very vulnerable at night. They have very poor night vision and their best defense is to stay still and quiet. I live in the mountains of Colorado where we have bears and mountain lions, coyotes, foxes, bobcats, raccoons, and weasles as well as hawks and owls. I've lost one pullet during daylight hours to a bobcat while my hens were out free ranging. I've never had a loss in my coop, which I built with a fully enclosed inner-run that is hardware cloth on the bottom and chicken wire on the top. The gates to the inner run are secured at night as well as the doors to the coop.
However I raise meat birds in the summer and have had several bad mornings, coming out to find several dead or missing birds. I believe they've all been lost to raccoons who reach through small openings, grab a sleeping bird, pull it towards the opening and start gnawing away on whatever part of the poor bird it can reach. One of my worst losses was four of my 15 dual purpose roosters, so don't count on roosters protection at night. I think they'll be just as vulnerable as a hen once the sun goes down. I've had to fortify every grow-out pen I've built. Next one will be like Fort Knox!
I purchased an Anatolian Shepherd puppy after the bobcat killed one of my pullets. He spends about 60 - 120 minutes a day outside either teathered or in his oversized run while the chickens are out free ranging. I haven't had any other day-time losses. However, in order to protect at night he'd need to be out there at night and I'd rather have hiim in guarding me.
BTW, whether you decide to purchase an Anatolian or a Great Pyr, remember that these dogs, while having great livestock guardian instincts still need considerable training and time to mature. I've seen so many ASDs, Pys and ASD/Pry crosses listed on rescue sites at 8 - 18 months of age with descriptions like "didn't work out as a livestock guardian" when the poor dogs were still puppies. These dogs don't fully mature until they are close to 4 years old and usually shouldn't be left alone with chickens until they are probably 2 years old and have been exposed and working with them all that time. Their puppy natures can lead them to "play" with the chickens, which doesn't always end well.
I went out free ranging every day with a baggie of chicken treats in one pocket and a bag of puppy treats in the other. I'd sit on a bench with a book and supervise free rangeing, calling the puppy and the chickens to me with treats. If the chickens ran or flapped or took a dust bath my puppy was allowed to investigate but not chase or pounce. At three years of age he is completely safe around chickens and turkeys, but he'd still might wander off and leave them if he was unconfined.
Good luck.
