We have a small backyard flock of ten chickens. They free range during the day and were all fine when we put them back into the coop two nights ago. The next morning, our four year old Cuckoo Maran hen was dead on the floor of the coop. I examined her everywhere and couldn't find any signs of injury. She was fat and healthy. No feathers missing or blood, no discharge from her eyes or mouth, nothing unusual about her vent. She was stiff except for her neck, which flopped over like it had been broken. Their coop is completely secure and all the other hens were fine--there's no way anything got in there overnight. Their roosts are only about 3 feet off the ground. We have a border collie who has been their protector for four years, helping us raise each new batch from chicks. He gets a little rough with them sometimes and pins them to the ground with his paw or his mouth. He's never injured one. Never drawn blood, never pulled out feathers. My biggest worry is that he somehow hurt this hen and she was able to walk to the coop normally, but died from her injury. Sorry this message is so long. But could a hen break her neck inside a small coop? Or did she just have a heart attack or something else sudden, and her neck was the only part that didn't show rigor mortis? I really want to believe my dog didn't hurt her.