I have one Buff Orpington roo who is almost a year old, and 17 hens. I added him to the older flock with 9 other young hens sometime over the summer. Everyone seemed to get along just fine. There were previously 3 roos that we had to sell because they were too aggressive with each other and the hens, so the girls were used to having obnoxious men in the house. And the new Buff roo has always been a kinder gentler sort...or so we thought. I didn't actually see this happen, and my husband only barely saw it, but the roo was mating with the one hen that I always called Big Mama. She was from our first batch of hens, and always at the top of the pecking order among the girls. Husband ignored them until it seemed to go on longer and more roughly than he would expect. The hen was off to the side in the coop and flapping her wings but could not get up. Rooster went over, pecking at her, looking like he was trying to revive her (according to husband) but then started pecking at her even more viciously. When husband went over to separate them, hen was clearly dead. Husband thinks she had a heart attack caused by old age, or rooster broke her neck by accident. She had no obvious wounds other than some missing feathers around her back end. I say she was probably the strongest of the old girls, and I don't think 4 years is very old for a chicken, but I also don't think this roo has a mean streak in him. What do you all think? Is it common for chickens who have been living peacefully for months to try to change the pecking order and attack each other? They are normally free ranging, but have been literally cooped up because of the 3 feet of snow on the ground for the last 2 or 3 months. They have a large coop (an old milking shed), but have been trapped in there most of the winter because of the snow. Doors are open during the day, but they can't make it outside very far... Only in the past week and a half has the snow started to melt and they have been venturing outside again. Even more reason that I am surprised this would happen now, now that they have more freedom.