I posted a question several days ago regarding the sudden death of my large Black Austrolorp. She had no sign of illness at all, very active and even laid her egg on schedule the day she died. In talking to some people who have been around the poultry business for years, they said in those very large birds, a heart attack can occur. When I explained that she was lying flat on her back, I was told that makes it even more likely. We were having terrible lightning and thunder and the animals have been quite agitated. I don't know if this is what occured but, at least it is a possible answer. It is a little easier to accept an answer like that when you lose what appears to be a very healthy bird suddenly.