Yesterday I let out my 18 hens and 2 roosters. Everyone came running out of the coop into their "play area" with their usual enthusiasm. I went back about 30 minutes later to give them their treats and one of my girls (light brahma) was laying dead in the pen. Everyone else was fine. She had no marks on her body and looked like she just laid down and died. I have not had any problems with them in the past (all about 1 year old). Nothing had been done any different than usual. Everyone is still fine today. Any guesses on what happened to her? Do chickens have heart attacks??