This morning when I opened the coop to let my chickens out, one of them was dead. She was laying on her side on the drop board under the roost and looks like she just fell asleep. I can't find one thing wrong with her. She doesn't have a mark on her anywhere. She had been dead a few hours. She was fine yesterday and has shown no signs of illness at all. All the other chickens (8 total) are acting perfectly fine. She was just a year old. Any ideas?