Yesterday Out 2 1/2 Year Old Barred Plymouth Rock Hen

Yesterday out 2 1/2 year old Barred Plymouth Rock hen sat on an outside roost all day not moving. She didn't come for her food or respond in any way. She was puffed up and had here head buried in her back neck feathers. She has always been a puppy dog concerning personality and human bond with us. Late evening her eyes were blank and glazed. She had no injuries that we could find. We knew she'd be gone in the morning. This morning she was up and scratching around like normal. She even went out with the others to free range today and ate her regular feed and treat of meal worms with crushed egg shells. Any ideas?