I don't know what happened, but I went out tonight to make sure my chicks were snug in their coop (as I do every night) and I noticed that Rosie was apart from the others and laying at an odd angle. I thought she had already died, but when I picked her up I saw she was barely breathing. Took her inside under the heat lamp and she was slightly convulsing and laying at an odd angle. No outward signs of injury or illness. Earlier today, I was out with them as they ran around the yard and she was fine. She'd come up to me twice and let me pick up and pet her. She didn't appear to be sick in any way at all. My other 3 chicks seem to be fine. Rosie took her last breath as my husband and I watched on. I love all my chicks, but Rosie was probably my favorite. She was so gentle and sweet.