I try to watch my flock (40 plus chickens) closely but this one got right past me. Yesterday morning (every morning) I went out and looked around at all the chickens for signs of injury, illness, looked for eggs on the coop floor, etc. I do this everyday. I,m sort of a nut with my camera and took this photo of one of my Red Stars. I know that they are relatively short lived hens and mine are all 3 years +. It's been over 100 degrees here, hotter every day and today it was 107. I go out 3 times a day minimum and spray cold water in the shade for them to lay in and cool off. I thought they were doing pretty well, but I found his exact girl dead at 7pm right next to the water. I honestly didn't see any issues with her. She ran to get treats, was eating, drinking and the only thing I can think of is the heat must have gotten her. She didn't have a name, but rest in peace little lady.
