Remember a heat index is how it feels for a human who sweats and is cooled by evaporation, Chickens don't sweat, a heat index is meaningless to them. They will ventilate to cool off. If you see a chicken on a really hot day they will be breathing very fast to cool off, and will be holding out their wings for shade. They do very well in actual 100 degree days. They will crawl under things for shade, or dig themselves into the dirt. I've had chickens my whole life and never have lost one due to heat. I have seen them walk into a water puddle, but never lie down in one. I always laugh when I go to the fair and see people who have fans blowing on their poultry. Fans only provide relief for things that perspire.
