I have read that 100 degrees is their limit. I keep a box fan running all summer, though my coop is large and very breezy. Mine sometimes stay in the coop during the heat of the day, for the shade, I think, but they have more outdoor shade this year so we'll see. We are too humid for a swamp cooler but you might be able to use one -- it is basically a fan blowing air through a rack of wet hay. When they start mouth breathing and holding their wings away from their bodies, they are suffering from the heat. I have actually bodily dunked them in a tub of water on awful days, I wet the soil, I shower water on them with the hose (which they hate) -- there are lots of ideas on here. I froze milk jugs of water for them to hang around, but they ignored them. For the most part mine survive on shade and lots of breeze. Every summer I read about people losing birds to the heat, though.