Keep them out of direct sun without obstructing air flow so they can dump heat by spreading wings. It is even better if you can keep so roof of where they are housed is either shiny / white or not hit directly by sunlight. Keep them in water so the can replace water lost through evaporative cooling / panting. Allowing them to have contact with cool ground enables dumping heat through feet like a dog does. Adding water to soil can help keep soil cool. Ideally soil in shade as well.
^^^ excellent advice. Along the west wall is also important, at least for me, in SD, the late afternoon is when it gets the hottest, and the sun is lower in the sky. I use a lattice panel along the west wall, gives a bit of shade, still lets the wind through.
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Deep all day shade is best, with good air flow.
Misters are great, if you're in an arid climate.
Lots of cool water, expect runnier poops as the fluids go right thru them to shed heat.
I put down large shallow pans(washing machine pans) of ice cubes in the late afternoon, they'll walk in them lay down in them, and sip the water as the ice melts.
I also put a box fan in the east windows of coop, which is shaded most the day, to help cool the coop down in late afternoon.
If we are going to have a long run of very hot days, I will put out a dose of chicksaver electrolytes. Just a half gallons worth, so everyone gets some, every 2-3 days. Had to use that stuff on a heat stroked hen one summer, with a soaked Qtip on the side if her beak, so gave some to the rest of the birds too....it really seemed to help.