it is not so much about the bathing when it is hot it is about laying around on cool ground. Otherwise it's like a swimming pool full of hot water in the sun(burn).
(1) Lots of shade. this is the primary necessity, trees do it the best but you can do it artificially read next bit.
(2) Passive cooling. If there is a coop with a regular roof it can become a furnace, so keep the radiation and heat away from the inside by ADDING an extra roof about 20cm (a foot) away from the old roof. It doesn't need to be waterproof or new, just needs to be shade. I use old roofing iron, really old, with holes galore in it. Radiation turns into hot air that shimmers up into the sky from the outside layer of tin. The layer is in no way continuous at all, air gaps are important so that the hot air escapes. The inside roof of the coop stays noticeably cooler, a LOT cooler. You can add an extra extra roof, but I find one is enough, and the same treatment for the sides that the sun hammers in the morning and afternoon. My chooks give the coop their seal of approval by preferring it as much as any other cold spot in the yard (they just find and hang out in the coolest areas naturally to escape the heat)
(3) If all else is going to fail, build them a bunker, dig a long hole, put whatever you can find to make a strong tunnel or cavity, then put the dirt on top. When it is a foot or more thick, they can escape the heat and cool down nicely. Similar to lightning ridge or coober pedy.
(4) Fast fix. put pallets on the roof of a flat coop. wooden pallets have shade, the parts that are most in the sun will turn the radiation into heated air and are spaced out from the roof by just the right amount to let air circulate. They won't last forever because they are wood, but they will do the job and it will work and lower the temperature by a great deal.
(5) These things all apply to houses and humans too.
