If trees aren't an option, consider a shade sail, a sheet of plywood or anything else you can muster up. Anything but direct sun, please!
Drinking water is absolutely vital, too, and like us chickens drink a lot more when the weather is hot. They really can't survive without water: it's important to make this clear to chicken-minders when you go on holiday.
I like a hanging drinker with a bulge that stops falling in the water. If it's hanging it can't be knocked over, and if it's positioned high enough it won't have straw or dirt kicked into it. Try to keep it out of direct sun so it stays cool.
My favourite water tip is to add just a dash of cider vinegar to the water. I only started doing this about a year ago, just with the cheap supermarket DYC version, and quickly realised that it almost completely stopped the water going green in the heat. Wonderful! The vinegar is also said to do good things for chickens' digestive health. I haven't noticed any difference, but then they were healthy to start with. (Except the one that died, oops - that was post-vinegar.)
Hopefully the care you give your birds is rewarding you in the form of many eggs! These long summer days are great for egg production, and the egg section of our fridge is overflowing. Quiche time, I think.