Honestly, I have only had my chickens since last Fall. I helped my grandmother with hers since I was a little girl and loved it, but this is my first summer with my own flock. I may change my mind later and say they need more help surviving the summer, I don't know. I know that so far, mine seem fine. They have plenty of shade and cool water. They have access to the whole back field, free range, and can pick where they need to be to keep comfortable. Most of them spend the heat of the day behind the stack of wood pallets, dust bathing. I think as long as you give your chickens plenty of clean water, shade, and dust bathing spots, they should be fine.
@Daisy: I don't know how your humidity compares to here, but it's been rainy all week which is adding to our usual humidity problems. (Though of course I appreciate the rain. After last drought- I mean summer, I hope I'm never ungrateful for rain.) A couple times I've walked outside and my glasses have steamed up. My chickens still seem to handle it fine.