It's just that is got so hot so fast. My chicks feather in the garage until it gets to be 70degrees all night and then I put them out near the others, but in the greenhouse, where I normally let them "look no touch" for several weeks until they are integrated. But this year the greenhouse got to 107 degrees in maybe 6 days time. So, of course, I had to punt. Right now they are in the middle of the big shady hen run under a picnic table that I have wired closed like a 4 X 8 foot play pen.
We usually have a little bit of a longer transition period from cool to not too terribly hot to OMG it's horrible out here and this year we went straight to the blazing hot.
Anyway, can you give them a hose that makes standing water or mud? Not like a gusher but make it wet and they will scratch around and peck in the cool. If you can cool their feet off they will stop panting. I have a fan blowing past the nesting boxes to cool those down since the hens are pretty much held to that spot for a while.
As you probably already know, it's from 1'ish to 5'ish that's most brutal. If you can cool them off just once during that time frame, it seems to help them. Others have more hints but that is what I do.