I have to ask the obvious question; are they too hot? Panting makes me wonder. Also, is it possible their feed has gone bad---and if you feed a wet mash, did it sour in the warm weather? If so, you may be seeing the effects of food poisoning (toxins). If the latter, I would feed them molasses water ( add enough food grade molasses to water to make the solution a medium caramel color) which acts as a chicken laxative, to try to move the toxins out of the GI system faster...but it's not a cure for toxins already affecting the body. Aside from that, is there ANY chance they could have eaten any other poisons, such as pesticide, garden fertilizer, any thing at all aside from their regular feed?