Most living animals, us included, are better adapted for keeping warm than for keeping cool. That's obvious at a glance when one sees the abundance of fur cats and dogs have (or in this case, feathers on your chickens). Although the domestic chicken is a descendant of the Red Asian Jungle Fowl (note the word "jungle", as in "hot, steamy, humid conditions"), even it is better able to keep warm than cool.
Have you ever stuck your fingers into a chicken's feathers and felt its skin? You might be surprised. The body temperature of the bird is about 106.5 degrees Fahrenheit. It keeps this temperature steady by converting energy found in its feed into heat. The less heat the chicken needs, the less feed it must eat. To get rid of excess body heat, the chicken doesn't perspire (since it has no sweat glands), but instead it pants like a dog.
The effect of reduced feed consumption, combined with the direct influence of strong heat on the metabolism of the chicken, produces several things.
First, a quick drop in production and efficiency as the heat rises. In meat birds, they'll stop gaining weight. In layer hens, the feed-to-egg ratio is reduced and they often completely stop laying altogether.
Second, heat stress. This is a quite obvious deduction.
Third, the chicken's system grinds to a halt. An observer will notice that the birds are just lying around like a bunch of limp socks...no clucking or preening at all.
And finally, death. This is when the owner begins frantically leaving messages on our poultry message boards in the hopes of finding some way to resurrect the poor creatures.
Please note that the chicken, thanks to God's wonderful creativity, will become acclimatized after a while. Experiments done by a panel of expert scientists have shown that, in layers, the body temperature returns to normal or stabilizes at a slightly higher temperature 3 to 5 days after the initial exposure to constant, extreme temperatures. Thus, if a chickens goes through varying high temperatures repeatedly, it will adapt and be able to survive at an ambient temperature 5 (F) degrees higher than before acclimatization.
The higher the relative humidity of the air, the less heat the mature birds can tolerate and the more stress it causes. However, high humidity has not been proven to affect the growth rate of young chickens.