Chickens don't sweat. So, they have to drink a lot of water to cool off their internal temperature on a hot day. If you are giving them dry feed, that will make a huge difference. I ferment their feed. So, while they are not drinking as much from the water founts, they are still taking in as much liquid over the course of the day. You should check to be sure that your waterers are level, and that there are no leaks. If the base is even just a little bit cross threaded or if they are not level they will spill all the water out onto the ground. If all of your birds are adults, you could give them a couple shallow buckets as well, so that if you did have a fount leak, they would still have an other water option.