We had a horrible heat wave last week and I had one chicken die and another that wasn't acting herself. She then died three days later. This morning I went out to the coop and there was another one lying dead under the roost. They start by isolating themselves from the rest of the flock, not eating and acting lethargic. At first I thought it was the heat and the fact that they are 3 1/2 years old now. But this week the weather is cool and the humidity lifted so I don't think it's heat related. What can I do to prevent any more losses?