Birds will perform and grow optimal in their thermo neutral zone, the temperature range where they don't have to put extra effort in loosing heat or producing heat to maintain their desired body temperature.

The thermo neutral zone depends on the heat production of the bird. The more heat a bird produces, the lower the temperature has to be to allow enough heat loss for the birds.

The heat production of birds depends on the digestion of feed. This means it depends on their feed intake and the type of feed they consume. Digestion of protein in the feed produces much more heat then digestion of fat. Diets that rely on fat as energy source are therefor better suitable in high temperature situations then high protein diets.

For a broiler of 2.2 kg, which consumes 180 grams of feed per day the average heat production per day is about 250 Kcal (12 BTU). This means that a broiler at the end of the growing period produces about 60 BTU or 150 Kcal per hour.

If a bird produces more heat than it loses (because for instance the environmental temperture is too high), the body temperature of the bird will increase. As a result, the bird will become inactive, stop eating and stop growing. On the other hand, If a bird loses more heat than it produces, it will put more energy (feed) in keeping up its body temperature, instead of directing it towards growth.

This means that maximum growth and optimum feed conversion will only be obtained if heat loss of the birds is in balance with maximum heat production.