Birds (as all animals) use feed to grow, produce and maintain their body functions. During the digestion of the feed, metabolic heat is produced.
This heat is used by the animal to maintain its body temperature. Warm blooded animals have an optimum body temperature at which their body functions are optimally performed. An animal will try to keep its body at this temperature.

As the heat production is determined by the amount of feed digested and metabolized, we can estimate the heat production roughly by the amount of feed consumed. A more precise estimation however is not easy to make as the exact feed formulation and the actual digestibility is not known as well as not all feed components give an equal heat production.

Heat production is not equal for every type of animal.
A broiler of 2.2 kg, consuming about 180 grams of feed, produces roughly 250 kcal (12 BTU) per kg of body weight.
A breeder bird of 3.5 kg, consuming about 165 grams of feed, produces about (?)
A rearing pullet of 1.5 kg, consuming 70 grams of feed, produces about (?)

For all animals, it holds that they will try not to produce more heat than they can lose otherwise, their body temperatures will rise and they will die. Every (warm-blooded) animal has an optimal body temperature, at which its biological processes will run at an optimal level. When the actual body temperature is slightly higher or lower than optimum, the animal can survive but will try to control its body temperature back to the optimal level. When the body temperature gets out of control, we say that it is outside the thermoneutral zone of the animal. If this condition gets too severe, biological processes will suffer and eventually the animal will die.