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The ability of birds and mammals to maintain a relatively stable internal body temperature while under a considerable range of ambient environmental temperatures results in distinct ecological and physiological advantages. A constant body temperature facili-tates the activity of homeothermic (warm-blooded) animals in cold environments because the many temperature-dependent physiological and biochemical processes of the body are un-impeded. Conversely, poikilothermic (cold-blooded) animals lack the ability for precise thermoregulation and can function at top physiological efficiency only when ambient environmental temperatures are within a rather narrow optimum range. Homeothermia is accomplished by a system of physiological feed-back mechanisms which maintain a thermal balance in the body by regulating the rate of heat production and the amount of heat loss. An optimum temperature range (thermoneutral zone) exists in which the expenditure of energy for heat production and heat loss is minimal. However, seldom is the temperature of the environment precisely that which is best suited for the physiological requirements of the animal. Thus, a homeotherm is nearly always either passing excess body heat to the environment, or is producing heat to replace that lost to the environment. Because both heat transfer and heat production require some degree of energy expenditure and, since the energy budget in birds is usually relatively small, excessive energy demands for thermoregulation can be lethal.