Mammals and birds are homeothermic and capable of sustaining their high body temperature in a cold environment. Endothermic homeothermy results both from aerobically supported high resting heat production rates in virtually all soft tissues, from thermoregulatory heat production and from prevention of excessive heat loss by insulation with fur or plumage (Ruben 1995). In contrast, the body temperature of poikilothermic animals corresponds passively with ambient temperature, ectothermic poikilothermy characterizing animals which have insufficient endogenous heat production for thermoregulation and which gain their internal heat from the environment.
The high resting metabolic rates of mammals and birds are thought to have evolved as a by-product of aerobically supported sustained activity. This sustained activity has been a beneficial characteristic in, for example, hunting a prey or escaping a predator, and it has been favoured by natural selection. Sustained activity was also a precondition for the postural change from sprawled to limb-supported upright posture in which continuous muscle twitches are utilized (Heath 1968). A high resting metabolic rate and high body temperature have not been advantageous features as such and indeed their maintenance has been a costly process in terms of energy consumed. Thus selection has not favoured high and controlled body temperature but it has certainly encouraged the sustained activity that such a temparature enables.
In juvenile mammals and birds, the ability to thermoregulate is rarely comparable to that found in adults. Smaller body size and weaker insulation lead to greater heat loss in cold, and regulatory heat production has a smaller capacity to increase body temperature. This thesis deals with thermoregulation in cold among young birds, especially with the development of heat production.