Thermoregulation
Maintenance of optimum body temperature, which in birds ranges from about 38°C to about 43°C according to species and its activity. As birds are 'warm-blooded' ('homoiothermal') they are able to keep themselves at an almost constant temperature independently of their environment, although in cold conditions they fluff out their feathers to keep warm by trapping an insulating layer of air, and in hot weather they pant to allow excess body heat to escape via the respiratory system (as birds are unable to sweat). Chicks lose heat rapidly because of their small size and need a certain amount of brooding, which is essential for nidicolous nestlings until their feathers begin to grow as they cannot regulate their temperature before they have reached this stage, being 'cold-blooded' ('poikilothermal') until this time.From Peter Weaver's Birdwatcher's Dictionary
Copyright © 1981 by Peter Weaver