The occurrence of non-shivering thermogenesis in birds has long been a controversial issue. Although birds are endothermic vertebrates, sharing with mammals (placental mammals and marsupials) a common ancestor, they do not possess brown adipose tissue or a similar type of tissue, unlike their mammalian counterparts. Some bird species are, however, able to withstand very low ambient temperatures (-70 degrees C) or undergo periods of heterothermia, and there is now good experimental evidence showing that non-shivering thermogenesis may indeed occur in birds under such conditions. The skeletal muscles of birds, particularly the flight muscles, occupy a significant fraction (approximately 30 %) of the total body mass, and recent results have shown that they are likely to be the main sites for non-shivering thermogenesis. The precise mechanisms involved in adaptive thermogenesis in birds are still not fully understood. The translocation of Ca(2+) between intracellular compartments and the cystosol mediated by the sarcoplasmic reticulum Ca(2+)-ATPase, uncoupled from ATP synthesis, is one mechanism whereby chemi-osmotic energy can be converted into heat, and it has been proposed as one of the possible mechanisms underlying non-shivering thermogenesis in birds on the basis of data obtained mainly from ducklings acclimatized to cold conditions. The recent characterization of an uncoupling protein homolog in avian skeletal muscle and the expression of its mRNA at different stages of the torpor/rewarming cycle of hummingbirds indicate that it has the potential to function as an uncoupling protein and could play a thermogenic role during rewarming in these birds.