As endothermic animals, birds allocate a significant part of their energy intake for maintaining homeothermy, especially in cold environments. Their body temperatures and metabolic rates are typically higher than in mammals of similar body mass. Therefore, food shortage and starvation pose a true energetic challenge for maintaining homeothermy in birds (McCue 2010). As most birds are small, their energy reserves in relation to metabolic rate are limited, which together with high surface-to-volume ratios exacerbates the effects of starvation. During food shortage, energy can be saved if the need for active thermogenesis can be reduced. Such a hypometabolic state can be achieved by increasing thermal insulation beyond the level found in fed birds or by reducing body temperature in a regulated manner or by combining the two mechanisms. Even in fed state, birds increase their insulation during the rest phase of the daily cycle. This is done mainly by ptiloerection and postural adjustments and there is probably not very much extra insulation available by these mechanisms in fasting birds. However, a significant decrease in heat loss can be achieved by regional heterothermia, where peripheral parts of the body are allowed to cool and only the core parts are held normothermic. Although this phenomenon is known from other physiological situations, it has not been studied extensively in fasting or food-restricted birds. A general decrease in core body temperature will naturally decrease the metabolic burden of homeothermy. More and more evidence for a capability to a regulated decrease of body temperature (rest-phase hypothermia, shallow torpor) during starvation in birds is accumulating (McKechnie & Lovegrove 2002). The decline of body temperature in shallow torpor is typically 3-10°C. While deep torpor triggered by starvation or even without energetic stress has been recognized for a long time in some avian groups (e.g., in humming-birds and swifts), recent studies show that basically all avian groups can save energy by entering starvation-induced shallow torpor during the rest-phase of their daily cycle. In deep torpor, birds lose their responsiveness to environmental stimuli, whereas in shallow torpor the react to external disturbances, which has implications ion vigilance and predator avoidance. So far, deep or shallow torpor has been found in at least 11 avian orders and 29 families. The metabolic savings in shallow torpor are obviously smaller than in deep torpor. The reduction in metabolic rate is, however, often larger than predicted by the decline in deep-body temperature (Hohtola 2012). This suggests that fasting birds are able to recruit otherwise unused insulative mechanisms. Regional heterothermy in peripheral tissues is one possibility, although it has not been studied in detail. It would save energy by two mechanisms: 1) By making the temperature gradient between body core and the environment less steep (resulting in lower conductance) and 2) By lowering thermostatic costs of the peripheral parts at lower tissue temperature. The thermoregulatory adjustments during fasting are tuned to the amount of remaining energy reserves. Thus, nocturnal hypothermia is deeper in birds that roost at lower body mass (Reinertsen 1996) and hypothermia is deeper on each successive fasting day during prolonged fasting (see Hohtola 2012). How starvation is translated to a neurohumoral thermoregulatory signal that results in a lower regulated body temperature and metabolic savings is, however, not known. These mechanisms, together with the observed large energy savings ensuing from the relatively small decline in body temperature warrant further studies on fasting-induced thermoregulatory adaptations.