Avian eggs usually experience temperatures of 30 to 40°C during the incubation period, but eggs often cool to much lower temperatures. Hyperthermia is less common. Passerines do not show higher incubation temperatures than other orders that have been studied extensively. Field measurements of incubation temperatures are usually lower than the optimal temperatures for development found in laboratory studies. Some species regulate egg temperatures closely throughout incubation; but in at least one penguin species, mean egg temperature increases and ranges of egg temperature decrease through the incubation period. Both the optimal temperature for continuous exposure and the range of temperatures producing high survivorship differ among species. Species also differ in their responses to temperature exposures of limited duration. Thus, the use of a "physiological zero" applicable to all species is not warranted. Penguins have both a lower optimum and a broader range of acceptable incubation temperatures than do other species studied. Age, duration, and temperature of exposure significantly affect survivorship. Hyperthermia is evidently more injurious to the developing embryo than is hypothermia. Resistance to cold exposure is a heritable trait, but the genetics and physiology of the response(s) are not known. For older embryos, the physiological effects of hyperthermia are similar to those of adult birds in terms of the organ systems that are first to fail. An estimate of thermal tolerance for short exposures in most species studied is 16 to 41°C and, for exposures lasting several hours, 36 to 39°C.

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Yellow-bellied marmots (Marmota flaviventris) were studied at three sites in central Oregon. Juveniles substantially reduced their foraging activity when equivalent black-body temperatures exceeded their upper critical temperature. Inclusion of heat stress into estimates of environmental harshness drastically reduced the differences in available foraging time between high elevation and low elevation sites.