Data from: Body temperature regulation in hot environments

Organisms in hot environments will not be able to passively dissipate
metabolically generated heat. Instead, they have to revert to evaporative
cooling, a process that is energetically expensive and promotes excessive
water loss. To alleviate these costs, birds in captivity let their body
temperature increase, thereby entering a state of hyperthermia. Here we
explore the use of hyperthermia in wild birds captured during the hot and
dry season in central Nigeria. We found pronounced hyperthermia in several
species with the highest body temperatures close to predicted lethal
levels. Furthermore, birds let their body temperature increase in direct
relation to ambient temperatures, increasing body temperature by 0.22°C
for each degree of increased ambient temperature. Thus to offset the costs
of thermoregulation in ambient temperatures above the upper critical
temperature, birds are willing to let their body temperatures increase by
up to 5°C above normal temperatures. This flexibility in body temperature
may be an important mechanism for birds to adjust to predicted increasing
ambient temperatures in the future.