The functional significance of facultative hyperthermia varies with body size and phylogeny in birds

Facultative hyperthermia, the elevation of body temperature above normothermic levels, during heat exposure, importantly affects the water economy and heat balance of terrestrial endotherms. We currently lack a mechanistic understanding of the benefits hyperthermia provides for avian taxa. Facultative hyperthermia has been proposed to minimize rates of water loss via three distinct mechanisms: M1) by maintaining body temperature (T-b) above environmental temperatures (T-e), heat can be lost non-evaporatively, saving water; M2) by minimizing the thermal gradient when T-e > T-b, environmental heat gain and evaporative water loss rates are reduced; and M3) by storing heat via increases in T-b which reduces evaporative heat loss demands and conserves water. Although individuals may benefit from all three mechanisms during heat exposure, the relative importance of each mechanism has not been quantified among species that differ in their body size, heat tolerance and mechanisms of evaporative heat dissipation. We measured resting metabolism, evaporative water loss and real-time T-b from 33 species of birds representing nine orders ranging in mass from 8 to 300 g and estimated the water savings associated with each proposed mechanism. We show that facultative hyperthermia varies in its benefits among species. Small songbirds with comparatively low evaporative cooling capacities benefit most from (M1), and hyperthermia maintains a thermal gradient that allows non-evaporative heat losses. Other species benefited most from (M2) minimizing evaporative losses via a reduced thermal gradient for heat gain at high T-e. We found that (M3), heat storage, only improved the water economy of the sandgrouse, providing little benefit to other species. We propose that differences in the frequency and magnitude of hyperthermia will drive taxon-specific differences in temperature sensitivity of tissues and enzymes and that the evolution of thermoregulatory mechanisms of evaporative heat dissipation may contribute to differences in basal metabolic rate among avian orders. Understanding the mechanistic basis of heat tolerance is essential to advance our understanding of the ecology of birds living in hot environments that are warming rapidly, where extreme heat events are already re-structuring avian communities.