Heat tolerances of temperate and tropical birds and their implications for susceptibility to climate warming

Characterizing heat tolerance is critical for predicting an organism’s vulnerability to climate warming. Recent studies of ectotherms report that impacts of climate warming are expected to be greater in the tropics, where ectotherms tend to have lower heat tolerances and experience air temperatures closer to their heat tolerance limits than their temperate counterparts. However, similar comparisons of heat tolerance are largely lacking for endotherms, and it remains an open question whether climate warming will also disproportionately affect tropical endotherms.
To address this empirical gap, we measured thermoregulatory responses to acute heat stress in 81 bird species (23 temperate, 58 tropical), assembling the largest comparative dataset of endothermic heat tolerances to date.
After controlling for body mass and experimental chamber humidity, temperate species had significantly higher heat tolerance limits (ΔHTL = 2.2 °C; 45.2 vs. 40 °C) and upper critical temperatures (ΔUCT =...