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 =
1.1 °C; 38.7 vs. 37.6 °C) on average than tropical species. Importantly,
however, these differences do not appear to impact vulnerability to
climate warming, as neither thermal safety margins (i.e. the difference
between UCT and maximum air temperature, Tmax) nor warming tolerances (
the difference between HTL and Tmax) differed between temperate and
tropical species. We also observed substantial variation in heat tolerance
among avian orders, with pigeons and doves (Columbiformes) being among the
most heat tolerant species in our dataset. Overall, our results suggest
that, from a physiological standpoint, tropical birds may not be
systematically more susceptible to climate warming than temperate birds,
contrasting previous studies of ectotherms. Furthermore, we show that
certain avian clades may be more resilient to warming irrespective of
local climate. However, because we only sampled at one temperate and one
tropical site, we caution that replication from other habitats and
localities are needed to evaluate the generality of our findings.