Physiological performance declines precipitously at high body temperature (Tb), but little attention has been paid to adaptive variation in upper Tb limits among endotherms. We hypothesized that avian maximum tolerable body temperature (T_bmax) has evolved in response to climate, with higher T_bmax in species exposed to high environmental heat loads or humidity-related constraints on evaporative heat dissipation. To test this hypothesis, we compared T_bmax and related variables among 53 bird species at multiple sites in South Africa with differing maximum air temperatures (T_air) and humidity using a phylogenetically-informed comparative framework. Birds in humid, lowland habitats had comparatively high T_bmax (mean ± SD = 45.60 ± 0.58°C) and low normothermic T_b (T_bnorm), with a significantly greater capacity for hyperthermia (T_bmax -T_bnorm gradient = 5.84 ± 0.77 °C) compared to birds occupying cool montane (4.97 ± 0.99 °C) or hot arid (4.11 ± 0.84 °C) climates. Unexpectedly, T_bmax was significantly lower among desert birds (44.65 ± 0.60°C), a surprising result in light of the functional importance of hyperthermia for water conservation. Our data reveal a macrophysiological pattern and support recent arguments that endotherms have evolved thermal generalization versus specialization analogous to the continuum among ectothermic animals. Specifically, a combination of modest hyperthermia tolerance and efficient evaporative cooling in desert birds is indicative of thermal specialization, whereas greater hyperthermia tolerance and less efficient evaporative cooling among species in humid lowland habitats suggests thermal generalization.

Body temperature (T_b), resting metabolic rate (RMR) and evaporative water loss (EWL) in birds at two study sites in South Africa collected over a range of air temperatures (T_air) using flow-through respirometry.

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