Mechanistic insights into avian and chiropteran thermal risk exposure in a warming world.

Climate change threatens biodiversity by challenging animals' ability to balance energy and water budgets, across timescales ranging from (i) acute-lethal (hours-to-days) to (ii) chronic-sublethal (days-to-weeks). However, predicting the effects of heat exposure over these timescales is complicated by inter- and intraspecific variability in traits, such as physiological tolerances, life-history strategies and the surrounding microhabitat. To address these challenges, we synthesize current knowledge of the physiological and behavioural mechanisms underpinning acute and chronic thermal risks in small endotherms, focusing on birds and bats. We review thermal risks and the key traits determining vulnerability from a biophysical modelling perspective. We then critically evaluate the growing use of biophysical models to predict thermal vulnerability, emphasizing their value but also the risk of oversimplification in the absence of empirical validation. Importantly, most models assume fixed physiological trait values and thermal thresholds and do not incorporate phenotypic plasticity. We also identify key knowledge gaps that constrain predictive capacity, including limited data on dehydration tolerance and heat tolerance thresholds, particularly in bats; incomplete understanding of reaction norms and acclimatory timescales; and poor resolution of how individual thermal trade-offs scale to population dynamics.