Integrating fine-scale behaviour and microclimate data into biophysical models highlights the risk of lethal hyperthermia and dehydration

Climate change threatens biodiversity by compromising the ability to balance energy and water, influencing animal behaviour, species interactions, distribution and ultimately survival. Predicting climate change effects on thermal physiology is complicated by interspecific variation in thermal tolerance limits, thermoregulatory behaviour and heterogenous thermal landscapes. We develop an approach for assessing thermal vulnerability for endotherms by incorporating behaviour and microsite data into a biophysical model. We parameterised the model using species-specific functional traits and published behavioural data on hotter (maximum daily temperature, T_max > 35°C) and cooler days (T_max < 35°C). Incorporating continuous time-activity focal observations of behaviour into the biophysical approach reveals that the three insectivorous birds modelled here are at greater risk of lethal hyperthermia than dehydration under climate change, contrary to previous thermal risk assessments. Southern yellow-billed hornbills Tockus leucomelas, southern pied babblers Turdoides bicolor and southern fiscals Lanius collaris are predicted to experience a risk of lethal hyperthermia on ~ 24, 65 and 40 more days year⁻¹, respectively, in 2100 relative to current conditions. Maintaining water balance may also become increasingly challenging. Babblers are predicted to experience a 57% increase (to ~186 days year⁻¹) in exposure to conditions associated with net negative 24 h water balance in the absence of drinking, with ~ 86 of those days associated with a risk of lethal dehydration. Hornbills and fiscals are predicted to experience ~ 84 and 100 days year⁻¹, respectively, associated with net negative 24 h water balance, with ≤ 20 of those days associated with a risk of lethal dehydration. Integrating continuous time-activity focal data is vital to understand and predict thermal challenges animals likely experience. We provide a comprehensive thermal risk assessment and emphasise the importance of thermoregulatory and drinking behaviour for endotherm persistence in coming decades.