Behavioural trade-offs between foraging and thermoregulation result in negative relationships between diurnal body mass (M-b) gain and daily maximum air temperature (T-max) in arid-zone birds. However, it remains unclear how these trade-offs are affected by habitat type or sexually dimorphic behaviour. Using the South African arid-zone endemic Red Lark (Calendulauda burra) as a model taxon and focusing solely on non-breeding adults, we evaluated predictions that a) time-activity budgets are more strongly affected by air temperature (T-a) than is the case for species occurring in arid savanna habitats with greater access to shaded microsites, and b) the effects of T-max on diurnal M-b gain differ between males and females. Behavioural observations revealed an effect of T-a on activity, and sex differences in the temperature-dependence of resting and wing-spreading. Moreover, M-b data obtained from habituated larks trained to perch on an electronic balance revealed that in males, but not females, diurnal M-b gain was negatively related to T-max, with diurnal M-b gain equivalent to average overnight M-b loss when T-max = 33.5 degrees C and zero diurnal M-b gain (i.e. net 24-hr M-b loss of similar to 5 %) when T-max = 40.0 degrees C. Under recent climate conditions, male Red Larks rarely, if ever, experienced consecutive days with T-max > 40.0 degrees C but, under an unmitigated climate change scenario, will experience 10-15 consecutive days per summer by the end of the 21(st)century. Our findings suggest that this threatened lark species is unlikely to persist across much of its current range by 2100.