Sublethal fitness costs of chronic exposure to hot weather vary between sexes in a threatened desert lark

Abstract

<p>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 (<i>Calendulauda burra</i>) 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 Mb gain equivalent to average overnight M_b loss when T_max = 33.5°C and zero diurnal Mb gain (i.e. net 24-hr Mb loss of ~ 5 %) when T_max = 40.0°C. Under recent climate conditions, male Red Larks rarely, if ever, experienced consecutive days with T_max > 40.0°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.</p>