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

Abstract

ABSTRACT 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°C and zero diurnal M b gain (i.e. net 24-hr M b 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 21st century. Our findings suggest that this threatened lark species is unlikely to persist across much of its current range by 2100.