Hot days are associated with short-term adrenocortical responses in a Southern African arid-zone passerine bird

Non-invasive methods for investigating the biological effects of environmental variables are invaluable for understanding potential impacts of climate change on behavioural and physiological stress responses of free-ranging animals. Foraging efficiency, body mass maintenance and breeding success are compromised in Southern pied babblers Turdoides bicolor exposed to air temperatures between ~ 35°C and ~ 40°C. We tested the hypothesis that these very hot days are acute stressors for free-ranging babblers by quantifying the relationship between daily maximum air temperature (T max ) and faecal glucocorticoid metabolite (fGCM) levels. We collected naturally-excreted droppings from free-ranging pied babblers and analysed fGCM levels. Levels of fGCMs in droppings collected after 3pm were independent of same-day T max < 38 °C and averaged 140.25 ng g −1 Dry Weight ± 56.92 ng g −1 DW (mean ± SD) over this range. Above an inflection T max = 38 °C, however, fGCM levels increased linearly with same-day T max and averaged 190.79 ng g −1 DW ± 70.13 ng g −1 DW. There was no relationship between T max on the previous day and fGCM levels in droppings collected the following morning. Group size, breeding stage, sex and rank did not predict variation in fGCM levels. These results suggest that very high T max may be linked to acute, but not chronic, heat stress responses. The fGCM levels we measured are likely to represent a biologically meaningful response to an environmental stressor (high T max ), suggesting a physiological mechanism underlying observed changes in behaviour and reproductive success at high temperatures in this species.