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

Abstract

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 and ~40 degrees celsius. 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 (Tmax) 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 Tmax < 38 degrees celsius and averaged 140.25 ng g-1 Dry Weight, plus-minus 56.92 ng g-1 DW (mean, plus-minus SD) over this range. Above an inflection Tmax = 38 degrees celsius, however, fGCM levels increased linearly with same-day Tmax and averaged 190.79 ng g-1 DW, plus-minus 70.13 ng g-1 DW. There was no relationship between Tmax 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 Tmax 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 Tmax), suggesting a physiological mechanism underlying observed changes in behaviour and reproductive success at high temperatures in this species.

Dehydration risk is associated with reduced nest attendance and hatching success in a cooperatively breeding bird, the southern pied babbler Turdoides bicolor

“…Physiologically, the capacity of small endotherms such as birds to tolerate heat exposure is governed by their ability to dissipate heat ( McKechnie and Wolf, 2019 ). In free-living birds, high air temperatures are associated with lower metabolic rates ( Bourne et al , 2019 ; Smit and McKechnie, 2015 ), dehydration ( Bourne, 2020 ; Sharpe et al , 2019 ), higher glucocorticid levels ( Moagi et al , 2021 ), impaired cognitive function ( Soravia et al , 2021 ) and even death ( Conradie et al , 2020 ; McKechnie et al , 2012 ). As incubating birds reach limits in their ability to tolerate high temperatures over long periods, they undertake more frequent or longer incubation recesses ( Bourne, 2020 ; Clauser and McRae, 2017 ) and may ultimately abandon their nests ( Clauser and McRae, 2017 ; Sharpe et al , 2019 ).…”

“…High average air temperatures during summer are associated with dramatically reduced survival probabilities in adult pied babblers, particularly when these occur in combination with drought ( Bourne et al , 2020b ; Ridley et al , 2021 ). Additionally, faecal glucocorticoid levels are elevated in pied babblers at air temperatures above 38°C ( Moagi et al , 2021 ), indicative of an acute physiological response to high temperatures.…”