The ability of some birds to survive and breed in the hottest and driest habitats on the planet, despite their diurnal habits and high mass-specific energy and water requirements, has intrigued ecological physiologists for nearly a century. Central to this field of enquiry has been the study of evaporative cooling, the only mechanism whereby birds can defend body temperature below environmental temperature. We developed a novel technique for quantifying the limits of avian heat tolerance and evaporative cooling capacity, and used this approach to investigate thermoregulatory capacity in arid-zone avifaunas from three continents. Our data reveal that the efficiency and scaling of heat tolerance vary greatly among higher taxa. We also found substantial variation within orders as well as intraspecific variation among populations. Some species can dissipate up to ~500 % of metabolic heat production via evaporation, and not all taxa showed an upper critical limit of thermoneutrality; in species with well-developed cutaneous evaporative heat dissipation and/or gular flutter, resting metabolic rate often remained at minimal levels even at air temperatures above 50 °C.
We have recently used these physiological data to model how water requirements for evaporative cooling will increase with climate change. By synthesizing data from physiological and behavioural studies of southern African arid-zone birds over the last decade, we have been able to model the risks of lethal acute heat exposure versus sublethal chronic exposure, and explore how the relative severity of these risk categories will vary across the southern African arid zone under current and future conditions. The risk of direct mortality during extreme heat events will remain low for most species, a pattern contrasting with projections for the American southwest and the scenario likely for Australia. However, the severity and spatial extent of sublethal effects of chronic heat exposure will increase substantially during the coming decades. By the 2080s, Southern Pied Babblers (Turdoides bicolor) will experience 20-30 consecutive days yr-1 on which maximum air temperature exceeds the threshold at which their diurnal mass gain is zero, a situation where each 24-hr period is associated with loss of ~4% of body mass. For Southern Yellow-billed Hornbills (Tockus leucomelas) average maximum temperatures over large parts of their southern African range will approach or exceed 35 °C, the threshold associated with the probability of successful breeding decreasing below 50 %. Many species currently occurring in southern Africa’s arid savanna biome will not be able to persist under future conditions, and we predict large losses of avian biodiversity by the end of the century.
Andrew McKechnie is an evolutionary physiologist whose research focuses on the thermal physiology of endotherms, particularly the development of mechanistic approaches to modelling the responses of arid-zone birds to climate change. He obtained his PhD from the University of Natal and then spent two years as a postdoctoral fellow at the University of New Mexico. He currently holds a dual appointment as Professor in the Department of Zoology and Entomology at the University of Pretoria and South African Research Chair in Conservation Physiology at the National Zoological Garden, and has been a core team member of the DST-NRF Centre of Excellence at the FitzPatrick Institute of African Ornithology at the University of Cape Town since the Centre was established in 2004. He is an Associate Editor for Emu – Austral Ornithology, serves on the editorial board of Journal of Comparative Physiology B, and is a scientific advisor to African BirdLife magazine.