Physiological and behavioural aspects of climate change vulnerability in ground-foraging passerines inhabiting South Africa’s Tankwa Karoo desert

In the face of anthropogenic global heating, understanding how endotherms persist both behaviorally and physiologically in response to temperature extremes has become increasingly important. For this reason, research has primarily focused on those species inhabiting desert environments, where harsh environmental conditions may leave arid-zone endotherms vulnerable to the effects of increasing air temperature (Tair). 

These data included several species from the Aluadidae, primarily from the genus Calendulauda, focusing on arid zones such as Namaqualand and the Kalahari. Physiological performance decreases in response to high Tb making maximum body temperature (MaxTb) an important metric in determining the maximum environmental temperature endotherms can tolerate. However, MaxTb was not included in the aforementioned analyses and has been found to vary in species occupying different climates. 

In addition to physiological responses, birds can also make use of thermoregulatory behaviours in response to temperature extremes, whether it be hot (i.e., panting, wing-spreading and shade-seeking) or cold (i.e., ptiloerection). While these behaviours may buffer individuals from the immediate effects of high and low Tair, the prioritization of these thermoregulatory behaviours has been found to force birds to compromise on time spent foraging and maintaining body condition. However, the effects of behavioural thermoregulatory trade-offs across summer and winter seasons and how these may be affected in the face of climate change remain unclear. 

First chapter, the researcher aimed to quantify the thermoregulatory performance and heat tolerance of a water-dependent lark species from the genus Galerida inhabiting the Tankwa Karoo. To do this,  thermoregulatory data was collected for 8 large-billed larks (Galerida magnirostris) using open flow-through respirometry, exposing individuals to steadily increasing Tair setpoints until they reached their HTL. Large-billed larks displayed a thermoregulatory capacity comparable to that associated with most desert birds studied to date, possessing a MaxTb of ~ 45°C and an HTL of ~50°C. Combining these data with that previously published, researcher expanded upon previous efforts investigating the links between drinking behaviour and thermoregulation by assessing whether thermal physiology differences between regularly drinking and occasional - /non-drinking species extend to MaxTb. To do this, I collated published data of MaxTb and the slope of body temperature (Tbslope) above the upper critical limit of thermoneutrality for 18 other southern African arid-zone passerines. Interspecific analyses revealed no differences between drinking and non-drinking species. Therefore, findings provide a quantitative thermal profile for large-billed larks inhabiting one of the hottest and driest areas in southern Africa, while my interspecific analyses reiterate the overall importance of evaporative cooling in the thermoregulation of arid-zone birds. In the face of predicted decreases in rainfall brought about by climate change, this chapter emphasizes the importance of the conservation of key water sources within arid zones and the role these can play in the persistence of desert avifauna.

Second chapter, it was hypothetically tested that thermoregulatory behaviours are traded off against foraging in response to seasonal temperature extremes, using Karoo chats (Emarginata schlegelii ) as a model species. To do this, researcher collected behavioural data over both summer and winter seasons for 20 individuals. The data indicated no trade-offs between foraging and thermoregulation during the winter season. However, during summer individuals were found to prioritize heat dissipation behaviours at the expense of time spent foraging in response to high Tair. To gain insight into how these trade-offs or lack thereof may be affected if current climate trends continue, researcher modelled maximum and minimum daily temperatures for the Tankwa Karoo over the last 38 years. These data revealed limited trends except for a 0.02°C increase in daily summer maximums. 

The results presented in the dissertation also provide a foundation on which future studies may build, by addressing topics such as behavioural thermoregulatory trade-offs and the impact these may have on the persistence of the Tankwa Karoo avifaunal community. The study chapters also provide insights into the possible impacts increasing Tair could have on the physiological and behavioural thermoregulatory mechanisms in arid-zone birds inhabiting the Tankwa Karoo, highlighting the need to conserve these landscapes in the face of advancing climate change.