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

Abstract

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). Due to their primarily diurnal foraging activity and small body size, arid-zone birds have been identified as being particularly vulnerable to the effects of climate change, and therefore the subject of much past and current research. Over the past decade, a multitude of studies have investigated the thermoregulatory performance of arid-zone birds in response to acute heat exposure, mainly quantifying evaporative cooling capacities as well as the maximum Tair species can withstand before the onset of severe hyperthermia (heat tolerance limits; HTL). Such studies have revealed that these variables are correlated with water dependence in southern African spp, with drinking species exhibiting larger evaporative cooling capacities and higher HTLs than their non-drinking counterparts. 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. . Therefore, there remains scope to evaluate members of other lark genera occupying other arid zones within South Africa, while assessing whether MaxTb may also be correlated to water dependence. 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. Therefore, my master’s dissertation aims to address the aforementioned gaps in the literature, providing a quantitative thermal profile for a water-dependent lark species inhabiting the Tankwa Karoo, while providing insights into behavioural thermoregulation in response to seasonal temperature extremes. In my first chapter, I 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, I collected thermoregulatory data 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, I 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. My interspecific analyses revealed no differences between drinking and non-drinking species. Therefore, my 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. In my second chapter, I tested the hypothesis 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, I collected behavioural data over both summer and winter seasons for 20 individuals. My 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, I 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. Therefore, chapter 2 highlights that while Karoo chats may not be constrained by Tair experienced during the winter season, they may be constrained by thermoregulatory trade-offs between heat dissipation behaviours and foraging during the summer season. These thermoregulatory trade-offs may persist and potentially be exacerbated if current summer warming trends for the Tankwa Karoo continue, posing a threat to the long-term persistence of Karoo chats and other members of the avifaunal community inhabiting this area. In conclusion, my dissertation fills gaps in the current literature regarding aspects of water dependence and thermoregulatory performance, as well as behavioural responses to seasonal temperature extremes. The results presented in my 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. My 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.