The costs of keeping cool in a warming world: implications of high temperatures for foraging, thermoregulation and body condition of an arid‐zone bird

Abstract

Recent mass mortalities of bats, birds and even humans highlight the substantial threats that rising global temperatures pose for endotherms. Although less dramatic, sublethal fitness costs of high temperatures may be considerable and result in changing population demographics. Endothermic animals exposed to high environmental temperatures can adjust their behaviour (e.g. reducing activity) or physiology (e.g. elevating rates of evaporative water loss) to maintain body temperatures within tolerable limits. The fitness consequences of these adjustments, in terms of the ability to balance water and energy budgets and therefore maintain body condition, are poorly known. We investigated the effects of daily maximum temperature on foraging and thermoregulatory behaviour as well as maintenance of body condition in a wild, habituated population of Southern Pied Babblers Turdoides bicolor. These birds inhabit a hot, arid area of southern Africa where they commonly experience environmental temperatures exceeding optimal body temperatures. Repeated measurements of individual behaviour and body mass were taken across days varying in maximum air temperature. Contrary to expectations, foraging effort was unaffected by daily maximum temperature. Foraging efficiency, however, was lower on hotter days and this was reflected in a drop in body mass on hotter days. When maximum air temperatures exceeded 35.5°C, individuals no longer gained sufficient weight to counter typical overnight weight loss. This reduction in foraging efficiency is likely driven, in part, by a trade-off with the need to engage in heat-dissipation behaviours. When we controlled for temperature, individuals that actively dissipated heat while continuing to forage experienced a dramatic decrease in their foraging efficiency. This study demonstrates the value of investigations of temperature-dependent behaviour in the context of impacts on body condition, and suggests that increasingly high temperatures will have negative implications for the fitness of these arid-zone birds.

Adaptive thermoregulation during summer in two populations of an arid‐zone passerine

“…Despite summer diurnal temperatures often exceeding 358C, nocturnal temperatures are frequently below 108C in the southern Kalahari Desert, especially during dry periods when re-radiation levels are high at night. By allowing T b to decrease with T air , desert birds would conserve energy on cool nights, which may offset diurnal reductions in foraging performance brought about by low food availability and/or high T air (du Plessis et al 2012). Daily heterothermy levels in semi-desert birds, on the other hand, showed a stronger relationship to T air during the dry season than during the wet season.…”

“…Desert birds are exposed to these T air fluctuations to a greater degree than taxa such as reptiles and small mammals because few diurnal avian species make use of thermally buffered microsites below ground (Bartholomew and Cade 1963, Wolf 2000. Limited food resources often force desert birds to forage during hot weather (Tieleman and Williams 2002), posing the potential risk of water and energy expenditure rates exceeding intake rates (du Plessis et al 2012). Moreover, the scarcity of surface water means that balancing water losses associated with high rates of thermoregulatory EWL may become costly during periods of very hot weather.…”

Effects of extreme weather on two sympatric Australian passerine bird species

“…In the few studies demonstrating fitness costs of high temperatures, negative effects on body condition and survival are associated with air temperatures more than or equal to 358C [3][4][5]7]. Studies of physiology show that for many passerines, the onset of panting, which facilitates evaporative cooling, occurs at slightly lower temperatures, at about 308C, but only when air temperatures approach body temperature (approx.…”

“…Prolonged exposure (i.e. over successive days) to high temperatures in the mid-30s8C can lead to reductions in adult body mass, as well as affect nestling development, presumably through a combination of dehydration via increased rates of evaporative cooling and energetic constraints that result from reduced foraging opportunities or provisioning effort [3][4][5][6][7].…”

Avian thermoregulation in the heat: scaling of heat tolerance and evaporative cooling capacity in three southern African arid-zone passerines

“…For tropical species at lower air temperatures and higher humidity the thermal endpoints may be significantly lower (Weathers, 1997). Studies of wild birds in arid habitats have identified important threshold T a values in the 30-40°C range for variables related to body condition (du Plessis et al, 2012) and provisioning rates during breeding (Cunningham et al, 2013), suggesting that in many cases, detailed models of specific determinants of survival and/or reproduction will be necessary to predict the effects of climate change. However, catastrophic mortality events during extreme heat waves in the arid zones of Australia and elsewhere (reviewed by McKechnie et al, 2012;McKechnie and Wolf, 2010), combined with predicted increases in the frequency and intensity of heat waves (IPCC, 2011), underscore the relevance of models of avian survival over time scales of hours during acute heat exposure, and the need for comparative data on the upper limits of avian heat tolerance.…”