Cooling requirements fueled the collapse of a desert bird community from climate change.

Climate change threatens global biodiversity by increasing extinction risk, yet few studies have uncovered a physiological basis of climate-driven species declines. Maintaining a stable body temperature is a fundamental requirement for homeothermic animals, and water is a vital resource that facilitates thermoregulation through evaporative cooling, especially in hot environments. Here, we explore the potential for thermoregulatory costs to underlie the community collapse of birds in the Mojave Desert over the past century in response to climate change. The probability of persistence was lowest for species occupying the warmest and driest sites, which imposed the greatest cooling costs. We developed a general model of heat flux to evaluate whether water requirements for evaporative cooling contributed to species' declines by simulating thermoregulatory costs in the Mojave Desert for 50 bird species representing the range of observed declines. Bird species' declines were positively associated with climate-driven increases in water requirements for evaporative cooling and exacerbated by large body size, especially for species with animal-based diets. Species exhibiting reductions in body size across their range saved up to 14% in cooling costs and experienced less decline than species without size reductions, suggesting total cooling costs as a mechanism underlying Bergmann's rule. Reductions in body size, however, are unlikely to offset the 50 to 78% increase in cooling costs threatening desert birds from future climate change. As climate change spreads warm, dry conditions across the planet, water requirements are increasingly likely to drive population declines, providing a physiological basis for climate-driven extinctions

Biophysical approaches to predicting species vulnerability

This article is a Commentary on Briscoe et al., https://doi.org/10.1111/gcb.16557We provide a commentary on Briscoe et al. (2022) highlighting the opportunity that biophysical models provide for predicting species' responses to climate change.http://www.wileyonlinelibrary.com/journal/gcbhj2024Zoology and EntomologySDG-13:Climate actio

The physiology of heat tolerance in small endotherms

Understanding the heat tolerances of small mammals and birds has taken on new urgency with the advent of climate change. Here, we review heat tolerance limits, pathways of evaporative heat dissipation that permit the defense of body temperature during heat exposure, and mechanisms operating at tissue, cellular, and molecular levels.The National Research Foundation of South Africa and the National Science Foundation.http://www.physiologyonline.org2020-09-01hj2020Zoology and Entomolog

Thermoregulation in desert birds : scaling and phylogenetic variation in heat tolerance and evaporative cooling

Evaporative heat dissipation is a key aspect of avian thermoregulation in hot environments.We quantified variation in avian thermoregulatory performance at high air temperatures (Ta) using published data on body temperature (Tb), evaporative water loss (EWL) and resting metabolic rate (RMR) measured under standardized conditions of very low humidity in 56 arid-zone species. Maximum Tb during acute heat exposure varied from 42.5±1.3°C in caprimulgids to 44.5±0.5°C in passerines. Among passerines, both maximum Tb and the difference between maximum and normothermic Tb decreased significantly with body mass (Mb). Scaling exponents for minimum thermoneutral EWL and maximum EWL were 0.825 and 0.801, respectively, even though evaporative scope (ratio of maximum to minimum EWL) varied widely among species. Upper critical limits of thermoneutrality (Tuc) varied by >20°C and maximumRMR during acute heat exposure scaled toMb 0.75 in both the overall data set and among passerines. The slope of RMR at Ta>Tuc increased significantly with Mb but was substantially higher among passerines, which rely on panting, comparedwith columbids, in which cutaneous evaporation predominates. Our analysis supports recent arguments that interspecific within-taxon variation in heat tolerance is functionally linked to evaporative scope and maximum ratios of evaporative heat loss (EHL) to metabolic heat production (MHP). We provide predictive equations for most variables related to avian heat tolerance. Metabolic costs of heat dissipation pathways, rather than capacity to increase EWL above baseline levels, appear to represent the major constraint on the upper limits of avian heat tolerance.The National Research Foundation and the National Science Foundation.http://jeb.biologists.orgam2022Zoology and Entomolog

Seasonal and geographical variation in heat tolerance and evaporative cooling capacity in a passerine bird

Intraspecific variation in avian thermoregulatory responses to heat stress has received little attention, despite increasing evidence that endothermic animals show considerable physiological variation among populations. We investigated seasonal (summer versus winter) variation in heat tolerance and evaporative cooling in an Afrotropical ploceid passerine, the white-browed sparrow-weaver (Plocepasser mahali; ∼ 47 g) at three sites along a climatic gradient with more than 10 °C variation in mid-summer maximum air temperature (Ta). We measured resting metabolic rate (RMR) and total evaporative water loss (TEWL) using open flow-through respirometry, and core body temperature (Tb) using passive integrated transponder tags. Sparrow-weavers were exposed to a ramped profile of progressively higher Ta between 30 and 52 °C to elicit maximum evaporative cooling capacity (N=10 per site per season); the maximum Ta birds tolerated before the onset of severe hyperthermia (Tb ≈ 44 °C) was considered to be their hyperthermia threshold Ta (Ta,HT). Our data reveal significant seasonal acclimatisation of heat tolerance, with a desert population of sparrow-weavers reaching significantly higher Ta in summer (49.5 ± 1.4 °C, i.e. higher Ta,HT) than in winter (46.8 ± 0.9 °C), reflecting enhanced evaporative cooling during summer. Moreover, desert sparrow-weavers had significantly higher heat tolerance and evaporative cooling capacity during summer compared with populations from more mesic sites (Ta,HT=47.3 ± 1.5 and 47.6 ± 1.3 °C). A better understanding of the contributions of local adaptation versus phenotypic plasticity to intraspecific variation in avian heat tolerance and evaporative cooling capacity is needed for modelling species' responses to changing climates.The Department of Science and Technology-National Research Foundation (DST-NRF) Centre of Excellence at the Percy FitzPatrick Institute (to A.E.M.), University of Pretoria (to A.E.M.), and the National Science Foundation [IOS-1122228 to B.O.W.].http://jeb.biologists.orghb2017Zoology and Entomolog

Feeling the heat : Australian land birds and climate change

No Abstracthttp://www.publish.csiro.au/journals/emuhb2013ab201

Seasonal metabolic acclimatization varies in direction and magnitude among populations of an afrotropical Passerine bird

Avian metabolic responses demonstrate considerable diversity under fluctuating environmental conditions, a well-studied example being the seasonal upregulation of basal metabolic rate (BMR) and summit metabolism (Msum) in temperate species experiencing harsh winters. Fewer studies have examined seasonal metabolic acclimatization in subtropical or tropical species. We investigated seasonal metabolic variation in an Afrotropical ploceid passerine, the white-browed sparrow-weaver (Plocepasser mahali; ∼47 g), at three sites along a climatic gradient of approximately 77C in winter minimum air temperature (Ta). We measured Msum (n ≥ 10 per site per season) in a helox atmosphere, BMR of the same birds at thermoneutrality (Ta ≈ 307C), and resting metabolic rates at 57C ≤ Ta ≤ 207C. Patterns of seasonal adjustments in BMR varied among populations in a manner not solely related to variation in seasonal Ta extremes, ranging from BMR ∼52% higher in winter than in summer to no seasonal difference. Greater cold tolerance was found in a population at a colder desert site, manifested as higher Msum (∼25% higher) and lower helox temperature at cold limit values compared with a milder, mesic site. Our results lend support to the idea that greater variance in the pattern of seasonal metabolic responses occurs in subtropical and tropical species compared with their temperatezone counterparts and that factors other than Ta extremes (e.g., food availability) maybe important in determining the magnitude and direction of seasonal metabolic adjustments in subtropical birds.The DST-NRF Centre of Excellence at the Percy FitzPatrick Institute and the University of Pretoria.http://www.press.uchicago.edu/ucp/journals/journal/pbz.html2018-03-31am2017Visual Art

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

Many birds can defend body temperature (Tb) far below air temperature (Ta) during acute heat exposure, but relatively little is known about how avian heat tolerance and evaporative cooling capacity varies with body mass (Mb), phylogeny or ecological factors. We determined maximum rates of evaporative heat dissipation and thermal end points (Tb and Ta associated with thermoregulatory failure) in three southern African ploceid passerines, the scalyfeathered weaver (Sporopipes squamifrons, Mb≈10 g), sociable weaver (Philetairus socius, Mb≈25 g) and white-browed sparrowweaver (Plocepasser mahali, Mb≈40 g). Birds were exposed to a ramped profile of progressively increasing Ta, with continuous monitoring of behaviour and Tb used to identify the onset of severe hyperthermia. The maximum Ta birds tolerated ranged from 48°C to 54°C, and was positively related to Mb. Values of Tb associated with severe heat stress were in the range of 44 to 45°C. Rates of evaporative water loss (EWL) increased rapidly when Ta exceeded Tb, and maximum evaporative heat dissipation was equivalent to 141–222% of metabolic heat production. Fractional increases in EWL between Ta<40°C and the highest Ta reached by each species were 10.8 (S. squamifrons), 18.4 (P. socius) and 16.0 (P. mahali). Resting metabolic rates increased more gradually with Ta than expected, probably reflecting the very low chamber humidity values we maintained. Our data suggest that, within a taxon, larger species can tolerate higher Ta during acute heat stress.National Science Foundation under IOS-1122228http://jeb.biologists.org2016-06-30hb2016Zoology and Entomolog

Differences in the use of surface water resources by desert birds are revealed using isotopic tracers

The scarcity of free-standing water sources is a key determinant of animal and plant community structure in arid environments, and an understanding of the extent to which particular species use surface water is vital for modeling the effects of climate change on desert avifauna. We investigated interspecific variation in the use of artificial water sources among birds in the Kalahari Desert, South Africa, by (i) observations at waterholes and (ii) tracing spatial water-use patterns during summer by isotopically labeled water sources and blood sampling. More than 50% of the avian community (primarily insectivores and omnivores) were not observed to drink. The majority (53%) of species drinking at waterholes were granivorous, and their use of surface water was best predicted by their relative abundance in the community. Species representing the remaining dietary guilds drank significantly more on hot days. Blood samples revealed that only 11 of 42 species (mostly granivores and a few omnivores) showed evidence of drinking at a waterhole with enriched deuterium values; on average, in the latter birds, water from the enriched waterhole accounted for ~38% of their body water pool. These findings illustrate that 2 methods employed in this study provide different, but complementary data on the relative importance of a water source for an avian community. Although our results suggest that most avian species are independent of surface water, drinking patterns on the hottest days during our study period suggest that free-standing water might become more important for some of the non-drinking species under hotter climatic conditions