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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.

We studied the plant resource use between and within populations of desert tortoise (Gopherus agassizii) across a precipitation gradient in the Sonoran Desert of Arizona. The carbon and nitrogen stable isotope values in animal tissues are a reflection of the carbon and nitrogen isotope values in diet, and consequently represent a powerful tool to study animal feeding ecology. We measured the δ(13)C and δ(15)N values in the growth rings on the shells of tortoises in different populations to characterize dietary specialization and track tortoise use of isotopically distinct C4/CAM versus C3 plant resources. Plants using C3 photosynthesis are generally more nutritious than C4 plants and these trait differences can have important growth and fitness consequences for consumers. We found that dietary specialization decreases in successively drier and less vegetated sites, and that broader population niche widths are accompanied by an increase in the dietary variability between individuals. Our results highlight how individual consumer plant resource use is bounded under a varying regime of precipitation and plant productivity, lending insight into how intra-individual dietary specialization varies over a spatial scale of environmental variability.

Extreme high environmental temperatures produce a variety of consequences for wildlife, including mass die-offs. Heat waves are increasing in frequency, intensity, and extent, and are projected to increase further under climate change. However, the spatial and temporal dynamics of die-off risk are poorly understood. Here, we examine the effects of heat waves on evaporative water loss (EWL) and survival in five desert passerine birds across the southwestern United States using a combination of physiological data, mechanistically informed models, and hourly geospatial temperature data. We ask how rates of EWL vary with temperature across species; how frequently, over what areas, and how rapidly lethal dehydration occurs; how EWL and die-off risk vary with body mass; and how dieoff risk is affected by climate warming. We find that smaller-bodied passerines are subject to higher rates of mass-specific EWL than larger-bodied counterparts and thus encounter potentially lethal conditions much more frequently, over shorter daily intervals, and over larger geographic areas. Warming by 4 °C greatly expands the extent, frequency, and intensity of dehydration risk, and introduces new threats for larger passerine birds, particularly those with limited geographic ranges. Our models reveal that increasing air temperatures and heat wave occurrence will potentially have important impacts on the water balance, daily activity, and geographic distribution of arid-zone birds. Impacts may be exacerbated by chronic effects and interactions with other environmental changes. This work underscores the importance of acute risks of high temperatures, particularly for small-bodied species, and suggests conservation of thermal refugia and water sources.

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.

Facultative hyperthermia, the elevation of body temperature above normothermic levels, during heat exposure, importantly affects the water economy and heat balance of terrestrial endotherms. We currently lack a mechanistic understanding of the benefits hyperthermia provides for avian taxa. Facultative hyperthermia has been proposed to minimize rates of water loss via three distinct mechanisms: M1) by maintaining body temperature (Tb) above environmental temperatures (Te), heat can be lost non‐evaporatively, saving water; M2) by minimizing the thermal gradient when Te > Tb, environmental heat gain and evaporative water loss rates are reduced; and M3) by storing heat via increases in Tb which reduces evaporative heat loss demands and conserves water. Although individuals may benefit from all three mechanisms during heat exposure, the relative importance of each mechanism has not been quantified among species that differ in their body size, heat tolerance and mechanisms of evaporative heat dissipation. We measured resting metabolism, evaporative water loss and real‐time Tb from 33 species of birds representing nine orders ranging in mass from 8 to 300 g and estimated the water savings associated with each proposed mechanism. We show that facultative hyperthermia varies in its benefits among species. Small songbirds with comparatively low evaporative cooling capacities benefit most from (M1), and hyperthermia maintains a thermal gradient that allows non‐evaporative heat losses. Other species benefited most from (M2) minimizing evaporative losses via a reduced thermal gradient for heat gain at high Te. We found that (M3), heat storage, only improved the water economy of the sandgrouse, providing little benefit to other species. We propose that differences in the frequency and magnitude of hyperthermia will drive taxon‐specific differences in temperature sensitivity of tissues and enzymes and that the evolution of thermoregulatory mechanisms of evaporative heat dissipation may contribute to differences in basal metabolic rate among avian orders. Understanding the mechanistic basis of heat tolerance is essential to advance our understanding of the ecology of birds living in hot environments that are warming rapidly, where extreme heat events are already re‐structuring avian communities. A plain language summary is available for this article. Plain Language Summary

Climate change threatens biodiversity by compromising the ability to balance energy and water, influencing animal behaviour, species interactions, distribution and ultimately survival. Predicting climate change effects on thermal physiology is complicated by interspecific variation in thermal tolerance limits, thermoregulatory behaviour and heterogenous thermal landscapes. We develop an approach for assessing thermal vulnerability for endotherms by incorporating behaviour and microsite data into a biophysical model. We parameterised the model using species-specific functional traits and published behavioural data on hotter (maximum daily temperature, Tmax > 35°C) and cooler days (Tmax < 35°C). Incorporating continuous time-activity focal observations of behaviour into the biophysical approach reveals that the three insectivorous birds modelled here are at greater risk of lethal hyperthermia than dehydration under climate change, contrary to previous thermal risk assessments. Southern yellow-billed hornbills Tockus leucomelas, southern pied babblers Turdoides bicolor and southern fiscals Lanius collaris are predicted to experience a risk of lethal hyperthermia on ~ 24, 65 and 40 more days year−1 , respectively, in 2100 relative to current conditions. Maintaining water balance may also become increasingly challenging. Babblers are predicted to experience a 57% increase (to ~186 days year−1 ) in exposure to conditions associated with net negative 24 h water balance in the absence of drinking, with ~ 86 of those days associated with a risk of lethal dehydration. Hornbills and fiscals are predicted to experience ~ 84 and 100 days year−1 , respectively, associated with net negative 24 h water balance, with ≤ 20 of those days associated with a risk of lethal dehydration. Integrating continuous time-activity focal data is vital to understand and predict thermal challenges animals likely experience. We provide a comprehensive thermal risk assessment and emphasise the importance of thermoregulatory and drinking behaviour for endotherm persistence in coming decades.

Environmental temperatures that exceed body temperature (Tb) force endothermic animals to rely solely on evaporative cooling to dissipate heat. However, evaporative heat dissipation can be drastically reduced by environmental humidity, imposing a thermoregulatory challenge. The goal of this study was to investigate the effects of humidity on the thermoregulation of desert birds and to compare the sensitivity of cutaneous and respiratory evaporation to reduced vapor density gradients. Rates of evaporative water loss, metabolic rate, and Tb were measured in birds exposed to humidities ranging from ∼2 to 30 g H2O m23 (0%–100% relative humidity at 307C) at air temperatures between 447 and 567C. In sociable weavers, a species that dissipates heat primarily through panting, rates of evaporative water loss were inhibited by as much as 36% by high humidity at 487C, and these birds showed a high degree of hyperthermia. At lower temperatures (407–447C), evaporative water loss was largely unaffected by humidity in this species. In Namaqua doves, which primarily use cutaneous evaporation, increasing humidity reduced rates of evaporative water loss, but overall rates of water loss were lower than those observed in sociable weavers. Our data suggest that cutaneous evaporation is more efficient than panting, requiring less water to maintain Tb at a given temperature, but panting appears less sensitive to humidity over the air temperature range investigated here.

Intense heat waves are occurring more frequently, with concomitant increases in the risk of catastrophic avian mortality events via lethal dehydration or hyperthermia. We quantified the risks of lethal hyperthermia and dehydration for 10 Australian aridzone avifauna species during the 21st century, by synthesizing thermal physiology data on evaporative water losses and heat tolerance limits. We evaluated risks of lethal hyperthermia or exceedance of dehydration tolerance limits in the absence of drinking during the hottest part of the day under recent climatic conditions, compared to those predicted for the end of this century across Australia. Increases in mortality risk via lethal dehydration and hyperthermia vary among the species modelled here but will generally increase greatly, particularly in smaller species (∼10–42 g) and those inhabiting the far western parts of the continent. By 2100 CE, zebra finches’ potential exposure to acute lethal dehydration risk will reach ∼ 100 d y−1 in the far northwest of Australia and will exceed 20 d y−1 over > 50% of this species’ current range. Risks of dehydration and hyperthermia will remain much lower for large non-passerines such as crested pigeons. Risks of lethal hyperthermia will also increase substantially for smaller species, particularly if they are forced to visit exposed water sources at very high air temperatures to avoid dehydration. An analysis of atlas data for zebra finches suggests that population declines associated with very hot conditions are already occurring in the hottest areas. Our findings suggest that the likelihood of persistence within current species ranges, and the potential for range shifts, will become increasingly constrained by temperature and access to drinking water. Our model adds to an increasing body of literature suggesting that arid environments globally will experience considerable losses of avifauna and biodiversity under unmitigated climate change scenarios.

Nightjars represent a model taxon for investigating physiological limits of heat tolerance because of their habit of roosting and nesting in sunlit sites during the heat of the day. We investigated the physiological responses of Rufous-cheeked nightjars ( Caprimulgus rufigena ) and Freckled nightjars ( Caprimulgus tristigma ) to high air temperatures ( T a ) by measuring body temperature ( T b ), resting metabolic rate (RMR) and total evaporative water loss (TEWL) at T a ranging from 10 to 56 °C. Both species became hyperthermic at T a  >  T b . Lower critical limits of thermoneutrality occurred at T a between 35 and 37 °C, whereas we detected no clear upper critical limits of thermoneutrality. Between T a  ≈ 37.0 and 39.9 °C, rates of TEWL increased rapidly with T a . At T a  ≥ 40 °C, fractional increases in mass-specific TEWL rates were 78–106% of allometric predictions. Increasing evaporative heat dissipation incurred only small metabolic costs, with the RMR of neither species ever increasing by more than 20% above thermoneutral values. Consequently, both species displayed extremely efficient evaporative cooling; maximum evaporative heat dissipation was equivalent to 515% of metabolic heat production (MHP) at T a  ≈ 56 °C in C. rufigena and 452% of MHP at T a  ≈ 52 °C in C. tristigma . Our data reiterate that caprimulgids have evolved an efficient mechanism of evaporative cooling via gular fluttering, which minimizes metabolic heat production at high T a and reduces total heat loads. This likely aids in reducing TEWL rates and helps nightjars cope with some of the most thermally challenging conditions experienced by any bird.

The deserts of the southwestern US are experiencing rapid warming, and climate models predict declining winter precipitation. The combined effects of higher air temperatures and drought are a reduction in productivity, which may importantly impact reproduction in consumers. Here, we investigate the effects of warming and drought on the reproductive timing and output in loggerhead shrikes (Lanius ludovicianus) in central New Mexico from 2007 to 2012. We found increases in air temperature of 3 °C during the breeding season (March–July) and highly variable winter and annual precipitation. With increasing spring temperatures, shrikes advanced nesting phenology by 20 days over 6 years, a much higher rate than is reported for any other bird species. During this period, the number of breeding pairs also increased from 25 to 37, and clutch size and the number offspring produced per successful nest did not vary. Nest success, however, was often very low and ranged from 11 to 46 %. Although our models indicated that low nest success was driven by precipitation and temperature, it was mediated indirectly through increased predation rates during the hot and dry periods.