).Severe heat waves have occasionally led to catastrophic avian mortality in hot desert environments. Climate change models predict increases in the intensity, frequency and duration of heat waves. A model of avian evaporative water requirements and survival times during the hottest part of day reveals that the predicted increases in maximum air temperatures will result in large fractional increases in water requirements (in small birds, equivalent to 150 -200 % of current values), which will severely reduce survival times during extremely hot weather. By the 2080s, desert birds will experience reduced survival times much more frequently during mid-summer, increasing the frequency of catastrophic mortality events.

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

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.avian ecology | physiological ecology | climate change | heat waves | water balance E xtreme weather events are increasingly seen as an important factor in ecology and conservation, with consequential effects on individuals, populations, communities, and ecosystems (1-3). Recent data indicate an increase in the incidence of heat waves and extreme high temperatures (4, 5). Despite difficulties in quantifying trends in mass mortality events, heat waves are known to have caused a number of large-scale die-offs among birds, pteropodid bats, and other taxa in recent years (6, 7). Moreover, current (8) and projected (9) increases in the frequency, duration, and severity of heat waves are likely to make these mortality events more common as the century progresses (10).Birds may be particularly susceptible to heat waves given their typically diurnal activity periods, small size, and high mass-specific rates of metabolism and water loss. Small birds also have a very limited capacity to store vital resources such as water, and consequently must balance their water budgets over time scales of minutes to hours during hot weather (10). Constraints on water availability and heat stress are known to produce changes in behavior, reproductive success, occupancy, and mortality in birds (11). Heat-related mortali...

Basal metabolic rate (BMR) is often predicted by allometric interpolation, but such predictions are critically dependent on the quality of the data used to derive allometric equations relating BMR to body mass (Mb). An examination of the metabolic rates used to produce conventional and phylogenetically independent allometries for avian BMR in a recent analysis revealed that only 67 of 248 data unambiguously met the criteria for BMR and had sample sizes with n>/=3. The metabolic rates that represented BMR were significantly lower than those that did not meet the criteria for BMR or were measured under unspecified conditions. Moreover, our conventional allometric estimates of BMR (W; logBMR=-1.461+0.669logMb) using a more constrained data set that met the conditions that define BMR and had n>/=3 were 10%-12% lower than those obtained in the earlier analysis. The inclusion of data that do not represent BMR results in the overestimation of predicted BMR and can potentially lead to incorrect conclusions concerning metabolic adaptation. Our analyses using a data set that included only BMR with n>/=3 were consistent with the conclusion that BMR does not differ between passerine and nonpasserine birds after taking phylogeny into account. With an increased focus on data mining and synthetic analyses, our study suggests that a thorough knowledge of how data sets are generated and the underlying constraints on their interpretation is a necessary prerequisite for such exercises.

Ecological processes in arid lands are often described by the pulse-reserve paradigm, in which rain events drive biological activity until moisture is depleted, leaving a reserve. This paradigm is frequently applied to processes stimulated by one or a few precipitation events within a growing season. Here we expand the original framework in time and space and include other pulses that interact with rainfall. This new hierarchical pulse-dynamics framework integrates space and time through pulse-driven exchanges, interactions, transitions, and transfers that occur across individual to multiple pulses extending from micro to watershed scales. Climate change will likely alter the size, frequency, and intensity of precipitation pulses in the future, and arid-land ecosystems are known to be highly sensitive to climate variability. Thus, a more comprehensive understanding of arid-land pulse dynamics is needed to determine how these ecosystems will respond to, and be shaped by, increased climate variability.

Species interactions play key roles in linking the responses of populations, communities, and ecosystems to environmental change. For instance, species interactions are an important determinant of the complexity of changes in trophic biomass with variation in resources. Water resources are a major driver of terrestrial ecology and climate change is expected to greatly alter the distribution of this critical resource. While previous studies have documented strong effects of global environmental change on species interactions in general, responses can vary from region to region. Dryland ecosystems occupy more than one-third of the Earth's land mass, are greatly affected by changes in water availability, and are predicted to be hotspots of climate change. Thus, it is imperative to understand the effects of environmental change on these globally significant ecosystems. Here, we review studies of the responses of population-level plant-plant, plant-herbivore, and predator-prey interactions to changes in water availability in dryland environments in order to develop new hypotheses and predictions to guide future research. To help explain patterns of interaction outcomes, we developed a conceptual model that views interaction outcomes as shifting between (1) competition and facilitation (plant-plant), (2) herbivory, neutralism, or mutualism (plant-herbivore), or (3) neutralism and predation (predator-prey), as water availability crosses physiological, behavioural, or population-density thresholds. We link our conceptual model to hypothetical scenarios of current and future water availability to make testable predictions about the influence of changes in water availability on species interactions. We also examine potential implications of our conceptual model for the relative importance of top-down effects and the linearity of patterns of change in trophic biomass with changes in water availability. Finally, we highlight key research needs and some possible broader impacts of our findings. Overall, we hope to stimulate and guide future research that links changes in water availability to patterns of species interactions and the dynamics of populations and communities in dryland ecosystems.

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The physical environmental factors (air temperature, solar radiation, wind speed) that define specific microclimates and their effects on water and energy budgets of small birds are of major importance to our understanding of avian thermal biology. We examined the effects of solar radiation, wind speed, and their interaction on metabolic rates in the Verdin, Auriparus flaviceps. Daytime resting metabolic rates and evaporative water loss rates as a function of air temperature, as well as basal metabolic rate, were also measured to allow estimation of water and energy flux rates in diverse microclimates. In the absence of solar radiation, as wind speed was increased from 0.4 to 3.0 m/s, metabolic rate increased 14%. Exposure to simulated solar radiation significantly reduced metabolic heat production at all wind speeds measured except 3.0 m/s. Solar heat gain (SHG) was estimated for an irradiance of 1000 W/m$2$, similar to that commonly observed in nature. At 0.4 m/s wind speed and 1000 W/m$2$ irradiance, SHG may reduce metabolic rate by 46%. SHG declines precipitously as wind speed is increased, and at 3.0 m/s, metabolic rate is only reduced by 3%. Analyses of changes in thermostatic costs associated with microclimate selection in winter suggest that Verdins may reduce metabolic rate by as much as 50% by shifting from a shaded, windy site to one protected from the wind and exposed to 1000 W/m$2$ solar radiation. Similar analyses for Verdins during the summer suggest that microsite selection can result in significant water savings. By remaining out of the sun and wind, Verdins can reduce their rate of evaporative water loss by at least a factor of four. This analysis clearly demonstrates the potential importance of daytime microclimate selection to balancing water and energy budgets in small birds.