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...

Recent evidence suggests that avian facultative hypothermic responses are more common, and occur in a wider variety of ecological contexts, than previously thought. The capacity for shallow hypothermia (rest-phase hypothermia) occurs throughout the avian phylogeny, but the capacity for pronounced hypothermia (torpor) appears to be restricted to certain taxa. Families in which torpor has been reported include the Todidae, Coliidae, Trochilidae, Apodidae, Caprimulgidae, and Columbidae. Facultative hypothermia occurs in species ranging in body mass (Mb) from <3 g to ca. 6500 g. Minimum body temperature (Tb) during hypothermia is continuously distributed from 4.3°C to ca. 38°C. The physiological distinction between torpor and rest-phase hypothermia is unclear. Whereas these two responses have traditionally been distinguished on the basis of Tb, we find little support for the biological reality of specific Tb limits. Instead, we argue that emphasis should be placed on understanding the relationship between metabolic and Tb reduction and the capacity to respond to external stimuli. Patterns of thermoregulation during avian hypothermic responses are relatively variable, and do not necessarily follow the entry–maintenance–arousal patterns that characterize mammalian responses. Avian hypothermic responses are determined by a suite of ecological and physiological determinants including food availability, ambient temperature, hormone levels, and breeding cycle. Respuestas Facultativas de la Hipotermia en Aves: Una Revisión Resumen. Evidencias recientes sugieren que las respuestas facultativas de la hipotermia aviar son más comunes y ocurren en una gran cantidad de contextos ecológicos, a diferencia de lo que anteriormente se pensaba. La capacidad de una hipotermia ligera (hipotermia de descanso) ocurre en toda la filogenia de las aves, pero la capacidad de mantener una hipotermia pronunciada (torpor) aparece sólo en ciertos taxones. El torpor ha sido reportado en las familias Todidae, Coliidae, Trochilidae, Apodidae, Caprimulgidae y Columbidae. La hipotermia facultativa ocurre en especies con un peso corporal (Mb) de <3 g hasta 6.5 kg. Durante la hipotermia, la temperatura mínima corporal (Tb) está distribuída contínuamente entre 4.3°C y 38°C. La diferencia fisiológica entre el torpor y la hipotermia de descanso no es clara. Tradicionalmente se ha reconocido que las dos respuestas se basan en la Tb. Sin embargo, nosotros encontramos pocas evidencias biológicas sobre límites específicos de la Tb. Por el contrario, nosotros argumentamos que el énfasis debe enfocarse en la relación entre la reducción metabólica y de Tb y la capacidad de responder a estímulos externos. Los patrones de termoregulación de las respuestas hipotérmicas de las aves son relativamente variables y no necesariamente siguen los patrones de entrada-mantenimiento-elevación que caracterizan estas respuestas en los mamíferos. Las respuestas de la hipotermia en aves están determinadas por la interacción entre factores ecológicos y fisiológicos como disponibilidad de alimentos, temperatura ambiental, niveles hormonales y ciclo reproductivo.

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Abstract. Heterothermy plays an important role in lowering the costs of thermoregulation in endotherms by reducing water and energy requirements. We tested predictions that birds in arid habitats should express fine-scale variation in their thermoregulatory patterns as a function of prevailing climatic conditions. We assessed effects of air temperature (T air ) and water vapor pressure deficit (D) on body temperature (T b ) in free-living White-browed Sparrow-Weavers (Plocepasser mahali ) during summer in two arid habitats in the Kalahari Desert, South Africa, using data from a dry period at a hot, desert site (n ¼ 7 birds), and during a dry period (n ¼ 4 birds) and a wet period (n ¼ 5 birds) at a milder, semi-desert site. The desert birds maintained a significantly higher set-point T b (41.58 6 0.28C, mean 6 SD) than semi-desert birds (40.28 6 0.28C). During the warmest part of day (12:00-18:00 hours), T b increased significantly during periods of high T air and/or high humidity, and mean and maximum T b were up to 1.48 and 2.38C, respectively, above normal levels. However, as T air increased, birds at the desert site maintained T b at or below set-point levels for a greater proportion of the time than birds at the semi-desert site. Birds at the desert site also expressed a greater magnitude of daily heterothermy (heterothermy index, HI ¼ 2.48 6 0.38C, mean 6 SD) than birds at the semi-desert site: the latter population showed a greater magnitude of heterothermy during a dry period (HI ¼ 2.18 6 0.38C) than during a wet period (HI ¼ 1.68 6 0.28C). Birds continued foraging throughout the warmest part of the day, despite the fact that heat dissipation (percentage of time spent panting and wing-spreading) increased significantly with increasing T air . Our findings reveal that populations can vary in their thermoregulatory responses in both space and time and suggest that small changes in T air can have significant effects on thermoregulation in free-ranging desert birds, even when T air , T b . These data have important implications for assessing vulnerability of species to climate change, suggesting that sensitivity should be assessed at the population, rather than species, level.