ABSTRACT Relatively little effort has been directed towards elucidating the role of physiological stress pathways in mediating avian responses to global heating. For free-ranging southern pied babblers, Turdoides bicolor , daily maximum air temperatures ( T max ) between ∼35 and ∼40°C result in reduced foraging efficiency, loss of body mass and compromised breeding success. We tested the hypothesis that very hot days are experienced as stressors by quantifying relationships between T max and faecal glucocorticoid metabolite (fGCM) levels in naturally excreted droppings. On days when T max <38°C, fGCM levels were independent of T max (mean±s.d. 140.25±56.92 ng g −1 dry mass). At T max >38°C, however, fGCM levels increased linearly with T max and averaged 190.79±70.13 ng g −1 dry mass. The effects of T max on fGCM levels did not carry over to the following morning, suggesting that very hot days are experienced as acute stressors.

ABSTRACT Evaporative heat dissipation is a key aspect of avian thermoregulation in hot environments. We quantified variation in avian thermoregulatory performance at high air temperatures ( T a ) using published data on body temperature ( T b ), evaporative water loss (EWL) and resting metabolic rate (RMR) measured under standardized conditions of very low humidity in 56 arid-zone species. Maximum T b 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 T b and the difference between maximum and normothermic T b decreased significantly with body mass ( M b ). 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 ( T uc ) varied by >20°C and maximum RMR during acute heat exposure scaled to M b 0.75 in both the overall data set and among passerines. The slope of RMR at T a > T uc increased significantly with M b but was substantially higher among passerines, which rely on panting, compared with 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.

ABSTRACT At a global scale, thermal physiology is correlated with climatic variables such as temperature and aridity. There is also evidence that thermoregulatory traits vary with fine-scale microclimate, but this has received less attention in endotherms. Here, we test the hypothesis that avian thermoregulation varies with microclimate and behavioural constraints in a non-passerine bird. Male and female southern yellow-billed hornbills ( Tockus leucomelas ) experience markedly different microclimates while breeding, with the female sealing herself into a tree cavity and moulting all her flight feathers during the breeding attempt, becoming entirely reliant on the male for provisioning. We examined interactions between resting metabolic rate (RMR), evaporative water loss (EWL) and core body temperature ( T b ) at air temperatures ( T a ) between 30°C and 52°C in male and female hornbills, and quantified evaporative cooling efficiencies and heat tolerance limits. At thermoneutral T a , neither RMR, EWL nor T b differed between sexes. At T a >40°C, however, RMR and EWL of females were significantly lower than those of males, by ∼13% and ∼17%, respectively, despite similar relationships between T b and T a , maximum ratio of evaporative heat loss to metabolic heat production and heat tolerance limits (∼50°C). These sex-specific differences in hornbill thermoregulation support the hypothesis that avian thermal physiology can vary within species in response to fine-scale microclimatic factors. In addition, Q 10 for RMR varied substantially, with Q 10 ≤2 in some individuals, supporting recent arguments that active metabolic suppression may be an underappreciated aspect of endotherm thermoregulation in the heat.

ABSTRACT Small birds inhabiting northern temperate and boreal latitudes typically increase metabolic rates during cold winters or acclimation to low air temperatures ( T accl ). Recent studies suggest considerable variation in patterns of seasonal metabolic acclimatization in birds from subtropical and tropical regions with milder winters, but there remains a dearth of acclimation studies investigating metabolic flexibility among lower-latitude birds. We used short-term thermal acclimation experiments to investigate phenotypic flexibility in basal metabolic rate (BMR), thermoneutral evaporative water loss (EWL) and summit metabolism ( M sum ) in three populations of white-browed sparrow-weavers ( Plocepasser mahali ) along a climatic and aridity gradient. We allocated individuals to one of three T accl treatments (5, 20 and 35°C; n =11 per population per T accl ) for 28 days, and measured post-acclimation BMR, EWL and M sum using flow-through respirometry. Our data reveal the expected pattern of lower BMR and EWL (∼12% and 25% lower, respectively) in birds at T accl =35°C compared with cooler T accl treatments, as observed in previous acclimation studies on subtropical birds. We found no variation in the reaction norms of BMR and EWL among populations in response to acclimation, suggesting previously documented differences in seasonal BMR acclimatization are the result of phenotypic flexibility. In contrast to higher-latitude species, M sum did not significantly vary in response to thermal acclimation. These findings support the idea that factors other than enhancing cold tolerance may be driving patterns of metabolic variation in subtropical birds.

ABSTRACT Evaporative cooling is a prerequisite for avian occupancy of hot, arid environments, and is the only avenue of heat dissipation when air temperatures ( T a ) exceed body temperature ( T b ). Whereas diurnal birds can potentially rehydrate throughout the day, nocturnal species typically forgo drinking between sunrise and sunset. We hypothesized that nocturnal birds have evolved reduced rates of evaporative water loss (EWL) and more economical evaporative cooling mechanisms compared with diurnal species, permitting nocturnal species to tolerate extended periods of intense heat without becoming lethally dehydrated. We used phylogenetically informed regressions to compare EWL and evaporative cooling efficiency [ratio of evaporative heat loss (EHL) and metabolic heat production (MHP); EHL/MHP] among nocturnal and diurnal birds at high T a . We analyzed variation in three response variables: (1) slope of EWL at T a between 40 and 46°C, (2) EWL at T a =46°C and (3) EHL/MHP at T a =46°C. Nocturnality emerged as a weak, negative predictor, with nocturnal species having slightly shallower slopes and reduced EWL compared with diurnal species of similar mass. In contrast, nocturnal activity was positively correlated with EHL/MHP, indicating a greater capacity for evaporative cooling in nocturnal birds. However, our analysis also revealed conspicuous differences among nocturnal taxa. Caprimulgids and Australian owlet-nightjars had shallower slopes and reduced EWL compared with similarly sized diurnal species, whereas owls had EWL rates comparable to those of diurnal species. Consequently, our results did not unequivocally demonstrate more economical cooling among nocturnal birds. Owls predominately select refugia with cooler microclimates, but the more frequent and intense heat waves forecast for the 21st century may increase microclimate temperatures and the necessity for active heat dissipation, potentially increasing owls' vulnerability to dehydration and hyperthermia.

ABSTRACT The thermoregulatory responses of owls to heat stress have been the subject of few studies. Although nocturnality buffers desert-dwelling owls from significant heat stress during activity, roost sites in tree and cactus cavities or in deep shade provide only limited refuge from high environmental temperatures during the day. We measured thermoregulatory responses to acute heat stress in two species of small owls, the elf owl ( Micrathene whitneyi ) and the western screech-owl ( Megascops kennicottii ), which occupy the Sonoran Desert of southwestern North America, an area of extreme heat and aridity. We exposed wild-caught birds to progressively increasing air temperatures ( T a ) and measured resting metabolic rate (RMR), evaporative water loss (EWL), body temperature ( T b ) and heat tolerance limits (HTL; the maximum T a reached). Comparatively low RMR values were observed in both species, T b approximated T a at 40°C and mild hyperthermia occurred as T a was increased toward the HTL. Elf owls and screech-owls reached HTLs of 48 and 52°C, respectively, and RMR increased to 1.5 and 1.9 times thermoneutral values. Rates of EWL at the HTL allowed for the dissipation of 167–198% of metabolic heat production (MHP). Gular flutter was used as the primary means of evaporative heat dissipation and produced large increases in evaporative heat loss (44–100%), accompanied by only small increases (<5%) in RMR. These small, cavity-nesting owls have thermoregulatory capacities that are intermediate between those of the open-ground nesting nightjars and the passerines that occupy the same ecosystem.

ABSTRACT Avian orders differ in their thermoregulatory capabilities and tolerance of high environmental temperatures. Evaporative heat loss, and the primary avenue whereby it occurs, differs amongst taxa. Although Australian parrots (Psittaciformes) have been impacted by mass mortality events associated with extreme weather events (heat waves), their thermoregulatory physiology has not been well characterized. We quantified the upper limits to thermoregulation under extremely hot conditions in two Australian parrots: the mulga parrot ( Psephotellus varius ; ∼55 g) and the galah ( Eolophus roseicapilla ; ∼265 g). At air temperatures ( T a ) exceeding body temperature ( T b ), both species showed increases in T b to maximum values around 43–44°C, accompanied by rapid increases in resting metabolic rate above clearly defined upper critical limits of thermoneutrality and increases in evaporative water loss to levels equivalent to 700–1000% of baseline rates at thermoneutral T a . Maximum cooling capacity, quantified as the fraction of metabolic heat production dissipated evaporatively, ranged from 1.71 to 1.79, consistent with the known range for parrots, similar to the corresponding range in passerines, and well below the corresponding ranges for columbids and caprimulgids. Heat tolerance limit (the maximum T a tolerated) ranged from 44 to 55°C, similar to the range reported for passerines, but lower than that reported for columbids and caprimulgids. Our data suggest that heat tolerance in parrots is similar to that in passerines. We argue that understanding how thermoregulatory capacity and heat tolerance vary across avian orders is vital for predicting how climate change and the associated increase in frequency of extreme weather events may impact avian populations in the future.

ABSTRACT Little is known about the phylogenetic variation of avian evaporative cooling efficiency and heat tolerance in hot environments. We quantified thermoregulatory responses to high air temperature ( T a ) in ∼100-g representatives of three orders, namely, the African cuckoo ( Cuculus gularis , Cuculiformes), lilac-breasted roller ( Coracias caudatus , Coraciiformes) and Burchell's starling ( Lamprotornis australis , Passeriformes). All three species initiated respiratory mechanisms to increase evaporative heat dissipation when body temperature ( T b ) approached 41.5°C in response to increasing T a , with gular flutter observed in cuckoos and panting in rollers and starlings. Resting metabolic rate and evaporative water loss increased by quantitatively similar magnitudes in all three species, although maximum rates of evaporative water loss were proportionately lower in starlings. Evaporative cooling efficiency [defined as the ratio of evaporative heat loss (EHL) to metabolic heat production (MHP)] generally remained below 2.0 in cuckoos and starlings, but reached a maximum of ∼3.5 in rollers. The high value for rollers reveals a very efficient evaporative cooling mechanism, and is similar to EHL/MHP maxima for similarly sized columbids which very effectively dissipate heat via cutaneous evaporation. This unexpected phylogenetic variation among the orders tested in the physiological mechanisms of heat dissipation is an important step toward determining the evolution of heat tolerance traits in desert birds.

ABSTRACT Birds in the order Caprimulgiformes (nightjars and allies) have a remarkable capacity for thermoregulation over a wide range of environmental temperatures, exhibiting pronounced heterothermy in cool conditions and extreme heat tolerance at high environmental temperatures. We measured thermoregulatory responses to acute heat stress in three species of Caprimulgiformes that nest in areas of extreme heat and aridity, the common poorwill ( Phalaenoptilus nuttallii : Caprimulgidae) and lesser nighthawk ( Chordeiles acutipennis : Caprimulgidae) in the Sonoran Desert of Arizona, and the Australian owlet-nightjar ( Aegotheles cristatus : Aegothelidae) in the mallee woodlands of South Australia. We exposed wild-caught birds to progressively increasing air temperatures ( T a ) and measured resting metabolic rate (RMR), evaporative water loss (EWL), body temperature ( T b ) and heat tolerance limit (HTL; the maximum T a reached). Comparatively low RMR values were observed in all species (0.35, 0.36 and 0.40 W for the poorwill, nighthawk and owlet-nightjar, respectively), with T b approximating T a at 40°C and mild hyperthermia occurring as T a reached the HTL. Nighthawks and poorwills reached HTLs of 60 and 62°C, respectively, whereas the owlet-nightjar had a HTL of 52°C. RMR increased gradually above minima at T a of 42, 42 and 35°C, and reached 1.7, 1.9 and 2.0 times minimum resting values at HTLs in the poorwill, nighthawk and owlet-nightjar, respectively. EWL increased rapidly and linearly as T a exceeded T b and resulted in maximum rates of evaporative heat dissipation equivalent to 237–424% of metabolic heat production. Bouts of gular flutter resulted in large transient increases in evaporative heat loss (50–123%) accompanied by only small increments in RMR (<5%). The cavity-nesting/roosting owlet-nightjar had a lower HTL and less efficient evaporative cooling compared with the species that nest and/or roost on open desert surfaces. The high efficiency of gular flutter for evaporative cooling, combined with mild hyperthermia, provides the physiological basis for defending T b well below T a in extreme heat and is comparable to the efficient cooling observed in arid-zone columbids in which cutaneous EWL is the predominant cooling pathway.

ABSTRACT We examined thermoregulatory performance in seven Sonoran Desert passerine bird species varying in body mass from 10 to 70 g – lesser goldfinch, house finch, pyrrhuloxia, cactus wren, northern cardinal, Abert's towhee and curve-billed thrasher. Using flow-through respirometry, we measured daytime resting metabolism, evaporative water loss and body temperature at air temperatures ( T air ) between 30 and 52°C. We found marked increases in resting metabolism above the upper critical temperature ( T uc ), which for six of the seven species fell within a relatively narrow range (36.2–39.7°C), but which was considerably higher in the largest species, the curve-billed thrasher (42.6°C). Resting metabolism and evaporative water loss were minimal below the T uc and increased with T air and body mass to maximum values among species of 0.38–1.62 W and 0.87–4.02 g H 2 O h −1 , respectively. Body temperature reached maximum values ranging from 43.5 to 45.3°C. Evaporative cooling capacity, the ratio of evaporative heat loss to metabolic heat production, reached maximum values ranging from 1.39 to 2.06, consistent with known values for passeriforms and much lower than values in taxa such as columbiforms and caprimulgiforms. These maximum values occurred at heat tolerance limits that did not scale with body mass among species, but were ∼50°C for all species except the pyrrhuloxia and Abert's towhee (48°C). High metabolic costs associated with respiratory evaporation appeared to drive the limited heat tolerance in these desert passeriforms, compared with larger desert columbiforms and galliforms that use metabolically more efficient mechanisms of evaporative heat loss.

ABSTRACT Evaporative heat loss pathways vary among avian orders, but the extent to which evaporative cooling capacity and heat tolerance vary within orders remains unclear. We quantified the upper limits to thermoregulation under extremely hot conditions in five Australian passerines: yellow-plumed honeyeater ( Lichenostomus ornatus ; ∼17 g), spiny-cheeked honeyeater ( Acanthagenys rufogularis ; ∼42 g), chestnut-crowned babbler ( Pomatostomus ruficeps ; ∼52 g), grey butcherbird ( Cracticus torquatus ; ∼86 g) and apostlebird ( Struthidea cinerea ; ∼118 g). At air temperatures ( T a ) exceeding body temperature ( T b ), all five species showed increases in T b to maximum values around 44–45°C, accompanied by rapid increases in resting metabolic rate above clearly defined upper critical limits of thermoneutrality and increases in evaporative water loss (EWL) to levels equivalent to 670–860% of baseline rates at thermoneutral T a . Maximum cooling capacity, quantified as the fraction of metabolic heat production dissipated evaporatively, ranged from 1.20 to 2.17, consistent with the known range for passerines, and well below the corresponding ranges for columbids and caprimulgids. Heat tolerance limit (HTL, the maximum T a tolerated) scaled positively with body mass, varying from 46°C in yellow-plumed honeyeaters to 52°C in a single apostlebird, but was lower than that of three southern African ploceid passerines investigated previously. We argue this difference is functionally linked to a smaller scope for increases in EWL above baseline levels. Our data reiterate the reliance of passerines in general on respiratory evaporative heat loss via panting, but also reveal substantial within-order variation in heat tolerance and evaporative cooling capacity.

ABSTRACT Climate change has the potential to trigger social change. As a first step towards understanding mechanisms determining the vulnerability of animal societies to rising temperatures, we investigated interactions between social rank and thermoregulation in three arid-zone bird species: fawn-coloured lark ( Mirafra africanoides , territorial); African red-eyed bulbul ( Pycnonotus nigricans , loosely social) and sociable weaver ( Philetairus socius , complex cooperative societies). We assessed relationships between body temperature ( T b ), air temperature ( T a ) and social rank in captive groups in the Kalahari Desert. Socially dominant weavers and bulbuls had lower mean T b than subordinate conspecifics, and dominant individuals of all species maintained more stable T b as T a increased. Dominant bulbuls and larks tended to monopolise available shade, but dominant weavers did not. Nevertheless, dominant weavers thermoregulated more precisely, despite expending no more behavioural effort on thermoregulation than subordinates. Increasingly unequal risks associated with heat stress may have implications for the stability of animal societies in warmer climates.

ABSTRACT Birds show phylogenetic variation in the relative importance of respiratory versus cutaneous evaporation, but the consequences for heat tolerance and evaporative cooling capacity remain unclear. We measured evaporative water loss (EWL), resting metabolic rate (RMR) and body temperature ( T b ) in four arid-zone columbids from southern Africa [Namaqua dove ( Oena capensis , ∼37 g), laughing dove ( Spilopelia senegalensis , ∼89 g) and Cape turtle dove ( Streptopelia capicola , ∼148 g)] and Australia [crested pigeon ( Ocyphaps lophotes ), ∼186 g] at air temperatures ( T a ) of up to 62°C. There was no clear relationship between body mass and maximum T a tolerated during acute heat exposure. Maximum T b at very high T a was 43.1±1.0, 43.7±0.8, 44.7±0.3 and 44.3±0.8°C in Namaqua doves, laughing doves, Cape turtle doves and crested pigeons, respectively. In all four species, RMR increased significantly at T a above thermoneutrality, but the increases were relatively modest with RMR at T a =56°C being 32, 60, 99 and 11% higher, respectively, than at T a =35°C. At the highest T a values reached, evaporative heat loss was equivalent to 466, 227, 230 and 275% of metabolic heat production. The maximum ratio of evaporative heat loss to metabolic production observed in Namaqua doves, 4.66, exceeds by a substantial margin previous values reported for birds. Our results support the notion that cutaneous evaporation provides a highly efficient mechanism of heat dissipation and an enhanced ability to tolerate extremely high T a .

ABSTRACT Sandgrouse (Pterocliformes) are quintessential examples of avian adaptation to desert environments, but relatively little is known about the limits to their heat tolerance and evaporative cooling capacity. We predicted that evaporative cooling in Burchell's sandgrouse ( Pterocles burchelli ) is highly efficient and provides the basis for tolerance of very high air temperature ( T a ). We measured body temperature ( T b ), resting metabolic rate (RMR) and evaporative water loss (EWL) at T a between 25°C and ∼58°C in birds exposed to successive increments in T a . Normothermic T b averaged 39.0°C, lower than typical avian values. At T a >34.5°C, T b increased linearly to a maximum of 43.6°C at T a =56°C. The upper critical limit of thermoneutrality ( T uc ) was T a =43.8°C, closely coinciding with the onset of panting and gular flutter. Above the T uc , RMR increased 2.5-fold to 2.89 W at T a =56°C, a fractional increase far exceeding that of many other species under comparable conditions. Rates of EWL increased rapidly at T a >42.9°C to 7.84±0.90 g h −1 at T a =56°C, an 11-fold increase above minimal levels. Maximum evaporative cooling efficiency (ratio of evaporative heat loss to metabolic heat production) was 2.03, but could be as high as 2.70 if our assumption that the birds were metabolising lipids is incorrect. Thermoregulation at very high T a in P. burchelli was characterised by large increases in RMR and EWL, and is much less efficient than in taxa such as columbids and caprimulgids.

ABSTRACT 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 ( T a ). We measured resting metabolic rate (RMR) and total evaporative water loss (TEWL) using open flow-through respirometry, and core body temperature ( T b ) using passive integrated transponder tags. Sparrow-weavers were exposed to a ramped profile of progressively higher T a between 30 and 52°C to elicit maximum evaporative cooling capacity ( N =10 per site per season); the maximum T a birds tolerated before the onset of severe hyperthermia ( T b ≈44°C) was considered to be their hyperthermia threshold T a ( T a,HT ). Our data reveal significant seasonal acclimatisation of heat tolerance, with a desert population of sparrow-weavers reaching significantly higher T a in summer (49.5±1.4°C, i.e. higher T a,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 ( T a,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.

ABSTRACT 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 scaly-feathered weaver ( Sporopipes squamifrons , M b ≈10 g), sociable weaver ( Philetairus socius , M b ≈25 g) and white-browed sparrow-weaver ( 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.

Summit metabolism ( M sum ), the maximum rate of resting metabolic thermogenesis, has been found to be broadly correlated with climatic variables and the use of heterothermy in some endotherms. Far less is known about M sum and metabolic expansibility [ME, the ratio of M sum to basal metabolic rate (BMR)] in bats compared with many other endotherm taxa. We measured BMR and M sum during winter and summer in captive and wild populations of a pteropodid from the southern subtropics, Wahlberg's epauletted fruit bat ( Epomophorus wahlbergi ) in Pretoria, South Africa. The M sum of fruit bats ranged from 5.178±0.611 W (captive, summer) to 6.006±0.890 W (captive, winter), and did not vary significantly between seasons. In contrast, BMR decreased by 17–25% in winter. The combination of seasonally stable M sum but flexible BMR resulted in ME being significantly higher in winter than in summer, ranging from 7.24±1.49 (wild, summer) to 13.11±2.14 (captive, winter). The latter value is well above the typical mammalian range. Moreover, both M sum and ME were significantly higher in captive bats than in wild individuals; we speculate this represents a phenotypic response to a reduction in exercise-associated heat production while in captivity. Our data for E. wahlbergi , combined with those currently available for other chiropterans, reveal that M sum in bats is highly variable compared with allometrically expected values for other mammals.

SUMMARY Many birds exhibit considerable phenotypic flexibility in maintenance energy requirements, and up- or downregulate basal metabolic rate (BMR) over time scales of days to weeks during thermal acclimation. However, the extent to which individual birds can reverse the direction of BMR adjustments over short time scales remains unknown. In this study, we examined metabolic responses to short-term thermal acclimation in laughing doves Streptopelia senegalensis . In 30 wild-caught doves (mean body mass=92.6 g) divided into three experimental groups of 10 birds each, initial BMR averaged 0.760±0.036 W. Thereafter, each group was acclimated to one of three acclimation air temperatures ( T acc =10, 22 or 35°C) for 21 days, during which time the doves were housed in individual cages. Following the first acclimation period (acclimation I), BMR (W) was significantly lower and was negatively and linearly related to T acc [BMR=0.714-0.005 T acc ]. Acclimation I BMR varied from 0.546±0.039 W in doves acclimated to T acc =35°C to 0.665±0.058 W at T acc =10°C. A second acclimation period of a further 21 days (acclimation II) revealed that the direction of BMR adjustments could be reversed within individuals, with acclimation II BMR again negatively and linearly related to T acc . The slope of the relationship between BMR and T acc following acclimation II was not significantly different to that following acclimation I. BMR exhibited consistent inter-individual variation, with a low but significant repeatability of 0.113. The within-individual BMR variation of up to 26% that we observed in laughing doves reveals that BMR is a highly flexible trait in this species, and reiterates the need to take phenotypic plasticity into account in comparative analyses of avian energetic parameters.

SUMMARY We investigated changes in the relative contributions of respiratory evaporative water loss (REWL) and cutaneous evaporative water loss (CEWL) to total evaporative water loss (TEWL) in response to short-term thermal acclimation in western white-winged doves Zenaida asiatica mearnsii . We measured REWL, CEWL, oxygen consumption and carbon dioxide production in a partitioned chamber using flow-through respirometry. In doves housed for 2-4 weeks in a room heated to ca . 43°C during the day, TEWL increased from 5.5±1.3 mg g -1 h -1 at an air temperature( T a ) of 35°C to 19.3±2.5 mg g -1 h -1 at T a =45°C. In doves housed at room temperature for the same period, TEWL increased from 4.6±1.1 mg g -1 h -1 at T a =35°C to 16.1±4.6 mg g -1 h -1 at T a =45°C. The CEWL of heat-acclimated doves increased from 3.6±1.2 mg g -1 h -1 (64% of TEWL) at 35°C to 15.0±2.1 mg g -1 h -1 (78% of TEWL) at T a =45°C. Cool-acclimated doves exhibited more modest increases in CEWL, from 2.7±0.7 mg g -1 h -1 at T a =35°C to 7.8±3.4 mg g -1 h -1 at T a =45°C, with the contribution of CEWL to TEWL averaging 53% over this T a range. Cool-acclimated doves became mildly hyperthermic (body temperature T b =42.9±0.4°C) and expended 35% more energy relative to heat-acclimated doves( T b =41.9±0.6°C) at T a =45°C, even though TEWL in the two groups was similar. In each of the two groups, metabolic rate did not vary with T a , and averaged 7.1±0.5 mW g -1 in cool-acclimated doves and 6.3±0.8 mW g -1 in heat-acclimated doves. The differences in TEWL partitioning we observed between the two experimental groups resulted from a consistently lower skin water vapour diffusion resistance ( r v ) in the heat-acclimated doves. At T a =45°C, r v in the cool-acclimated doves was 120±81 s cm -1 , whereas r v in the heat-acclimated doves was 38±8 s cm -1 . Our data reveal that in Z. a. mearnsii , TEWL partitioning varies in response to short-term thermal acclimation.