Thermoregulatory responses in seasonally acclimatized captive Southern White-faced Scops-owls

Seasonal thermoregulatory responses that are associated with cold tolerance have been reported for many species that inhabit regions where winters are severe (e.g. Holarctic), but relatively few studies have focused on species from regions where the climate is more unpredictable (e.g. Southern Africa). In this study, metabolic rate (VO₂) and body temperature (T_b) was measured during summer and winter in captive Southern White-faced Scops-owl (Ptilopsis granti), to test for thermoregulatory responses representing energy conservation in winter. During winter the Southern White-faced Scops-owls increased resting metabolic rate (RMR) by 45% to regulate a set point T_b—a result similar to what had been shown in small passerines from the Holarctic region. Increased RMR and increased conductance at cold T_a's are suggestive of improved cold tolerance. Basal metabolic rate (BMR) was 0.60 mL O₂ g⁻¹ h⁻¹ and showed no seasonal flexibility. Thus, contrary to expectation, the Southern White-faced Scops-owls showed seasonal thermoregulatory responses that are unlikely to represent energy conservation which was expected for a medium-sized bird inhabiting unpredictable climates in Southern Africa.     

Metabolic and ventilatory acclimatization to cold stress in house sparrows (Passer domesticus)

Passerines that overwinter in temperate climates undergo seasonal acclimatization that is characterized by metabolic adjustments that may include increased basal metabolic rate (BMR) and cold-induced summit metabolism (M(sum)) in winter relative to summer. Metabolic changes must be supported by equivalent changes in oxygen transport. While much is known about the morphology of the avian respiratory system, little is known about respiratory function under extreme cold stress. We examined seasonal variation in BMR, M(sum), and ventilation in seasonally acclimatized house sparrows from Wisconsin. BMR and M(sum) increased significantly in winter compared with summer. In winter, BMR increased 64%, and M(sum) increased 29% over summer values. The 64% increase in winter BMR is the highest recorded for birds. Metabolic expansibility (M(sum)/BMR) was 9.0 in summer and 6.9 in winter birds. The metabolic expansibility of 9.0 in summer is the highest yet recorded for birds. Ventilatory accommodation under helox cold stress was due to changes in breathing frequency (f), tidal volume, and oxygen extraction efficiency in both seasons. However, the only significant difference between summer and winter ventilation measures in helox cold stress was f. Mean f in helox cold stress for winter birds was 1.23 times summer values.     

Seasonal effects on metabolism and thermoregulation abilities of the Red-winged Starling (Onychognathus morio)

Basal metabolic rate (BMR) of birds is beginning to be viewed as a highly flexible physiological trait influenced by environmental fluctuations, and in particular changes in ambient temperatures (T_a). Southern Africa is characterized by an unpredictable environment with daily and seasonal variation. This study sought to evaluate the effects of seasonal changes in T_a on mass-specific resting metabolic rate (RMR), BMR and body temperature (T_b) of Red-winged Starlings (Onychognathus morio). They have a broad distribution, from Ethiopia to the Cape in South Africa and are medium-sized frugivorous birds. Metabolic rate (VO₂) and T_b were measured in wild caught Red-winged Starlings after a period of summer and winter acclimatization in outdoor aviaries. RMR and BMR were significantly higher in winter than summer. Body mass of Starlings was significantly higher in winter compared with summer. The increased RMR and BMR in winter indicate improved ability to cope with cold and maintenance of a high T_b. These results show that the metabolism of Red-winged Starlings are not constant, but exhibit a pronounced seasonal phenotypic flexibility with maintenance of a high T_b.     

Seasonal metabolic acclimatization in mountain chickadees and juniper titmice

Mountain chickadees and juniper titmice from northern Utah were examined to determine metabolic and body-composition characteristics associated with seasonal acclimatization. These species use behavioral adaptations and nocturnal hypothermia, which reduce energetic costs. These adjustments could reduce the need for extensive metabolic adjustments typically found in small passerines that overwinter in cold regions. In addition, these species live at higher altitudes, which may also decrease metabolic acclimatization found in birds. Winter birds tolerated colder test temperatures than summer birds. This improved cold tolerance was associated with an increase in maximal thermogenic capacity or summit metabolism (M(sum)). Winter M(sum) exceeded summer M(sum) by 26.1% in chickadees and 16.2% in titmice. Basal metabolic rates (BMR) were also significantly higher in winter birds compared with summer birds. Pectoralis wet muscle mass increased 33.3% in chickadees and 24.1% in titmice in winter and paralleled the increased M(sum) and BMR. Dry mass of contour plumage increased in winter for both species and was associated with decreased thermal conductance in winter chickadees compared to summer chickadees. Chickadees and titmice show metabolic acclimatization similar to other temperate species.     

Phenotypic flexibility of body composition associated with seasonal acclimatization in passerine birds

Improved winter cold tolerance is widespread among small birds overwintering in cold climates and is associated with improved shivering endurance and elevated summit metabolic rate (M_sum). Phenotypic flexibility resulting in elevated M_sum could result from either increased skeletal muscle mass (perhaps with support from similar adjustments in “nutritional organs”) and/or cellular metabolic intensity. We investigated seasonal changes in body composition of three species of passerine birds resident in cold winter climates, all of which show large seasonal variations in M_sum (>25%); white-breasted nuthatch (Sitta carolinensis), black-capped chickadee (Poecile atricapillus), and house sparrow (Passer domesticus). All three species displayed significant winter increases in pectoralis and heart masses, and supracoracoideus mass also increased in winter chickadees. Gizzard mass increased in winter for all three species, but masses of other nutritional organs did not vary consistently with season. These data suggest that winter increases in pectoralis and heart masses are important contributors to elevated thermogenic capacity and cold tolerance, but seasonal variation in nutritional organ masses, other than gizzard, which is likely associated with dietary changes, are not universally associated with seasonal phenotypes. The winter increases in pectoralis and heart masses are consistent with data from other small passerines showing marked seasonal changes in cold tolerance and support the Variable Maximum Model of seasonal phenotypic flexibility, where physiological adjustments that promote improved cold tolerance, also result in elevated M_sum.     

Seasonal thermoregulation in the burrowing parrot (Cyanoliseus patagonus)

Birds exposed to seasonal environments are faced with the problem of maintaining thermogenic homoeostasis. Previous studies have established that birds native to the Holarctic increase their Resting Metabolic Rate at different ambient temperatures (RMR_Ta) and Basal Metabolic Rate (BMR) in winter as an adaptation to cold temperature since winters are more severe, while their non-Holarctic counterparts generally decrease their winter BMR as an energy saving mechanism during unproductive and dry winter months. In this study, we examined seasonal thermoregulation in the burrowing parrot (Cyanoliseus patagonus), a colonial psittacine native to the Patagonian region of Argentina, a region with an unpredictable environment. We found significantly higher mass specific RMR_Ta and BMR in summer than in winter. Both summer and winter BMR of the species fell within the predicted 95% confident interval for a parrot of its size. Body mass was significantly higher in winter than in summer. The burrowing parrot had broad thermo-neutral zones in winter and summer. The circadian rhythm of core body temperature (T_b) of burrowing parrots was not affected by season, showing that this species regulated its T_b irrespective of season. These results suggest that the burrowing parrots' seasonal thermoregulatory responses represent that of energy conservation which is important in an unpredictable environment.     

树麻雀代谢率和器官重量在季节驯化中表型的可塑性变化

动物能量代谢的生理生态特征与物种的分布和丰富度密切相关,基础代谢率(BMR)是内温动物能量预算的重要组成部分。北温带的小型鸟类,通过增加产热来适应低温环境。增加BMR的基础之一是中心器官(代谢机器)发生明显的变化。本研究中我们测定了树麻雀(Passermontanus)的BMR、体重和各器官的重量,分析了麻雀各器官的季节性变化及与BMR的关系。方差分析表明:麻雀的BMR存在明显的季节性变化,在冬季和秋季较高。麻雀内部器官的变化同样有明显的季节性,冬季和秋季麻雀的肝脏、心脏、肌胃、小肠、直肠和整体消化道的重量,都有明显的增加。相关分析表明:麻雀的BMR与肝脏、心脏和消化道等内部器官存在明显的相关性。我们的结果验证了“中心限制假说”,即麻雀体内存在着与BMR相关的“代谢机器”,中心器官是提高麻雀BMR的基础之一。     

Seasonal variation in metabolic rate of a medium-sized frugivore, the Knysna Turaco (Tauraco corythaix)

Many seasonal thermoregulation studies have been conducted on Holarctic birds that live in predictable, highly seasonal climates with severe winters. However, relatively few studies have been conducted on their southern hemisphere Afrotropical counterparts that encounter less predictable climates with milder winters. These latter birds are expected to conserve energy in winter by downregulating their metabolic rates. Therefore in this study, metabolic rate was measured during summer and winter in Knysna Turaco, Tauraco corythaix (Musophagiformes, Musophagidae) (c. 310 g), a non-passerine, in order to test whether there is energy conservation in winter. No overall significant differences in resting metabolic rates over a range of ambient temperatures were observed between winter and summer. However, whole-organism basal metabolic rates were 18.5% higher (p=0.005) in winter than in summer (210.83±15.97 vs. 186.70±10.52 O₂ h⁻¹). Knysna Turacos had broad thermoneutral zones ranging from 20 to 28 °C in winter and 10 to 30 °C in summer. These results suggest that Knysna Turacos show seasonal thermoregulatory responses that represent cold defense rather than energy conservation, which is contrary to what was expected.     

Seasonal acclimatization of metabolism in Eurasian tree sparrows (Passer montanus)

Acclimatization to winter conditions is an essential prerequisite for survival of small passerines of the northern temperate zone. Changes in photoperiod, ambient temperature and food availability trigger seasonal acclimatization in physiology and behavior of many birds. In the present study, seasonal adjustments in several physiological, hormonal, and biochemical markers were examined in wild-captured Eurasian tree sparrows (Passer montanus) from the Heilongjiang Province in China. In winter sparrows had higher body mass and basal metabolic rate (BMR). Consistently, the dry mass of liver, heart, gizzard, small intestine, large intestine and total digestive tract were higher in winter than in that in summer. The contents of mitochondrial protein in liver, and state-4 respiration and cytochrome c oxidase (COX) activity in liver and muscle increased significantly in winter. Circulating level of serum triiodothyronine (T₃) was significantly higher in winter than in summer. Together, these data suggest that tree sparrows mainly coped with cold by enhancing thermogenic capacities through increased organ masses and heightened activity of respiratory enzymes activities. The results support the view that prominent winter increases in BMR are manifestations of winter acclimatization in tree sparrows and that seasonal variation in metabolism in sparrows is similar to that in other small temperate-wintering birds.     

Effects of short-term acclimation on thermoregulatory responses of the rock kestrel, Falco rupicolus

The effects of a short-term acclimation period on basal metabolic rate (BMR) and resting metabolic rate (RMR) were measured in captive-bred Rock Kestrels (Falco rupicolus). Birds were exposed to winter conditions (pre-acclimation) in a semi-natural environment before they were acclimated for a period of 3 weeks at a constant temperature of 25 °C and a constant light:dark cycle (12:12 h) (post-acclimation). After acclimation the kestrels showed changes in RMR, BMR and the width of the thermoneutral zone. There was inter- and intra-individual phenotypic plasticity in BMR and RMR both pre- and post-acclimation. However, more inter-individual variation was seen after acclimation. This study concurs with recent suggestions that phenotypic plasticity in BMR is prevalent in avian physiology, and thus a single-species-specific BMR value may not be representative. Furthermore, comparative avian studies of BMR need to account for phenotypic plasticity.     

Phenotypic flexibility in basal metabolic rate and the changing view of avian physiological diversity: a review

Comparative analyses of avian energetics often involve the implicit assumption that basal metabolic rate (BMR) is a fixed, taxon-specific trait. However, in most species that have been investigated, BMR exhibits phenotypic flexibility and can be reversibly adjusted over short time scales. Many non-migrants adjust BMR seasonally, with the winter BMR usually higher than the summer BMR. The data that are currently available do not, however, support the idea that the magnitude and direction of these adjustments varies consistently with body mass. Long-distance migrants often exhibit large intra-annual changes in BMR, reflecting the physiological adjustments associated with different stages of their migratory cycles. Phenotypic flexibility in BMR also represents an important component of short-term thermal acclimation under laboratory conditions, with captive birds increasing BMR when acclimated to low air temperatures and vice versa. The emerging view of avian BMR is of a highly flexible physiological trait that is continually adjusted in response to environmental factors such as temperature. The within-individual variation observed in avian BMR demands a critical re-examination of approaches used for comparisons across taxa. Several key questions concerning the shapes and other properties of avian BMR reaction norms urgently need to be addressed, and hypotheses concerning metabolic adaptation should explicitly account for phenotypic flexibility.     

Physiological responses in rufous-collared sparrows to thermal acclimation and seasonal acclimatization

A large number of physiological acclimation studies assume that flexibility in a certain trait is both adaptive and functionally important for organisms in their natural environment; however, it is not clear how an organism’s capacity for temperature acclimation translates to the seasonal acclimatization that these organisms must accomplish. To elucidate this relationship, we measured BMR and TEWL rates in both field-acclimatized and laboratory-acclimated adult rufous-collared sparrows (Zonotrichia capensis). Measurements in field-acclimatized birds were taken during the winter and summer seasons; in the laboratory-acclimated birds, we took our measurements following 4 weeks at either 15 or 30°C. Although BMR and TEWL rates did not differ between winter and summer in the field-acclimatized birds, laboratory-acclimated birds exposed to 15°C exhibited both a higher BMR and TEWL rate when compared to the birds acclimated to 30°C and the field-acclimatized birds. Because organ masses seem to be similar between field and cold-acclimated birds whereas BMR is higher in cold-acclimated birds, the variability in BMR cannot be explained completely by adjustments in organ masses. Our findings suggest that, although rufous-collared sparrows can exhibit thermal acclimation of physiological traits, sparrows do not use this capacity to cope with minor to moderate fluctuations in environmental conditions. Our data support the hypothesis that physiological flexibility in energetic traits is a common feature of avian metabolism.     

Phenotypic flexibility in passerine birds: Seasonal variation of aerobic enzyme activities in skeletal muscle

Improved winter cold tolerance is widespread among small passerines resident in cold climates and is generally associated with elevated summit metabolic rate (M_sum=maximum thermoregulatory metabolic rate) and improved shivering endurance with increased reliance on lipids as fuel. Elevated M_sum and improved cold tolerance may result from greater metabolic intensity, due to mass-specific increase in oxidative enzyme capacity, or increase in the masses of thermogenic tissues. To examine the mechanisms underlying winter increases in M_sum, we investigated seasonal changes in mass-specific and total activities of the key aerobic enzymes citrate synthase (CS) and β-hydroxyacyl CoA-dehydrogenase (HOAD) in pectoralis, supracoracoideus and mixed leg muscles of three resident passerine species, black-capped chickadee (Poecile atricapillus), house sparrow (Passer domesticus), and white-breasted nuthatch (Sitta carolinensis). Activities of CS were generally higher in winter than in summer muscles for chickadees and house sparrows, but not nuthatches. Mass-specific HOAD activity was significantly elevated in winter relative to summer in all muscles for chickadees, but did not vary significantly with season for sparrows or nuthatches, except for sparrow leg muscle. These results suggest that modulation of substrate flux and cellular aerobic capacity in muscle contribute to seasonal metabolic flexibility in some species and tissues, but such changes play varying roles among small passerines resident in cold climates.     

Shorebirds' seasonal adjustments in thermogenic capacity are reflected by changes in body mass: how preprogrammed and instantaneous acclimation work together

Phenotypic flexibility in shorebirds has been studied mainly in the context of adjustments to migration and to quality of food; little is known on how birds adjust their phenotype to harsh winter conditions. We showed earlier that red knot (Calidris canutus islandica) can acclimate to cold by elevating body mass. This goes together with larger pectoral muscles, i.e., greater shivering machinery, and thus, better thermogenic capacity. Here, we present results of a yearlong experiment with indoor captive knots to determine whether this strategy is part of their natural seasonal phenotypic cycle. We maintained birds under three thermal regimes: constant cold (5 °C), constant thermoneutrality (25 °C) and natural seasonal variation between these extremes (9-22 °C). Each month we measured variables related to the birds' endurance to cold and physiological maintenance [body mass, thickness of pectoral muscles, summit metabolic rate (M(sum)), food intake, gizzard size, basal metabolic rate (BMR)]. Birds from all treatments expressed synchronized and comparable variation in body mass in spite of thermal treatments, with a 17-18% increase between the warmest and coldest months of the year; which appeared regulated by an endogenous driver. In addition, birds living in the cold exhibited a 10% higher average body mass than did those maintained at thermoneutrality. Thickness of the pectoral muscle tracked changes in body mass in all treatments and likely contributed to greater capacity for shivering in heavier birds. Consequently, M(sum) was 13% higher in cold-acclimated birds compared to those experiencing no thermoregulation costs. However, our data also suggest that part of maximal heat production comes from nonshivering processes. Birds facing cold conditions ate up to 25% more food than did birds under thermoneutral conditions, yet did not develop larger gizzards. Seasonal variation in BMR followed changes in body mass, probably reflecting changes in mass of metabolically active tissues. Just as cold-exposed birds, red knots in the variable treatment increased body mass in winter, thereby improving cold endurance. During summer, however, they maintained a lower body mass and thermogenic capacity compared to cold-exposed birds, similar to individuals kept at thermoneutrality. We conclude that red knots acclimate to seasonal variations in ambient temperature by modulating body mass, combining a preprogrammed increase in mass during winter with a capacity for fine-tuning body mass and thermogenic capacity to temperature variations.     

Thermal history can affect the short-term thermal acclimation of basal metabolic rate in the passerine Zonotrichia capensis

The obligatory cost of living for endotherms is measured by basal metabolic rate (BMR), a variable that is known to change after thermal acclimation. However, the relative timing between variation in ambient temperature and BMR is not well understood. In this study, we addressed this problem in the sparrow Zonotrichia capensis, studying whether previous thermal history affects the response of BMR to a new acclimation temperature. We found that after 4 weeks of acclimation either to 30 or 15 °C birds exhibited significant differences in BMR from pre-acclimation levels. Nevertheless, after a re-acclimation to the opposite treatment for six additional weeks, in the group previously acclimated to warm conditions the change in BMR was significantly greater than in the group previously acclimated to cold. We also found differences in the mass of the small intestine between groups but constancy in the mass of liver, kidney and heart masses at the end of the experiments. Our results indicate that the thermal history affects metabolic adjustments and highlights the importance of considering this when evaluating the plasticity of metabolic traits in small birds.     

Seasonal variation in thermal energetics of the Australian owlet-nightjar (Aegotheles cristatus)

Many birds living in regions with seasonal fluctuations in ambient temperatures (T_a) typically respond to cold by increasing insulation and adjusting metabolic rate. Seasonal variation in thermal physiology has not been studied for the Caprimulgiformes, an order of birds that generally have basal metabolic rates (BMR) lower than predicted for their body mass. We measured the metabolic rate and thermal conductance of Australian owlet-nightjars (Aegotheles cristatus) during summer and winter using open-flow respirometry. Within the thermoneutral zone (TNZ; 31.3 to 34.8 °C), there was no seasonal difference in BMR or thermal conductance (C), but body temperature was higher in summer- (38.2 ± 0.3 °C) than winter-acclimatized (37.1 ± 0.5 °C) birds. Below the TNZ, resting metabolic rate (RMR) increased linearly with decreasing T_a, and RMR and C were higher for summer- than winter-acclimatized birds. The mean mass-specific BMR of owlet-nightjars (1.27 mL O₂ g^− 1 h^− 1) was close to the allometrically predicted value for a 45 g Caprimulgiformes, but well below that predicted for birds overall. These results suggest that owlet-nightjars increase plumage insulation to cope with low winter T_a, which is reflected in the seasonal difference in RMR and C below the TNZ, rather than adjusting BMR.     

Seasonal effects on thermoregulatory responses of the Rock Kestrel, Falco rupicolis

Thermoregulatory responses are known to differ seasonally in endotherms and this is often dependent on the environment and region they are resident. Holarctic animals are exposed to severe winters and substantial seasonal variation in ambient temperature. In contrast, those in the Afrotropics have less severe winters, but greater variation in temperature, rainfall and net primary production. These environmental factors place different selection pressures on physiological responses in endotherms. In this study, metabolic rate (VO₂) and body temperature (T_b) were measured in captive bred Rock Kestrels (Falco rupicolus) from the Afrotropics after a period of summer and winter acclimatisation. Resting metabolic rate was significantly lower after the winter acclimatisation period than after the summer acclimatisation period, and there was a shift in the thermoneutral zone from 20–33 °C in summer to 15–30 °C in winter. However, no significant difference in basal metabolic rate between summer and winter was found. The results show that Rock Kestrels reduce energy expenditure at low ambient temperatures in winter as expected in an Afrotropical species.     

Seasonal effects on thermoregulatory abilities of the Wahlberg's epauletted fruit bat (Epomophorus wahlbergi) in KwaZulu-Natal,South Africa

Seasonal variations in ambient temperature (T_a) require changes in thermoregulatory responses of endotherms. These responses vary according to several factors including taxon and energy constraints. Despite a plethora of studies on chiropteran variations in thermoregulation, few have examined African species. In this study, we used the Wahlberg's epauletted fruit bat (Epomophorus wahlbergi, body mass≈115 g) to determine how the thermoregulatory abilities of an Afrotropical chiropteran respond to seasonal changes in T_a. Mass specific Resting Metabolic Rates (RMR_Ta) and basal metabolic rate (BMR) were significantly higher in winter than in summer. Furthermore, winter body mass was significantly higher than summer body mass. A broad thermoneutral zone (TNZ) was observed in winter (15–35 °C) compared with summer (25–30 °C). This species exhibited heterothermy (rectal and core body temperature) during the photophase (bats' rest-phase) particularly at lower T_as and had a low tolerance of high T_as. Overall, there was a significant seasonal variation in the thermoregulatory abilities of E. wahlbergi. The relative paucity of data relating to the seasonal thermoregulatory abilities of Afrotropical bats suggest further work is needed for comparison and possible effects of climate change, particularly extreme hot days.     

Seasonal variation in body composition in an Afrotropical passerine bird: increases in pectoral muscle mass are,unexpectedly, associated with lower thermogenic capacity

Phenotypic flexibility in avian metabolic rates and body composition have been well-studied in high-latitude species, which typically increase basal metabolic rate (BMR) and summit metabolism (M_sum) when acclimatized to winter conditions. Patterns of seasonal metabolic acclimatization are more variable in lower-latitude birds that experience milder winters, with fewer studies investigating adjustments in avian organ and muscle masses in the context of metabolic flexibility in these regions. We quantified seasonal variation (summer vs winter) in the masses of organs and muscles frequently associated with changes in BMR (gizzard, intestines and liver) and M_sum (heart and pectoral muscles), in white-browed sparrow-weavers (Plocepasser mahali). We also measured pectoral muscle thickness using a portable ultrasound system to determine whether we could non-lethally estimate muscle size. A concurrent study measured seasonal changes in BMR and M_sum in the same population of sparrow-weavers, but different individuals. There was no seasonal variation in the dry masses of the gizzard, intestines or liver of sparrow-weavers, and during the same period, BMR did not vary seasonally. We found significantly higher heart (~ 18% higher) and pectoral muscle (~ 9% higher) dry mass during winter, although ultrasound measurements did not detect seasonal changes in pectoral muscle size. Despite winter increases in pectoral muscle mass, M_sum was ~ 26% lower in winter compared to summer. To the best of our knowledge, this is the first study to report an increase in avian pectoral muscle mass but a concomitant decrease in thermogenic capacity.

     

Differences between temperate and tropical birds in seasonal acclimatization of thermoregulatory traits

Phenotypic flexibility can be an important determinant of fitness in variable environments. The climatic variability hypothesis (CVH) predicts that phenotypic flexibility in thermoregulatory traits will be greater in temperate species than tropical species as a means of coping with increased temperature seasonality at higher latitudes. However, support for the CVH has been mixed, and recent studies suggest that tropical birds are capable of substantial phenotypic flexibility. To test the generality of the CVH, we used flow‐through respirometry to quantify seasonal acclimatization in thermoregulatory traits in suites of temperate (n = 6) and tropical (n = 41) birds. We used W/S ratios (winter/summer trait values) to quantify the direction and magnitude of seasonal change (W/S ratio of 1 means no seasonal change). Temperate species exhibited coordinated changes in thermoregulatory traits in winter, including large increases in thermoneutral zone (TNZ) breadth and reductions in heat loss below the lower limit of the TNZ. Conversely, tropical species exhibited idiosyncratic seasonal thermoregulatory responses, and mean W/S ratios were close to 1 for all traits, indicative of little seasonal change and consistent with predictions of the CVH. Nevertheless, mean W/S ratios did not differ significantly between temperate and tropical species for either M_b or BMR, demonstrating that tropical birds can also exhibit substantial thermoregulatory flexibility. Our results highlight the need for complementary acclimation experiments to determine if latitudinal differences in seasonal acclimatization are due to inherent differences in capacity for flexibility.