How does flexibility in body composition relate to seasonal changes in metabolic performance in a small passerine wintering at northern latitude?

Effects of sublethal methylmercury and food stress on songbird energetic performance: metabolic rates, molt and feather quality

Organisms regularly adjust their physiology and energy balance in response to predictable seasonal environmental changes. Stressors and contaminants have the potential to disrupt these critical seasonal transitions. No studies have investigated how simultaneous exposure to the ubiquitous toxin methylmercury (MeHg) and food stress affects birds' physiological performance across seasons. We quantified several aspects of energetic performance in song sparrows, Melospiza melodia, exposed or not to unpredictable food stress and MeHg in a 2×2 experimental design, over 3 months during the breeding season, followed by 3 months post-exposure. Birds exposed to food stress had reduced basal metabolic rate and non-significant higher factorial metabolic scope during the exposure period, and had a greater increase in lean mass throughout most of the experimental period. Birds exposed to MeHg had increased molt duration, and increased mass:length ratio of some of their primary feathers. Birds exposed to the combined food stress and MeHg treatment often had responses similar to the stress-only or MeHg-only exposure groups, suggesting these treatments affected physiological performance through different mechanisms and resulted in compensatory or independent effects. Because the MeHg and stress variables were selected in candidate models with a ΔAICc lower than 2 but the 95% confidence interval of these variables overlapped zero, we found weak support for MeHg effects on all measures except basal metabolic rate, and for food stress effects on maximum metabolic rate, factorial metabolic scope and feather mass:length ratio. This suggests that MeHg and food stress effects on these measures are statistically identified but not simple and/or were too weak to be detected via linear regression. Overall, combined exposure to ecologically relevant MeHg and unpredictable food stress during the breeding season does not appear to induce extra energetic costs for songbirds in the post-exposure period. However, MeHg effects on molt duration could carry over across multiple annual cycle stages.

Phenotypic flexibility in metabolic adjustments and digestive function in white-shouldered starlings: responses to short-term temperature acclimation

Changing the intrinsic rate of metabolic heat production is the main adaptive strategy for small birds to cope with different ambient temperatures. In this study, we tested the hypothesis that the small passerine the white-shouldered starling (Sturnus sinensis) can modulate basal metabolism under temperature acclimation by changing the morphological, physiological and biochemical state of its tissues and organs. We measured the effects of temperature on body mass, basal metabolic rate (BMR), wet mass of various internal organs, state 4 respiration (S4R) and cytochrome c oxidase (CCO) activity in the pectoral muscle and organs, metabolites in the pectoral muscle, energy intake, histological dynamics and the activity of duodenal digestive enzymes. Warm acclimation decreased BMR to a greater extent than cold acclimation. At the organ level, birds in the cold-acclimated group had significantly heavier intestines but significantly lighter pectoral muscles. At the cellular level, birds in the cold-acclimated group showed significantly higher S4R in the liver and heart and CCO activity in the liver and kidney at both the mass-specific and whole-organ levels. A metabolomic analysis of the pectoral tissue revealed significantly higher lipid decomposition, amino acid degradation, ATP hydrolysis, and GTP and biotin synthesis in cold-acclimated birds. Acclimation to cold significantly increased the gross energy intake (GEI), feces energy (FE) and digestive energy intake (DEI) but significantly decreased the digestive efficiency of these birds. Furthermore, cold-acclimated birds had a higher maltase activity and longer villi in the duodenum. Taken together, these data show that white-shouldered starlings exhibit high phenotypic flexibility in metabolic adjustments and digestive function under temperature acclimation, consistent with the notion that small birds cope with the energy challenges presented by a cold environment by modulating tissue function in a way that would affect BMR.

Great tits (Parus major) in a west European temperate forest show little seasonal variation in metabolic energy requirements

Understanding how birds annually allocate energy to cope with changing environmental conditions and physiological states is a crucial question in avian ecology. There are several hypotheses to explain species' energy allocation. One prominent hypothesis suggests higher energy expenditure in winter due to increased thermoregulatory costs. The "reallocation" hypothesis suggests no net difference in seasonal energy requirements, while the "increased demand" hypothesis predicts higher energy requirements during the breeding season. Birds are expected to adjust their mass and/or metabolic intensity in ways that are consistent with their energy requirements. Here, we look for metabolic signatures of seasonal variation in energy requirements of a resident passerine of a temperate-zone (great tit, Parus major). To do so, we measured whole-body and mass-independent basal (BMR), summit (Msum), and field (FMR) metabolic rates during late winter and during breeding in Belgian great tits. During the breeding season, birds had on average 10% higher whole-body BMR and FMR compared to winter, while their Msum decreased by 7% from winter to breeding. Mass-independent metabolic rates showed a 10% increase in BMR and a 7% decrease in Msum from winter to breeding. Whole-body BMR was correlated with Msum, but this relationship did not hold for mass-independent metabolic rates. The modest seasonal change we observed suggests that great tits in our temperature study area maintain a largely stable energy budget throughout the year, which appears mostly consistent with the reallocation hypothesis.

Plasticity of mitochondrial function safeguards phosphorylating respiration during in vitro simulation of rest-phase hypothermia

Many animals downregulate body temperature to save energy when resting (rest-phase hypothermia). Small birds that winter at high latitudes have comparatively limited capacity for hypothermia and so pay large energy costs for thermoregulation during cold nights. Available evidence suggests this process is fueled by adenosine triphosphate (ATP)-dependent mechanisms. Most ATP is produced by oxidative phosphorylation in the mitochondria, but mitochondrial respiration may be lower during hypothermia because of the temperature dependence of biological processes. This can create conflict between increased organismal ATP demand and a lower mitochondrial capacity to provide it. We studied this in blood cell mitochondria of wild great tits (Parus major) by simulating rest-phase hypothermia via a 6°C reduction in assay temperature in vitro. The birds had spent the night preceding the experiment in thermoneutrality or in temperatures representing mild or very cold winter nights, but night temperatures never affected mitochondrial respiration. However, across temperature groups, endogenous respiration was 14% lower in hypothermia. This did not reflect general thermal suppression of mitochondrial function because phosphorylating respiration was unaffected by thermal state. Instead, hypothermia was associated with a threefold reduction of leak respiration, from 17% in normothermia to 4% in hypothermia. Thus, the coupling of total respiration to ATP production was 96% in hypothermia, compared to 83% in normothermia. Our study shows that the thermal insensitivity of phosphorylation combined with short-term plasticity of leak respiration may safeguard ATP production when endogenous respiration is suppressed. This casts new light on the process by which small birds endure harsh winter cold and warrants future tests across tissues in vivo.

Seasonal variation in thermoregulatory capacity of three closely related Afrotropical Estrildid finches introduced to Europe

A species' potential geographical range is largely determined by how the species responds physiologically to its changing environment. It is therefore crucial to study the physiological mechanisms that species use to maintain their homeothermy in order to address biodiversity conservation challenges, such as the success of invasions of introduced species. The common waxbill Estrilda astrild, the orange-cheeked waxbill E. melpoda, and the black-rumped waxbill E. troglodytes are small Afrotropical passerines that have established invasive populations in regions where the climate is colder than in their native ranges. As a result, they are highly suitable species for studying potential mechanisms for coping with a colder and more variable climate. Here, we investigated the magnitude and direction of seasonal variation in their thermoregulatory traits, such as basal (BMR), summit (M_sum) metabolic rates and thermal conductance. We found that, from summer to autumn, their ability to resist colder temperatures increased. This was not related to larger body masses or higher BMR and M_sum, but instead, species downregulated BMR and M_sum toward the colder season, suggesting energy conservation mechanisms to increase winter survival. BMR and M_sum were most strongly correlated with temperature variation in the week preceding the measurements. Common waxbill and black-rumped waxbill, whose native ranges encompass the highest degree of seasonality, showed the most flexibility in metabolic rates (i.e., stronger downregulation toward colder seasons). This ability to adjust thermoregulatory traits, combined with increased cold tolerance, may facilitate their establishment in areas characterized by colder winters and less predictable climates.

Mediterranean songbirds show pronounced seasonal variation in thermoregulatory traits

Addressing the patterns of variation in thermal traits is crucial to better predict the potential effects of climate change on organisms. Here, we assessed seasonal (winter vs summer) adjustments in key thermoregulatory traits in eight Mediterranean-resident songbirds. Overall, songbirds increased whole-animal (by 8%) and mass-adjusted (by 9%) basal metabolic rate and decreased (by 56%) thermal conductance below the thermoneutral zone during winter. The magnitude of these changes was within the lower values found in songbirds from northern temperate areas. Moreover, songbirds increased (by 11%) evaporative water loss within the thermoneutral zone during summer, while its rate of increase above the inflection point of evaporative water loss (i.e., the slope of evaporative water loss versus temperature) decreased by 35% during summer - a value well above that reported for other temperate and tropical songbirds. Finally, body mass increased by 5% during winter, a pattern similar to that found in many northern temperate species. Our findings support the idea that physiological adjustments might enhance the resilience of Mediterranean songbirds to environmental changes, with short-term benefits by saving energy and water under thermally stressful conditions. Nevertheless, not all species showed the same patterns, suggesting different strategies in their thermoregulatory adaptations to seasonal environments.

Skeletal muscle and metabolic flexibility in response to changing energy demands in wild birds

Phenotypically plastic responses of animals to adjust to environmental variation are pervasive. Reversible plasticity (i.e., phenotypic flexibility), where adult phenotypes can be reversibly altered according to prevailing environmental conditions, allow for better matching of phenotypes to the environment and can generate fitness benefits but may also be associated with costs that trade-off with capacity for flexibility. Here, we review the literature on avian metabolic and muscle plasticity in response to season, temperature, migration and experimental manipulation of flight costs, and employ an integrative approach to explore the phenotypic flexibility of metabolic rates and skeletal muscle in wild birds. Basal (minimum maintenance metabolic rate) and summit (maximum cold-induced metabolic rate) metabolic rates are flexible traits in birds, typically increasing with increasing energy demands. Because skeletal muscles are important for energy use at the organismal level, especially to maximum rates of energy use during exercise or shivering thermogenesis, we consider flexibility of skeletal muscle at the tissue and ultrastructural levels in response to variations in the thermal environment and in workloads due to flight exercise. We also examine two major muscle remodeling regulatory pathways: myostatin and insulin-like growth factor -1 (IGF-1). Changes in myostatin and IGF-1 pathways are sometimes, but not always, regulated in a manner consistent with metabolic rate and muscle mass flexibility in response to changing energy demands in wild birds, but few studies have examined such variation so additional study is needed to fully understand roles for these pathways in regulating metabolic flexibility in birds. Muscle ultrastrutural variation in terms of muscle fiber diameter and associated myonuclear domain (MND) in birds is plastic and highly responsive to thermal variation and increases in workload, however, only a few studies have examined ultrastructural flexibility in avian muscle. Additionally, the relationship between myostatin, IGF-1, and satellite cell (SC) proliferation as it relates to avian muscle flexibility has not been addressed in birds and represents a promising avenue for future study.

How does mitochondrial function relate to thermogenic capacity and basal metabolic rate in small birds?

We investigated the role of mitochondrial function in the avian thermoregulatory response to a cold environment. Using black-capped chickadees (Poecile atricapillus) acclimated to cold (-10°C) and thermoneutral (27°C) temperatures, we expected to observe an upregulation of pectoralis muscle and liver respiratory capacity that would be visible in mitochondrial adjustments in cold-acclimated birds. We also predicted that these adjustments would correlate with thermogenic capacity (Msum) and basal metabolic rate (BMR). Using tissue high-resolution respirometry, mitochondrial performance was measured as respiration rate triggered by proton leak and the activity of complex I (OXPHOSCI) and complex I+II (OXPHOSCI+CII) in the liver and pectoralis muscle. The activity of citrate synthase (CS) and cytochrome c oxidase (CCO) was also used as a marker of mitochondrial density. We found 20% higher total CS activity in the whole pectoralis muscle and 39% higher total CCO activity in the whole liver of cold-acclimated chickadees relative to that of birds kept at thermoneutrality. This indicates that cold acclimation increased overall aerobic capacity of these tissues. Msum correlated positively with mitochondrial proton leak in the muscle of cold-acclimated birds while BMR correlated with OXPHOSCI in the liver with a pattern that differed between treatments. Consequently, this study revealed a divergence in mitochondrial metabolism between thermal acclimation states in birds. Some functions of the mitochondria covary with thermogenic capacity and basal maintenance costs in patterns that are dependent on temperature and body mass.

Breathing in the Cold: Seasonal Changes in the Ventilatory Pattern in a Small Boreal Passerine Bird

Small passerine birds in the north need to take advantage of several behavioral and physiological mechanisms to maintain energy balance during the winter characterized by low food supply, low ambient temperatures, and short days. Here we test if the breathing pattern of a non-migratory species, the great tit (Parus major), show seasonal variation that could help the species keeping a positive energy balance in the winter. To this aim, we measured oxygen consumption and ventilatory variables (tidal volume and respiratory frequency) in summer- and winter-acclimatized great tits exposed to ambient temperatures between –15 and 30°C. Winter-acclimatized great tits had a higher resting metabolic rate and a different breathing pattern compared to the summer-acclimatized birds. During the winter the great tits utilized a breathing pattern, consisting of an increased respiratory frequency to tidal volume ratio compared to summer-acclimatized birds at all temperatures. The higher oxygen uptake and the altered breathing pattern in the winter-acclimatized tits resulted in a higher lung oxygen extraction. However, during acute cold exposure neither the winter- nor summer-acclimatized great tits increased the oxygen extraction at low ambient temperature. The higher lung oxygen extraction in the winter-acclimatized tits implies that the birds will save on the minute ventilation, which reduces the evaporative water loss through respiration. The daily water loss saved can be more than 1 g of water per day. This is a substantial saving corresponding to a saving in evaporative heat loss corresponding to between 4 and 8% of the resting metabolic rate. This might be significant in keeping an energy balance, and the altered breathing pattern in the winter, ensuring an increased oxygen extraction, may therefore represents an additional physiological mechanism making it possible for small passerine birds to survive the northern winter.

Metabolic Profiling and Integration of Metabolomic and Transcriptomic Data From Pectoralis Muscle Reveal Winter-Adaptive Metabolic Responses of Black-Capped Chickadee and American Goldfinch

Seasonal changes, such as alterations in food availability or type and cold conditions, present challenges to free-living birds living in highly seasonal climates. Small birds respond to such challenges through seasonal metabolic flexibility, which better matches seasonal metabolic phenotypes to environmental conditions and can improve fitness. To better understand the mechanistic basis of this metabolic flexibility, we conducted a large-scale metabolic profiling of pectoralis muscle in black-capped chickadees (Poecile atricapillus) and American goldfinches (Spinus tristis), which are small, year-round bird species of temperate-zones. We analyzed muscle samples using non-biased, global metabolomics profiling technology based on UHLC/MS/MS2 platforms. A total of 582 metabolites was characterized for summer and winter season samples. Chickadees showed greater seasonal separation of global metabolite profiles than goldfinches, which is consistent with previous transcriptomic studies of pectoralis muscle in these two species. Reduced levels of amino acids during winter occurred in both species and might reflect decreasing dietary protein intake, amino acid shuttling to other pathways for thermogenesis and/or elevated rates of protein turnover in the pectoralis muscle. Concomitant decreased abundances in tricarboxylic acid cycle (TCA) metabolites suggest faster cycling of the oxidative phosphorylation pathway in winter to meet the metabolic demands of thermogenesis. Accordingly, chickadees displayed shifts toward lipid oxidation in winter, whereas goldfinches showed winter declines in ketone bodies, which suggests increased energy demand or subtle changes in substrate availability. Beyond the winter-specific changes in metabolite abundances, integration of the metabolomic and the transcriptomic data revealed a landscape of gene–metabolite associations related to the winter-adaptive metabolic response. This landscape of gene–metabolite pairs was overrepresented by pathways associated with transport of small molecules, metabolism of amino acids and derivatives, activation and biosynthesis of fatty acid derivatives, and biosynthesis and metabolism of nicotinate and nicotinamide derivatives. Collectively, our results suggest that increased levels of NADH and its derivatives in the pectoralis muscle are a potential novel mechanism for increasing winter metabolic output, fueled by lipids, for thermogenesis during winter.

Snow Buntings Maintain Winter-Level Cold Endurance While Migrating to the High Arctic

Arctic breeding songbirds migrate early in the spring and can face winter environments requiring cold endurance throughout their journey. One such species, the snow bunting (Plectrophenax nivalis), is known for its significant thermogenic capacity. Empirical studies suggest that buntings can indeed maintain winter cold acclimatization into the migratory and breeding phenotypes when kept captive on their wintering grounds. This capacity could be advantageous not only for migrating in a cold environment, but also for facing unpredictable Arctic weather on arrival and during preparation for breeding. However, migration also typically leads to declines in the sizes of several body components linked to metabolic performance. As such, buntings could also experience some loss of cold endurance as they migrate. Here, we aimed to determine whether free-living snow buntings maintain a cold acclimatized phenotype during spring migration. Using a multi-year dataset, we compared body composition (body mass, fat stores, and pectoralis muscle thickness), oxygen carrying capacity (hematocrit) and metabolic performance (thermogenic capacity – Msum and maintenance energy expenditure – BMR) of birds captured on their wintering grounds (January–February, Rimouski, QC, 48°N) and during pre-breeding (April–May) in the Arctic (Alert, NU, 82°). Our results show that body mass, fat stores and Msum were similar between the two stages, while hematocrit and pectoralis muscle thickness were lower in pre-breeding birds than in wintering individuals. These results suggest that although tissue degradation during migration may affect flight muscle size, buntings are able to maintain cold endurance (i.e., Msum) up to their Arctic breeding grounds. However, BMR was higher during pre-breeding than during winter, suggesting higher maintenance costs in the Arctic.

Seasonal changes in the hippocampal formation of hoarding and non-hoarding tits

The hippocampal formation (HF) processes spatial memories for cache locations in food-hoarding birds. Hoarding is a seasonal behavior, and seasonal changes in the HF have been described in some studies, but not in others. One potential reason is that birds may have been sampled during the seasonal hoarding peak in some studies, but not in others. In this study, we investigate the seasonal changes in hoarding and HF in willow tits (Poecile montanus). We compare this to seasonal changes in HF in a closely related non-hoarding bird, the great tit (Parus major). Willow tits near Oulu, Finland, show a seasonal hoarding peak in September and both HF volume and neuron number show a similar peak. HF neuronal density also increases in September, but then remains the same throughout winter. Unexpectedly, the great tit HF also changes seasonally, although in a different pattern: the great tit telencephalon increases in volume from July to August and decreases again in November. Great tit HF volume follows suit, but with a delay. Great tit HF neuron number and density also increase from August to September and stay high throughout winter. We hypothesize that seasonal changes in hoarding birds’ HF are driven by food-hoarding experience (e.g., the formation of thousands of memories). The seasonal changes in great tit brains may also be due to experience-dependent plasticity, responding to changes in the social and spatial environment. Large-scale experience-dependent neural plasticity is therefore probably not an adaptation of food-hoarding birds, but a general property of the avian HF and telencephalon.

Metabolic Flexibility in Response to Within-Season Temperature Variability in House Sparrows

The climatic variability hypothesis (CVH) posits that more flexible phenotypes should provide a fitness advantage for organisms experiencing more variable climates. While typically applied across geographically separated populations, whether this principle applies across seasons or other conditions (e.g., open vs. sheltered habitats) which differ in climatic variability remains essentially unstudied. In north-temperate climates, climatic variability in winter usually exceeds that in summer, so extending the CVH to within-population seasonal variation predicts that winter phenotypes should be more flexible than summer phenotypes. We tested this prediction of the within-season extension of the CVH by acclimating summer and winter-collected house sparrows (Passer domesticus) to 24, 5, and -10°C and measuring basal metabolic rate (BMR) and summit metabolic rate (M_sum = maximum cold-induced metabolic rate) before and after acclimation (Accl). To examine mechanistic bases for metabolic variation, we measured flight muscle and heart masses and citrate synthase and β-hydroxyacyl coA-dehydrogenase activities. BMR and M_sum were higher for cold-acclimated than for warm-acclimated birds, and BMR was higher in winter than in summer birds. Contrary to our hypothesis of greater responses to cold Accl in winter birds, metabolic rates generally decreased over the Accl period for winter birds at all temperatures but increased at cold temperatures for summer birds. Flight muscle and heart masses were not significantly correlated with season or Accl treatment, except for supracoracoideus mass, which was lower at -10°C in winter, but flight muscle and heart masses were positively correlated with BMR and flight muscle mass was positively correlated with M_sum. Catabolic enzyme activities were not clearly related to metabolic variation. Thus, our data suggest that predictions of the CVH may not be relevant when extended to seasonal temperature variability at the within-population scale. Indeed, these data suggest that metabolic rates are more prominently upregulated in summer than in winter in response to cold. Metabolic rates tended to decrease during Accl at all temperatures in winter, suggesting that initial metabolic rates at capture (higher in winter) influence metabolic Accl for captive birds.

Wintering Snow Buntings Elevate Cold Hardiness to Extreme Levels but Show No Changes in Maintenance Costs

AbstractResident temperate passerines adjust their phenotypes to cope with winter constraints, with peak performance in metabolic traits typically occurring during the coldest months. However, it is sparsely known whether cold-adapted northern species make similar adjustments when faced with variable seasonal environments. Life in near-constant cold could be associated with limited flexibility in traits underlying cold endurance. We investigated this by tracking individual physiological changes over five consecutive winters in snow buntings (Plectrophenax nivalis), an Arctic-breeding migratory passerine typically confronted with nearly constant cold. Buntings were held in an outdoor aviary and exposed to seasonal temperature variation typical of temperate zone climates. We measured phenotypic changes in body composition (body, fat, and lean mass, pectoralis muscle thickness), oxygen transport capacity (hematocrit), metabolic performance (basal metabolic rate [BMR] and summit metabolic rate [M_sum]), thermogenic endurance (time to reach M_sum), and cold tolerance (temperature at M_sum). Snow buntings showed flexibility in functions underlying thermogenic capacity and cold endurance comparable to that observed in temperate resident passerines wintering at similar latitudes. Specifically, they increased body mass (13%), fat mass (246%), hematocrit (23%), pectoralis muscle thickness (8%), and M_sum (27%). We also found remarkable cold tolerance in these birds, with individuals reaching M_sum in helox at temperatures equivalent to less than -90°C in air. However, in contrast with resident temperate passerines, lean mass decreased by 12%, and there was no clear increase in maintenance costs (BMR). Our results show that the flexibility of traits underlying thermal acclimatization in a cold-adapted northern species is comparable to that of temperate resident species living at lower latitudes and is therefore not limited by life in near-constant cold.

Aminopeptidase-N modulation assists lean mass anabolism during refuelling in the white-throated sparrow

Songbirds meet the extreme metabolic demands of migration by burning both stored fat and protein. However, catabolizing these endogenous tissues for energy leads to organ atrophy, and reductions in gastrointestinal tissue can be as great as 50% of the pre-flight mass. Remarkably, during stopover refuelling birds quickly regain digestive mass and performance. Aminopeptidase-N (APN) is a brush-border enzyme responsible for late-stage protein digestion and may critically assist tissue reconstruction during the stopover, thus compensating for reduced gut size. We hypothesized that birds recovering from a fast would differentially upregulate APN activity relative to disaccharidases to rapidly process and assimilate dietary protein into lean mass. We fasted 23 wild-caught migratory white-throated sparrows (Zonotrichia albicollis) for 48 h to mimic mass reductions experienced during migratory flight and measured intestinal APN activity before the fast, immediately after the fast, and during recovery at 24 h and 48 h post-fast. Total fat mass, lean mass and basal metabolic rate were measured daily. We show that fasted birds maintain APN activity through the fast, despite a 30% reduction in intestine mass, but during refuelling, APN activity increases nearly twofold over pre-fasted individuals. This suggests that dynamically regulating APN may be necessary for rapid protein reconstruction during the stopover.

Structural plasticity of the avian pectoralis: a case for geometry and the forgotten organelle

The avian pectoralis muscle demonstrates incredible plasticity. This muscle is the sole thermogenic organ of small passerine birds, and many temperate small passerines increase pectoralis mass in winter, potentially to increase heat production. Similarly, this organ can double in size prior to migration in migratory birds. In this Commentary, following the August Krogh principle, I argue that the avian pectoralis is the perfect tissue to reveal general features of muscle physiology. For example, in both mammals and birds, skeletal muscle fiber diameter is generally accepted to be within 10–100 µm. This size constraint is assumed to include reaction-diffusion limitations, coupled with metabolic cost savings associated with fiber geometry. However, avian muscle fiber structure has been largely ignored in this field, and the extensive remodeling of the avian pectoralis provides a system with which to investigate this. In addition, fiber diameter has been linked to whole-animal metabolic rates, although this has only been addressed in a handful of bird studies, some of which demonstrate previously unreported levels of plasticity and flexibility. Similarly, myonuclei, which are responsible for protein turnover within the fiber, have been forgotten in the avian literature. The few studies that have addressed myonuclear domain (MND) changes in avian muscle have found rates of change not previously seen in mammals. Both fiber diameter and MND have strong implications for aging rates; most aging mammals demonstrate muscular atrophy (a decrease in fiber diameter) and changes in MND. As I discuss here, these features are likely to differ in birds.

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.

Slowing down the metabolic engine: impact of early-life corticosterone exposure on adult metabolism in house sparrows (Passer domesticus)

Whole-organism metabolism is an integrative process that determines not only the energy cost of living but also the energy output that is available for behavioral and physiological processes during the life cycle. Developmental challenge is known to affect growth, development of several organs, and several physiological mechanisms (such as HPA responsiveness, oxidative stress or immunity), which may altogether affect adult metabolism. All of these developmental effects are likely to be mediated by glucocorticoids, but the impact of developmental glucocorticoid exposure on adult metabolism has rarely been studied and the results are equivocal. In this study, we examined the impact of developmental exposure to corticosterone (CORT, the main avian glucocorticoid hormone) on resting metabolic rate (RMR, measured in thermoneutrality, 25°C) and thermoregulatory metabolic rate (TMR, measured in cold challenge conditions, 5°C) in the house sparrow. Following experimental administration of CORT at the nestling stage, house sparrows were kept in captivity until adulthood, when their metabolism was measured. We found that post-natal CORT exposure decreased both RMR and TMR in adult sparrows. This CORT-mediated reduction of metabolism was also associated with a reduced overnight body mass loss. Therefore, our results suggest that developmental CORT exposure can orient the phenotype towards an energy-saving strategy, which may be beneficial in a constraining environmental context.

Seasonal muscle ultrastructure plasticity and resistance of muscle structural changes during temperature increases in resident black-capped chickadees and rock pigeons

Resident birds in temperate zones respond to seasonally fluctuating temperatures by adjusting their physiology, such as changes in basal metabolic rate or peak metabolic rate during cold exposure, or altering their organ sizes, so as to match the thermogenic requirements of their current environment. Climate change is predicted to cause increases in the frequency of heat and cold wave events, which could increase the likelihood that birds will face an environmental mismatch. Here, we examined seasonality and the effects of acute and chronic heat shock to 33°C and subsequent recovery from heat shock on the ultrastructure of the superficial pectoralis muscle fiber diameter, myonuclear domain (MND) and capillary density in two temperate bird species of differing body mass, the black-capped chickadee (Poecile atricapillus) and the rock pigeon (Columba livia). We found that muscle fiber ultrastructure did not change with heat treatment. However, in black-capped chickadees, there was a significant increase in fiber diameter in spring phenotype birds compared with summer phenotype birds. In rock pigeons, we saw no differences in fiber diameter across seasons. Capillary density did not change as a function of fiber diameter in black-capped chickadees, but did change seasonally, as did MND. Across seasons, as fiber diameter decreased, capillary density increased in the pectoralis muscle of rock pigeons. For both species in this study, we found that as fiber diameter increased, so did MND. Our findings imply that these two temperate birds employ different muscular growth strategies that may be metabolically beneficial to each.

Within-winter flexibility in muscle and heart lipid transport and catabolism in passerine birds

Small birds in cold climates may show within-winter metabolic flexibility to match metabolic capacities to prevailing weather conditions. This flexibility may occur over periods of days to weeks, but the underlying mechanisms for such flexibility are not well understood. Because lipids are the primary fuel for sustained thermogenesis, we examined whether lipid transport and catabolism can mediate within-winter metabolic flexibility in two small temperate-zone wintering passerine birds, dark-eyed juncos (Junco hyemalis) and house sparrows (Passer domesticus). We used simple and multiple regression analyses to test for correlations of several lipid transporters in pectoralis muscle (plasma membrane-bound and cytosolic fatty acid-binding proteins, FABP; fatty acyl translocase, FAT/CD36) and regulatory enzymes (carnitine acyl transferase, CPT; β-hydroxyacyl CoA dehydrogenase, HOAD) in pectoralis and heart with short-term (ST, 0-7 days), medium-term (MT, 14-30 days) and long-term (LT, 30-year mean daily and extreme minimum temperatures, day of winter season) temperature variables. We hypothesized negative correlations between these regulators and temperature variables. Juncos showed negative correlations for FABPs with ST or MT temperature variables, but other lipid transporters and regulatory enzymes showed positive correlations with ST or MT temperatures for juncos, suggesting no consistent pathway-wide response to within-winter temperatures. LT temperature variables showed several significant associations with lipid transporters and enzymes for juncos, but also not in consistent directions. House sparrows showed the expected negative correlations with LT temperatures for FABPpm, but positive correlations with temperature variables for FABPc, CPT and HOAD. Different species-specific patterns of variation and the absence of consistent pathway-wide responses to temperature suggest that the lipid transport and catabolism pathway is not a uniform mediator of within-winter metabolic flexibility among small birds.

Overwinter temperature has no effect on problem solving abilities or responses to novelty in Black-capped Chickadees (Poecile atricapillus)

Birds overwintering at northern latitudes face challenging environments in which refined cognitive and behavioural responses to environmental stimuli could be a benefit. Populations of the same species from different latitudes have been shown to differ in their cognitive and behavioural responses, and these differences have been attributed to local adaptation. However, individuals overwintering at intermediate latitudes experience great breadth and variation in environmental conditions, and thus it is reasonable that these individuals would alter their responses based on current conditions. To determine within-species responses to environmental conditions we sampled birds from a single population at an intermediate latitude and assessed their problem solving abilities and their responses to novelty. We held birds overwinter in one of three experimental temperature regimes and assessed problem solving abilities and responses to novel stimuli in the spring. We found that overwinter temperature had no effect on problem solving ability. We also show that overwinter temperature had no effect on an individual's response to novelty. These findings strengthen the argument that differences in these behaviours seen at the population level are in fact driven by local adaptation, and that current environmental condition may have limited effects on these behaviours.

Mass or pace? Seasonal energy management in wintering boreal passerines

Research on winter energy management in small vertebrates has focused on the regulation of body mass (BM) within a framework of starvation-predation trade-off. Winter-acclimatized birds exhibit a seasonal increase in both BM and basal metabolic rate (BMR), although the patterns of co-variation between the two traits remain unknown. We studied this co-variation in three different species of wild titmice, great, blue and willow tits, originating from two boreal regions at different latitudes. Seasonal change in BM and BMR was inter-dependent, particularly in the great tit; however, by contrast, no seasonal change was observed in the willow tit. BMR changed non-linearly in concert with BM with a peak in midwinter for both blue and great tits, whereas such non-linear pattern in willow tit was opposite and independent of BM. Surprisingly, BMR appears to be more sensitive to ambient temperatures than BM in all three species studied. Energy management is a multifaceted strategy that cannot be fully understood without considering reserve levels and energy expenditure simultaneously. Thus, our study indicates that the prevailing conceptual framework based on variation in BM alone is insufficient to understand seasonal energy management in small wintering passerines.

Large muscles are beneficial but not required for improving thermogenic capacity in small birds

It is generally assumed that small birds improve their shivering heat production capacity by developing the size of their pectoralis muscles. However, some studies have reported an enhancement of thermogenic capacity in the absence of muscle mass variation between seasons or thermal treatments. We tested the hypothesis that an increase in muscle mass is not a prerequisite for improving avian thermogenic capacity. We measured basal (BMR) and summit (M_sum) metabolic rates of black capped chickadees (Poecile atricapillus) acclimated to thermoneutral (27 °C) and cold (-10 °C) temperatures and obtained body composition data from dissections. Cold acclimated birds consumed 44% more food, and had 5% and 20% higher BMR and M_sum, respectively, compared to individuals kept at thermoneutrality. However, lean dry pectoralis and total muscle mass did not differ between treatments, confirming that the improvement of thermogenic capacity did not require an increase in skeletal muscle mass. Nevertheless, within temperature treatments, M_sum was positively correlated with the mass of all measured muscles, including the pectoralis. Therefore, for a given acclimation temperature individuals with large muscles do benefit from muscle size in term of heat production but improving thermogenic capacity during cold acclimation likely requires an upregulation of cell functions.

The performing animal: causes and consequences of body remodeling and metabolic adjustments in red knots facing contrasting thermal environments

Using red knots ( Calidris canutus) as a model, we determined how changes in mass and metabolic activity of organs relate to temperature-induced variation in metabolic performance. In cold-acclimated birds, we expected large muscles and heart as well as improved oxidative capacity and lipid transport, and we predicted that this would explain variation in maximal thermogenic capacity (Msum). We also expected larger digestive and excretory organs in these same birds and predicted that this would explain most of the variation in basal metabolic rate (BMR). Knots kept at 5°C were 20% heavier and maintained 1.5 times more body fat than individuals kept in thermoneutral conditions (25°C). The birds in the cold also had a BMR up to 32% higher and a Msum 16% higher than birds at 25°C. Organs were larger in the cold, with muscles and heart being 9–20% heavier and digestive and excretory organs being 21–36% larger than at thermoneutrality. Rather than the predicted digestive and excretory organs, the cold-induced increase in BMR correlated with changes in mass of the heart, pectoralis, and carcass. Msum varied positively with the mass of the pectoralis, supracoracoideus, and heart, highlighting the importance of muscles and cardiac function in cold endurance. Cold-acclimated knots also expressed upregulated capacity for lipid transport across mitochondrial membranes [carnitine palmitoyl transferase (CPT)] in their pectoralis and leg muscles, higher lipid catabolism capacity in their pectoralis muscles [β-hydroxyacyl CoA-dehydrogenase (HOAD)], and elevated oxidative capacity in their liver and kidney (citrate synthase). These adjustments may have contributed to BMR through changes in metabolic intensity. Positive relationships among Msum, CPT, and HOAD in the heart also suggest indirect constraints on thermogenic capacity through limited cardiac capacity.

Body temperature responses to handling stress in wintering Black-capped Chickadees (Poecile atricapillus L.)

Body temperature variation in response to acute stress is typically characterized by peripheral vasoconstriction and a concomitant increase in core body temperature (stress-induced hyperthermia). It is poorly understood how this response differs between species and within individuals of the same species, and how it is affected by the environment. We therefore investigated stress-induced body temperature changes in a non-model species, the Black-capped Chickadee, in two environmental conditions: outdoors in low ambient temperature (mean: -6.6°C), and indoors, in milder ambient temperature close to thermoneutrality (mean: 18.7°C). Our results show that the change in body temperature in response to the same handling stressor differs in these conditions. In cold environments, we noted a significant decrease in core body temperature (-2.9°C), whereas the response in mild indoor conditions was weak and non-significant (-0.6°C). Heat loss in outdoor birds was exacerbated when birds were handled for longer time. This may highlight the role of behavioral thermoregulation and heat substitution from activity to body temperature maintenance in harsh condition. Importantly, our work also indicates that changes in the physical properties of the bird during handling (conductive cooling from cold hands, decreased insulation from compression of plumage and prevention of ptiloerection) may have large consequences for thermoregulation. This might explain why females, the smaller sex, lost more heat than males in the experiment. Because physiological and physical changes during handling may carry over to affect predation risk and maintenance of energy balance during short winter days, we advice caution when designing experimental protocols entailing prolonged handling of small birds in cold conditions.

How low can you go? An adaptive energetic framework for interpreting basal metabolic rate variation in endotherms

Adaptive explanations for both high and low body mass-independent basal metabolic rate (BMR) in endotherms are pervasive in evolutionary physiology, but arguments implying a direct adaptive benefit of high BMR are troublesome from an energetic standpoint. Here, we argue that conclusions about the adaptive benefit of BMR need to be interpreted, first and foremost, in terms of energetics, with particular attention to physiological traits on which natural selection is directly acting. We further argue from an energetic perspective that selection should always act to reduce BMR (i.e., maintenance costs) to the lowest level possible under prevailing environmental or ecological demands, so that high BMR per se is not directly adaptive. We emphasize the argument that high BMR arises as a correlated response to direct selection on other physiological traits associated with high ecological or environmental costs, such as daily energy expenditure (DEE) or capacities for activity or thermogenesis. High BMR thus represents elevated maintenance costs required to support energetically demanding lifestyles, including living in harsh environments. BMR is generally low under conditions of relaxed selection on energy demands for high metabolic capacities (e.g., thermoregulation, activity) or conditions promoting energy conservation. Under these conditions, we argue that selection can act directly to reduce BMR. We contend that, as a general rule, BMR should always be as low as environmental or ecological conditions permit, allowing energy to be allocated for other functions. Studies addressing relative reaction norms and response times to fluctuating environmental or ecological demands for BMR, DEE, and metabolic capacities and the fitness consequences of variation in BMR and other metabolic traits are needed to better delineate organismal metabolic responses to environmental or ecological selective forces.

Context-dependent regulation of pectoralis myostatin and lipid transporters by temperature and photoperiod in dark-eyed juncos

A prominent example of seasonal phenotypic flexibility is the winter increase in thermogenic capacity (=summit metabolism, [Formula: see text]) in small birds, which is often accompanied by increases in pectoralis muscle mass and lipid catabolic capacity. Temperature or photoperiod may be drivers of the winter phenotype, but their relative impacts on muscle remodeling or lipid transport pathways are little known. We examined photoperiod and temperature effects on pectoralis muscle expression of myostatin, a muscle growth inhibitor, and its tolloid-like protein activators (TLL-1 and TLL-2), and sarcolemmal and intracellular lipid transporters in dark-eyed juncos Junco hyemalis. We acclimated winter juncos to four temperature (3 °C or 24 °C) and photoperiod [short-day (SD) = 8L:16D; long-day (LD) = 16L:8D] treatments. We found that myostatin, TLL-1, TLL-2, and lipid transporter mRNA expression and myostatin protein expression did not differ among treatments, but treatments interacted to influence lipid transporter protein expression. Fatty acid translocase (FAT/CD36) levels were higher for cold SD than for other treatments. Membrane-bound fatty acid binding protein (FABPpm) levels, however, were higher for the cold LD treatment than for cold SD and warm LD treatments. Cytosolic fatty acid binding protein (FABP_c) levels were higher on LD than on SD at 3 °C, but higher on SD than on LD at 24 °C. Cold temperature groups showed upregulation of these lipid transporters, which could contribute to elevated M_sum compared to warm groups on the same photoperiod. However, interactions of temperature or photoperiod effects on muscle remodeling and lipid transport pathways suggest that these effects are context-dependent.

Temperature and photoperiod as environmental cues affect body mass and thermoregulation in Chinese bulbuls, Pycnonotus sinensis

Seasonal changes in temperature and photoperiod are important environmental cues used by small birds to adjust their body mass (M_b) and thermogenesis. However, the relative importance of these cues with respect to seasonal adjustments in M_b and thermogenesis is difficult to distinguish. In particular, the effects of temperature and photoperiod on energy metabolism and thermoregulation are not well known in many passerines. To address this problem, we measured the effects of temperature and photoperiod on M_b, energy intake, resting metabolic rate (RMR), organ mass and physiological and biochemical markers of metabolic activity in the Chinese bulbul (Pycnonotus sinensis). Groups of Chinese bulbuls were acclimated in a laboratory to the following conditions: (1) warm and long photoperiod, (2) warm and short photoperiod, (3) cold and long photoperiod, and (4) cold and short photoperiod, for 4 weeks. The results indicate that Chinese bulbuls exhibit adaptive physiological regulation when exposed to different temperatures and photoperiods. M_b, RMR, gross energy intake and digestible energy intake were higher in cold-acclimated than in warm-acclimated bulbuls, and in the short photoperiod than in the long photoperiod. The resultant flexibility in energy intake and RMR allows Chinese bulbuls exposed to different temperatures and photoperiods to adjust their energy balance and thermogenesis accordingly. Cold-acclimated birds had heightened state-4 respiration and cytochrome c oxidase activity in their liver and muscle tissue compared with warm-acclimated birds indicating the cellular mechanisms underlying their adaptive thermogenesis. Temperature appears to be a primary cue for adjusting energy budget and thermogenic ability in Chinese bulbuls; photoperiod appears to intensify temperature-induced changes in energy metabolism and thermoregulation.

Uncoupling Basal and Summit Metabolic Rates in White-Throated Sparrows: Digestive Demand Drives Maintenance Costs, but Changes in Muscle Mass Are Not Needed to Improve Thermogenic Capacity

Avian basal metabolic rate (BMR) and summit metabolic rate (M_sum) vary in parallel during cold acclimation and acclimatization, which implies a functional link between these variables. However, evidence suggests that these parameters may reflect different physiological systems acting independently. We tested this hypothesis in white-throated sparrows (Zonotrichia albicollis) acclimated to two temperatures (-8° and 28°C) and two diets (0% and 30% cellulose). We expected to find an uncoupling of M_sum and BMR where M_sum, a measure of maximal shivering heat production, would reflect muscle and heart mass variation and would respond only to temperature, while BMR would reflect changes in digestive and excretory organs in response to daily food intake, responding to both temperature and diet. We found that the gizzard, liver, kidneys, and intestines responded to treatments through a positive relationship with food intake. BMR was 15% higher in cold-acclimated birds and, as expected, varied with food intake and the mass of digestive and excretory organs. In contrast, although M_sum was 19% higher in cold-acclimated birds, only heart mass responded to temperature (+18% in the cold). Pectoral muscles did not change in mass with temperature but were 8.2% lighter on the cellulose diet. Nevertheless, M_sum varied positively with the mass of heart and skeletal muscles but only in cold-acclimated birds. Our results therefore suggest that an upregulation of muscle metabolic intensity is required for cold acclimation. This study increases support for the hypothesis that BMR and M_sum reflect different physiological systems responding in parallel to constraints associated with cold environments.

Within-Winter Flexibility in Muscle Masses, Myostatin, and Cellular Aerobic Metabolic Intensity in Passerine Birds

Metabolic rates of passerine birds are flexible traits that vary both seasonally and among and within winters. Seasonal variation in summit metabolic rates (M_sum = maximum thermoregulatory metabolism) in birds is consistently correlated with changes in pectoralis muscle and heart masses and sometimes with variation in cellular aerobic metabolic intensity, so these traits might also be associated with shorter-term, within-winter variation in metabolic rates. To determine whether these mechanisms are associated with within-winter variation in M_sum, we examined the effects of short-term (ST; 0-7 d), medium-term (MT; 14-30 d), and long-term (LT; 30-yr means) temperature variables on pectoralis muscle and heart masses, pectoralis expression of the muscle-growth inhibitor myostatin and its metalloproteinase activators TLL-1 and TLL-2, and pectoralis and heart citrate synthase (CS; an indicator of cellular aerobic metabolic intensity) activities for two temperate-zone resident passerines, house sparrows (Passer domesticus) and dark-eyed juncos (Junco hyemalis). For both species, pectoralis mass residuals were positively correlated with ST temperature variables, suggesting that cold temperatures resulted in increased turnover of pectoralis muscle, but heart mass showed little within-winter variation for either species. Pectoralis mRNA and protein expression of myostatin and the TLLs were only weakly correlated with ST and MT temperature variables, which is largely consistent with trends in muscle masses for both species. Pectoralis and heart CS activities showed weak and variable trends with ST temperature variables in both species, suggesting only minor effects of temperature variation on cellular aerobic metabolic intensity. Thus, neither muscle or heart masses, regulation by the myostatin system, nor cellular aerobic metabolic intensity varied consistently with winter temperature, suggesting that other factors regulate within-winter metabolic variation in these birds.

Short photoperiod increases energy intake, metabolic thermogenesis and organ mass in silky starlings Sturnus sericeus

Environmental cues play important roles in the regulation of an animal's physiology and behavior. One such cue, photoperiod, plays an important role in the seasonal acclimatization of birds. It has been demonstrated that an animal's body mass, basal metabolic rate (BMR), and energy intake, are all affected by photoperiod. The present study was designed to examine photoperiod induced changes in the body mass, metabolism and metabolic organs of the silky starling, Sturnus sericeus. Captive silky starlings increased their body mass and BMR during four weeks of acclimation to a short photoperiod. Birds acclimated to a short photoperiod also increased the mass of certain organs (liver, gizzard and small intestine), and both gross energy intake (GEI) and digestible energy intake (DEI), relative to those acclimated to a long photoperiod. Furthermore, BMR was positively correlated with body mass, liver mass, GEI and DEI. These results suggest that silky starlings increase metabolic thermogenesis when exposed to a short photoperiod by increasing their body and metabolic organ mass, and their GEI and DEI. These findings support the hypothesis that bird species from temperate climates typically display high phenotypic flexibility in thermogenic capacity.

Phenotypic flexibility of thermogenesis in the hwamei (Garrulax canorus): responses to cold acclimation

Cold acclimation in birds involves a comprehensive array of physiological and morphological adjustment ranging from changes in aerobic enzyme activity to metabolic rate and organ mass. In the present study, we investigated phenotypic variation in thermogenic activity in the hwamei (Garrulax canorus) under normal (35°C) or cold (15°C) ambient temperature conditions. Acclimation to an ambient temperature of 15°C for 4 wk significantly increased the body mass, basal metabolic rate (BMR), and energy intake, including both gross energy intake and digestible energy intake, compared with birds kept at 35°C. Furthermore, birds acclimated to 15°C increased the dry mass of their liver and kidneys, but not their heart and pectoral muscles, and displayed higher state-4 respiration in the liver, kidneys, heart, and pectoral muscles, and higher cytochrome-c oxidase (COX) activity in liver, kidney, and pectoral muscle, compared with those kept at 35°C. There was a positive correlation between BMR and state-4 respiration in all of the above organs except the liver, and between BMR and COX activity in all of the above organs. Taken together, these data illustrate the morphological, physiological, and enzymatic changes associated with cold acclimation, and support the notion that the hwamei is a bird species from temperate climates that exhibits high phenotypic flexibility of thermogenic capacity.

Summer-to-Winter Phenotypic Flexibility of Fatty Acid Transport and Catabolism in Skeletal Muscle and Heart of Small Birds

Prolonged shivering in birds is mainly fueled by lipids. Consequently, lipid transport and catabolism are vital for thermogenic performance and could be upregulated along with thermogenic capacity as part of the winter phenotype. We investigated summer-to-winter variation in lipid transport and catabolism by measuring mRNA expression, protein levels, and enzyme activities for several key steps of lipid transport and catabolic pathways in pectoralis muscle and heart in two small temperate-zone resident birds, American goldfinches (Spinus tristis) and black-capped chickadees (Poecile atricapillus). Cytosolic fatty acid binding protein (FABPc; a key component of intramyocyte lipid transport) mRNA and/or protein levels were generally higher in winter for pectoralis muscle and heart for both species. However, seasonal variation in plasma membrane lipid transporters, fatty acyl translocase, and plasma membrane fatty acid binding protein in pectoralis and heart differed between the two species, with winter increases for chickadees and seasonal stability or summer increases for goldfinches. Catabolic enzyme activities generally showed limited seasonal differences for both tissues and both species. These data suggest that FABPc is an important target of upregulation for the winter phenotype in pectoralis and heart of both species. Plasma membrane lipid transporters and lipid catabolic capacity were also elevated in winter for chickadees but not for goldfinches. Because the two species show differential regulation of distinct aspects of lipid transport and catabolism, these data are consistent with other recent studies documenting that different bird species or populations employ a variety of strategies to promote elevated winter thermogenic capacity.

Phenotypic flexibility of skeletal muscle and heart masses and expression of myostatin and tolloid-like proteinases in migrating passerine birds

Migrant birds require large flight muscles and hearts to enhance aerobic capacity and support sustained flight. A potential mechanism for increasing muscle and heart masses during migration in birds is the muscle growth inhibitor myostatin and its metalloproteinase activators, tolloid-like proteinases (TLL-1 and TLL-2). We hypothesized that myostatin, TLL-1 and TLL-2 are downregulated during migration in pectoralis and hearts of migratory passerines to promote hypertrophy. We measured seasonal variation of tissue masses, mRNA expression of myostatin, TLL-1, and TLL-2, and myostatin protein levels in pectoralis muscle and heart for yellow warblers (Setophaga petechia), warbling vireos (Vireo gilvus), and yellow-rumped warblers (Setophaga coronata). Pectoralis mass was greatest in spring for warbling vireos and yellow warblers, but was stable between spring and fall for yellow-rumped warblers. Heart mass was higher in spring than in fall for yellow-rumped warblers, lowest in fall for warbling vireos, and seasonally stable for yellow warblers. Pectoralis and heart mRNA expression of myostatin and the TLLs did not differ significantly for any of the three species, offering little support for our hypothesis for a prominent role for myostatin in regulating migration-induced variation in pectoralis and heart masses. In contrast, pectoralis myostatin protein levels were lowest in spring for all three species, consistent with our hypothesis. Myostatin protein levels in heart, however, were seasonally stable for warbling vireos and yellow warblers, and increased in spring relative to fall for yellow-rumped warblers. These data offer mixed support for our hypothesis for the pectoralis, but suggest that myostatin is not a prominent regulator of migration-induced heart hypertrophy. Moreover, the different seasonal patterns for pectoralis mRNA and protein expression suggest that post-transcriptional modification of myostatin may contribute to pectoralis mass regulation during migration.

Reaction norms in natural conditions: how does metabolic performance respond to weather variations in a small endotherm facing cold environments?

Reaction norms reflect an organisms' capacity to adjust its phenotype to the environment and allows for identifying trait values associated with physiological limits. However, reaction norms of physiological parameters are mostly unknown for endotherms living in natural conditions. Black-capped chickadees (Poecile atricapillus) increase their metabolic performance during winter acclimatization and are thus good model to measure reaction norms in the wild. We repeatedly measured basal (BMR) and summit (Msum) metabolism in chickadees to characterize, for the first time in a free-living endotherm, reaction norms of these parameters across the natural range of weather variation. BMR varied between individuals and was weakly and negatively related to minimal temperature. Msum varied with minimal temperature following a Z-shape curve, increasing linearly between 24°C and −10°C, and changed with absolute humidity following a U-shape relationship. These results suggest that thermal exchanges with the environment have minimal effects on maintenance costs, which may be individual-dependent, while thermogenic capacity is responding to body heat loss. Our results suggest also that BMR and Msum respond to different and likely independent constraints.

Mechanistic drivers of flexibility in summit metabolic rates of small birds

Flexible metabolic phenotypes allow animals to adjust physiology to better fit ecological or environmental demands, thereby influencing fitness. Summit metabolic rate (Msum = maximal thermogenic capacity) is one such flexible trait. Skeletal muscle and heart masses and myocyte metabolic intensity are potential drivers of Msum flexibility in birds. We examined correlations of skeletal muscle and heart masses and pectoralis muscle citrate synthase (CS) activity (an indicator of cellular metabolic intensity) with Msum in house sparrows (Passer domesticus) and dark-eyed juncos (Junco hyemalis) to determine whether these traits are associated with Msum variation. Pectoralis mass was positively correlated with Msum for both species, but no significant correlation remained for either species after accounting for body mass (Mb) variation. Combined flight and leg muscle masses were also not significantly correlated with Msum for either species. In contrast, heart mass was significantly positively correlated with Msum for juncos and nearly so (P = 0.054) for sparrows. Mass-specific and total pectoralis CS activities were significantly positively correlated with Msum for sparrows, but not for juncos. Thus, myocyte metabolic intensity influences Msum variation in house sparrows, although the stronger correlation of total (r = 0.495) than mass-specific (r = 0.378) CS activity with Msum suggests that both pectoralis mass and metabolic intensity impact Msum. In contrast, neither skeletal muscle masses nor pectoralis metabolic intensity varied with Msum in juncos. However, heart mass was associated with Msum variation in both species. These data suggest that drivers of metabolic flexibility are not uniform among bird species.