Heterothermic Migration Strategies in Flying Vertebrates

Migration is a widespread and highly variable trait among animals. Population-level patterns arise from individual-level decisions, including physiological and energetic constraints. Many aspects of migration are influenced by behaviors and strategies employed during periods of stopover, where migrants may encounter variable or unpredictable conditions. Thermoregulation can be a major cost for homeotherms which largely encounter ambient temperatures below the lower critical temperature during migration, especially during the rest phase of the daily cycle. In this review we describe the empirical evidence, theoretical models, and potential implications of bats and birds that use heterothermy to reduce thermoregulatory costs during migration. Torpor-assisted migration is a strategy described for migrating temperate insectivorous bats, whereby torpor can be used during periods of inactivity to drastically reduce thermoregulatory costs and increase net refueling rate, leading to shorter stopover duration, reduced fuel load requirement, and potential consequences for broad-scale movement patterns and survival. Hummingbirds can adopt a similar strategy, but most birds are not capable of torpor. However, there is an increasing recognition of the use of more shallow heterothermic strategies by diverse bird species during migration, with similarly important implications for migration energetics. A growing body of published literature and preliminary data from ongoing research indicate that heterothermic migration strategies in birds may be more common than traditionally appreciated. We further take a broad evolutionary perspective to consider heterothermy as an alternative to migration in some species, or as a conceptual link to consider alternatives to seasonal resource limitations. There is a growing body of evidence related to heterothermic migration strategies in bats and birds, but many important questions related to the broader implications of this strategy remain.

Avian Heterothermy: A Review of Patterns and Processes

Many birds reduce rest-phase energy demands through heterothermy, physiological responses involving facultative, reversible reductions in metabolic rate and body temperature (Tb). Here, we review the phylogenetic distribution and ecological contexts of avian heterothermy. Heterothermy has been reported in 140 species representing 15 orders and 39 families. Recent work supports the view that deep heterothermy is most pronounced in phylogenetically older taxa whereas heterothermy in passerines and other recently diverged taxa is shallower and confined to minimum Tb > 20°C. The reasons why deep heterothermy is absent in passerines remain unclear; we speculate an evolutionary trade-off may exist between the capacity to achieve low heterothermic Tb and the tolerance of hyperthermic Tb. Inter- and intraspecific variation in heterothermy is correlated with factors including foraging ecology (e.g., territoriality and defense of food resources among hummingbirds), food availability and foraging opportunities (e.g., lunar phase predicts torpor use in caprimulgids), and predation risk. Heterothermy also plays a major role before and during migration. Emerging questions include the magnitude of energy savings associated with heterothermy among free-ranging birds, the role phylogenetic variation in the capacity for heterothermy has played in evolutionary radiations into extreme habitats, and how the capacity for heterothermy affects avian vulnerability to rapid anthropogenic climate change.

Variation in movement strategies: Capital versus income migration

Animal migrations represent the regular movements of trillions of individuals. The scale of these movements has inspired human intrigue for millennia and has been intensively studied by biologists. This research has highlighted the diversity of migratory strategies seen across and within migratory taxa: while some migrants temporarily express phenotypes dedicated to travel, others show little or no phenotypic flexibility in association with migration. However, a vocabulary for describing these contrasting solutions to the performance trade-offs inherent to the highly dynamic lifestyle of migrants (and strategies intermediate between these two extremes) is currently missing. We propose a taxon-independent organising framework based on energetics, distinguishing between migrants that forage as they travel (income migrants) and those that fuel migration using energy acquired before departure (capital migrants). Not only does our capital:income continuum of migratory energetics account for the variable extent of phenotypic flexibility within and across migrant populations, but it also aligns with theoreticians' treatment of migration and clarifies how migration impacts other phases of the life cycle. As such, it provides a unifying scale and common vacabulary for comparing the migratory strategies of divergent taxa.

A conceptual framework to integrate cold-survival strategies: torpor, resistance and seasonal migration

Freezing temperatures are inherently challenging for life, which is water based. How species cope with these conditions fundamentally shapes ecological and evolutionary processes. Despite this, there is no comprehensive conceptual framework for cold-survival strategies-seasonal migration, cold resistance and torpor. Here, I propose a framework with four components for conceptualizing and quantifying cold-survival strategies. Cold-survival strategies are (i) collectively encompassed by the proposed framework, and that this full breadth of strategies should be considered in focal species or systems (comprehensive consideration). These strategies also (ii) exist on a spectrum, such that species can exhibit partial use of strategies, (iii) are non-exclusive, such that some species use multiple strategies concurrently (combined use) and (iv) should collectively vary inversely and proportionally with one another when controlling for the external environment (e.g. when considering species that occur in sympatry in their summer range), such that use of one strategy reduces, collectively, the use of others (proportional use). This framework is relevant to understanding fundamental patterns and processes in evolution, ecology, physiology and conservation biology.

How Birds During Migration Maintain (Oxidative) Balance

Animals dynamically adjust their physiology and behavior to survive in changing environments, and seasonal migration is one life stage that demonstrates these dynamic adjustments. As birds migrate between breeding and wintering areas, they incur physiological demands that challenge their antioxidant system. Migrating birds presumably respond to these oxidative challenges by up-regulating protective endogenous systems or accumulating dietary antioxidants at stopover sites, although our understanding of the pre-migration preparations and mid-migration responses of birds to such oxidative challenges is as yet incomplete. Here we review evidence from field and captive-bird studies that address the following questions: (1) Do migratory birds build antioxidant capacity as they build fat stores in preparation for long flights? (2) Is oxidative damage an inevitable consequence of oxidative challenges such as flight, and, if so, how is the extent of damage affected by factors such as the response of the antioxidant system, the level of energetic challenge, and the availability of dietary antioxidants? (3) Do migratory birds ‘recover’ from the oxidative damage accrued during long-duration flights, and, if so, does the pace of this rebalancing of oxidative status depend on the quality of the stopover site? The answer to all these questions is a qualified ‘yes’ although ecological factors (e.g., diet and habitat quality, geographic barriers to migration, and weather) affect how the antioxidant system responds. Furthermore, the pace of this dynamic physiological response remains an open question, despite its potential importance for shaping outcomes on timescales ranging from single flights to migratory journeys. In sum, the antioxidant system of birds during migration is impressively dynamic and responsive to environmental conditions, and thus provides ample opportunities to study how the physiology of migratory birds responds to a changing and challenging world.

Testing Predictions of Optimal Migration Theory in Migratory Bats

Optimal migration theory is a framework used to evaluate trade-offs associated with migratory strategies. Two strategies frequently considered by migration theory are time minimizing, whereby migration is completed as quickly as possible, and energy minimizing, whereby migration is completed as energetically efficiently as possible. Despite extensive literature dedicated to generating analytical predictions about these migratory strategies, identifying appropriate study systems to empirically test predictions is difficult. Theoretical predictions that compare migratory strategies are qualitative, and empirical tests require that both time-minimizers and energy-minimizers are present in the same population; spring migrating silver-haired (Lasionycteris noctivagans) and hoary bats (Lasiurus cinereus) provide such a system. As both species mate in the fall, spring-migrating males are thought to be energy-minimizers while females benefit from early arrival to summering grounds, and are thought to be time-minimizers. Thermoregulatory expression also varies between species during spring migration, as female silver-haired bats and males of both species use torpor while female hoary bats, which implant embryos earlier, are thought to avoid torpor use which would delay pregnancy. Based on optimal migration theory, we predicted that female silver-haired bats and hoary bats would have increased fuel loads relative to males and the difference between fuel loads of male and female hoary bats would be greater than the difference between male and female silver-haired bats. We also predicted that females of both species would have a greater stopover foraging proclivity and/or assimilate nutrients at a greater rate than males. We then empirically tested our predictions using quantitative magnetic resonance to measure fuel load, δ13C isotope breath signature analysis to assess foraging, and 13C–labeled glycine to provide an indicator of nutrient assimilation rate. Optimal migration theory predictions of fuel load were supported, but field observations did not support the predicted refueling mechanisms, and alternatively suggested a reliance on increased fuel loads via carry-over effects. This research is the first to validate a migration theory prediction in a system of both time and energy minimizers and uses novel methodological approaches to uncover underlying mechanisms of migratory stopover use.

Circadian and Seasonal Patterns of Body Temperature in Arctic Migratory and Temperate Non-migratory Geese

Arctic migration presents unique challenges to circadian physiology. In addition to the metabolic cost of maintaining a relatively high body temperature (Tb) above ambient temperature, migratory birds are also exposed to rapidly changing light conditions as they transition between light-dark cycles and a 24-hour polar day. A previous study suggested that Arctic-migratory barnacle geese (Branta leucopsis) may utilise adaptive heterothermy (i.e., a controlled decrease in core Tb) during and around the autumn migratory period in order to minimise the metabolic cost of migration, but the impact of seasonally changing daylight conditions on other parameters of the circadian profile of Tb in these geese remained obscure. Here, we provide a detailed comparative analysis on the circadian rhythm of Tb and its seasonal development in free-living barnacle geese from three study populations that differ in their migratory behaviour and in the environments they occupy. We recorded abdominal Tb in non-migratory geese from a temperate breeding colony in Netherlands and in migratory geese from a colony in the Russian low Arctic, and analysed these data together with previously published Tb data on geese from a migratory colony in the high Arctic of Svalbard. We found that the circadian Tb profile in the barnacle goose was well aligned with the daily and seasonally changing daylight conditions. In the migratory populations, a fast re-entrainment of the rhythm and its phase was observed when zeitgeber conditions changed during migratory movements. The circadian rhythmicity of Tb was lost once the geese encountered permanent daylight at their northern staging and breeding sites. Circadian Tb rhythmicity was re-established when the period of permanent daylight ended, at rates corresponding to rates of seasonal changes in daylength in the high and low Arctic. Although our data corroborated findings of a decrease in daily mean Tb before autumn migration in both migratory populations in this study, the pre-migratory decrease in Tb was less drastic than previously reported. Moreover, in contrast to previous study, the decrease in Tb stopped at the onset of migration. Overall, our data reveal no evidence that heterothermy in the barnacle goose is functionally linked to migration.

Heat loss in sleeping garden warblers (Sylvia borin) during migration

For small songbirds, energy is often a limiting factor during migration and, for this reason, they are forced to alternate nocturnal flights with stopovers to rest and replenish energy stores. Stopover duration has a key role for a successful migration and may have an important impact on fitness. Thus, migrants need to optimize their energy consumption at this stage to reduce their permanence at the site. A recent study has shown that lean individuals reduce their metabolic rate when tucking the head in the feathers during sleep. The underlying mechanism is very likely a reduction in conductance, but the thermoregulatory benefit of the increased insulation has never been quantified yet. Here, we compared heat loss in individual migratory birds while sleeping in different postures. Using a thermal camera and a within-individual approach, we estimated that Garden Warblers can reduce their rate of heat loss by 54% by sleeping with the head tucked in the feathers. This energy saving has a relevant impact on the individual's energy balance because it can account for up to 8.69% of daily energy expenditure during stopover. Our study provides novel and important information to understand the fundamental role of thermoregulatory strategies on bird's energy management.

Metabolic reduction after long-duration flight is not related to fat-free mass loss or flight duration in a migratory passerine

Migratory birds catabolize large quantities of protein during long flights, resulting in dramatic reductions in organ and muscle mass. One of the many hypotheses to explain this phenomenon is that decrease in lean mass is associated with reduced resting metabolism, saving energy after flight during refueling. However, the relationship between lean body mass and resting metabolic rate remains unclear. Furthermore, the coupling of lean mass with resting metabolic rate and with peak metabolic rate before and after long-duration flight have not previously been explored. We flew migratory yellow-rumped warblers (Setophaga coronata) in a wind tunnel under one of two humidity regimes to manipulate the rate of lean mass loss in flight, decoupling flight duration from total lean mass loss. Before and after long-duration flights, we measured resting and peak metabolism, and also measured fat mass and lean body mass using quantitative magnetic resonance. Flight duration ranged from 28 min to 600 min, and birds flying under dehydrating conditions lost more fat-free mass than those flying under humid conditions. After flight, there was a 14% reduction in resting metabolism but no change in peak metabolism. Interestingly, the reduction in resting metabolism was unrelated to flight duration or to change in fat-free body mass, indicating that protein metabolism in flight is unlikely to have evolved as an energy-saving measure to aid stopover refueling, but metabolic reduction itself is likely to be beneficial to migratory birds arriving in novel habitats.

Seasonal Expression of Avian and Mammalian Daily Torpor and Hibernation: Not a Simple Summer-Winter Affair†

Daily torpor and hibernation (multiday torpor) are the most efficient means for energy conservation in endothermic birds and mammals and are used by many small species to deal with a number of challenges. These include seasonal adverse environmental conditions and low food/water availability, periods of high energetic demands, but also reduced foraging options because of high predation pressure. Because such challenges differ among regions, habitats and food consumed by animals, the seasonal expression of torpor also varies, but the seasonality of torpor is often not as clear-cut as is commonly assumed and differs between hibernators and daily heterotherms expressing daily torpor exclusively. Hibernation is found in mammals from all three subclasses from the arctic to the tropics, but is known for only one bird. Several hibernators can hibernate for an entire year or express torpor throughout the year (8% of species) and more hibernate from late summer to spring (14%). The most typical hibernation season is the cold season from fall to spring (48%), whereas hibernation is rarely restricted to winter (6%). In hibernators, torpor expression changes significantly with season, with strong seasonality mainly found in the sciurid and cricetid rodents, but seasonality is less pronounced in the marsupials, bats and dormice. Daily torpor is diverse in both mammals and birds, typically is not as seasonal as hibernation and torpor expression does not change significantly with season. Torpor in spring/summer has several selective advantages including: energy and water conservation, facilitation of reproduction or growth during development with limited resources, or minimisation of foraging and thus exposure to predators. When torpor is expressed in spring/summer it is usually not as deep and long as in winter, because of higher ambient temperatures, but also due to seasonal functional plasticity. Unlike many other species, subtropical nectarivorous blossom-bats and desert spiny mice use more frequent and pronounced torpor in summer than in winter, which is related to seasonal availability of nectar or water. Thus, seasonal use of torpor is complex and differs among species and habitats.

Sleeping Unsafely Tucked in to Conserve Energy in a Nocturnal Migratory Songbird

Each spring and fall, millions of normally diurnal birds switch to migrating at night. Most of these are small songbirds (passerine) migrating long distances that need to alternate their migratory flights with refueling stopovers [1, 2], which can account for up to 80% of the total migratory period [3]. After a long nocturnal flight, these birds face the contrasting needs to recover sleep and refill depleted energy stores, all while vulnerable to predation [4, 5]. Here, we investigated how garden warblers at a Mediterranean stopover site modulate their sleep behavior in relation to their metabolic state. At night, garden warblers in poor metabolic condition sleep more and exhibit less migratory restlessness than birds in good condition do. In addition, rather than sleeping with their head facing forward, birds in poor condition prefer to sleep with their head turned and tucked in their feathers. We further show that sleep with the head tucked is associated with lower respiratory and metabolic rates and reduced heat loss mediated by hiding the head-the body part with the highest heat dissipation-under the feathers. However, the benefit of conserving energy while sleeping with the head tucked was countered by reduced anti-predator vigilance. Birds presented with a sound simulating the approach of a predator responded more slowly when the head was tucked than when it was untucked. Consequently, our study demonstrates that through changing their sleep position and intensity, migrating songbirds can negotiate a previously unknown trade-off between sleep-mediated energy conservation and anti-predatory vigilance.

Obese super athletes: fat-fueled migration in birds and bats

Migratory birds are physiologically specialized to accumulate massive fat stores (up to 50-60% of body mass), and to transport and oxidize fatty acids at very high rates to sustain flight for many hours or days. Target gene, protein and enzyme analyses and recent -omic studies of bird flight muscles confirm that high capacities for fatty acid uptake, cytosolic transport, and oxidation are consistent features that make fat-fueled migration possible. Augmented circulatory transport by lipoproteins is suggested by field data but has not been experimentally verified. Migratory bats have high aerobic capacity and fatty acid oxidation potential; however, endurance flight fueled by adipose-stored fat has not been demonstrated. Patterns of fattening and expression of muscle fatty acid transporters are inconsistent, and bats may partially fuel migratory flight with ingested nutrients. Changes in energy intake, digestive capacity, liver lipid metabolism and body temperature regulation may contribute to migratory fattening. Although control of appetite is similar in birds and mammals, neuroendocrine mechanisms regulating seasonal changes in fuel store set-points in migrants remain poorly understood. Triacylglycerol of birds and bats contains mostly 16 and 18 carbon fatty acids with variable amounts of 18:2n-6 and 18:3n-3 depending on diet. Unsaturation of fat converges near 70% during migration, and unsaturated fatty acids are preferentially mobilized and oxidized, making them good fuel. Twenty and 22 carbon n-3 and n-6 polyunsaturated fatty acids (PUFA) may affect membrane function and peroxisome proliferator-activated receptor signaling. However, evidence for dietary PUFA as doping agents in migratory birds is equivocal and requires further study.

Specialist-generalist model of body temperature regulation can be applied at the intraspecific level

According to theoretical predictions, endothermic homeotherms can be classified as either thermal specialists or thermal generalists. In high cost environments, thermal specialists are supposed to be more prone to using facultative heterothermy than generalists. We tested this hypothesis at the intraspecific level using male laboratory mice (C57BL/cmdb) fasted under different thermal conditions (20 and 10°C) and for different time periods (12-48 h). We predicted that variability of body temperature (T_b) and time spent with T_b below normothermy would increase with the increase of environmental demands (duration of fasting and cold). To verify the above prediction, we measured T_b and energy expenditure of fasted mice. We did not record torpor bouts but we found that variations in T_b and time spent in hypothermia increased with environmental demands. In response to fasting, mice also decreased their energy expenditure. Moreover, animals that showed more precise thermoregulation when fed had more variable T_b when fasted. We postulate that the prediction of the thermoregulatory generalist-specialist trade-off can be applied at the intraspecific level, offering a valid tool for identifying mechanistic explanations of the differences in animal responses to variations in energy supply.

Bats on a budget: torpor-assisted migration saves time and energy

Bats and birds must balance time and energy budgets during migration. Migrating bats face similar physiological challenges to birds, but nocturnality creates special challenges for bats, such as a conflict between travelling and refueling, which many birds avoid by feeding in daylight and flying at night. As endothermic animals, bats and birds alike must expend substantial amounts of energy to maintain high body temperatures. For migratory birds refueling at stopovers, remaining euthermic during inactive periods reduces the net refuelling rate, thereby prolonging stopover duration and delaying subsequent movement. We hypothesized that bats could mitigate similar ambient-temperature dependent costs by using a torpor-assisted migration strategy. We studied silver-haired bats Lasionycteris noctivagans during autumn migration using a combination of respirometry and temperature-sensitive radiotelemetry to estimate energy costs incurred under ambient temperature conditions, and the energy that bats saved by using torpor during daytime roosting periods. All bats, regardless of sex, age, or body condition used torpor at stopover and saved up to 91% of the energy they would have expended to remain euthermic. Furthermore, bats modulated use of torpor depending on ambient temperature. By adjusting the time spent torpid, bats achieved a rate of energy expenditure independent of the ambient temperature encountered at stopover. By lowering body temperature during inactive periods, fuel stores are spared, reducing the need for refuelling. Optimal migration models consider trade-offs between time and energy. Heterothermy provides a physiological strategy that allows bats to conserve energy without paying a time penalty as they migrate. Although uncommon, some avian lineages are known to use heterothermy, and current theoretical models of migration may not be appropriate for these groups. We propose that thermoregulatory strategies should be an important consideration of future migration studies of both bats and birds.

Selection for high activity-related aerobic metabolism does not alter the capacity of non-shivering thermogenesis in bank voles

An intriguing question is how the capacity of non-shivering thermogenesis (NST)-a special mechanism supporting endothermic thermoregulation in mammals-is affected by selection for high exercise metabolism. It has been proposed that high NST could be a mechanism to compensate for a low basal production of heat. On the other hand, high basal or activity metabolism is associated with physiological characteristics such as high performance of the circulatory system, which are also required for achieving a high NST. Here we tested whether selection for high aerobic exercise performance, which correlates with an increased basal metabolic rate, led to a correlated evolution of maximum and facultative NST. Therefore, we measured the NST of bank voles, Myodes (= Clethrionomys) glareolus, from lines selected for 13-14 generations (n=46) for high aerobic metabolism achieved during swimming and from unselected, control lines (n=46). Open-flow respirometry was used to measure the rate of oxygen consumption (V(·)O2) in anesthetized bank voles injected with noradrenaline (NA). After adjusting for body mass, maximum NST (maximum V(·)O2 recorded after injection of NA) did not differ between the selected (2.38±0.08 mLO2min(-1)) and control lines (2.36±0.08 mLO2min(-1); P=0.891). Facultative NST (= maximum NST minus resting metabolic rate of anesthetized animals) did not differ between the selected (1.49±0.07 mLO2min(-1)) and control lines (1.50±0.07 mLO2min(-1); P=0.985), either. Therefore, our results suggest that NST capacity is not strongly linked to maximum activity-related aerobic metabolic rate.

Increased activity correlates with reduced ability to mount immune defenses to endotoxin in zebra finches

When suffering from infection, animals experience behavioral and physiological alterations that potentiate the immune system's ability to fight pathogens. The behavioral component of this response, termed "sickness behavior," is characterized by an overall reduction in physical activity. A growing number of reports demonstrate substantial flexibility in these sickness behaviors, which can be partially overcome in response to mates, intruders and parental duties. Since it is hypothesized that adopting sickness behaviors frees energetic resources for mounting an immune response, we tested whether diminished immune responses coincided with reduced sickness behaviors by housing male zebra finches (Taeniopygia guttata) in social conditions that alter their behavioral response to an endotoxin. To facilitate our data collection, we developed and built a miniaturized sensor capable of detecting changes in dorsoventral acceleration and categorizing them as different behaviors when attached to the finches. We found that the immune defenses (quantified as haptoglobin-like activity, ability to change body temperature and bacterial killing capacity) increased as a function of increased time spent resting. The findings indicate that when animals are sick attenuation of sickness behaviors may exact costs, such as reduced immune function. The extent of these costs depends on how relevant the affected components of immunity are for fighting a specific infection.

Melatonin reduces migratory restlessness in Sylvia warblers during autumnal migration

INTRODUCTION

A remarkable aspect of bird migration is its nocturnality, particularly common in Passeriformes. The switch in activity from purely diurnal to also nocturnal is evident even in caged birds that during migratory periods develop an intense nocturnal restlessness, termed Zugunruhe. The mechanisms that control this major change in activity are mostly unknown. Previous work with Sylvia warblers suggested an involvement of melatonin, a hormone associated with day-night cycles in most vertebrates. In a recent study we found no effects of melatonin administration on Zugunruhe during spring migration. However, previous studies indicated that the response to melatonin manipulation could differ between spring and autumn migration, which are in fact separate life history stages. Here we tested whether a non-invasive treatment with melatonin can alter Zugunruhe in wild garden warblers S. borin and blackcaps S. atricapilla subject to temporary captivity at an autumnal stopover site. Food availability in the cage (yes/no) was added as a second factor because previous work showed that it enhanced Zugunruhe.

RESULTS

The melatonin treatment significantly decreased the amount of Zugunruhe, while the availability of food only tended to increase the amount of Zugunruhe. Fuel deposits also had a strong effect on the amount of nocturnal activity: lean birds with a fat score of 1 showed significantly less Zugunruhe than fatter birds. The change in body mass during the time spent in the recording cage depended on food availability, but not on any of the other factors.

CONCLUSIONS

This study shows that the migratory programme of two Sylvia warblers can be manipulated by administration of exogenous melatonin and confirms that this hormone is involved in the control of migratory behaviour. To our knowledge, this is one of the first demonstrations that the autumn migratory programme can be altered by hormonal manipulation in migrating birds. The comparison with a similar study carried out with the same modalities during spring migration suggests that there are seasonal differences in the sensitivity of the migratory programme to hormonal factors. In birds breeding in the northern hemisphere, the importance of a timely arrival to the breeding sites could explain why the control of the migratory programme is more rigid in spring.

Contributions of endocrinology to the migration life history of birds

Migration is a key life cycle stage in nearly 2000 species of birds and is a greatly appreciated phenomenon in both cultural and academic arenas. Despite a long research tradition concerning many aspects of migration, investigations of hormonal contributions to migratory physiology and behavior are more limited and represent a comparatively young research field. We review advances in our understanding of the hormonal mechanisms of migration with particular emphasis on the sub-stages of the migration life history: development, departure, flight and arrival. These sub-stages vary widely in their behavioral, ecological and physiological contexts and, as such, should be given appropriate individual consideration.

Phenotypic flexibility in migrating bats: seasonal variation in body composition, organ sizes and fatty acid profiles

Many species of bats migrate long distances, but the physiological challenges of migration are poorly understood. We tested the hypothesis that migration is physiologically demanding for bats by examining migration-related phenotypic flexibility. Both bats and birds are endothermic, flying vertebrates; therefore, we predicted that migration would result in similar physiological trade-offs. We compared hoary bats (Lasiurus cinereus) during spring migration and summer non-migratory periods, comparing our results with previous observations of birds. Migrating bats had reduced digestive organs, enlarged exercise organs, and fat stores had higher proportions of polyunsaturated fatty acids (PUFAs). These results are consistent with previous studies of migrating birds; however, we also found sex differences not typically associated with bird migration. Migrating female hoary bats increased the relative size of fat stores by reducing lean body components, while males maintained the same relative amount of fat in both seasons. The ratio of n-6 to n-3 PUFA in flight muscle membrane increased in migrating males and decreased in migrating females, consistent with males using torpor more frequently than females during spring migration. Enlarged exercise organs, reduced digestive organs and changes in adipose tissue composition reflect the elevated energetic demands of migration. Sex-specific patterns of fat storage and muscle membrane composition likely reflect challenges faced by females that migrate while pregnant. Our results provide some of the first insights into the physiological demands of bat migration and highlight key differences between bats and birds.

Seasonal variation in energy expenditure is not related to activity level or water temperature in a large diving bird

There is considerable interest in understanding how the energy budget of an endotherm is modulated from a physiological and ecological point of view. In this paper, we used the heart rate method and daily heart rate (DHR), as a proxy of DEE across seasons, to test the effect of locomotion activity and water temperature on the energy budget of a large diving bird. DHR was monitored continuously in common eiders (Somateria mollissima) during seven months together with measures of time spent flying and time spent feeding. DHR varied substantially during the recording period with numerous increases and decreases that occurred across seasons although we could not find any relationship between DHR and the time spent active (feeding and flying). However, inactive heart rate (IHR) decreased as locomotion activity increases suggesting common eiders were using behavioural compensation when under a high work load. We were also unable to detect a negative relationship between water temperature and resting heart rate, a proxy of resting metabolic rate. This was unexpected based on the assumption that high thermoregulation costs would be associated with cold waters. We showed that high level of energy expenditure coincided with feather moult and warm waters, which suggest that the observed variable pattern of seasonal DEE was driven by feather growth and possibly by other productive costs. Nevertheless, our results indicate that behavioural compensation and possibly the timing of moult may be used as mechanisms to reduce seasonal variation in energy expenditure.

Migratory stopover in the long-distance migrant silver-haired bat, Lasionycteris noctivagans

1. Some bat species make long-distance latitudinal migrations between summer and winter grounds, but because of their elusive nature, few aspects of their biology are well understood. The need for migratory stopover sites to rest and refuel, such as used by birds, has been repeatedly suggested, but not previously tested empirically in bats. 2. We studied migrating silver-haired bats (Lasionycteris noctivagans) at Long Point, ON, Canada. We used digital radio-transmitters to track 30 bats using an array of five towers that effectively covered the entire region (c. 20 × 40 km). We measured stopover duration and departure direction, and documented movement patterns, foraging activity and roost sites. We measured body composition on arrival using quantitative magnetic resonance and simulated long-distance migration using observed body composition to predict migration range and rate. 3. Migration occurred in two waves (late August and mid-September). Most bats stayed 1-2 days, although two remained >2 weeks. One third of the bats foraged while at the site, many foraging opportunistically on nights when rain precluded continued migration. Bats roosted in a variety of tree species and manmade structures in natural and developed areas. Half of the bats departed across Lake Erie (minimum crossing distance c. 38 km) while half departed along the shoreline. 4. Simulations predicted a migration rate of c. 250-275 km per day and suggest that all but one of the bats in our study carried sufficient fuel stores to reach the putative wintering area (estimated distance 1500 km) without further refuelling. 5. Our results suggest that migrating bats stopover for sanctuary or short-term rest as opposed to extended rest and refuelling as in many songbirds. Daily torpor could reduce energy costs when not in flight, minimizing the need for extended stopovers and allowing bats to potentially complete their migration at a fraction of the time and energy cost of similar sized birds.

Heterothermy, and the energetic consequences of huddling in small migrating passerine birds

The success of migration of small passerine birds depends largely on effective refueling at stopover sites. In our previous studies, we found that hypothermia facilitates accumulation of fuel at the beginning of a stopover. Later we found that blackcaps, Sylvia atricapilla, might further reduce their energy expenditure by huddling while at rest. Here, we report experimental results supporting our hypothesis that huddling is beneficial to small migrating passerines both from energetic and thermoregulatory points of view. To test this hypothesis we measured metabolic rates and body temperatures of seven blackcaps placed in respirometry chambers overnight, either solitarily or in groups of three or four at ambient temperatures of 5, 10, and 15°C. Concurring with our predictions, huddling blackcaps maintained higher body temperatures than did solitary birds, but had mass-specific metabolic rates lower by ∼30% than those of solitary individuals. Based on our previous studies, we estimated energy savings through huddling to be comparable to energy savings through hypothermia in solitary blackcaps and suggest that huddling may be an important way of saving energy for small passerine birds resting at migratory stopovers. At the same time it might offer the additional benefit of lower risk of predation. In this light, we predict that huddling occurs frequently in nature, leading to significant savings of energy, faster accumulation of fuel, presumably lower risk of becoming a prey, more successful migration, and eventually increased fitness.

Food availability but not melatonin affects nocturnal restlessness in a wild migrating passerine

A large number of passerine species migrate at night, although most of them are diurnal outside the migratory seasons. This diurnal-to-nocturnal transition is a major life-history event, yet little is known about its physiological control. Previous work showed that during the migratory periods captive birds showing nocturnal migratory restlessness (Zugunruhe) have reduced concentrations of circulating melatonin at night compared to non-migratory periods. This suggested that the hormone melatonin, a main component of the avian circadian system, is involved in the expression of Zugunruhe. Other studies demonstrated that the relationship between low melatonin levels and Zugunruhe is not a seasonal correlation. When Zugunruhe was interrupted by exposing birds to a fasting-and-refeeding protocol, melatonin levels increased. Here we studied whether melatonin and food availability influence the intensity of Zugunruhe in wild migrating garden warblers (Sylvia borin) at a stopover site. Birds were held in recording cages overnight, with or without food available, and either bled to determine melatonin concentrations or treated transdermally with melatonin. We found that melatonin levels at night were correlated with the intensity of diurnal locomotor activity and with condition, but were not correlated with Zugunruhe. Similarly, the melatonin treatment did not have effects on Zugunruhe, whereas food availability increased it. Our study shows that the nocturnal melatonin levels in migrating warblers depend on food availability and are correlated with condition. In addition, it suggests that melatonin does not control Zugunruhe and might rather be involved in energy conservation and/or clock synchronization during migration.

Reduced body mass gain in small passerines during migratory stopover under simulated heat wave conditions

For birds that migrate long distances, maximizing the rate of refueling at stopovers is advantageous, but ambient conditions may adversely influence this vital process. We simulated a 3-day migratory stopover for garden warblers (Sylvia borin) and compared body temperatures (T(b)) and rates of refueling under conditions of a heat wave (T(a)=40 °C by day, and 15 °C at night) with those under more moderate conditions (T(a)=27 °C by day, and 15 °C at night). We measured T(b) with implanted thermo-sensitive radio transmitters. Birds had significantly lower rates of body mass gain on the first day of stopover (repeated measures mixed model ANOVA, p=0.002) affecting body mass during the entire stopover (p=0.034) and higher maximum T(b) during the day when exposed to high T(a) than when exposed to moderate T(a) (p=0.002). In addition, the birds exposed to high T(a) by day had significantly lower minimum T(b) at night than those exposed to moderate daytime T(a) (p=0.048), even though T(a) at night was the same for both groups. We interpret this lower nighttime T(b) to be a means of saving energy to compensate for elevated daytime thermoregulatory requirements, while higher T(b) by day may reduce protein turnover. All effects on T(b) were significantly more pronounced during the first day of stopover than on days two and three, which may be linked to the rate of renewal of digestive function during stopover. Our results suggest that environmental factors, such as high T(a), constrain migratory body mass gain. Extreme high T(a) and heat waves are predicted to increase due to global climate change, and thus are likely to pose increasing constraints on regaining body mass during stopover and therefore migratory performance in migratory birds.

Long-term warm or cold acclimation elicits a specific transcriptional response and affects energy metabolism in zebrafish

Organisms are often forced to acclimate to changing environmental temperature. Temperature compensation mechanisms have been reported, which enable organisms to minimize some of the temperature related effects. To investigate this process, zebrafish (Danio rerio) were acclimated to a control (26 degrees C), an increased (34 degrees C) or a decreased (18 degrees C) temperature for 4, 14 and 28 days. In general, warm acclimation depleted energy stores and decreased the condition factor, while cold acclimation increased both. The energy parameters as well as the transcriptional responses (investigated using printed 15k microarrays and real time PCR) indicated that warm acclimation was particularly stressful. However, after 28 days of warm acclimation, energy stores had recovered from the initial depletion. This could have been facilitated by the observed downregulation of transcripts involved in catabolic processes. Transcriptional regulation seemed to be an important means of coordinating the temperature compensation process. We could distinguish an early response which was independent of the direction of the temperature change and a direction specific long-term response. The early response was characterized by the upregulation of defence mechanisms, tissue regeneration and hemopoiesis. In the long-term response there was a strong emphasis on compensating for the altered metabolic rate as well as cell structure and replacement.