What Can Birds Tell Us about the Migration Physiology of Bats?

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.

Relating wing morphology and immune function to patterns of partial and differential bat migration using stable isotopes

Migration is energetically expensive and is predicted to drive similar morphological adaptations and physiological trade-offs in migratory bats and birds. Previous studies suggest that fixed traits like wing morphology vary among species and individuals according to selective pressures on flight, while immune defenses can vary flexibly within individuals as energy is variably reallocated throughout the year. We assessed intraspecific variation in wing morphology and immune function in silver-haired bats (Lasionycteris noctivagans), a species that follows both partial and differential migration patterns. We hypothesized that if bats experience energy constraints associated with migration, then wing morphology and immune function should vary based on migratory tendency (sedentary or migratory) and migration distance. We predicted that long-distance migrants would have reduced immune function and more migration-adapted wing shapes compared to resident or short-distance migrating bats. We estimated breeding latitude of spring migrants using stable hydrogen isotope techniques. Our sample consisted primarily of male bats, which we categorized as residents, long-distance northern migrants, short-distance northern migrants, and southern migrants (apparent breeding location south of capture site). Controlling for individual condition and capture date, we related wing characteristics and immune indices among groups. Some, but not all, aspects of wing form and immune function varied between migrants and residents. Long-distance northern migrants had larger wings than short-distance northern migrants and lower wing loading than southern migrants. Compared with resident bats, short-distance northern migrants had reduced IgG while southern migrants had heightened neutrophils and neutrophil-to-lymphocyte ratios. Body fat, aspect ratio, wing tip shape, and bacteria killing ability did not vary with migration status or distance. In general, male silver-haired bats do not appear to mediate migration costs by substantially downregulating immune defenses or to be under stronger selection for wing forms adapted for fast, energy-efficient flight. Such phenotypic changes may be more adaptive for female silver-haired bats, which migrate farther and are more constrained by time in spring than males. Adaptations for aerial hawking and the use of heterothermy by migrating bats may also reduce the energetic cost of migration and the need for more substantial morphological and physiological trade-offs.

Offshore Occurrence of a Migratory Bat, Pipistrellus nathusii, Depends on Seasonality and Weather Conditions

Simple Summary

Migratory bats regularly fly over the North Sea, where the number of offshore wind farms will increase rapidly in the next decades. Information is urgently needed on the timing and the conditions bats can be expected offshore, since windfarms can cause fatalities amongst bats. We therefore collected acoustic data on the presence of bats at four nearshore locations at sea between 2012 and 2016. Modelling the occurrence of Nathusius’ pipistrelle for 480 nights in autumn showed that its migration is strongest in early September, with east-northeasterly tailwinds, low wind speeds, and relatively high temperatures. The species’ migration did not show a strong relationship with other factors, i.e., moon phase, cloud cover, atmospheric pressure, rain, and visibility. Our results provide valuable input to policy-makers to prescribe mitigation measures to reduce bat fatalities in offshore wind farms.

Abstract

Bats regularly migrate over the North Sea, but information on the environmental conditions when this occurs is scarce. Detailed information is urgently needed on the conditions under which bats can be expected offshore, as the number of offshore windfarms that can cause fatalities amongst bats in the North Sea is increasing rapidly. We performed ultrasonic acoustic monitoring at multiple nearshore locations at sea between 2012 and 2016 for, in total, 480 monitoring nights. We modelled the offshore occurrence of Nathusius’ pipistrelle in autumn as a function of weather conditions, seasonality, and the lunar cycle using a generalized additive mixed model (GAMM). We investigated which covariates are important using backward selection based on a likelihood ratio test. Our model showed that important explanatory variables for the offshore occurrence of Nathusius’ pipistrelle are seasonality (night in year), wind speed, wind direction, and temperature. The species’ migration is strongest in early September, with east-northeasterly tailwinds, wind speeds < 5 m/s, and temperatures > 15 °C. Lunar cycle, cloud cover, atmospheric pressure, atmospheric pressure change, rain, and visibility were excluded during the model selection. These results provide valuable input to reduce bat fatalities in offshore wind farms by taking mitigation measures.

Body size affects immune cell proportions in birds and non-volant mammals, but not bats

Powered flight has evolved several times in vertebrates and constrains morphology and physiology in ways that likely have shaped how organisms cope with infections. Some of these constraints probably have impacts on aspects of immunology, such that larger fliers might prioritize risk reduction and safety. Addressing how the evolution of flight may have driven relationships between body size and immunity could be particularly informative for understanding the propensity of some taxa to harbor many virulent and sometimes zoonotic pathogens without showing clinical disease. Here, we used a comparative framework to quantify scaling relationships between body mass and the proportions of two types of white blood cells - lymphocytes and granulocytes (neutrophils/heterophils) - across 63 bat species, 400 bird species and 251 non-volant mammal species. By using phylogenetically informed statistical models on field-collected data from wild Neotropical bats and from captive bats, non-volant mammals and birds, we show that lymphocyte and neutrophil proportions do not vary systematically with body mass among bats. In contrast, larger birds and non-volant mammals have disproportionately higher granulocyte proportions than expected for their body size. Our inability to distinguish bat lymphocyte scaling from birds and bat granulocyte scaling from all other taxa suggests there may be other ecological explanations (i.e. not flight related) for the cell proportion scaling patterns. Future comparative studies of wild bats, birds and non-volant mammals of similar body mass should aim to further differentiate evolutionary effects and other aspects of life history on immune defense and its role in the tolerance of (zoonotic) infections.

Bats and birds as viral reservoirs: A physiological and ecological perspective

The birds (class Aves) and bats (order Chiroptera, class Mammalia) are well known natural reservoirs of a diverse range of viruses, including some zoonoses. The only extant volant vertebrates, bats and birds have undergone dramatic adaptive radiations that have allowed them to occupy diverse ecological niches and colonize most of the planet. However, few studies have compared the physiology and ecology of these ecologically, and medically, important taxa. Here, we review convergent traits in the physiology, immunology, flight-related ecology of birds and bats that might enable these taxa to act as viral reservoirs and asymptomatic carriers. Many species of birds and bats are well adapted to urban environments and may host more zoonotic pathogens than species that do not colonize anthropogenic habitats. These convergent traits in birds and bats and their ecological interactions with domestic animals and humans increase the potential risk of viral spillover transmission and facilitate the emergence of novel viruses that most likely sources of zoonoses with the potential to cause global pandemics.

Potential foraging niche release in insectivorous bat species relatively unaffected by white-nose syndrome?

White-nose syndrome (WNS) has rendered four of Ontario’s species endangered, while leaving the other four species relatively unaffected. The causes and extent of the declines have been widely studied. The influence on remaining bat species has not. Comparing acoustic data recorded ∼10 years apart, we evaluated how species in southeastern Ontario, Canada, use different foraging habitats pre- and post-WNS detection. We observed activity declines in now-endangered species over open fields (small-footed myotis, Myotis leibii (Audubon and Bachman, 1842); little brown bat, Myotis lucifugus (Le Conte, 1831); northern myotis, Myotis septentrionalis (Trouessart, 1897); tricolored bat, Perimyotis subflavus (F. Cuvier, 1832)) and speculate that the reduction of the once most common species (M. lucifugus) may have resulted in other species searching for prey in habitat once dominated by M. lucifugus. That is, these changes may have allowed greater presence in open field and clutter or edge environments by the big brown bat (Eptesicus fuscus (Palisot de Beauvois, 1796)) and three migratory species (silver-haired bat, Lasionycteris noctivagans (Le Conte, 1831); red bat, Lasiurus borealis (Müller, 1776); hoary bat, Lasiurus cinereus (Palisot de Beauvois, 1796)). However, our results also suggest that (i) while the decline of most resident bat species due to WNS may have relaxed competition for relatively unaffected species in some, but not all habitats, that (ii) sensory and biomechanical constraints may limit prey exploitation by these less-affected bat species in these habitats.

PRINCIPLES AND PATTERNS OF BAT MOVEMENTS: FROM AERODYNAMICS TO ECOLOGY

Movement ecology as an integrative discipline has advanced associated fields because it presents not only a conceptual framework for understanding movement principles but also helps formulate predictions about the consequences of movements for animals and their environments. Here, we synthesize recent studies on principles and patterns of bat movements in context of the movement ecology paradigm. The motion capacity of bats is defined by their highly articulated, flexible wings. Power production during flight follows a U-shaped curve in relation to speed in bats yet, in contrast to birds, bats use mostly exogenous nutrients for sustained flight. The navigation capacity of most bats is dominated by the echolocation system, yet other sensory modalities, including an iron-based magnetic sense, may contribute to navigation depending on a bat's familiarity with the terrain. Patterns derived from these capacities relate to antagonistic and mutualistic interactions with food items. The navigation capacity of bats may influence their sociality, in particular, the extent of group foraging based on eavesdropping on conspecifics' echolocation calls. We infer that understanding the movement ecology of bats within the framework of the movement ecology paradigm provides new insights into ecological processes mediated by bats, from ecosystem services to diseases.

Energetics of migratory bats during stopover: a test of the torpor-assisted migration hypothesis

The torpor-assisted migration hypothesis posits that migration is facilitated in bats by the use of torpor during stopover roosting periods, and predicts that at stopover bats regulate time in torpor facultatively so that daily energy expenditure is independent of ambient roosting temperature. Energy savings can thus be directed to migratory movement. However, direct measurements of total roosting energy expenditure in relation to ambient and body temperature are lacking. We captured migratory silver-haired bats (Lasionycteris noctivagans; ∼11g) at Long Point, Ontario, Canada in spring and fall 2016. We used quantitative magnetic resonance analysis to measure body composition change and energy expenditure over a twelve-hour roosting period in a ventilated incubator at 10, 17 and 25°C. We assessed the effects of season, body mass, sex and age on energy expenditure. We found that daily energy expenditure was independent of roosting temperature, and that this was achieved by flexible use of torpor. Variation in body mass at capture was driven mainly by differences in fat, and the amount of body fat was negatively related to torpor use, particularly in spring. Season, sex and age also affected torpor use and energy expenditure, notably with pregnant females being generally fatter and using less torpor than males in spring. We estimate that stopover contributes only 15-20% to the total energy costs of migration in bats compared to 70% or more in typical birds. This study provides support for the torpor-assisted migration hypothesis, and furthers our understanding of the energy budgets of migratory bats.

Variations in Copepod Proteome and Respiration Rate in Association with Diel Vertical Migration and Circadian Cycle

The diel vertical migration of zooplankton is a process during which individuals spend the night in surface waters and retreat to depth during the daytime, with substantial implications for carbon transport and the ecology of midwater ecosystems. The physiological consequences of this daily pattern have, however, been poorly studied beyond investigations of speed and the energetic cost of swimming. Many other processes are likely influenced, such as fuel use, energetic trade-offs, underlying diel (circadian) rhythms, and antioxidant responses. Using a new reference transcriptome, proteomic analyses were applied to compare the physiological state of a migratory copepod, Pleuromamma xiphias, immediately after arriving to the surface at night and six hours later. Oxygen consumption was monitored semi-continuously to explore underlying cyclical patterns in metabolic rate under dark-dark conditions. The proteomic analysis suggests a distinct shift in physiology that reflects migratory exertion and changes in metabolism. These proteomic analyses are supported by the respiration experiments, which show an underlying cycle in metabolic rate, with a peak at dawn. This project generates molecular tools (transcriptome and proteome) that will allow for more detailed understanding of the underlying physiological processes that influence and are influenced by diel vertical migration. Further, these studies suggest that P. xiphias is a tractable model for continuing investigations of circadian and diel vertical migration influences on plankton physiology. Previous studies did not account for this cyclic pattern of respiration and may therefore have unrepresented respiratory carbon fluxes from copepods by about 24%.

Migratory flight imposes oxidative stress in bats

Many animal species migrate over long distances, but the physiological challenges of migration are poorly understood. It has recently been suggested that increased molecular oxidative damage might be one important challenge for migratory animals. We tested the hypothesis that autumn migration imposes an oxidative challenge to bats by comparing values of 4 blood-based markers of oxidative status (oxidative damage and both enzymatic and nonenzymatic antioxidants) between Nathusius’ bats Pipistrellus nathusii that were caught during migration flights with those measured in conspecifics after resting for 18 or 24 h. Experiments were carried out at Pape Ornithological Station in Pape (Latvia) in 2016 and 2017. Our results show that flying bats have a blood oxidative status different from that of resting bats due to higher oxidative damage and different expression of both nonenzymatic and enzymatic antioxidants (glutathione peroxidase). The differences in oxidative status markers varied between sampling years and were independent from individual body condition or sex. Our work provides evidence that migratory flight might impose acute oxidative stress to bats and that resting helps animals to recover from oxidative damage accrued en route. Our data suggest that migrating bats and birds might share similar strategies of mitigating and recovering from oxidative stress.

Do Bar-Headed Geese Train for High Altitude Flights?

SYNOPSIS

Exercise at high altitude is extremely challenging, largely due to hypobaric hypoxia (low oxygen levels brought about by low air pressure). In humans, the maximal rate of oxygen consumption decreases with increasing altitude, supporting progressively poorer performance. Bar-headed geese (Anser indicus) are renowned high altitude migrants and, although they appear to minimize altitude during migration where possible, they must fly over the Tibetan Plateau (mean altitude 4800 m) for much of their annual migration. This requires considerable cardiovascular effort, but no study has assessed the extent to which bar-headed geese may train prior to migration for long distances, or for high altitudes. Using implanted loggers that recorded heart rate, acceleration, pressure, and temperature, we found no evidence of training for migration in bar-headed geese. Geese showed no significant change in summed activity per day or maximal activity per day. There was also no significant change in maximum heart rate per day or minimum resting heart rate, which may be evidence of an increase in cardiac stroke volume if all other variables were to remain the same. We discuss the strategies used by bar-headed geese in the context of training undertaken by human mountaineers when preparing for high altitude, noting the differences between their respective cardiovascular physiology.

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.

The Metabolic Flexibility of Hovering Vertebrate Nectarivores

Foraging hummingbirds and nectar bats oxidize both glucose and fructose from nectar at exceptionally high rates. Rapid sugar flux is made possible by adaptations to digestive, cardiovascular, and metabolic physiology affecting shared and distinct pathways for the processing of each sugar. Still, how these animals partition and regulate the metabolism of each sugar and whether this occurs differently between hummingbirds and bats remain unclear.

Wing morphology, flight type and migration distance predict accumulated fuel load in birds

Birds often accumulate large fat and protein reserves to fuel long-distance flights. While it is well known that species that fly the longest accumulate the largest amounts of fuel, considerable cross-species variation in fuel load is seen after controlling for overall migration distance. It remains unclear whether this variation can be explained by aerodynamic attributes of different species, despite obvious ecological and conservation implications. Here we collected data on wing morphology, flight type, migration distance and fuel load from 213 European bird species and explored three questions: (1) Does maximum fuel load relate to migration distance across species?; (2) Does wing morphology, as described by wing aspect ratio and wing loading, influence maximum fuel load, and; (3) Does flight type influence maximum fuel load? Our results indicate that maximum fuel load increases with migration across species, but residual variance is high. Our results indicate that maximum fuel load is also correlated with migration distance, but again residual variance is high. The latter variance is explained by aspect ratio and flight type, while wing loading and body mass explain little variance. Birds with slender wings accumulate less fuel than species with low wing aspect ratio when covering a similar migration distance. Continuously flapping species accumulate the largest amounts of fuel, followed by flapping and soaring, flapping and gliding species, while the smallest fuel loads were observed in birds with passerine-type flight. These results highlight complex eco-evolutionary adaptations to migratory behaviour, pointing toward the importance of energy-minimisation.

Determinants of spring migration departure decision in a bat

Migratory decisions in birds are closely tied to environmental cues and fat stores, but it remains unknown if the same variables trigger bat migration. To learn more about the rare phenomenon of bat migration, we studied departure decisions of female common noctules (Nyctalus noctula) in southern Germany. We did not find the fattening period that modulates departure decisions in birds. Female noctules departed after a regular evening foraging session, uniformly heading northeast. As the day of year increased, migratory decisions were based on the interactions among wind speed, wind direction and air pressure. As the migration season progressed, bats were likely to migrate on nights with higher air pressure and faster tail winds in the direction of travel, and also show high probability of migration on low-pressure nights with slow head winds. Common noctules thus monitor complex environmental conditions to find the optimal migration night.

Tracking post-hibernation behavior and early migration does not reveal the expected sex-differences in a "female-migrating" bat

Long-distance migration is a rare phenomenon in European bats. Genetic analyses and banding studies show that females can cover distances of up to 1,600 km, whereas males are sedentary or migrate only short distances. The onset of this sex-biased migration is supposed to occur shortly after rousing from hibernation and when the females are already pregnant. We therefore predicted that the sexes are exposed to different energetic pressures in early spring, and this should be reflected in their behavior and physiology. We investigated this in one of the three Central European long-distance migrants, the common noctule (Nyctalus noctula) in Southern Germany recording the first individual partial migration tracks of this species. In contrast to our predictions, we found no difference between male and female home range size, activity, habitat use or diet. Males and females emerged from hibernation in similar body condition and mass increase rate was the same in males and females. We followed the first migration steps, up to 475 km, of radio-tagged individuals from an airplane. All females, as well as some of the males, migrated away from the wintering area in the same northeasterly direction. Sex differences in long-distance migratory behavior were confirmed through stable isotope analysis of hair, which showed greater variation in females than in males. We hypothesize that both sexes faced similarly good conditions after hibernation and fattened at maximum rates, thus showing no differences in their local behavior. Interesting results that warrant further investigation are the better initial condition of the females and the highly consistent direction of the first migratory step in this population as summering habitats of the common noctule occur at a broad range in Northern Europe. Only research focused on individual strategies will allow us to fully understand the migratory behavior of European bats.

Flight muscle carnitine palmitoyl transferase activity varies with substrate chain length and unsaturation in the hoary bat (Lasiurus cinereus)

Fat is an important fuel for bats to support high metabolic rates in extended periods of flight. The fatty acid composition of adipose stores could affect whole animal exercise performance, as fatty acids vary in rates of mobilization and oxidation. A key step in the fatty acid oxidation pathway is transporting fatty acids from the cytosol into mitochondria, mediated by the enzyme carnitine palmitoyl transferase (CPT). Therefore, understanding the substrate preference patterns of CPT in bats is important for interpreting the consequences of adipose fatty acid profiles. We measured CPT activity with eight different fatty acyl CoA substrates (16:0, 16:1ω7, 18:0, 18:1ω9, 18:2ω6, 18:3ω3, 20:4ω6, and 22:6ω3) in the pectoralis muscle of migrating and nonmigrating hoary bats (Lasiurus cinereus (Beauvois, 1796)). The pattern of substrate preference was similar to the patterns previously reported for birds and rats and was not affected by migration. Generally, activity increased with the number of double bonds and was higher with 16 carbon fatty acids compared with 18 carbon fatty acids. Given the observed substrate variation in CPT activity, there is no evidence to suggest that recently reported seasonal changes in the adipose fatty acid composition of migrating hoary bats would lead to increased lipid oxidation rate, and may instead be a consequence of seasonal shifts in diet.

Diet of the insectivorous bat Pipistrellus nathusii during autumn migration and summer residence

Migration is widespread among vertebrates, yet bat migration has received little attention and only in the recent decades has a better understanding of it been gained. Migration can cause significant changes in behaviour and physiology, due to increasing energy demands and aerodynamic constraints. Dietary shifts, for example, have been shown to occur in birds before onset of migration. For bats, it is not known if a change in diet occurs during migration, although breeding season-related dietary preference has been documented. It is known that a diet rich in fats and the accumulation of fat deposits do increase the flight range of migratory bats. Some bat species can be regarded as long-distance migrants, covering up to 2000 km between summer and winter roosting areas. Pipistrellus nathusii (Vespertilionidae), a European long-distant migrant, travels each year along the Baltic Sea from north-eastern Europe to hibernate in central and southern Europe. This study presents data on the dietary habits of migrating Pipistrellus nathusii compared with those during the breeding season. We analysed faecal samples from bats on fall migration caught at the Ornithological Field Station in Pape, Latvia and from samples collected in North-Latvian summer roosts. We applied both morphological identification and molecular methods, as morphological methods also recognize life stages of prey and can contribute frequency data. The diets of bats on migration and breeding bats were similar, with Diptera and Lepidoptera comprising the major prey categories. However, certain prey groups could be explained by the different hunting habitats exploited during migration vs. summer residence.

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.

The insectivorous bat Pipistrellus nathusii uses a mixed-fuel strategy to power autumn migration

In contrast to birds, bats are possibly limited in their capacity to use body fat as an energy source for long migrations. Here, we studied the fuel choice of migratory Pipistrellus nathusii (approximate weight: 8 g) by analysing the stable carbon isotope ratio (δ(13)C(V-PDB)) of breath and potential energy sources. Breath δ(13)C(V-PDB) was intermediate between δ(13)C(V-PDB) of insect prey and adipocyte triacylglycerols, suggesting a mixed-fuel use of P. nathusii during autumn migration. To clarify the origin of oxidized fatty acids, we performed feeding experiments with captive P. nathusii. After an insect diet, bat breath was enriched in (13)C relative to the bulk and fat portion of insects, but not deviating from the non-fat portion of insects, suggesting that bats oxidized exogenous proteins and carbohydrates, but not exogenous fatty acids. A feeding experiment with (13)C-labelled substrates confirmed these findings. In conclusion, migratory P. nathusii oxidized dietary proteins directly from insects captured en route in combination with endogenous fatty acids from adipocytes, and replenished their body reserves by routing dietary fatty acids to their body reserves.

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.

Light enough to travel: migratory bats have smaller brains, but not larger hippocampi, than sedentary species

Migratory bird species have smaller brains than non-migratory species. The behavioural flexibility/migratory precursor hypothesis suggests that sedentary birds have larger brains to allow the behavioural flexibility required in a seasonally variable habitat. The energy trade-off hypothesis proposes that brains are heavy, energetically expensive and therefore, incompatible with migration. Here, we compared relative brain, neocortex and hippocampus volume between migratory and sedentary bats at the species-level and using phylogenetically independent contrasts. We found that migratory bats had relatively smaller brains and neocortices than sedentary species. Our results support the energy trade-off hypothesis because bats do not exhibit the same degree of flexibility in diet selection as sedentary birds. Our results also suggest that bat brain size differences are subtler than those found in birds, perhaps owing to bats' shorter migration distances. Conversely, we found no difference in relative hippocampus volume between migratory and sedentary species, underscoring our limited understanding of the role of the hippocampus in bats.