The paradox of extreme high-altitude migration in bar-headed geese Anser indicus

Population genetics of Artemia urmiana species complex (Crustacea, Anostraca): A group with asymmetrical dispersal and gene flow mediated by migratory waterfowl

Bird-mediated dispersal of resting eggs is the main mechanism for Artemia dispersal among catchments. The bisexual populations of Artemia urmiana species complex, which is here considered to be a collection of Artemia genetically close to the so-called "Western Asian Lineage", are mostly distributed in central and western Asia (i.e., in regions falling into the Central Asian Flyway of migratory birds) and live in diversified habitats. Little is known about the genetic relationships among these populations. Aiming to understand the population genetic characteristics and the roles of migratory birds on the dispersal and gene flow of this Artemia group, we evaluated the genetic diversity, genetic differentiation, and gene flow among 14 populations, with their altitudes ranging from 540 to 4870 m above sea level, using 13 microsatellite markers. Almost all populations exhibited high genetic diversity and heterozygote excess, which may be a consequence of combined effects of dispersal and hybridization. The global genetic differentiation (FST) value was 0.092, the pairwise FST values were 0.003-0.246. Discriminant analysis of principal components identified three genetic clusters, consisting of Urmia Lake (Iran), Zhundong (Xinjiang, China), and 12 Qinghai-Tibet Plateau populations, respectively. The among-population genetic differentiation seems to be a consequence of isolation by distance and adaptation to diversified habitats induced by altitudinal gradient. Historical gene flows are asymmetrical, and show an evolutionary source-sink dynamics, with Jingyu Lake (Xinjiang, China) population being the major source. These results support our hypothesis that in Qinghai-Tibet Plateau and surrounding areas the bird-mediated dispersal of Artemia may be biased towards from north to south and/or from higher altitude to lower altitude.

Invited review: The speed-duration relationship across the animal kingdom

The parameters of the hyperbolic speed-duration relationship (the asymptote critical speed, CS, and the curvature constant, D') provide estimates of the maximal steady state speed (CS) and the distance an animal can run, swim, or fly at speeds above CS before it is forced to slow down or stop (D'). The speed-duration relationship has been directly studied in humans, horses, mice and rats. The technical difficulties with treadmill running in dogs and the relatively short greyhound race durations means that, perhaps surprisingly, it has not been assessed in dogs. The endurance capabilities of lizards, crabs and salamanders has also been measured, and the speed-duration relationship can be calculated from these data. These analyses show that 1) raising environmental temperature from 25 °C to 40 °C in lizards can double the CS with no change in D'; 2) that lungless salamanders have an extremely low critical speed due, most likely, to O₂ diffusion limitations associated with cutaneous respiration; and 3) the painted ghost crab possesses the highest endurance parameter ratio (D'/CS) yet recorded (470 s), allowing it to maintain high speeds for extended periods. Although the speed-duration relationship has not been measured in fish, the sustainable swimming speed has been quantified in a range of species and is conceptually similar to the maximal steady state in humans. The high aerobic power of birds and low metabolic cost of transport during flight permits the extreme feats of endurance observed in bird migrations. However, the parameters of the avian speed-duration relationship have not been quantified.

Effects of Developmental Hypoxia on the Vertebrate Cardiovascular System

Developmental hypoxia has profound and persistent effects on the vertebrate cardiovascular system, but the nature, magnitude, and long-term outcome of the hypoxic consequences are species specific. Here we aim to identify common and novel cardiovascular responses among vertebrates that encounter developmental hypoxia, and we discuss the possible medical and ecological implications.

Distribution of birds in the high-altitude area of Mount Everest

The species and abundance of birds in the high altitude area of Mount Everest decreased sharply with the increase of altitude. Alpine Choughs forage at altitudes of up to 8000 m and are the highest distribution of birds.

Chromosome-level genome assembly of the bar-headed goose (Anser indicus)

Bar-headed geese (Anser indicus) are adaptable to plateau environments. In this study, we sequenced and assembled a high-quality chromosome-level genome of the bar-headed goose using PacBio long reads and Hi-C technique, and generated 115.73 Gb of Illumina short-reads and 95.89 Gb of PacBio long-reads. The assembled bar-headed goose genome, with a contig N50 of 5.734 Mb and a scaffold N50 of 65.77 Mb, is 1.129 Gb in length and includes 33 chromosomes and 451 fragments. BUSCO assessment yielded a completeness score of 94.4%. In total, 15,376 protein-coding genes were predicted, of which 94.95% had homologs in protein databases. We identified 78 positively selected genes (PSGs) in the bar-headed goose genome, which were mainly enriched in calcium ion and ATP-binding. This bar-headed goose genome will be an important resource for increasing our understanding regarding the genetic basis of adaptation to life at a high altitude.

Black Kites on a flyway between Western Siberia and the Indian Subcontinent

The Black Kite (Milvus migrans) is one of the most widespread raptors in the world. The Palaearctic is populated by two migrating subspecies, Milvus migrans migrans and Milvus migrans lineatus, in the western and eastern part of this realm, respectively. There is a large intergradation zone of M. m. migrans/M. m. lineatus in-between. Although the migration routes of M. m. migrans from Europe to Sub-Saharan Africa and the Middle East are well known, detailed information about migration routes of Black Kites from intergradation zone are missing. Using satellite telemetry we are able to fill this gap in our knowledge of these birds. We tagged with GPS/SMS/GPRS telemetry loggers 13 and 6 Black Kite pulli in lowland around Biysk (Altai Krai, Russia) and in mountains around Kosh-Agach (Altai Republic, Russia), respectively. After fledging, Black Kites from both subpopulations stayed in a small, non-overlapping areas and then migrated to southern Asia through narrow corridors. Black Kites originating from Biysk migrated through the Western Circum-Himalayan Corridor. Black Kites originating from Kosh-Agach used the Trans-Himalayan Corridor crossing the Himalayas in altitudes of up to 6256 m asl. The average total distance travelled of Black Kites from both subpopulations was 9166 km without any significant differences between these subpopulations. Timing of both spring and autumn migration did not vary along different age groups. Black Kites from both subpopulations wintered in low elevations of Pakistan and India. Birds wintered on average for 190 days, and the mean area of individual home ranges in winter was 4704 km². During the breeding period, birds dwelled in south-western Siberia, where they spent on average 125 days with an average home range size 3537 km². We found that ontogenetic shifts in migratory behaviour of Black Kites from Eastern Russia differ from those in population/subspecies in Europe. Black Kites crossing the Himalayas fly and, moreover, stay for hours resting at night in the environment of mountains at altitudes over 5000 m.

A migratory divide spanning two continents is associated with genomic and ecological divergence

Migratory divides are contact zones between breeding populations with divergent migratory strategies during the nonbreeding season. These locations provide an opportunity to evaluate the role of seasonal migration in the maintenance of reproductive isolation, particularly the relationship between population structure and features associated with distinct migratory strategies. We combine light-level geolocators, genomic sequencing, and stable isotopes to investigate the timing of migration and migratory routes of individuals breeding on either side of a migratory divide coinciding with genomic differentiation across a hybrid zone between barn swallow (Hirundo rustica) subspecies in China. Individuals west of the hybrid zone, with H. r. rustica ancestry, had comparatively enriched stable-carbon and hydrogen isotope values and overwintered in eastern Africa, whereas birds east of the hybrid zone, with H. r. gutturalis ancestry, had depleted isotope values and migrated to southern India. The two subspecies took divergent migratory routes around the high-altitude Karakoram Range and arrived on the breeding grounds over 3 weeks apart. These results indicate that assortative mating by timing of arrival and/or selection against hybrids with intermediate migratory traits may maintain reproductive isolation between the subspecies, and that inhospitable geographic features may have contributed to the diversification of Asian avifauna by influencing migratory patterns.

Differences in on-ground and aloft conditions explain seasonally different migration paths in Demoiselle crane

BACKGROUND

Although some migratory birds may take different routes during their outbound and inbound migration, the factors causing these differential migrations to and from the breeding grounds, have rarely been investigated. In Northeast Asia, Demoiselle crane (Anthropoides virgo) performs one of the most extreme "loop" migrations known to date. During outbound migration, they cross the Himalayas to non-breeding sites in northwest India. Contrastingly, during inbound migration to the breeding grounds, they fly around the western end of the Himalayas. We hypothesise that differences in prevailing environmental conditions aloft and/or on-ground during both seasonal migrations are at the core of this phenomenon.

METHODS

Based on the tracking data of 16 individuals of tagged Demoiselle crane, we compared conditions during actual migration with those of simulated "reverse" migration (i.e. by adding 180 degrees to the flight direction and adding and subtracting half a year to the timestamps of outbound and inbound migration, respectively).

RESULTS

The comparison of actual and simulated "reverse" migration indicated that cranes would have encountered poorer aloft (wind support and thermal uplift) and on-ground conditions (temperature) if they had migrated in a reverse outbound migration and poorer on-ground conditions (Normalised Difference Vegetation Indexes [NDVI]) if they had migrated in a reverse inbound direction.

CONCLUSIONS

Our analyses suggest that both on-ground and aloft conditions play a key role in explaining Demoiselle cranes' loop migration, during the periods that they chose to use these alternative routes. Knowledge on the determinants of (differential) migration routes allow predicting migration decisions and may be critical in mitigating global change effects on animal migrations.

Introduction to the theme issue: Measuring physiology in free-living animals

By describing where animals go, biologging technologies (i.e. animal attached logging of biological variables with small electronic devices) have been used to document the remarkable athletic feats of wild animals since the 1940s. The rapid development and miniaturization of physiologging (i.e. logging of physiological variables such as heart rate, blood oxygen content, lactate, breathing frequency and tidal volume on devices attached to animals) technologies in recent times (e.g. devices that weigh less than 2 g mass that can measure electrical biopotentials for days to weeks) has provided astonishing insights into the physiology of free-living animals to document how and why wild animals undertake these extreme feats. Now, physiologging, which was traditionally hindered by technological limitations, device size, ethics and logistics, is poised to benefit enormously from the on-going developments in biomedical and sports wearables technologies. Such technologies are already improving animal welfare and yield in agriculture and aquaculture, but may also reveal future pathways for therapeutic interventions in human health by shedding light on the physiological mechanisms with which free-living animals undertake some of the most extreme and impressive performances on earth. This article is part of the theme issue 'Measuring physiology in free-living animals (Part I)'.

Far eastern curlew and whimbrel prefer flying low - wind support and good visibility appear only secondary factors in determining migratory flight altitude

BACKGROUND

In-flight conditions are hypothesized to influence the timing and success of long-distance migration. Wind assistance and thermal uplift are thought to reduce the energetic costs of flight, humidity, air pressure and temperature may affect the migrants' water balance, and clouds may impede navigation. Recent advances in animal-borne long-distance tracking enable evaluating the importance of these factors in determining animals' flight altitude.

METHODS

Here we determine the effects of wind, humidity, temperature, cloud cover, and altitude (as proxy for climbing costs and air pressure) on flight altitude selection of two long-distance migratory shorebirds, far eastern curlew (Numenius madagascariensis) and whimbrel (Numenius phaeopus). To reveal the predominant drivers of flight altitude selection during migration we compared the atmospheric conditions at the altitude the birds were found flying with conditions elsewhere in the air column using conditional logistic mixed effect models.

RESULTS

Our results demonstrate that despite occasional high-altitude migrations (up to 5550 m above ground level), our study species typically forego flying at high altitudes, limiting climbing costs and potentially alleviating water loss and facilitating navigation. While mainly preferring migrating at low altitude, notably in combination with low air temperature, the birds also preferred flying with wind support to likely reduce flight costs. They avoided clouds, perhaps to help navigation or to reduce the risks from adverse weather.

CONCLUSIONS

We conclude that the primary determinant of avian migrant's flight altitude selection is a preference for low altitude, with wind support as an important secondary factor. Our approach and findings can assist in predicting climate change effects on migration and in mitigating bird strikes with air traffic, wind farms, power lines, and other human-made structures.

Flight muscle and heart phenotypes in the high-flying ruddy shelduck

Ruddy shelduck migrate from wintering grounds in lowland India and Myanmar to breeding grounds in central China and Mongolia, sustaining flight over the Himalayas, where oxygen availability is greatly reduced. We compared phenotypes of the pectoralis muscle and the ventricle of the heart from ruddy shelduck and common shelduck (a closely related low-altitude congener) that were raised in common conditions at sea level, predicting that oxidative capacity would be greater in ruddy shelduck to support high-altitude migration. Fibre-type composition of the pectoralis and the maximal activity of eight enzymes involved in mitochondrial energy metabolism in the pectoralis and heart, were compared between species. Few differences distinguished ruddy shelduck from common shelduck in the flight muscle, with the exception that ruddy shelduck had higher activities of complex II and higher ratios of complex IV (cytochrome c oxidase) and complex II when expressed relative to citrate synthase activity. There were no species differences in fibre-type composition, so these changes in enzyme activity may reflect an evolved modification in the functional properties of muscle mitochondria, potentially influencing mitochondrial respiratory capacity and/or oxygen affinity. Ruddy shelduck also had higher lactate dehydrogenase activity concurrent with lower pyruvate kinase and hexokinase activity in the left ventricle, which likely reflects an increased capacity for lactate oxidation by the heart. We conclude that changes in pathways of mitochondrial energy metabolism in the muscle and heart may contribute to the ability of ruddy shelduck to fly at high altitude.

Physiological Genomics of Adaptation to High-Altitude Hypoxia

Population genomic studies of humans and other animals at high altitude have generated many hypotheses about the genes and pathways that may have contributed to hypoxia adaptation. Future advances require experimental tests of such hypotheses to identify causal mechanisms. Studies to date illustrate the challenge of moving from lists of candidate genes to the identification of phenotypic targets of selection, as it can be difficult to determine whether observed genotype-phenotype associations reflect causal effects or secondary consequences of changes in other traits that are linked via homeostatic regulation. Recent work on high-altitude models such as deer mice has revealed both plastic and evolved changes in respiratory, cardiovascular, and metabolic traits that contribute to aerobic performance capacity in hypoxia, and analyses of tissue-specific transcriptomes have identified changes in regulatory networks that mediate adaptive changes in physiological phenotype. Here we synthesize recent results and discuss lessons learned from studies of high-altitude adaptation that lie at the intersection of genomics and physiology.

Shorebirds wintering in Southeast Asia demonstrate trans-Himalayan flights

Many birds wintering in the Indian subcontinent fly across the Himalayas during migration, including Bar-headed Geese (Anser indicus), Demoiselle Cranes (Anthropoides virgo) and Ruddy Shelducks (Tadorna ferruginea). However, little is known about whether shorebirds migrate across the Himalayas from wintering grounds beyond the Indian subcontinent. Using geolocators and satellite tracking devices, we demonstrate for the first time that Common Redshanks (Tringa totanus) and Whimbrels (Numenius phaeopus) wintering in Singapore can directly fly over the Himalayas to reach breeding grounds in the Qinghai-Tibet Plateau and north-central Russia respectively. The results also show that migratory shorebirds wintering in Southeast Asia can use both the Central Asian Flyway and the East Asian-Australasian Flyway. For Redshanks, westerly-breeding birds crossed the Himalayas while more easterly breeders on the Plateau migrated east of the Himalayas. For Whimbrels, an individual that crossed the Himalayas was probably from a breeding population that was different from the others that migrated along the coast up the East Asian-Australasian Flyway. The minimum required altitude of routes of trans-Himalayan Redshanks were no higher on average than those of eastern migrants, but geolocator temperature data indicate that birds departing Singapore flew at high elevations even when not required to by topography, suggesting that the Himalayan mountain range may be less of a barrier than assumed.

Comparative morphometric analysis of lungs of the semifossorial giant pouched rat (Cricetomys gambianus) and the subterranean Nigerian mole rat (Cryptomys foxi)

Lungs of the rodent species, the African giant pouched rat (Cricetomys gambianus) and the Nigerian mole rat (Cryptomys foxi) were investigated. Significant morphometric differences exist between the two species. The volume of the lung per unit body mass was 2.7 times larger; the respiratory surface area 3.4 times greater; the volume of the pulmonary capillary blood 2 times more; the harmonic mean thickness of the blood-gas (tissue) barrier (τht) ~29% thinner and; the total pulmonary morphometric diffusing capacity (DLo2) for O2 2.3 times more in C. foxi. C. gambianus occupies open burrows that are ventilated with air while C. foxi lives in closed burrows. The less morphometrically specialized lungs of C. gambianus may be attributed to its much larger body mass (~6 times more) and possibly lower metabolic rate and its semifossorial life whereas the 'superior' lungs of C. foxi may largely be ascribed to the subterranean hypoxic and hypercapnic environment it occupies. Compared to other rodents species that have been investigated hitherto, the τht was mostly smaller in the lungs of the subterranean species and C. foxi has the highest mass-specific DLo2. The fossorial- and the subterranean rodents have acquired various pulmonary structural specializations that relate to habitats occupied.

Tackling the Tibetan Plateau in a down suit: insights into thermoregulation by bar-headed geese during migration

Birds migrating through extreme environments can experience a range of challenges while meeting the demands of flight, including highly variable ambient temperatures, humidity and oxygen levels. However, there has been limited research into avian thermoregulation during migration in extreme environments. This study aimed to investigate the effect of flight performance and high altitude on body temperature (T _b) of free-flying bar-headed geese (Anser indicus), a species that completes a high-altitude trans-Himalayan migration through very cold, hypoxic environments. We measured abdominal T _b, along with altitude (via changes in barometric pressure), heart rate and body acceleration of bar-headed geese during their migration across the Tibetan Plateau. Bar-headed geese vary the circadian rhythm of T _b in response to migration, with peak daily T _b during daytime hours outside of migration but early in the morning or overnight during migration, reflecting changes in body acceleration. However, during flight, changes in T _b were not consistent with changes in flight performance (as measured by heart rate or rate of ascent) or altitude. Overall, our results suggest that bar-headed geese are able to thermoregulate during high-altitude migration, maintaining T _b within a relatively narrow range despite appreciable variation in flight intensity and environmental conditions.

Reduced metabolism supports hypoxic flight in the high-flying bar-headed goose (Anser indicus)

The bar-headed goose is famed for migratory flight at extreme altitude. To better understand the physiology underlying this remarkable behavior, we imprinted and trained geese, collecting the first cardiorespiratory measurements of bar-headed geese flying at simulated altitude in a wind tunnel. Metabolic rate during flight increased 16-fold from rest, supported by an increase in the estimated amount of O2 transported per heartbeat and a modest increase in heart rate. The geese appear to have ample cardiac reserves, as heart rate during hypoxic flights was not higher than in normoxic flights. We conclude that flight in hypoxia is largely achieved via the reduction in metabolic rate compared to normoxia. Arterial [Formula: see text] was maintained throughout flights. Mixed venous PO2 decreased during the initial portion of flights in hypoxia, indicative of increased tissue O2 extraction. We also discovered that mixed venous temperature decreased during flight, which may significantly increase oxygen loading to hemoglobin.

The Mitochondrial Basis for Adaptive Variation in Aerobic Performance in High-Altitude Deer Mice

Mitochondria play a central role in aerobic performance. Studies aimed at elucidating how evolved variation in mitochondrial physiology contributes to adaptive variation in aerobic performance can therefore provide a unique and powerful lens to understanding the evolution of complex physiological traits. Here, we review our ongoing work on the importance of changes in mitochondrial quantity and quality to adaptive variation in aerobic performance in high-altitude deer mice. Whole-organism aerobic capacity in hypoxia (VO2max) increases in response to hypoxia acclimation in this species, but high-altitude populations have evolved consistently greater VO2max than populations from low altitude. The evolved increase in VO2max in highlanders is associated with an evolved increase in the respiratory capacity of the gastrocnemius muscle. This appears to result from highlanders having more mitochondria in this tissue, attributed to a higher proportional abundance of oxidative fiber-types and a greater mitochondrial volume density within oxidative fibers. The latter is primarily caused by an over-abundance of subsarcolemmal mitochondria in high-altitude mice, which is likely advantageous for mitochondrial O2 supply because more mitochondria are situated adjacent to the cell membrane and close to capillaries. Evolved changes in gastrocnemius phenotype appear to be underpinned by population differences in the expression of genes involved in energy metabolism, muscle development, and vascular development. Hypoxia acclimation has relatively little effect on respiratory capacity of the gastrocnemius, but it increases respiratory capacity of the diaphragm. However, the mechanisms responsible for this increase differ between populations: lowlanders appear to adjust mitochondrial quantity and quality (i.e., increases in citrate synthase [CS] activity, and mitochondrial respiration relative to CS activity) and they exhibit higher rates of mitochondrial release of reactive oxygen species, whereas highlanders only increase mitochondrial quantity in response to hypoxia acclimation. In contrast to the variation in skeletal muscles, the respiratory capacity of cardiac muscle does not appear to be affected by hypoxia acclimation and varies little between populations. Therefore, evolved changes in mitochondrial quantity and quality make important tissue-specific contributions to adaptive variation in aerobic performance in high-altitude deer mice.

Why implantation of bio-loggers may improve our understanding of how animals cope within their natural environment

Bio-loggers are miniaturized autonomous devices that record quantitative data on the state of free-ranging animals (e.g. behavior, position and physiology) and their natural environment. This is especially relevant for species where direct visual observation is difficult or impossible. Today, ongoing technical development allows the monitoring of numerous parameters in an increasing range of species over extended periods. However, the external attachment of devices might affect various aspects of animal performance (energetics, thermoregulation, foraging as well as social and reproductive behavior), which ultimately affect fitness. External attachment might also increase entanglement risk and the conspicuousness of animals, leaving them more vulnerable to predation. By contrast, implantation of devices can mitigate many of these undesirable effects and might be preferable, especially for long-term studies, provided that the many challenges associated with surgical procedures can be mastered. Implantation may then allow us to gather data that would be impossible to obtain otherwise and thereby may provide new and ecologically relevant insights into the life of wild animals. Here, we: (i) discuss the pros and cons of attachment methods; (ii) highlight recent field studies that used implanted bio-loggers to address eco-physiological questions in a wide range of species; and (iii) discuss logger implantation in light of ethical considerations.

Investigating Home Range, Movement Pattern, and Habitat Selection of Bar-headed Geese during Breeding Season at Qinghai Lake, China

The Bar-headed Goose is the only true goose species or Anserinae to migrate solely within the Central Asian Flyway, and thus, it is an ideal species for observing the effects of both land use and climate change throughout the flyway. In this paper, we investigate the home range, movement pattern, and habitat selection of Bar-headed Geese (Anser indicus) during the breeding season at Qinghai Lake, which is one of their largest breeding areas and a major migration staging area in the flyway. We identified several areas used by the geese during the breeding season along the shoreline of Qinghai Lake and found that most geese had more than one core use area and daily movements that provided insight into their breeding activity. We also observed the intensive use of specific wetlands and habitats near Qinghai Lake. These data provide interesting insights into the movement ecology of this important species and also provide critical information for managers seeking to understand and respond to conservation concerns threatening Bar-headed Geese, such as landscape and habitat changes.

Allosteric mechanisms underlying the adaptive increase in hemoglobin-oxygen affinity of the bar-headed goose

The high blood-O2 affinity of the bar-headed goose (Anser indicus) is an integral component of the biochemical and physiological adaptations that allow this hypoxia-tolerant species to undertake migratory flights over the Himalayas. The high blood-O2 affinity of this species was originally attributed to a single amino acid substitution of the major hemoglobin (Hb) isoform, HbA, which was thought to destabilize the low-affinity T state, thereby shifting the T-R allosteric equilibrium towards the high-affinity R state. Surprisingly, this mechanistic hypothesis has never been addressed using native proteins purified from blood. Here, we report a detailed analysis of O2 equilibria and kinetics of native major HbA and minor HbD isoforms from bar-headed goose and greylag goose (Anser anser), a strictly lowland species, to identify and characterize the mechanistic basis for the adaptive change in Hb function. We find that HbA and HbD of bar-headed goose have consistently higher O2 affinities than those of the greylag goose. The corresponding Hb isoforms of the two species are equally responsive to physiological allosteric cofactors and have similar Bohr effects. Thermodynamic analyses of O2 equilibrium curves according to the two-state Monod-Wyman-Changeaux model revealed higher R-state O2 affinities in the bar-headed goose Hbs, associated with lower O2 dissociation rates, compared with the greylag goose. Conversely, the T state was not destabilized and the T-R allosteric equilibrium was unaltered in bar-headed goose Hbs. The physiological implication of these results is that increased R-state affinity allows for enhanced O2 saturation in the lungs during hypoxia, but without impairing O2 delivery to tissues.

How much can we trust GPS wildlife tracking? An assessment in semi-free-ranging Crested Ibis Nipponia nippon

GPS tracking has been increasingly used for wildlife studies in recent decades, but its performance has not been fully assessed, especially for newly developed lightweight transmitters. We assessed the performance of eight GPS transmitters developed in China by attaching them to Crested Ibises Nipponia nippon confined to two acclimation cages mimicking real habitats. We calculated the distance between GPS locations and the centroid of the cages as the positioning error, and used the 95% (95th percentile) positioning errors to define the accuracy. The positioning success averaged 92.0%, which is much higher than that of previous studies. Locations were not evenly distributed by Location Class (LC), with the LC A and B locations accounting for 88.7%. The observed 95% positioning error in the locations of LC A (9-39 m) and B (11-41 m) was quite accurate, while up to 6.9-8.8% of poor-quality locations were detected in LC C and D with >100 m or even >1, 000 m positioning error. Positioning success and accuracy were different between the test sites, probably due to the difference in vegetation structure. Thus, we argue that the tested transmitters could provide a large proportion of high-quality data for fine-scale studies, and a number of poor-quality locations that need attention. We suggest that the HPOD (horizontal dilution of precision) or PDOP (positional dilution of precision) be reported instead of the LC as a measurement of location accuracy for each location to ensure identification and filtering of implausible locations.

High-altitude shorebird migration in the absence of topographical barriers: avoiding high air temperatures and searching for profitable winds

Nearly 20% of all bird species migrate between breeding and nonbreeding sites annually. Their migrations include storied feats of endurance and physiology, from non-stop trans-Pacific crossings to flights at the cruising altitudes of jetliners. Despite intense interest in these performances, there remains great uncertainty about which factors most directly influence bird behaviour during migratory flights. We used GPS trackers that measure an individual's altitude and wingbeat frequency to track the migration of black-tailed godwits (Limosa limosa) and identify the abiotic factors influencing their in-flight migratory behaviour. We found that godwits flew at altitudes above 5000 m during 21% of all migratory flights, and reached maximum flight altitudes of nearly 6000 m. The partial pressure of oxygen at these altitudes is less than 50% of that at sea level, yet these extremely high flights occurred in the absence of topographical barriers. Instead, they were associated with high air temperatures at lower altitudes and increasing wind support at higher altitudes. Our results therefore suggest that wind, temperature and topography all play a role in determining migratory behaviour, but that their relative importance is context dependent. Extremely high-altitude flights may thus not be especially rare, but they may only occur in very specific environmental contexts.

Molecular basis of hemoglobin adaptation in the high-flying bar-headed goose

During the adaptive evolution of a particular trait, some selectively fixed mutations may be directly causative and others may be purely compensatory. The relative contribution of these two classes of mutation to adaptive phenotypic evolution depends on the form and prevalence of mutational pleiotropy. To investigate the nature of adaptive substitutions and their pleiotropic effects, we used a protein engineering approach to characterize the molecular basis of hemoglobin (Hb) adaptation in the high-flying bar-headed goose (Anser indicus), a hypoxia-tolerant species renowned for its trans-Himalayan migratory flights. To test the effects of observed substitutions on evolutionarily relevant genetic backgrounds, we synthesized all possible genotypic intermediates in the line of descent connecting the wildtype bar-headed goose genotype with the most recent common ancestor of bar-headed goose and its lowland relatives. Site-directed mutagenesis experiments revealed one major-effect mutation that significantly increased Hb-O2 affinity on all possible genetic backgrounds. Two other mutations exhibited smaller average effect sizes and less additivity across backgrounds. One of the latter mutations produced a concomitant increase in the autoxidation rate, a deleterious side-effect that was fully compensated by a second-site mutation at a spatially proximal residue. The experiments revealed three key insights: (i) subtle, localized structural changes can produce large functional effects; (ii) relative effect sizes of function-altering mutations may depend on the sequential order in which they occur; and (iii) compensation of deleterious pleiotropic effects may play an important role in the adaptive evolution of protein function.

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.

What makes Alpine swift ascend at twilight? Novel geolocators reveal year-round flight behaviour

Abstract

Studying individual flight behaviour throughout the year is indispensable to understand the ecology of a bird species. Recent development in technology allows now to track flight behaviour of small long-distance bird migrants throughout its annual cycle. The specific flight behaviour of twilight ascents in birds has been documented in a few studies, but only during a short period of the year, and never quantified on the individual level. It has been suggested that twilight ascents might be a role in orientation and navigation. Previous studies had reported the behaviour only near the breeding site and during migration. We investigated year-round flight behaviour of 34 individual Alpine swifts (Apus melba) of four different populations in relation to twilight ascents. We recorded twilight ascents all around the year and found a twofold higher frequency in ascents during the non-breeding residence phase in Africa compared to all other phases of the year. Dawn ascents were twice as common as dusk ascents and occurred mainly when atmospheric conditions remained stable over a 24-h period. We found no conclusive support that twilight ascents are essential for recalibration of compass cues and landmarks. Data on the wing flapping intensity revealed that high activity at twilight occurred more regularly than the ascents. We therefore conclude that alpine swift generally increase flight activity—also horizontal flight—during the twilight period and we suppose that this increased flight activity, including ascents, might be part of social interactions between individuals.

Significance statement

Year-round flight altitude tracking with a light-weight multi-sensor tag reveals that Alpine swifts ascend several hundred meters high at twilight regularly. The reason for this behaviour remains unclear and the low-light conditions at this time of the day preclude foraging as a possibility. The frequency and altitude of twilight ascents were highest during the non-breeding period, intermediate during migration and low for active breeders during the breeding phase. We discuss our findings in the context of existing hypotheses on twilight ascent and we propose an additional hypothesis which links twilight ascent with social interaction between flock members. Our study highlights how flight behaviour of individuals of a migratory bird species can be studied even during the sparsely documented non-breeding period.

Electronic supplementary material

The online version of this article (10.1007/s00265-017-2438-6) contains supplementary material, which is available to authorized users.

Detours in long-distance migration across the Qinghai-Tibetan Plateau: individual consistency and habitat associations

Migratory birds often follow detours when confronted with ecological barriers, and understanding the extent and the underlying drivers of such detours can provide important insights into the associated cost to the annual energy budget and the migration strategies. The Qinghai-Tibetan Plateau is the most daunting geographical barrier for migratory birds because the partial pressure of oxygen is dramatically reduced and flight costs greatly increase. We analyzed the repeated migration detours and habitat associations of four Pallas’s Gulls Larus ichthyaetus across the Qinghai-Tibetan Plateau over 22 migration seasons. Gulls exhibited notable detours, with the maximum distance being more than double that of the expected shortest route, that extended rather than reduced the passage across the plateau. The extent of longitudinal detours significantly increased with latitude, and detours were longer in autumn than in spring. Compared with the expected shortest routes, proximity to water bodies increased along autumn migration routes, but detour-habitat associations were weak along spring migration routes. Thus, habitat availability was likely one, but not the only, factor shaping the extent of detours, and migration routes were determined by different mechanisms between seasons. Significant between-individual variation but high individual consistency in migration timing and routes were revealed in both seasons, indicating a stronger influence of endogenous schedules than local environmental conditions. Gulls may benefit from repeated use of familiar routes and stopover sites, which may be particularly significant in the challenging environment of the Qinghai-Tibetan Plateau.

Respiratory mechanics and morphology of Tibetan and Andean high-altitude geese with divergent life histories

High-altitude bar-headed geese (Anser indicus) and Andean geese (Chloephaga melanoptera) have been shown to preferentially increase tidal volume over breathing frequency when increasing ventilation during exposure to hypoxia. Increasing tidal volume is a more effective breathing strategy but is also thought to be more mechanically and metabolically expensive. We asked whether there might be differences in the mechanics or morphology of the respiratory systems of high-altitude transient bar-headed geese and high-altitude resident Andean geese that could minimize the cost of breathing more deeply. We compared these two species with a low-altitude migratory species, the barnacle goose (Branta leucopsis). We ventilated anesthetized birds to measure mechanical properties of the respiratory system and used CT scans to quantify respiratory morphology. We found that the respiratory system of Andean geese was disproportionately larger than that of the other two species, allowing use of a deeper breathing strategy for the same energetic cost. The relative size of the respiratory system, especially the caudal air sacs, of bar-headed geese was also larger than that of barnacle geese. However, when normalized to respiratory system size, the mechanical cost of breathing did not differ significantly among these three species, indicating that deeper breathing is enabled by morphological but not mechanical differences between species. The metabolic cost of breathing was estimated to be <1% of basal metabolic rate at rest in normoxia. Because of differences in the magnitude of the ventilatory response, the cost of breathing was estimated to increase 7- to 10-fold in bar-headed and barnacle geese in severe hypoxia, but less than 1-fold in Andean geese exposed to the same low atmospheric P_O2.

The physiological basis of bird flight

Flapping flight is energetically more costly than running, although it is less costly to fly a given body mass a given distance per unit time than it is for a similar mass to run the same distance per unit time. This is mainly because birds can fly faster than they can run. Oxygen transfer and transport are enhanced in migrating birds compared with those in non-migrators: at the gas-exchange regions of the lungs the effective area is greater and the diffusion distance smaller. Also, migrating birds have larger hearts and haemoglobin concentrations in the blood, and capillary density in the flight muscles tends to be higher. Species like bar-headed geese migrate at high altitudes, where the availability of oxygen is reduced and the energy cost of flapping flight increased compared with those at sea level. Physiological adaptations to these conditions include haemoglobin with a higher affinity for oxygen than that in lowland birds, a greater effective ventilation of the gas-exchange surface of the lungs and a greater capillary-to-muscle fibre ratio. Migrating birds use fatty acids as their source of energy, so they have to be transported at a sufficient rate to meet the high demand. Since fatty acids are insoluble in water, birds maintain high concentrations of fatty acid-binding proteins to transport fatty acids across the cell membrane and within the cytoplasm. The concentrations of these proteins, together with that of a key enzyme in the β-oxidation of fatty acids, increase before migration.This article is part of the themed issue 'Moving in a moving medium: new perspectives on flight'.

Effects of hypoxia at different life stages on locomotory muscle phenotype in deer mice native to high altitudes

Animals native to high altitude must overcome the constraining effects of hypoxia on tissue O₂ supply to support routine metabolism, thermoregulation in the cold, and exercise. Deer mice (Peromyscus maniculatus) native to high altitude have evolved an enhanced aerobic capacity in hypoxia, along with increased capillarity and oxidative capacity of locomotory muscle. Here, we examined whether exposure to chronic hypoxia during development or adulthood affects muscle phenotype. Deer mice from a highland population were bred in captivity at sea level, and exposed to normoxia or one of four treatments of hypobaric hypoxia (12kPa O₂, simulating hypoxia at ~4300m): adult hypoxia (6-8weeks), post-natal hypoxia (birth to adulthood), pre-natal hypoxia (before conception to adulthood), and parental hypoxia (in which mice were conceived and raised in normoxia, but their parents were previously exposed to hypoxia). Litter size was similar across treatments, and pups survived the hypoxia exposures and grew to similar body masses at ~6-8months of age. Hypoxia had no effect on the masses of gastrocnemius and soleus muscles. There was a strong concordance between two distinct histological methods for staining capillaries in the gastrocnemius - alkaline phosphatase activity and binding of Griffonia simplicifolia lectin I - each of which showed that capillarity and muscle fibre size were largely unaffected by hypoxia. Maximal activities of several metabolic enzymes (cytochrome c oxidase, citrate synthase, isocitrate dehydrogenase, and lactate dehydrogenase) in the gastrocnemius were also largely unaffected by hypoxia. Therefore, the evolved muscle phenotype of high-altitude deer mice is relatively insensitive to hypoxia across life stages.

Atmospheric conditions create freeways, detours and tailbacks for migrating birds

The extraordinary adaptations of birds to contend with atmospheric conditions during their migratory flights have captivated ecologists for decades. During the 21st century technological advances have sparked a revival of research into the influence of weather on migrating birds. Using biologging technology, flight behaviour is measured across entire flyways, weather radar networks quantify large-scale migratory fluxes, citizen scientists gather observations of migrant birds and mechanistic models are used to simulate migration in dynamic aerial environments. In this review, we first introduce the most relevant microscale, mesoscale and synoptic scale atmospheric phenomena from the point of view of a migrating bird. We then provide an overview of the individual responses of migrant birds (when, where and how to fly) in relation to these phenomena. We explore the cumulative impact of individual responses to weather during migration, and the consequences thereof for populations and migratory systems. In general, individual birds seem to have a much more flexible response to weather than previously thought, but we also note similarities in migratory behaviour across taxa. We propose various avenues for future research through which we expect to derive more fundamental insights into the influence of weather on the evolution of migratory behaviour and the life-history, population dynamics and species distributions of migrant birds.

Bio-logging, new technologies to study conservation physiology on the move: a case study on annual survival of Himalayan vultures

Bio-logging, the on-animal deployment of miniaturised electronic data recorders, allows for the study of location, body position, and physiology of individuals throughout their ontogeny. For terrestrial animals, 1 Hz GPS-position, 3D-body acceleration, and ambient temperature provide standard data to link to the physiology of life histories. Environmental context is added at ever finer scales using remote sensing earth observation data. Here we showcase the use of such bio-logging approaches in a conservation physiology study on endangered Himalayan vultures (Gyps himalayensis). We determine environmental, behavioural, and physiological causes of survival in immature birds that roam from wintering sites in India, Bhutan, and Nepal towards summer areas in Tibet and Mongolia. Five of 18 immature griffons died during one year. Individuals that died had failed to migrate sufficiently far northward (>1500 km) in spring. Individuals likely died if they flew against headwinds from the north or were less able to find thermal updrafts. Surviving individuals migrated to cold and dry areas with low population density. We highlight flight experience, long distance movements, and remote places with low human population as factors critical for the survival of Himalayan vultures. High-resolution bio-logging studies can advance conservation management by pinpointing where and why migratory animals have problems and die.

Actogram analysis of free-flying migratory birds: new perspectives based on acceleration logging

The use of accelerometers has become an important part of biologging techniques for large-sized birds with accelerometer data providing information about flight mode, wing-beat pattern, behaviour and energy expenditure. Such data show that birds using much energy-saving soaring/gliding flight like frigatebirds and swifts can stay airborne without landing for several months. Successful accelerometer studies have recently been conducted also for free-flying small songbirds during their entire annual cycle. Here we review the principles and possibilities for accelerometer studies in bird migration. We use the first annual actograms (for red-backed shrike Lanius collurio) to explore new analyses and insights that become possible with accelerometer data. Actogram data allow precise estimates of numbers of flights, flight durations as well as departure/landing times during the annual cycle. Annual and diurnal rhythms of migratory flights, as well as prolonged nocturnal flights across desert barriers are illustrated. The shifting balance between flight, rest and different intensities of activity throughout the year as revealed by actogram data can be used to analyse exertion levels during different phases of the life cycle. Accelerometer recording of the annual activity patterns of individual birds will open up a new dimension in bird migration research.

Mass Transport: Circulatory System with Emphasis on Nonendothermic Species

Mass transport can be generally defined as movement of material matter. The circulatory system then is a biological example given its role in the movement in transporting gases, nutrients, wastes, and chemical signals. Comparative physiology has a long history of providing new insights and advancing our understanding of circulatory mass transport across a wide array of circulatory systems. Here we focus on circulatory function of nonmodel species. Invertebrates possess diverse convection systems; that at the most complex generate pressures and perform at a level comparable to vertebrates. Many invertebrates actively modulate cardiovascular function using neuronal, neurohormonal, and skeletal muscle activity. In vertebrates, our understanding of cardiac morphology, cardiomyocyte function, and contractile protein regulation by Ca2+ highlights a high degree of conservation, but differences between species exist and are coupled to variable environments and body temperatures. Key regulators of vertebrate cardiac function and systemic blood pressure include the autonomic nervous system, hormones, and ventricular filling. Further chemical factors regulating cardiovascular function include adenosine, natriuretic peptides, arginine vasotocin, endothelin 1, bradykinin, histamine, nitric oxide, and hydrogen sulfide, to name but a few. Diverse vascular morphologies and the regulation of blood flow in the coronary and cerebral circulations are also apparent in nonmammalian species. Dynamic adjustments of cardiovascular function are associated with exercise on land, flying at high altitude, prolonged dives by marine mammals, and unique morphology, such as the giraffe. Future studies should address limits of gas exchange and convective transport, the evolution of high arterial pressure across diverse taxa, and the importance of the cardiovascular system adaptations to extreme environments. © 2017 American Physiological Society. Compr Physiol 7:17-66, 2017.

Mitochondrial physiology in the skeletal and cardiac muscles is altered in torrent ducks, Merganetta armata, from high altitudes in the Andes

Torrent ducks inhabit fast-flowing rivers in the Andes from sea level to altitudes up to 4500 m. We examined the mitochondrial physiology that facilitates performance over this altitudinal cline by comparing the respiratory capacities of permeabilized fibers, the activities of 16 key metabolic enzymes and the myoglobin content in muscles between high- and low-altitude populations of this species. Mitochondrial respiratory capacities (assessed using substrates of mitochondrial complexes I, II and/or IV) were higher in highland ducks in the gastrocnemius muscle - the primary muscle used to support swimming and diving - but were similar between populations in the pectoralis muscle and the left ventricle. The heightened respiratory capacity in the gastrocnemius of highland ducks was associated with elevated activities of cytochrome oxidase, phosphofructokinase, pyruvate kinase and malate dehydrogenase (MDH). Although respiratory capacities were similar between populations in the other muscles, highland ducks had elevated activities of ATP synthase, lactate dehydrogenase, MDH, hydroxyacyl CoA dehydrogenase and creatine kinase in the left ventricle, and elevated MDH activity and myoglobin content in the pectoralis. Thus, although there was a significant increase in the oxidative capacity of the gastrocnemius in highland ducks, which correlates with improved performance at high altitudes, the variation in metabolic enzyme activities in other muscles not correlated to respiratory capacity, such as the consistent upregulation of MDH activity, may serve other functions that contribute to success at high altitudes.

How bar-headed geese fly over the Himalayas

Bar-headed geese cross the Himalayas on one of the most iconic high-altitude migrations in the world. Heart rates and metabolic costs of flight increase with elevation and can be near maximal during steep climbs. Their ability to sustain the high oxygen demands of flight in air that is exceedingly oxygen-thin depends on the unique cardiorespiratory physiology of birds in general along with several evolved specializations across the O2 transport cascade.

Stable Isotopes Suggest Low Site Fidelity in Bar-headed Geese (Anser indicus) in Mongolia: Implications for Disease Transmission

Population connectivity is an important consideration in studies of disease transmission and biological conservation, especially with regard to migratory species. Determining how and when different subpopulations intermingle during different phases of the annual cycle can help identify important geographical regions or features as targets for conservation efforts and can help inform our understanding of continental-scale disease transmission. In this study, stable isotopes of hydrogen and carbon in contour feathers were used to assess the degree of molt-site fidelity among Bar-headed Geese (Anser indicus) captured in north-central Mongolia. Samples were collected from actively molting Bar-headed Geese (n = 61), and some individual samples included both a newly grown feather (still in sheath) and an old, worn feather from the bird's previous molt (n = 21). Although there was no difference in mean hydrogen isotope ratios for the old and new feathers, the isotopic variance in old feathers was approximately three times higher than that of the new feathers, which suggests that these birds use different and geographically distant molting locations from year to year. To further test this conclusion, online data and modeling tools from the isoMAP website were used to generate probability landscapes for the origin of each feather. Likely molting locations were much more widespread for old feathers than for new feathers, which supports the prospect of low molt-site fidelity. This finding indicates that population connectivity would be greater than expected based on data from a single annual cycle, and that disease spread can be rapid even in areas like Mongolia where Bar-headed Geese generally breed in small isolated groups.

Oxygen in demand: How oxygen has shaped vertebrate physiology

In response to varying environmental and physiological challenges, vertebrates have evolved complex and often overlapping systems. These systems detect changes in environmental oxygen availability and respond by increasing oxygen supply to the tissues and/or by decreasing oxygen demand at the cellular level. This suite of responses is termed the oxygen transport cascade and is comprised of several components. These components include 1) chemosensory detectors that sense changes in oxygen, carbon dioxide, and pH in the blood, and initiate changes in 2) ventilation and 3) cardiac work, thereby altering the rate of oxygen delivery to, and carbon dioxide clearance from, the tissues. In addition, changes in 4) cellular and systemic metabolism alters tissue-level metabolic demand. Thus the need for oxygen can be managed locally when increasing oxygen supply is not sufficient or possible. Together, these mechanisms provide a spectrum of responses that facilitate the maintenance of systemic oxygen homeostasis in the face of environmental hypoxia or physiological oxygen depletion (i.e. due to exercise or disease). Bill Milsom has dedicated his career to the study of these responses across phylogenies, repeatedly demonstrating the power of applying the comparative approach to physiological questions. The focus of this review is to discuss the anatomy, signalling pathways, and mechanics of each step of the oxygen transport cascade from the perspective of a Milsomite. That is, by taking into account the developmental, physiological, and evolutionary components of questions related to oxygen transport. We also highlight examples of some of the remarkable species that have captured Bill's attention through their unique adaptations in multiple components of the oxygen transport cascade, which allow them to achieve astounding physiological feats. Bill's research examining the oxygen transport cascade has provided important insight and leadership to the study of the diverse suite of adaptations that maintain cellular oxygen content across vertebrate taxa, which underscores the value of the comparative approach to the study of physiological systems.

The roller coaster flight strategy of bar-headed geese conserves energy during Himalayan migrations

The physiological and biomechanical requirements of flight at high altitude have been the subject of much interest. Here, we uncover a steep relation between heart rate and wingbeat frequency (raised to the exponent 3.5) and estimated metabolic power and wingbeat frequency (exponent 7) of migratory bar-headed geese. Flight costs increase more rapidly than anticipated as air density declines, which overturns prevailing expectations that this species should maintain high-altitude flight when traversing the Himalayas. Instead, a "roller coaster" strategy, of tracking the underlying terrain and discarding large altitude gains only to recoup them later in the flight with occasional benefits from orographic lift, is shown to be energetically advantageous for flights over the Himalayas.

Control of breathing and the circulation in high-altitude mammals and birds

Hypoxia is an unremitting stressor at high altitudes that places a premium on oxygen transport by the respiratory and cardiovascular systems. Phenotypic plasticity and genotypic adaptation at various steps in the O2 cascade could help offset the effects of hypoxia on cellular O2 supply in high-altitude natives. In this review, we will discuss the unique mechanisms by which ventilation, cardiac output, and blood flow are controlled in high-altitude mammals and birds. Acclimatization to high altitudes leads to some changes in respiratory and cardiovascular control that increase O2 transport in hypoxia (e.g., ventilatory acclimatization to hypoxia). However, acclimatization or development in hypoxia can also modify cardiorespiratory control in ways that are maladaptive for O2 transport. Hypoxia responses that arose as short-term solutions to O2 deprivation (e.g., peripheral vasoconstriction) or regional variation in O2 levels in the lungs (i.e., hypoxic pulmonary vasoconstriction) are detrimental at in chronic high-altitude hypoxia. Evolved changes in cardiorespiratory control have arisen in many high-altitude taxa, including increases in effective ventilation, attenuation of hypoxic pulmonary vasoconstriction, and changes in catecholamine sensitivity of the heart and systemic vasculature. Parallel evolution of some of these changes in independent highland lineages supports their adaptive significance. Much less is known about the genomic bases and potential interactive effects of adaptation, acclimatization, developmental plasticity, and trans-generational epigenetic transfer on cardiorespiratory control. Future work to understand these various influences on breathing and circulation in high-altitude natives will help elucidate how complex physiological systems can be pushed to their limits to maintain cellular function in hypoxia.

Taxonomic diversity in the biology of migration

Migration is widespread across taxa and highly variable both within and among groups. There have been multiple efforts to synthesize migratory theory across groups and these frameworks benefit from the inclusion of diverse taxa. The objective of this partial series of the Canadian Journal of Zoology was to highlight taxonomic diversity in migration by featuring reviews of the current state of migration research in a range of taxa (freshwater fishes, mammals, and amphibians). This series provides an opportunity to compare and contrast the biology of migration among multiple taxa. The reviews include discussions of migration at both the individual and population levels, focusing on a range of topics including the evolution of migration, behavioural and physiological adaptations for migration, consequences of migration, and methods for modelling animal movement.

Maximum running speed of captive bar-headed geese is unaffected by severe hypoxia

While bar-headed geese are renowned for migration at high altitude over the Himalayas, previous work on captive birds suggested that these geese are unable to maintain rates of oxygen consumption while running in severely hypoxic conditions. To investigate this paradox, we re-examined the running performance and heart rates of bar-headed geese and barnacle geese (a low altitude species) during exercise in hypoxia. Bar-headed geese (n = 7) were able to run at maximum speeds (determined in normoxia) for 15 minutes in severe hypoxia (7% O₂; simulating the hypoxia at 8500 m) with mean heart rates of 466±8 beats min⁻¹. Barnacle geese (n = 10), on the other hand, were unable to complete similar trials in severe hypoxia and their mean heart rate (316 beats.min⁻¹) was significantly lower than bar-headed geese. In bar-headed geese, partial pressures of oxygen and carbon dioxide in both arterial and mixed venous blood were significantly lower during hypoxia than normoxia, both at rest and while running. However, measurements of blood lactate in bar-headed geese suggested that anaerobic metabolism was not a major energy source during running in hypoxia. We combined these data with values taken from the literature to estimate (i) oxygen supply, using the Fick equation and (ii) oxygen demand using aerodynamic theory for bar-headed geese flying aerobically, and under their own power, at altitude. This analysis predicts that the maximum altitude at which geese can transport enough oxygen to fly without environmental assistance ranges from 6,800 m to 8,900 m altitude, depending on the parameters used in the model but that such flights should be rare.

An implantable instrument for studying the long-term flight biology of migratory birds

The design of an instrument deployed in a project studying the high altitude Himalayan migrations of bar-headed geese (Anser indicus) is described. The electronics of this archival datalogger measured 22 × 14 × 6.5 mm, weighed 3 g, was powered by a ½AA-sized battery weighing 10 g and housed in a transparent biocompatible tube sealed with titanium electrodes for electrocardiography (ECG). The combined weight of 32 g represented less than 2% of the typical bodyweight of the geese. The primary tasks of the instrument were to continuously record a digitised ECG signal for heart-rate determination and store 12-bit triaxial accelerations sampled at 100 Hz with 15% coverage over each 2 min period. Measurement of atmospheric pressure provided an indication of altitude and rate of ascent or descent during flight. Geomagnetic field readings allowed for latitude estimation. These parameters were logged twice per minute along with body temperature. Data were stored to a memory card of 8 GB capacity. Instruments were implanted in geese captured on Mongolian lakes during the breeding season when the birds are temporarily flightless due to moulting. The goal was to collect data over a ten month period, covering both southward and northward migrations. This imposed extreme constraints on the design's power consumption. Raw ECG can be post-processed to obtain heart-rate, allowing improved rejection of signal interference due to strenuous activity of locomotory muscles during flight. Accelerometry can be used to monitor wing-beat frequency and body kinematics, and since the geese continued to flap their wings continuously even during rather steep descents, act as a proxy for biomechanical power. The instrument enables detailed investigation of the challenges faced by the geese during these arduous migrations which typically involve flying at extreme altitudes through cold, low density air where oxygen availability is significantly reduced compared to sea level.

The environmental-data automated track annotation (Env-DATA) system: linking animal tracks with environmental data

BACKGROUND

The movement of animals is strongly influenced by external factors in their surrounding environment such as weather, habitat types, and human land use. With advances in positioning and sensor technologies, it is now possible to capture animal locations at high spatial and temporal granularities. Likewise, scientists have an increasing access to large volumes of environmental data. Environmental data are heterogeneous in source and format, and are usually obtained at different spatiotemporal scales than movement data. Indeed, there remain scientific and technical challenges in developing linkages between the growing collections of animal movement data and the large repositories of heterogeneous remote sensing observations, as well as in the developments of new statistical and computational methods for the analysis of movement in its environmental context. These challenges include retrieval, indexing, efficient storage, data integration, and analytical techniques.

RESULTS

This paper contributes to movement ecology research by presenting a new publicly available system, Environmental-Data Automated Track Annotation (Env-DATA), that automates annotation of movement trajectories with ambient atmospheric observations and underlying landscape information. Env-DATA provides a free and easy-to-use platform that eliminates technical difficulties of the annotation processes and relieves end users of a ton of tedious and time-consuming tasks associated with annotation, including data acquisition, data transformation and integration, resampling, and interpolation. The system is illustrated with a case study of Galapagos Albatross (Phoebastria irrorata) tracks and their relationship to wind, ocean productivity and chlorophyll concentration. Our case study illustrates why adult albatrosses make long-range trips to preferred, productive areas and how wind assistance facilitates their return flights while their outbound flights are hampered by head winds.

CONCLUSIONS

The new Env-DATA system enhances Movebank, an open portal of animal tracking data, by automating access to environmental variables from global remote sensing, weather, and ecosystem products from open web resources. The system provides several interpolation methods from the native grid resolution and structure to a global regular grid linked with the movement tracks in space and time. The aim is to facilitate new understanding and predictive capabilities of spatiotemporal patterns of animal movement in response to dynamic and changing environments from local to global scales.

High thermal sensitivity of blood enhances oxygen delivery in the high-flying bar-headed goose

The bar-headed goose (Anser indicus) crosses the Himalaya twice a year at altitudes where oxygen (O2) levels are less than half those at sea level and temperatures are below -20°C. Although it has been known for over three decades that the major hemoglobin (Hb) component of bar-headed geese has an increased affinity for O2, enhancing O2 uptake, the effects of temperature and interactions between temperature and pH on bar-headed goose Hb-O2 affinity have not previously been determined. An increase in breathing of the hypoxic and extremely cold air experienced by a bar-headed goose at altitude (due to the enhanced hypoxic ventilatory response in this species) could result in both reduced temperature and reduced levels of CO2 at the blood-gas interface in the lungs, enhancing O2 loading. In addition, given the strenuous nature of flapping flight, particularly in thin air, blood leaving the exercising muscle should be warm and acidotic, facilitating O2 unloading. To explore the possibility that features of blood biochemistry in this species could further enhance O2 delivery, we determined the P50 (the partial pressure of O2 at which Hb is 50% saturated) of whole blood from bar-headed geese under conditions of varying temperature and [CO2]. We found that blood-O2 affinity was highly temperature sensitive in bar-headed geese compared with other birds and mammals. Based on our analysis, temperature and pH effects acting on blood-O2 affinity (cold alkalotic lungs and warm acidotic muscle) could increase O2 delivery by twofold during sustained flapping flight at high altitudes compared with what would be delivered by blood at constant temperature and pH.