Long-distance migratory songbirds cross ecological barriers such as deserts and seas as part of their journeys. Yet, the behavioural adaptations used to perform these crossings remain poorly understood. Using accelerometer data from multisensor loggers carried by free-flying thrush nightingales (Luscinia luscinia), a nocturnal migratory passerine, we quantified fine-scale activity patterns during the entire migratory journey, including desert crossings during both spring and autumn migration. When migrating over ecological barriers, the birds performed consecutive nocturnal migratory flights, followed by daytime layovers when the birds remained inactive until the next sunset, when resuming migratory flights. The absence of diurnal locomotion while crossing desert environments suggests that thrush nightingales do not refuel while stopping within the barriers but instead rely on energy reserves accumulated prior to barrier entry. This multi-day behavioural inactivity highlights physiological and energetic constraints faced by small migratory birds when crossing ecological barriers and illustrates the careful allocation of energy reserves prior to the barrier crossing to endure several consecutive migratory flights without refuelling. Our results provide new detailed insights into how long-distance migrants cross ecological barriers, improving our understanding of behavioural adaptations in bird migration ecology.

Bats are the only mammals capable of powered flight, allowing them to cover relatively long distances in a short time. However, the general inability of mammals to fuel endurance exercise solely by oxidizing fatty acids may prevent bats from undertaking long-distance intercontinental migrations-like birds do. Here, we conducted untargeted metabolomics to reveal the oxidative fuels used by wild caught Nathusius' pipistrelles. We investigated polar metabolites and lipids in whole blood from bats flying under controlled wind tunnel or field conditions and how metabolites respond to the physiological challenge. Around 70% of detected acyl carnitines were significantly elevated after flight in the wind tunnel compared to resting bats. The phospholipid levels varied; some increased while others decreased significantly after flight, and most did not return to resting levels within 1 h of recovery. During migration season, we observed a significant increase of phosphatidylethanolamines with unsaturated fatty acids and a bulk increase of several phosphatidylcholines and their lyso-derivatives. While migration had a clear effect on phospholipids, recovering after flight in both seasons was less pronounced and only 24% of acyl carnitines were increased after 1 h of rest. We conclude that endurance exercise such as migration has a greater influence on lipid composition and their abundance than short flights, which indicates a relatively high relevance of fatty acid oxidation to fuel migration in bats.

Power required to fly for a bird generally follows a U-shaped function of airspeed, with higher cost at both low and high speeds. Because power required increases with body mass faster than power available from flight muscles, larger birds may experience restricted flight speed ranges and climbing capabilities. Previous studies found limited flight performance in cormorants. Adapted for both flight and sub-surface swimming, they trade off larger flight muscles for powerful leg muscles used for diving. Our study tested whether the flight performance of greater cormorants is constrained by measuring airspeed under various seasonal and wind conditions. If flight muscles severely limit the range of flight speeds, cormorants would not be able to adopt ecologically relevant speeds between seasons and not increase speed in headwinds to minimize cost of transport. Results suggest that cormorants can achieve airspeeds beyond minimum power speed, selecting speeds near maximum range during autumn migration and exceeding this range on spring migration and during foraging flights. However, expected speed adjustments to headwinds were inconsistent, with some situations lacking the anticipated responses. The cormorants demonstrated partial wind drift compensation by adjusting flight headings along coastlines, though airspeed adjustments were not always observed. Although greater cormorants appear capable of reaching ecologically relevant speeds in many contexts, the overall scope of their flight speeds remains relatively narrow compared with smaller bird species. These findings indicate that greater cormorants have muscle power for adaptive behaviour in some cases, despite the influence of physiological constraints on their flight performance.

Many birds make migratory flights lasting several days. Migrants do not only repeatedly make small-scale shifts in altitude; some of them also climb and sink several thousand metres in a diel cycle. This is a seemingly energetically costly behaviour, adding to an already high energetic cost of flight. Here, we describe the climbing and sinking behaviour of migratory great snipes, Gallinago media, and discuss whether their diel altitude cycle could represent an additional energy cost to their migration. Making use of tracking data from multisensor data loggers, we quantified altitude variation and compared climb and sink rates from non-stop migratory flights lasting up to 4 days (84 h). In general, great snipes climb and sink at comparatively slow rates, averaging 0.21 m s-1 and -0.22 m s-1, respectively, almost invariably within ±0.7 m s-1, and hardly ever close to a theoretical maximum climb rate of 1.8 m s-1. Great snipes do not climb faster at night than in the day and climb and sink rates are similarly low for all long non-stop flights. The additional cost of shifting flight altitude for great snipes is apparently negligible compared with the cost of forward flight (<1%). This is largely explained by the birds flapping during a slow descent, thereby turning most of the potential energy built up during previous climbs into aerodynamic work. We conclude that the overall cost of shifting flight is probably low for most migratory birds, given that climb and sink rates are low.

Behavioral, physiological, and life-history adaptations to the lunar cycle are global phenomena across trophic levels and ecosystems yet remain poorly understood because of the challenges of studying free-living organisms at night. We show that the lunar cycle regulates both daily foraging activity and foraging success in the red-necked nightjar-a nocturnal avian insectivore-and that moonless periods lead to energy deficits that trigger synchronized energy-conservation responses. These cyclical imbalances cascade into fluctuations in fuel reserves and influence the timing of key annual life-history events, including migration and reproduction, but not molt. Despite adaptations to offset lunar constraints, nightjars' annual cycle remains governed by the moon's monthly rhythm, underscoring its pervasive influence on nocturnal life.

Most bird species are diurnal but drastically change their diel cycle of activity to migrate at night. Nocturnal migration has been documented using different methods (e.g., experiments, radar and radio tracking, and acoustic monitoring), but accurately quantifying the proportion of nocturnal versus diurnal flight at the individual and species levels, and understanding how this behavior evolved across the avian tree, has remained methodologically challenging.1,2,3 Such uncertainty is not only of theoretical importance but also limits our ability to mitigate conservation threats, particularly from light pollution and building collisions.4,5,6 With multi-sensor geolocators recording light, barometric pressure, and activity,7,8 we reconstructed high-resolution migratory trajectories9,10 for 411 individuals from 56 small- or medium-sized landbird species across four continents and measured the proportion of each flight that occurred during night or day. Our species-level quantification confirmed nocturnal migration as the dominant strategy among small landbirds, while also providing precise flight proportion estimates across a broad taxonomic sample and refining the classification of several species previously described as partial or facultative diurnal migrants. We found that birds initiated and ended migratory flights near civil dusk and dawn, thereby maximizing nocturnal travel. The phylogenetic signal we detected indicates that nocturnal migration is largely conserved within lineages. While nocturnal migration confers multiple advantages, the relative importance of the proposed drivers remains to be determined.11,12 Our study provides species-specific quantification of nocturnal migration, highlights tracking gaps across taxa and regions, and opens new avenues for studying the evolutionary, ecological, and sensory drivers of nocturnal migratory flights.

Dragonflies' exceptional climb capability plays a crucial role in essential survival behaviors such as predation and evasion. However, the climb performance in aerial vehicles remains relatively weak, posing challenges in achieving efficient and stable vertical ascent. To investigate the body motion patterns and wing flapping strategies during climbing flight, this study first conducts biological observations to obtain basic biological characteristics and key climbing parameters. Based on the observational data, numerical simulations are performed to explore aerodynamic performance during ascent. The flow field structures are analyzed through flow visualization. The results show that the dragonfly's climbing process can be divided into four stages, with the average flapping frequency being 28.0 Hz and the total duration being approximately 0.82 s. The ascent is predominantly vertical, with slight backward flight and turning near the end. The angle of attack is essential for regulating flight posture and speed. Meanwhile, by adjusting the flapping plane tilt angle, deviation angle, and amplitude, the dragonfly maintains high lift and stable climbing through the synergistic effects of typical vortex structures such as leading-edge vortices. During the rapid-ascent phase, a significant pressure difference occurs on the wing surfaces along with delayed stall, and rotational circulation is also formed, both of which are key mechanisms for enhanced aerodynamic force generation. Furthermore, the dragonfly dynamically balances lift and flight maneuverability by modulating the phase difference between the forewings and hindwings, with their interaction resulting in a maximum aerodynamic force increase of 25.1%, highlighting the sophisticated aerodynamic strategies during climbing flight.

Accelerated biodiversity loss has destabilized functional links within and between ecosystems. Species that cross different ecosystems during migration between breeding and nonbreeding sites are particularly sensitive to global change because they are exposed to various, often ecosystem-specific, threats. Because these threats have lethal and nonlethal effects on populations, many migratory species are declining, making this group especially vulnerable to global change. To mitigate their decline, research at a continental and flyway scale is required to adequately monitor changes in the migratory and demographic processes of populations during all parts of the annual cycle. The Motus Wildlife Tracking System (Motus) could provide a solution to data gaps that exist for small, migratory species. Motus is an automated telemetry system for animal tracking that uses a single very-high-frequency radio signal to track tagged individuals. Motus can provide information on movements made by individuals of small migrant species, thereby aiding the understanding of aspects of their migration that could affect demographic parameters. Conservation-focused research opportunities related to Motus include identification of critical stopover sites that support and connect multiple species and insight into migratory decisions in small migrant birds related to environmental stressors, such as artificial light at night. Examples of stopover studies from the existing network that demonstrate its utility include identification of a high-conservation-value stopover area for the blackpoll warbler (Setophaga striata) in the eastern United States. Geographical gaps in the network across the Mediterranean region and across eastern Europe need to be filled to track continent-wide movements. Motus can provide individual-level migration information for a variety of small-bodied taxa, and a drive to expand the network will improve its ability to direct conservation plans for such species.

Albeit costly, flight allows birds to travel great distances in a short time, making it a highly effective mode of locomotion, especially during migration.1,2 Understanding how birds use energy during flight is essential for studying their flight ecology.3 To fly, birds flap their wings, accelerating surrounding air and generating flight forces, where the rate of energy added to the wake represents flight mechanical power (Pmech).4 For flapping, birds utilize chemical energy in their flight muscles, which, along with the metabolism of other body functions, constitutes the flight metabolic power (Pmet).1,5 The ratio between Pmech and Pmet is the energy conversion efficiency (ɳ),6,7 which depends on the muscle's ability to convert fuel into work (the rest being dissipated as heat) and on the energy losses during aerodynamic force production. Due to lack of direct measurements, ɳ has been assumed constant across speeds (23%) or relied upon for modeling.4,7 Here, we estimated, in vivo, ɳ from direct measurements of Pmet and Pmech using the 13C-labeled sodium bicarbonate method and particle image velocimetry, respectively, in thrush nightingales flown in a wind tunnel. We found that ɳ varied as a concave function with flight speed, with a maximum ɳ of 15.3% within the range of 7-8 m s-1, occurring at ecologically relevant flight speeds. Our findings suggest tuning of performance to speeds most relevant for efficient transportation, with implications for modeling flight power,4,8 as ɳ, a fundamental attribute in bird flight energetics, varies across flight speeds.

The loss of intertidal foraging habitats at stopover sites is the primary cause of population declines in many migratory shorebirds. However, the absence of high‐tide roosts can exacerbate this impact by increasing the energy consumed for alarm flights and longer commutes between foraging and roosting sites, which have been poorly quantified during time‐constrained spring stopovers. We monitored alarm flights at roosts using camera traps and tracked the commutes between roosting and foraging sites of great knots ( Calidris tenuirostris ) and bar‐tailed godwits ( Limosa lapponica ) at Yalu Jiang Estuary, a critical stopover site for shorebirds in the North Yellow Sea. We analysed the energy costs of alarm flights by human disturbances and commuting using flight energetic models. Camera trap monitoring revealed that 20 of 67 (29.9%) roosting records at aquaculture ponds recorded human disturbance that resulted in alarm flights. Birds travelled 2.8 times farther than the optimal commuting distance when nearby high‐tide roosts were available. Flight energetic models estimated that alarm flights and extended commuting resulted in an additional energy expenditure of 355.9–902.3 kJ, equivalent to 3–4 days of foraging effort based on food intake rates. Synthesis and applications . Foraging sites and nearby high‐tide roosts are integral habitat components for shorebirds to acquire energetic surpluses, especially at migration staging areas. While protecting intertidal flats to provide foraging habitats for shorebirds, human activities should be minimised and disturbance should be avoided at aquaculture sites during the high‐tide periods of spring tides to provide safe roosts for shorebirds. Artificial roost structures (floating roosts, roost islands, etc.,) could also be an effective solution for providing additional roosting habitats for shorebirds.

Animal locomotion is constrained by Newtonian laws of motion and therefore biomechanics is a useful approach for quantitative analysis of force and power requirements. Aerial locomotion in vertebrates is no exception, and arguably the most significant developments are to be found in this journal. Evolutionary birds and bats are very successful groups, doubtless largely because of their ability to shift location in a short time. This has enabled birds and to a lesser extent bats to perform seasonal long-distance migrations between habitats suitable for reproduction and survival. Power required to fly and potential flight range in relation to fuel load are two fundamental relationships derived from flight mechanics, which both serve as a foundation for the development of optimal migration theory. From this framework where biomechanics, energetics and ecology combine, we can analyse which of the alternative strategies migrants adopt. Such adaptive behaviours include the selection of optimal flight speed and the migratory travel itinerary. However, despite decades of research efforts, there are still many unsolved problems concerning flight mechanics and energetics of vertebrate flight. One such is how the power-speed relationship maps onto metabolic rate during flight, the so-called energy conversion efficiency. There is conflicting empirical evidence concerning how energy conversion possibly varies with flight speed, body mass and body size. As ultimately it is the metabolic energy consumption that is under selection pressure, this is an urgent question for the utility of flight mechanical principles in ecology. In this Review, I discuss this and other knowledge gaps in vertebrate flight and migration.

BACKGROUND:The degree to which avian migrants revisit the same sites to replicate routes from previous years has received more and more attention as the possibilities of tracking small to medium-size birds over multiple annual cycles have improved. Repeated measurements of individuals with an appropriate sampling resolution can potentially inform about their navigation and migration strategies and to what extent observed variation within and between individuals may reflect the selective potential in the population. METHODS:We analysed the annual space-use of European nightjars Caprimulgus europaeus tracked with GPS-loggers in multiple years between northern Europe and southern Africa. We quantified spatial consistency of stationary sites and variation, repeatability, and latitudinal correlations in route choice and also investigated barrier-associated changes of within- and between-individual longitudinal variation in flight routes. RESULTS:We found that the nightjars consistently used the same breeding and wintering sites. In contrast, the birds generally varied their migration routes between years, and we could only rarely confirm site fidelity to stopover sites. Nevertheless, route variation within individuals remained low for most of both autumn and spring migration, while the between individual variation generally was larger, resulting in a high repeatability in flight routes. Although we found extensive spatial autocorrelation in both seasons across latitudes, we detected significant changes in longitudinal variation associated with the passage of ecological barriers enroute. Potential intermediate goal areas were visited prior to the crossing of the Mediterranean Sea and the Sahara Desert in both seasons. In spring, within-individual route variability dropped to a few tens of kilometres at the initiation of the Sahara crossing but increased to maximum over the barrier. CONCLUSIONS:The nightjars incorporate individual-specific space use within their annual cycle that allows for a degree of flexibility during migration, possibly driven by the energetic benefits of allowing adaptive wind drift while airborne. Our data demonstrate how topography and spatial autocorrelation of positions influence flight path variability that may diminish or reinforce individuality in route choice. Hence, this study highlights that identifying and quantifying past and present external influences on emergence of realised routes can be critical for distinguishing the genetic basis and environmental variation in migration.

Long-distance migratory songbirds alternate between flights and stopovers to complete their journeys. While migration is typically divided into spring and autumn, detailed insights into their migratory behaviour across and within seasons remain limited. Here we use multisensor data loggers in a long-distance migratory songbird, the thrush nightingale, to explore the structure of flight and stopover periods, the variation in fuelling intensity and the fuel load requirements across and within seasons. We evaluated the migratory behaviour data in light with migration strategies predictions derived from optimal migration theory. All 12 tracked individuals followed similar migratory routes, with both autumn and spring migrations segmented into distinctive flight and stopovers periods. Diurnal activity at stopovers, a proxy for fuelling intensity, varied with latitude, with almost zero activity when stopping during daytime across ecological barriers, reflecting limited fuelling opportunities. While crossing the Sahara Desert in autumn, the birds did not minimise pure time or energy strategies, indicating that survival is prioritised. During the last spring migratory period, from the Horn of Africa until the breeding grounds, the birds performed up to 22 consecutive nocturnal flights. After crossing the Arabian Peninsula, they switched strategy and adopted a sprint migration approach, refuelling intensively during daytime to sustain back-to-back nocturnal flights. The behavioural patterns observed demonstrate that spring and autumn migrations are not governed by a single strategy (time or energy minimiser). Instead, birds flexibly adjust their flight and stopover behaviour depending on the specific demands along the route, balancing energy, timing and survival risk. This study used multisensor data loggers to provide detailed insights into how a migratory songbird species structures its long-distance annual migrations, enhancing our understanding of the complexity of its migratory behaviour.

Flight behaviours of birds have been extensively studied from different angles such as their kinematics, aerodynamics and, more generally, their migration patterns. Nevertheless, much is still unknown about the daily foraging flight activity and behaviour of breeding birds, and potential differences among males and females. The recent development of miniaturized accelerometers allows us a glimpse into the daily life of a songbird. Here, we tagged 13 male and 13 female pied flycatchers (Ficedula hypoleuca) with accelerometers and used machine learning approaches to analyse their flight activity and effort during the chick rearing period. We found that during 2 h of foraging, chick-rearing pied flycatchers were flying on average 13.7% of the time. Almost all flights (>99%) were short flights lasting less than 10 s. Flight activity changed throughout the day and was highest in the morning and lowest in the early afternoon. Male pied flycatchers had lower wing loading than females, and in-flight accelerations were inversely correlated with wing loading. Despite this, we found no significant differences in flight duration and intensity between sexes. This suggests that males possess a higher potential flight performance, which they did not fully utilize during foraging flights.

Accelerated biodiversity loss during the Anthropocene has destabilised functional links within and between ecosystems. Migratory species that cross different ecosystems on their repeated journeys between breeding and non-breeding sites are particularly sensitive to global change because they are exposed to various, often ecosystem-specific threats. As these bring both lethal and non-lethal population impacts, many migratory species are declining, making this group especially vulnerable to global change. To mitigate their decline, research at a continental and flyway scale is required to adequately monitor changes in the demographic processes of populations and understand the needs of migratory species, during all parts of the annual cycle. The Motus Wildlife Tracking System (Motus) could provide a solution to data gaps that exist particularly for small and migratory species. Motus is an automated telemetry system for animal tracking, which originated in North America. It provides a collaborative network by using the same VHF radio frequency for all tracked individuals, in combination with an individual tag identifier. Motus can provide information on movements made by individuals of the smallest bird and bat, and even larger insect species, thus aiding our understanding of aspects of their migration that could impact demographic parameters. Here we emphasise conservation-focused research opportunities, with a particular lense on European migrant taxa. We highlight examples from the existing network, and identify geographical gaps in the network which need to be filled to track continent-wide movements. We conclude that Motus is a useful tool to produce individual-level migration information for a variety of small-bodied taxa, and that a drive to expand the network will improve its ability to conservation plans for such species.

Optimal migration theory prescribes adaptive strategies of energy, time or mortality minimization. To test alternative hypotheses of energy- and time-minimization migration we used multisensory data loggers that record time-resolved flight activity and light for positioning by geolocation in a long-distance migratory shorebird, the little ringed plover, Charadrius dubius . We could reject the hypothesis of energy minimization based on a relationship between stopover duration and subsequent flight time as predicted for a time minimizer. We found seasonally diverging slopes between stopover and flight durations in relation to the progress (time) of migration, which follows a time-minimizing policy if resource gradients along the migration route increase in autumn and decrease in spring. Total flight duration did not differ significantly between autumn and spring migration, although spring migration was 6% shorter. Overall duration of autumn migration was longer than that in spring, mainly owing to a mid-migration stop in most birds, when they likely initiated moult. Overall migration speed was significantly different between autumn and spring. Migratory flights often occurred as runs of two to seven nocturnal flights on adjacent days, which may be countering a time-minimization strategy. Other factors may influence a preference for nocturnal migration, such as avoiding flight in turbulent conditions, heat stress and diurnal predators.

Leapfrog migration is a common migration pattern in birds where the breeding and wintering latitudes between populations are in reversed latitudinal sequence. Competition for wintering and breeding sites has been suggested to be an ultimate factor, and several competitor‐based hypotheses have been proposed to explain this pattern. If wintering sites close to the breeding sites are favored, competitive exclusion could force subdominant individuals to winter further away. Competitive exclusion could be mediated either through body size or by prior occupancy. The alternative “spring predictability” hypothesis assumes competition for sufficiently close wintering areas, allowing the birds to use autocorrelated weather cues to optimally time spring migration departure. To test predictions and assumptions of these hypotheses, we combined morphometrics, migration, and weather data from four populations of common ringed plover breeding along a latitudinal (56–68° N) and climatic gradient (temperate to Arctic). Critical for our evaluation was that two populations were breeding on the same latitude in subarctic Sweden with the same distance to the closest potential wintering site, but differed in breeding phenology, and wintered in West Africa and Europe, respectively. Thus, while breeding on the same latitude, their winter distribution overlapped with that of an Arctic and temperate population. Body size was largest within the temperate population, but there was no size difference between the two subarctic populations. Populations wintering in Europe arrived there before populations wintering in Africa. The largest variation in the arrival of meteorological spring occurred at the temperate breeding site, while there was almost no difference among the other sites. In general, temperatures at the northernmost wintering area correlated well with each breeding site prior to breeding site‐specific spring arrival. Based on these observations, we conclude that competitive exclusion through body‐size‐related dominance cannot explain leapfrog migration. Furthermore, the assumptions on which the “spring predictability” hypothesis is based did not match the observed wintering ranges either. However, we could not reject the hypothesis that competitive exclusion mediated by prior occupancy in the wintering area could lead to leapfrog migration, and therefore, this hypothesis should be retained as working hypothesis for further work.