How do energy stores and changes in these affect departure decisions by migratory birds? A critical view on stopover ecology studies and some future perspectives

A conceptual framework on the role of magnetic cues in songbird migration ecology

Migrating animals perform astonishing seasonal movements by orienting and navigating over thousands of kilometres with great precision. Many migratory species use cues from the sun, stars, landmarks, olfaction and the Earth's magnetic field for this task. Among vertebrates, songbirds are the most studied taxon in magnetic-cue-related research. Despite multiple studies, we still lack a clear understanding of when, where and how magnetic cues affect the decision-making process of birds and hence, their realised migratory behaviour in the wild. This understanding is especially important to interpret the results of laboratory experiments in an ecologically appropriate way. In this review, we summarise the current findings about the role of magnetic cues for migratory decisions in songbirds. First, we review the methodological principles for orientation and navigation research, specifically by comparing experiments on caged birds with experiments on free-flying birds. While cage experiments can show the sensory abilities of birds, studies with free-flying birds can characterise the ecological roles of magnetic cues. Second, we review the migratory stages, from stopover to endurance flight, in which songbirds use magnetic cues for their migratory decisions and incorporate this into a novel conceptual framework. While we lack studies examining whether and when magnetic cues affect orientation or navigation decisions during flight, the role of magnetic cues during stopover is relatively well studied, but mostly in the laboratory. Notably, many such studies have produced contradictory results so that understanding the biological importance of magnetic cues for decisions in free-flying songbirds is not straightforward. One potential explanation is that reproducibility of magnetic-cue experiments is low, probably because variability in the behavioural responses of birds among experiments is high. We are convinced that parts of this variability can be explained by species-specific and context-dependent reactions of birds to the study conditions and by the bird's high flexibility in whether they include magnetic cues in a decision or not. Ultimately, this review should help researchers in the challenging field of magnetoreception to design experiments meticulously and interpret results of such studies carefully by considering the migration ecology of their focal species.

Pre-migratory flights in migrant songbirds: the ecological and evolutionary importance of understudied exploratory movements

Across the animal kingdom, from honeybees to cranes to beavers, exploratory movements to exploit resources, scout prospective territories, or otherwise gain valuable experiences and information that promote fitness have been documented. For example, exploratory movements to investigate potential dispersal targets have been observed in roe deer, Northern cardinals, and tigers alike. However, despite how widespread these movements are, a cohesive definition of exploratory movements has been lacking. We first provide a clear definition of exploratory movements, and use one particular group-migratory songbirds-to catalogue exploratory movements across the annual cycle. The exceptional mobility of migratory songbirds results in exploratory movements not only at a local scale, but also on a regional scale, both in and out of the breeding season. We review the extent to which these movements are made within this group, paying particular attention to how such movements confer fitness benefits, as by securing high-quality territories, prospecting for extra-pair paternity, or even exploiting ephemeral resources. We then zoom in one step further to a particular exploratory movement that has been, to date, almost completely overlooked within this group: that of pre-migratory flights. These flights, which occur during the transitional period between the stationary breeding period and the onset of migration, occur at night and may not be made by all individuals in a population-reasons why these flights have been heretofore critically understudied. We provide the first definition for this behaviour, summarise the current knowledge of this cryptic movement, and hypothesise what evolutionary/ecological advantages conducting it may confer to the individuals that undertake it. As these flights provide experience to the individuals that undertake them, we expect that birds that make pre-migratory flights are better equipped to survive migration (direct fitness benefits) and, due to orientation/navigation abilities, may also reach preferred territories on breeding and wintering grounds faster (indirect fitness benefits). We hope to encourage ecologists to consider such hidden movements in their research concepts and to enhance the framework of movement ecology by this behaviour due to its presumed high biological importance to the annual cycle of birds.

The cost of movement: assessing energy expenditure in a long-distant ectothermic migrant under climate change

Migration is an energetically taxing phenomenon as animals move across vast, heterogeneous landscapes where the cost of transport is impacted by permissible ambient conditions. In this study, we assessed the energetic demands of long-distance migration in a multigenerational ectothermic migrant, the monarch butterfly (Danaus plexippus). We tested the hypotheses that temperature-dependent physiological processes reduce energy reserves faster during migration than previously estimated, and that increasing climatic temperatures resulting from the climate crisis will intensify baseline daily energy expenditure. First, we reared monarchs under laboratory conditions to assess energy and mass conversion from fifth instar to adult stages, as a baseline for migratory adult mass and ontogenetic shifts in metabolic rate from larvae to adult. Then, using historical tag-recapture data, we estimated the movement propensity and migratory pace of autumn migrants using computer simulations and subsequently calculated energy expenditure. Finally, we estimated the energy use of monarchs based on these tag-recapture data and used this information to estimate daily energy expenditure over a 57 year period. We found support for our two hypotheses, noting that incorporating standard metabolic rate into estimates of migratory energy expenditure shows higher energy demand and that daily energy expenditure has been gradually increasing over time since 1961. Our study shows the deleterious energetic consequences under current climate change trajectories and highlights the importance of incorporating energetic estimates for understanding migration by small, ectothermic migrants.

Intent matters: how flow and forms of information impact collective navigation

The phenomenon of collective navigation has received considerable interest in recent years. A common line of thinking, backed by theoretical studies, is that collective navigation can improve navigation efficiency through the 'many-wrongs' principle, whereby individual error is reduced by comparing the headings of neighbours. When navigation takes place in a flowing environment, each individual's trajectory is influenced by drift. Consequently, a potential discrepancy emerges between an individual's intended heading and its actual heading. In this study, we develop a theoretical model to explore whether collective navigation benefits are altered according to the form of heading information transmitted between neighbours. Navigation based on each individual's intended heading is found to confer robust advantages across a wide spectrum of flows, via both a marked improvement in migration times and a capacity for a group to overcome flows unnavigable by solitary individuals. Navigation based on individual's actual headings is far less effective, only offering an improvement under highly favourable currents. For many currents, sharing actual heading information can even lead to journey times that exceed those of individual navigators.

The oxidative balance and stopover departure decisions in a medium- and a long-distance migrant

BACKGROUND

Birds have extremely elevated metabolic rates during migratory endurance flight and consequently can become physiologically exhausted. One feature of exhaustion is oxidative damage, which occurs when the antioxidant defense system is overwhelmed by the production of damaging reactive oxygen species (ROS). Migrating birds have been shown to decrease the amount of oxidative lipid damage during stopovers, relatively stationary periods in between migratory flights. It has therefore been argued that, in addition to accumulating fuel, one of the functions of stopover is to restore the oxidative balance. If this is so, we would expect that migrating birds are unlikely to resume migration from stopover when they still have high amounts of lipid damage.

METHODS

To test this hypothesis, we measured parameters of the oxidative balance and related these to stopover departure decisions of song thrushes (Turdus philomelos) and northern wheatears (Oenanthe oenanthe), a medium- and long-distance songbird migrant, respectively. We measured malondialdehyde (MDA) concentration, a biomarker for oxidative lipid damage, and total non-enzymatic antioxidant capacity (AOX), an overall biomarker of protection against ROS. Stopover departure decisions were determined using a fully automated telemetry system set-up on our small island study site.

RESULTS

The decision to resume migration was not related with MDA concentration in either study species, also not when this was corrected for circulating fatty acid concentrations. Similarly, AOX did not affect this decision, also not when corrected for uric-acid concentration. The time within the night when birds departed also was not affected by MDA concentration or AOX. However, confirming earlier observations, we found that in both species, fat individuals were more likely to depart than lean individuals, and fat northern wheatears departed earlier within the night than lean conspecifics. Northern wheatears additionally departed earlier in spring with more southerly winds.

CONCLUSIONS

We found no support for the idea that stopovers departure decisions are influenced by parameters of the oxidative balance. We discuss possible reasons for this unexpected finding.

Autumn migrating passerines at a desert edge: Do birds depart for migration after reaching a threshold fuel load or vary it according to the rate of fuel deposition?

Fuel deposition rate is predicted to determine departure fuel load during stopover in two models of optimal behavior of migrating birds. Yet, near ecological barriers, such as wide deserts, birds may switch to a different strategy of departing with just enough fuel to enable the long cross-barrier flight, thus reaching a threshold of fuel load regardless of the rate of fuel deposition. To test these predictions we studied autumn migrating Red-backed Shrikes (Lanius collurio) before they departed for a ∼2,000 km journey across the Sahara Desert. The body mass of fourteen individuals was measured on a daily basis throughout their stopover using field-deployed scales while being tracked by the ATLAS biotelemetry system in the Hula Valley, Israel. Statistical analysis found that the natural log of departure fuel load was positively related to both the capture fuel load and the fuel deposition rate. Hence, the results of this analysis suggest that bird condition at departure depended on the rate of fuel deposition, as predicted by models of time-minimization migration and the minimization of the total energy cost of migration. Departure fuel load and stopover duration were negatively related to each other as birds that remained for a long time in stopover departed with relatively low fuel loads. These findings suggest that even near a wide ecological barrier, departure fuel load is sensitive to the rate of fuel deposition, especially at lower values of fuel deposition rate. Birds that were able to accumulate fuel at higher rates showed a nearly constant departure fuel load and as such we could not exclude the possibility that the birds were trying to reach a certain threshold of fuel stores. Randomized 1,000 repeats of the aforementioned correlation suggest that the correlation between fuel deposition rate and the log of departure fuel load is valid and does not represent a spurious result. Following bird migration simulation using the program Flight, we conclude that fuel loads allowed most individual to accomplish the journey across the desert. Our findings suggest high between-individual variation in stopover parameters with likely consequences for bird migration performance and survival.

Early and Late Migrating Avian Individuals Differ in Constitutive Immune Function and Blood Parasite Infections – But Patterns Depend on the Migratory Strategy

Billions of birds migrate every year. To conduct a successful migration, birds undergo a multitude of physiological adaptions. One such adaptation includes adjustments of immune function, however, little is known about intraspecies (between-individual) and interspecies (between-species) variation in immune modulations during migration. Here, we explore if early and late migrating individuals differ in their immune function, and if such patterns differ among species with short- vs. long-distance migration strategies. We quantified three parameters of baseline (constitutive) innate immune function and one parameter of baseline (constitutive) acquired immune function in 417 individuals of 10 species caught during autumn migration at Falsterbo (Sweden). Early and late migrating individuals differed in lysis and total immunoglobulins (IgY), but the patterns show different directions in long-distance migrants (LDMs) (wintering in Africa) as compared to short-distance migrants (SDMs) (wintering within Europe). Specifically, early migrating LDMs had lower lysis but higher immunoglobulin levels than late migrating individuals. In short distance migrants, there was no difference in lysis between early and late migrating individuals, but immunoglobulin levels were higher in late migrating individuals. We found no correlation between timing of migration and haptoglobin, but LDMs had lower levels of haptoglobin than SDMs. We also found that the prevalence of haemosporidian blood parasite infections decreased in LDMs, but increased in SDMs, as the autumn progressed. Taken together, our study suggests that the investment into immune function depends on the migratory strategy (short- vs. long-distance migrants), and that early and late migrating individuals of a migration strategy might invest differently in baseline immune function, potentially driven by differences in the trade-offs with timing and speed of migration. Our study highlights the potential adaptations of immune function that could help explain trade-offs with other physiological systems, and behavioural responses during migration.

Can differential fatty acid composition help migrating birds to limit oxidative lipid damage?

During migratory endurance flights, which are energetically very demanding, migrants have to deal with prolonged elevated generation of reactive oxygen species (ROS). To limit the damaging actions that ROS have on lipids and proteins, migrating birds are known to upregulate their antioxidant defence system. However, there may be additional ways to limit oxidative damage incurred from flying. Migratory endurance flights are fuelled mainly with fatty acids (FAs), and the risk of their peroxidation (resulting in oxidative lipid damage) increases with the number of double bonds in a FA, with polyunsaturated FAs (2 or more double bonds, PUFAs) being most peroxidation-prone. By fuelling their flights with relatively few PUFAs, migratory birds could thus limit oxidative lipid damage. Within migratory birds, there is considerable variation in the length of their flights, with nocturnal migrants making lengthier flight bouts, thus more likely to experience lengthier periods of elevated ROS production, than diurnal migrants. However, whether migrants making lengthier flights incur more oxidative lipid damage is unknown. Neither is it known whether flight length and FA composition are associated. Therefore, we determined plasmatic malondialdehyde level, a marker of oxidative lipid damage, and FA composition of three nocturnal and two diurnal migrant species caught at an autumn stopover site. We found little inter-specific variation in malondialdehyde level, indicating that the amount of oxidative lipid damage was comparable across the species. In contrast, the species strongly differed in their plasmatic FA composition. The nocturnal migrants had significantly lower relative PUFA levels than both diurnal migrants, an effect mainly attributable to linoleic acid, an essential (strictly dietary) FA. Consequently, the susceptibility of plasmatic FAs to lipid peroxidation was significantly lower in the nocturnal than diurnal migrants. Because in birds, energy expenditure during flight decreases with the degree of FA unsaturation, we interpret our observation of lower PUFA levels in nocturnal migrants as support for the idea that utilizing PUFA-poor fuel can help migrating birds to curb oxidative lipid damage.

Experimental ghrelin administration affects migratory behaviour in a songbird

Twice a year, billions of birds take on drastic physiological and behavioural changes to migrate between breeding and wintering areas. On migration, most passerine birds regularly stop over along the way to rest and refuel. Endogenous energy stores are not only the indispensable fuel to complete long distance flights, but are also important peripheral signals that once integrated in the brain modulate crucial behavioural decisions, such as the decision to resume migration after a stopover. A network of hormones signals metabolic fuel availability to the brain in vertebrates, including the recently discovered gut-hormone ghrelin. Here, we show that ghrelin takes part in the control of migratory behaviour during spring migration in a wild migratory passerine. We manipulated blood concentrations of ghrelin of 53 yellow-rumped warblers (Setophaga coronata coronata) caught during stopover and automatically radio-tracked their migratory behaviour following release. We found that injections of acylated and unacylated ghrelin rapidly induced movements away from the release site, indicating that the ghrelin system acts centrally to mediate stopover departure decisions. The effects of the hormone manipulation declined within 8 h following release, and did not affect the overall rate of migration. These results provide experimental evidence for a pivotal role of ghrelin in the modulation of behavioural decisions during migration. In addition, this study offers insights into the regulatory functions of metabolic hormones in the dialogue between gut and brain in birds.

Fat Stores and Antioxidant Capacity Affect Stopover Decisions in Three of Four Species of Migratory Passerines With Different Migration Strategies: An Experimental Approach

During migratory stopovers, birds must make decisions about when and where to travel and these decisions are likely contingent on their fuel stores, food availability, and antioxidant capacity as well as seasonal changes in key environmental factors. We conducted a field experiment on an offshore stopover site (Block Island, Rhode Island, United States: 41°130N, 71°330W) during autumn migration to test the hypothesis that birds with greater fuel stores and non-enzymatic antioxidant capacity have shorter stopovers than lean birds with low antioxidant capacity, and to determine the extent to which this depends on migration strategy. We used a 2 × 2 factorial field experiment (two levels each of available food and dietary polyphenols) with four species of songbirds kept in captivity for 3–5 days to produce experimental groups with different fuel stores and antioxidant capacity. We attached digital VHF transmitters to assess stopover duration and departure direction using automated telemetry. Non-enzymatic antioxidant capacity increased during refueling for Red-eyed Vireos (Vireo olivaceus) and Blackpoll Warblers (Setophaga striata) fed ad lib diets, and for ad lib fed Hermit Thrushes (Catharus guttatus) supplemented with polyphenols, but not for Yellow-rumped Warblers (Setophaga coronata coronata). Glutathione peroxidase (GPx) decreased during captivity and was influenced by dietary treatment only in Red-eyed Vireos. Oxidative damage decreased during captivity for all species except Yellow-rumped Warblers. Stopover duration was shorter for Vireos and Blackpolls fed ad lib as compared to those fed maintenance. Ad lib fed Hermit Thrushes supplemented with polyphenols had shorter stopovers than those fed ad lib, as did thrushes fed at maintenance and supplemented with polyphenols compared with those fed at maintenance alone. There was no influence of condition on stopover duration for Yellow-rumped Warblers. Departure direction was not strongly related to condition, and birds primarily reoriented north when departing Block Island. Thus, fat stores and oxidative status interacted to influence the time passerines spent on stopover, and condition-dependent departure decisions were related to a bird’s migration strategy. Therefore, seasonal variation in macro- and micro-nutrient resources available for refueling at stopover sites can affect body condition and antioxidant capacity and in turn influence the timing and success of migration.

Drivers of migrant passerine composition at stopover islands in the western Mediterranean

Clues used by migrant birds to select sites for stopover are much less known than their reasons for leaving. Habitat characteristics and geographical location may affect the decision to use an island as a stopover site in different ways for different species. Thus, abundance and composition of migrants may be expected to differ between islands. Using standardized ringing from 9 western Mediterranean islands we evaluate drivers of abundance of trans-Saharan migrant passerines, specifically the role of species continental abundance, island characteristics and geographical location. Although continental abundance is a main driver of migrant composition on all islands migrant composition differs between them. Redundancy analysis and species response models revealed that the main drivers were distance to the nearest land toward the south, which has a positive effect on the number of migrants of most species, and island area, which appears as an important cue used for selecting a stopover island. Species whose abundance is positively related to island area have more pointed wings while species affected by distance to land toward the south have relatively more rounded wings. This suggests a hypothesis on the mechanism that may generate differences in passerine migrant composition between islands based on better efficiency of more pointed wings for long-distance flight.

Understanding the ecological and evolutionary function of stopover in migrating birds

Global movement patterns of migratory birds illustrate their fascinating physical and physiological abilities to cross continents and oceans. During their voyages, most birds land multiple times to make so-called 'stopovers'. Our current knowledge on the functions of stopover is mainly based on the proximate study of departure decisions. However, such studies are insufficient to gauge fully the ecological and evolutionary functions of stopover. If we study how a focal trait, e.g. changes in energy stores, affects the decision to depart from a stopover without considering the trait(s) that actually caused the bird to land, e.g. unfavourable environmental conditions for flight, we misinterpret the function of the stopover. It is thus important to realise and acknowledge that stopovers have many different functions, and that not every migrant has the same (set of) reasons to stop-over. Additionally, we may obtain contradictory results because the significance of different traits to a migrant is context dependent. For instance, late spring migrants may be more prone to risk-taking and depart from a stopover with lower energy stores than early spring migrants. Thus, we neglect that departure decisions are subject to selection to minimise immediate (mortality risk) and/or delayed (low future reproductive output) fitness costs. To alleviate these issues, we first define stopover as an interruption of migratory endurance flight to minimise immediate and/or delayed fitness costs. Second, we review all probable functions of stopover, which include accumulating energy, various forms of physiological recovery and avoiding adverse environmental conditions for flight, and list potential other functions that are less well studied, such as minimising predation, recovery from physical exhaustion and spatiotemporal adjustments to migration. Third, derived from these aspects, we argue for a paradigm shift in stopover ecology research. This includes focusing on why an individual interrupts its migratory flight, which is more likely to identify the individual-specific function(s) of the stopover correctly than departure-decision studies. Moreover, we highlight that the selective forces acting on stopover decisions are context dependent and are expected to differ between, e.g. K-/r-selected species, the sexes and migration strategies. For example, all else being equal, r-selected species (low survival rate, high reproductive rate) should have a stronger urge to continue the migratory endurance flight or resume migration from a stopover because the potential increase in immediate fitness costs suffered from a flight is offset by the expected higher reproductive success in the subsequent breeding season. Finally, we propose to focus less on proximate mechanisms controlling landing and departure decisions, and more on ultimate mechanisms to identify the selective forces shaping stopover decisions. Our ideas are not limited to birds but can be applied to any migratory species. Our revised definition of stopover and the proposed paradigm shift has the potential to stimulate a fruitful discussion towards a better evolutionary ecological understanding of the functions of stopover. Furthermore, identifying the functions of stopover will support targeted measures to conserve and restore the functionality of stopover sites threatened by anthropogenic environmental changes. This is especially important for long-distance migrants, which currently are in alarming decline.

The role of ketogenesis in the migratory fattening of the northern wheatear Oenanthe oenanthe

The fuelling capacity of migratory birds and their ability to avoid health conditions derived from the subsequent fat overload are exceptional among vertebrates. In this work, we screen the expression of the genes involved in the production of ketone bodies (KB) in the liver of northern wheatears (Oenanthe oenanthe) during the development and resolution of migratory fattening. Thirteen genes were found to be regulated among the migratory stages. Based on the dynamics of gene expression, we concluded that KB play a versatile role in wheatears' energy metabolism homeostasis. The ketogenic pathway can adaptively: (i) provide carbon equivalents for lipogenesis, speeding up fuelling; (ii) replace glucose during long-distance flights using lipids as the substrate; (iii) act as a floodgate to avoid steatosis; and (iv) might provide a metabolic solution to defatting in captive birds.

Avian Use of Agricultural Areas as Migration Stopover Sites: A Review of Crop Management Practices and Ecological Correlates

An estimated 17% of migratory bird species are threatened or near threatened with extinction. This represents an enormous potential loss of biodiversity and cost to human societies due to the economic benefits that birds provide through ecosystem services and ecotourism. Conservation of migratory bird species presents many unique challenges, as these birds rely on multiple geographically distinct habitats, including breeding grounds, non-breeding grounds, and stopover sites during migration. In particular, stopover habitats are seldom studied relative to breeding and non-breeding habitats, despite their importance as refueling stations for migratory birds. In this study, we summarize the current research on the use of temporary primary crops by birds during migration and we assess the species characteristics and agricultural practices most often associated with the use of cropland as stopover habitat. First, we conducted a systematic review of the literature to document the effects various farming practices and crop types have on the abundance and diversity of migratory birds using agricultural areas for stopovers. Second, we analyzed the ecological correlates of bird species in the Northern Hemisphere that predict which species may use these areas while migrating. We ran a GLMM to test whether primary diet, diet breadth, primary habitat, habitat breadth, or realm predicted stopover use of agricultural areas. Our review suggests that particular crop types (principally rice, corn, and sunflower), as well as farming practices that result in higher non-cultivated plant diversity, encourage the use of agricultural areas by migrating birds. We found that cropland is used as stopover habitat by bird species that can utilize a large breadth of habitats, as well as species with preferences for habitat similar in structure to agricultural areas.

The avian lightweights: Trans-Saharan migrants show lower lean body mass than short-/medium-distance migrants

Avian trans-Saharan migrants travelling long distances and crossing ecological barriers experience different constraints in terms of time, energy and safety than short-/medium-distance migrants without barrier-crossings. As such, natural selection shapes the aerodynamic properties of these groups differently. Yet, to the best of our knowledge, we lack information on whether natural selection has contributed to reducing energetic flight costs through generally lower body mass in trans-Saharan migrants. To fill parts of this gap, we investigated this eco-morphological pattern in 5,410 individuals of 22 Palearctic songbird species ranging from short-/medium-distance to trans-Saharan migrants. We used individual size-independent scaled lean body mass values based on wing length as a measure of body size and, for the first time, precisely determined lean body mass values by direct measurements via quantitative magnetic resonance technology. Scaled lean body mass for a given body size was significantly higher in short-/medium-distance migrants than in trans-Saharan migrants. Although scaled lean body mass significantly decreased with increasing migration distance in short-/medium-distance migrants, no such effect was found in trans-Saharan migrants. Our results thus show an eco-morphological pattern relating species' lean body mass not only to migration distance but also to migration group. This suggests that selective effects of the presence/absence of ecological barriers and/or of a threshold level for migration distance on migrant birds may be more important than the linear continuum of migration distance per se.

Temperature change is an important departure cue in nocturnal migrants: controlled experiments with wild-caught birds in a proof-of-concept study

The decision-making process of migrating birds at stopover sites is a complex interplay of the innate migration program and both intrinsic and extrinsic factors. While it is well studied how variation in precipitation, wind and air pressure influence this process, there is less evidence of the effects of temperature changes on the departure decision. Thus, we lack knowledge on how the predicted changes due to global climate change in temperature alone may affect the decision-making process during migration. Aiming to fill parts of this gap, we conducted a proof-of-concept study by manipulating the ambient temperature of temporarily confined wild-caught migrant songbirds under constant feeding conditions. In spring, departure probability increased with a 20°C rise in temperature for both a medium-distance migrant (European robin, Erithacus rubecula) and a long-distance migrant (northern wheatear, Oenanthe oenanthe), and in autumn, departure probabilities of the long-distance migrant both decreased with a 20°C rise and increased with a 20°C drop. Consequently, the temperature is an important departure cue influencing the decision-making process of migrating songbirds. Incorporating causal relationships between changes in temperature and departure probability in migration models could substantially improve our ability to predict the effects of climate change on the phenology of migratory birds.

Are movements of daytime and nighttime passerine migrants as different as day and night?

Even after decades of research, the migration of songbirds still holds numerous secrets. Distinct stopover and routing behavior of diurnally and nocturnally migrating songbirds has been stated in the 1960s, but empirical confirmation is yet lacking widely. We studied the behavior of individual diurnally migrating dunnocks and nocturnally migrating blackcaps by means of large-scale automated radio-telemetry. Birds were radio-tagged during their stopover at the German North Sea coast. Our data indicate longer initial stopover duration in the diurnally migrating dunnocks, opposing the hypothesis of nocturnal migrants needing more time to recover due to their longer migratory flights. Nonetheless, dunnocks stopped over more often along their tracks as when compared to the nocturnally migrating blackcaps. Behavior en route did not differ as clearly between species challenging the general view of contrasting routings of diurnal and nocturnal migrants with regard to landscape and open water. Our results imply additional factors of relevance other than differences in species or daily migration timing per se. We discuss and highlight the need of detailed and individual based data to better understand stopover and routing behavior of songbirds in the environmental context.

Developing a Stopover-CORT hypothesis: Corticosterone predicts body composition and refueling rate in Gray Catbirds during migratory stopover

Migratory flight is energetically challenging, requiring alternating phases of fuel catabolism and fuel accumulation, accompanied by dramatic changes in body composition and behavior. Baseline corticosterone (CORT; the primary glucocorticoid in birds) is thought to underlie transitions between fuel catabolism during flight, fuel deposition during stopover, and the initiation of migratory flight. However, studies of CORT on stopover physiology and behavior remain disparate efforts, lacking the cohesion of a general hypothesis. Here we develop a Stopover-CORT hypothesis formalizing the relationships among CORT, body condition, and refueling rate in migratory birds. First we expect body mass to increase with triglycerides (TRIG) as birds refuel. Second, based on a synthesis of previous literature, we predict a U-shaped CORT curve over the course of stopover, postulating that elevated CORT at arrival is reactive, responding to poor body condition, while CORT elevation before departure is preparative, driving changes in behavior and body condition. We tested these predictions in Gray Catbirds (Dumetella carolinensis) following a trans-Gulf flight during spring migration. We found baseline CORT was negatively correlated with body condition and TRIG, corresponding with our predictions for arriving and refueling-but not departing-birds. It is possible catbirds undergo regional habitat translocations rather than complete the entire stopover phase at our study site. We propose the Stopover-CORT hypothesis as a useful predictive framework for future studies of the mechanistic basis of stopover physiology. By studying the regulation of stopover refueling and departure, we may better understand physiological limitations to overall migration rate and improve assessments of habitat quality for refueling birds.

A quasi-experimental approach using telemetry to assess migration-strategy-specific differences in the decision-making processes at stopover

Background

Migrant birds travel between their breeding areas and wintering grounds by alternating energetically and physiologically demanding flights with periods of rest and fuelling, so-called stopovers. An important intrinsic factor influencing the decision to resume migration is the amount of energy stores available for the next flight. Correlative studies with free-flying birds and experimental studies with caged birds have shown that the amount of energy stores affects the day-to-day, within-day and the directional decision of departure. The methodological advantages of both the correlative and experimental approach are combined when radio-tagging many individuals on the same day and subsequently determining the departure decisions at a high spatiotemporal resolution. Making use of such a quasi-experimental approach with an automated radio-tracking system at stopover, we studied the effect of energy stores on departure decisions and whether they vary between species of different migration strategies experiencing contrasting time constraints. For this, we chose a long-distance migrant, the common redstart (Phoenicurus phoenicurus), and a medium-distance migrant, the European robin (Erithacus rubecula), because the former has to travel at relatively higher speed to reach its wintering ground in a reasonable time at the expense of relatively higher energetic costs for travelling than the latter.

Results

Common redstarts with higher energy stores were more likely to resume migration than their conspecifics with lower energy stores, whereas this pattern was absent in the European robins. The amount of energy stores significantly affected the timing of departure within the day, with large energy stores yielding early departures in both species. Departure directions from the stopover site during the first night after capture were oriented towards the seasonally appropriate direction but were not affected by variation in energy stores.

Conclusions

We demonstrate the importance of variation in energy stores on the departure decisions and that it may affect species with different migration strategies dissimilarly in autumn. Nevertheless, knowledge of other intrinsic factors, such as feeding conditions, health status and physiological consequences of previous flights, is additionally required to better understand the departure decisions of migrants, as this is the key to providing an overall assessment of the decision-making process.

Diel variation in corticosterone and departure decision making in migrating birds

Animals usually show distinct periods of diel activity and non-activity. Circulating baseline levels of glucocorticoid hormones (corticosterone and cortisol) often peak just before or at the transition from the non-active to the active period of the day. This upregulation of glucocorticoids may function to mobilize stored energy and prepare an animal for increased activity. Usually, the alternation of active and non-active periods is highly predictable; however, there is one group of animals for which this is not always the case. Many otherwise diurnal birds show nocturnal activity during the migration seasons. Nocturnal migratory flights are alternated with stopover periods during which the birds refuel and rest. Stopovers vary in length, meaning that nocturnal migrants are inactive in some nights (when they continue their stopover) but extremely active in other nights (when they depart and fly throughout the night). This provides an ideal natural situation for testing whether glucocorticoids are upregulated in preparation for an increase in activity, which we used in this study. We found that in northern wheatears (Oenanthe oenanthe), corticosterone levels peaked in the few hours before sunset in birds departing from stopover that night, and, importantly, that this peak was absent in birds continuing stopover. This indicates that corticosterone is upregulated in the face of an increase in energy demands, underlining corticosterone's preparative metabolic function (energy mobilization). The timing of upregulation of corticosterone also gives a first insight in when during the day nocturnally migrating birds decide whether or not to resume migration.

Energy Stores, Oxidative Balance, and Sleep in Migratory Garden Warblers (Sylvia borin) and Whitethroats (Sylvia communis) at a Spring Stopover Site

Little is known about how songbirds modulate sleep during migratory periods. Due to the alternation of nocturnal endurance flights and diurnal refueling stopovers, sleep is likely to be a major constraint for many migratory passerine species. Sleep may help to increase the endogenous antioxidant capacity that counteracts free radicals produced during endurance flight and reduces energy expenditure. Here, we investigated the relationship between sleep behavior, food intake, and two markers of physiological condition-the amount of energy reserves and oxidative status-in two migratory songbird species, the garden warbler (Sylvia borin) and the whitethroat (Sylvia communis). In garden warblers, birds with high energy stores were more prone to sleep during the day, while this condition-dependent sleep pattern was not present in whitethroats. In both species, birds with low energy stores were more likely to sleep with their head tucked in the feathers during nocturnal sleep. Moreover, we found a positive correlation between food intake and the extent of energy reserves in garden warblers, but not in whitethroats. Finally, we did not find significant correlations between oxidative status and sleep, or oxidative status and energy stores. Despite our study was not comparative, it suggests that different species might use different strategies to manage their energy during stopover and, additionally, it raises the possibility that migrants have evolved physiological adaptations to deal with oxidative damage produced during migration.

The physiology of movement

Movement, from foraging to migration, is known to be under the influence of the environment. The translation of environmental cues to individual movement decision making is determined by an individual's internal state and anticipated to balance costs and benefits. General body condition, metabolic and hormonal physiology mechanistically underpin this internal state. These physiological determinants are tightly, and often genetically linked with each other and hence central to a mechanistic understanding of movement. We here synthesise the available evidence of the physiological drivers and signatures of movement and review (1) how physiological state as measured in its most coarse way by body condition correlates with movement decisions during foraging, migration and dispersal, (2) how hormonal changes underlie changes in these movement strategies and (3) how these can be linked to molecular pathways. We reveale that a high body condition facilitates the efficiency of routine foraging, dispersal and migration. Dispersal decision making is, however, in some cases stimulated by a decreased individual condition. Many of the biotic and abiotic stressors that induce movement initiate a physiological cascade in vertebrates through the production of stress hormones. Movement is therefore associated with hormone levels in vertebrates but also insects, often in interaction with factors related to body or social condition. The underlying molecular and physiological mechanisms are currently studied in few model species, and show -in congruence with our insights on the role of body condition- a central role of energy metabolism during glycolysis, and the coupling with timing processes during migration. Molecular insights into the physiological basis of movement remain, however, highly refractory. We finalise this review with a critical reflection on the importance of these physiological feedbacks for a better mechanistic understanding of movement and its effects on ecological dynamics at all levels of biological organization.

Stopover departure decisions in songbirds: do long-distance migrants depart earlier and more independently of weather conditions than medium-distance migrants?

BACKGROUND

Songbirds following distinct migration strategies (e.g. long- vs. short- to medium-distance migrants) often differ in their speed of migration during autumn and, thus, are assumed to face different time constraints. During migration, most songbird species alternate migratory flights with stopover periods. Many of them restrict these migratory flights to the night, i.e., they are nocturnal migrants. At stopover, nocturnal migrants need to select a specific night (night-to-night decision) and time of night (within-night decision) to resume migration. These departure decisions, which largely determine the speed of migration, are jointly affected by a set of intrinsic and extrinsic factors, i.e., departure cues. Here we aim to assess whether the set of intrinsic and extrinsic factors and the magnitude of their respective effects on stopover departure decisions differs between nocturnally migrating songbird species, depending on their migration strategy and associated time constraints.

METHODS

We radio-tracked migrating Northern Wheatears (Oenanthe oenanthe; long-distance migrant), European robins (Erithacus rubecula) and Common Blackbirds (Turdus merula; both medium-distance migrants) during autumn stopover and analysed their night-to-night and within-night departure timing in relation to intrinsic and extrinsic factors.

RESULTS

Species generally differed in their departure timing on both temporal scales, with shortest stopovers and earliest nocturnal departures in the long-distance migrant. Some factors, such as day of year, fuel load, cloud cover and crosswind, had consistent effects on stopover departure decisions in all three species. However, species differed in the effects of tailwind assistance, change in atmospheric pressure and air temperature on their stopover departure decisions. Whereas night-to-night decisions were affected by these extrinsic factors in either both or one of the medium-distance migrants, such effects were not found in the long-distance migrant.

CONCLUSIONS

Our results suggest that the general timing of departures in songbirds is affected by the species-specific migration strategy and associated time constraints. Further, they imply that the assessment and usage of specific extrinsic factors, i.e., weather conditions, as departure cues is adjusted based on this migration strategy, with the long-distance migrants being least selective at departure. Other intrinsic and extrinsic factors, however, seem to be used as departure cues independent of migration strategy.

The amount of available food affects diurnal locomotor activity in migratory songbirds during stopover

Migratory passerine birds fly long distances twice a year alternating nocturnal flights with stopovers to rest and replenish energy stores. The duration of each stopover depends on several factors including internal clocks, meteorological conditions, and environmental factors such as availability of food. Foraging entails energetic costs, and if birds need to refuel efficiently, they should modulate their activity in relation to food availability. We investigated how food availability influences locomotor activity in migrating birds of six passerine species at a spring stopover site in the central Mediterranean Sea. We selected birds with low fat scores which we expected to be strongly motivated to refuel. We simulated stopover sites of different quality by providing temporarily caged birds with different amounts of food to simulate scarce to abundant food. We analysed the diurnal locomotory activity as a proxy for food searching effort. Low food availability resulted in an increased diurnal locomotor activity in almost all species, while all birds showed low intensity of nocturnal migratory restlessness. In conclusion, our study shows that food availability in an important determinant of behaviour of migratory birds at stopover sites.

Not just fuel: energy stores are correlated with immune function and oxidative damage in a long-distance migrant

In many animals, catabolic and anabolic periods are temporally separated. Migratory birds alternate energy expenditure during flight with energy accumulation during stopover. The size of the energy stores at stopover affects the decision to resume migration and thus the temporal organization of migration. We now provide data suggesting that it is not only the size of the energy stores per se that may influence migration scheduling, but also the physiological consequences of flying. In two subspecies of the northern wheatear Oenanthe oenanthe, a long-distance migrant, estimated energy stores at a stopover during autumn migration were positively related with both constitutive innate and acquired immune function, and negatively related with oxidative damage to lipids. In other words, migrants' physiological condition was associated with their energetic condition. Although time spent at stopover before sampling may have contributed to this relationship, our results suggest that migrants have to trade-off the depletion of energy stores during flight with incurring physiological costs. This will affect migrants' decisions when to start and when to terminate a migratory flight. The physiological costs associated with the depletion of energy stores may also help explaining why migrants often arrive at and depart from stopover sites with larger energy stores than expected. We propose that studies on the role of energy stores as drivers of the temporal organization of (avian) migration need to consider physiological condition, such as immunological and oxidative states.

Breeding latitude predicts timing but not rate of spring migration in a widespread migratory bird in South America

Identifying the processes that determine avian migratory strategies in different environmental contexts is imperative to understanding the constraints to survival and reproduction faced by migratory birds across the planet.We compared the spring migration strategies of Fork-tailed Flycatchers (Tyrannus s. savana) that breed at south-temperate latitudes (i.e., austral migrants) vs. tropical latitudes (i.e., intratropical migrants) in South America. We hypothesized that austral migrant flycatchers are more time-selected than intratropical migrants during spring migration. As such, we predicted that austral migrants, which migrate further than intratropical migrants, will migrate at a faster rate and that the rate of migration for austral migrants will be positively correlated with the onset of spring migration.We attached light-level geolocators to Fork-tailed Flycatchers at two tropical breeding sites in Brazil and at two south-temperate breeding sites in Argentina and tracked their movements until the following breeding season.Of 286 geolocators that were deployed, 37 were recovered ~1 year later, of which 28 provided useable data. Rate of spring migration did not differ significantly between the two groups, and only at one site was there a significantly positive relationship between date of initiation of spring migration and arrival date.This represents the first comparison of individual migratory strategies among conspecific passerines breeding at tropical vs. temperate latitudes and suggests that austral migrant Fork-tailed Flycatchers in South America are not more time-selected on spring migration than intratropical migrant conspecifics. Low sample sizes could have diminished our power to detect differences (e.g., between sexes), such that further research into the mechanisms underpinning migratory strategies in this poorly understood system is necessary.

Extreme climatic variability during migration invokes physiological and dietary plasticity among spring migrating ducks

Environmental stochasticity encountered during migration can have negative consequences for individuals and population demographics through direct reductions in survival or cross-seasonal impacts. We took advantage of substantial interannual variation in spring migration conditions over a 4 year field study to examine physiological and dietary variation among two species of migrant ducks. We collected female Lesser Scaup (Aythya affinis (Eyton, 1838)) and Blue-winged Teal (Spatula discors (Linnaeus, 1766)) during spring migration and measured lipid and protein reserves, an index of recent lipid metabolism based on concentrations of lipid metabolites in plasma, and diets. We documented systematic interannual variation among these metrics in both species, contrasting primarily the warmest, earliest spring and the coldest, latest spring. Lesser Scaup had reduced lipid and protein reserves and consumed less energy-rich prey during the coldest and latest spring but showed no interannual variation in the index of lipid metabolism. Blue-winged Teal similarly had reduced protein reserves in the cold, late spring but maintained constant lipid reserves among years, likely facilitated by increased consumption of energy-rich seeds reflected in diets and lipid metabolism. Our results reveal impacts of environmental stochasticity on migrants and suggest that recruitment may be impacted by variable conditions encountered during migration during extreme weather events.

The start of migration correlates with arrival timing, and the total speed of migration increases with migration distance in migratory songbirds: a cross-continental analysis

BACKGROUND

Anthropogenic changes in the climate and environment have globally affected ecological processes such that the spatiotemporal occurrence of the main annual cycle events (i.e., breeding, wintering, moulting, and migration) has shifted in migratory birds. Variation in arrival timing at migratory destinations can be proximately caused by an altered start of migration, total migration distance, and/or total speed of migration. Quantifying the relative contributions of these causes is important because this will indicate the mechanisms whereby birds could potentially adjust their annual cycle in response to global change. However, we have relatively little quantitative information about how each of these factors contributes to variation in arrival timing. My main aims are to estimate how arrival timing is correlated with variation in the start of migration and the total migration distance and how the total speed of migration may change with the total migration distance and body mass in a comprehensive analysis including multiple species.

METHODS

For this purpose, I considered individual tracks covering complete migrations from multiple species and distinguished between within- and between-species effects.

RESULTS

Assuming that the within- and between-species effects quantified under this approach agree with the effects acting at the individual level, starting migration one day later or increasing the total migration distance by 1000 km would result in later arrival timing by 0.4-0.8 days or 2-5 days, respectively. The generality with which the start of migration is correlated with arrival timing within species suggests that this is the general biological mechanism regulating arrival timing, rather than the total migration distance. The total speed of migration was positively correlated with the total migration distance but not with the bird's body mass.

CONCLUSIONS

As the start of migration is endogenously controlled and/or affected by hatching date, directional selection can probably act on existing within-species/within-population variation to alter arrival timing. This factor and the importance of variation in the start of migration for arrival timing suggest that migratory species/populations in which there is sufficient variation in the start of migration and transgenerational processes affect the corresponding timing may present an advantage over others in coping with anthropogenic-induced global changes.

Genomes to space stations: the need for the integrative study of migration for avian conservation

Ongoing changes to global weather patterns and human modifications of the environment have altered the breeding and non-breeding ranges of migratory species, the timing of their migrations, and even whether they continue to migrate at all. Animal movements are arguably one of the most difficult behaviours to study, particularly in smaller birds that migrate tens to thousands of kilometres seasonally, often moving hundreds of kilometres each day. The recent miniaturization of tracking and logging devices has led to a radical transformation in our understanding of avian migratory behaviour and migratory connectivity. While advances in technology have altered the way researchers study migratory behaviour in the field, advances in techniques related to the study of physiological and genetic mechanisms underlying migratory behaviour have rarely been integrated into field studies of tracking. To predict the capacity of migrants to adjust to a changing planet, it is essential that we combine avian migration data with physiological and genetic measurements taken at key time points prior to, during and after migration.

Immune function and blood parasite infections impact stopover ecology in passerine birds

Stopovers play a crucial role for the success of migrating animals and are key to optimal migration theory. Variation in refuelling rates, stopover duration and departure decisions among individuals has been related to several external factors. The physiological mechanisms shaping stopover ecology are, however, less well understood. Here, we explore how immune function and blood parasite infections relate to several aspects of stopover behaviour in autumn migrating short- and long-distance migrating songbirds. We blood sampled individuals of six species and used an automated radio-telemetry system in the stopover area to subsequently quantify stopover duration, 'bush-level' activity patterns (~ 0.1-30 m), landscape movements (~ 30-6000 m), departure direction and departure time. We show that complement activity, the acute phase protein haptoglobin and blood parasite infections were related to prolonged stopover duration. Complement activity (i.e., lysis) and total immunoglobulins were negatively correlated with bush-level activity patterns. The differences partly depended on whether birds were long-distance or short-distance migrants. Birds infected with avian malaria-like parasites showed longer landscape movements during the stopover than uninfected individuals, and birds with double blood parasite infections departed more than 2.5 h later after sunset/sunrise suggesting shorter flight bouts. We conclude that variation in baseline immune function and blood parasite infection status affects stopover ecology and helps explain individual variation in stopover behaviour. These differences affect overall migration speed, and thus can have significant impact on migration success and induce carry-over effects on other annual-cycle stages. Immune function and blood parasites should, therefore, be considered as important factors when applying optimal migration theory.

A mimicked bacterial infection prolongs stopover duration in songbirds-but more pronounced in short- than long-distance migrants

Migration usually consists of intermittent travel and stopovers, the latter being crucially important for individuals to recover and refuel to successfully complete migration. Quantifying how sickness behaviours influence stopovers is crucial for our understanding of migration ecology and how diseases spread. However, little is known about infections in songbirds, which constitute the majority of avian migrants. We experimentally immune-challenged autumn migrating passerines (both short- and long-distance migrating species) with a simulated bacterial infection. Using an automated radiotelemetry system in the stopover area, we subsequently quantified stopover duration, "bush-level" activity patterns (0.1-30 m) and landscape movements (30-6,000 m). We show that compared to controls, immune-challenged birds prolonged their stopover duration by on average 1.2 days in long-distance and 2.9 days in short-distance migrants, respectively (100%-126% longer than controls, respectively). During the prolonged stopover, the immune-challenged birds kept a high "bush-level" activity (which was unexpected) but reduced their local movements, independent of migration strategy. Baseline immune function, but not blood parasite infections prior to the immune challenge, had a prolonging effect on stopover duration, particularly in long-distance migrants. We conclude that a mimicked bacterial infection does not cause lethargy, per se, but restricts landscape movements and prolongs stopover duration, and that this behavioural response also depends on the status of baseline immune function and migration strategy. This adds a new level to the understanding of how acute inflammation affect migration behaviour and hence the ecology and evolution of migration. Accounting for these effects of bacterial infections will also enable us to fine-tune and apply optimal migration theory. Finally, it will help us predicting how migrating animals may respond to increased pathogen pressure caused by global change.

Cues, corticosterone and departure decisions in a partial migrant

Most migrating birds make multiple stopovers to fuel and/or rest. The decision to resume migration from stopover is based on various cues, such as time within the season and wind conditions. There are hints that the strength of these departure cues shapes corticosterone level, which in its turn appears to regulate the timing of departure. We here provide results that very strongly indicate that indeed departure cues jointly shape corticosterone level of migrants at stopover. We compared corticosterone level between migrating and sedentary common blackbirds (Turdus merula) sampled simultaneously at the same location during autumn migration. As expected, in migrating individuals corticosterone level was positively associated with time within the season and with current wind conditions. The latter was only apparent in adult birds and not in 1st year migrants, thus matching the observation that 1st year autumnal migrants are less wind selective than adults. In contrast to the migrants, in sedentary blackbirds these "cues" did not explain variation in corticosterone level. Furthermore, stopover departure seemed more likely and to occur earlier in the night in migrants with high corticosterone level. Our unique comparative study thus supports the newly developed concept that corticosterone mediates between departure cues and stopover departure timing in avian migrants.

Nocturnal departure timing in songbirds facing distinct migratory challenges

Most migratory songbirds travel between their breeding areas and wintering grounds through a series of nocturnal flights. The timing of their departures defines the potential flight duration and thus the distance covered during a migratory night. Yet, migratory songbirds show substantial variation in their nocturnal departure timing. With this study, we aim to assess whether the respective challenges of the migration route, namely its distance and nature, help to explain this variation. At a stopover site, we caught Northern Wheatears (Oenanthe oenanthe) of two subspecies that differ in distance and nature of their onward migration route in spring, but not in autumn. We determined the start of their nocturnal migratory restlessness during short-term captivity, and radiotracked their nocturnal departure timing after release in both migration seasons. Northern Wheatears started their nocturnal migratory restlessness earlier when facing a long remaining migration distance and an extended sea barrier in spring. Individual departure directions generally affected the nocturnal departure timing with early departures being directed towards the respective migratory destination. In spring, this pattern was predominantly found in birds carrying relatively large fuel stores, but was absent in lean birds. At the same time, birds facing a short remaining migration distance and no extended sea barrier strongly reacted to relatively large fuel stores by an early start of nocturnal migratory behaviour (migratory restlessness and departure timing), whereas this reaction was not found in birds facing a long remaining migration distance and sea barrier. These results suggest that the basic diel schedule of birds' migratory activity is adapted to the onward migration route. Further, they suggest that birds adjust their behavioural response, that is start of nocturnal migratory behaviour, to fuel stores in relation to their impending migratory challenges. This is a substantial step in understanding variation of nocturnal departure timing and its adjustments in migratory songbirds. Further, it emphasizes the importance of interpreting birds' nocturnal migratory behaviour in the respective ecological context.

Assessing seasonal changes in animal diets with stable-isotope analysis of amino acids: a migratory boreal songbird switches diet over its annual cycle

Tools to study seasonal changes in animal diets are needed to address a wide range of ecological questions. This is especially true of migratory animals that experience distinct environments where diets may be substantially different. However, tracking diets of individuals that move vast distances has proven difficult. Compound-specific isotope analysis has emerged as a valuable tool to study diets but has been little used to study dietary changes of migratory animals. Using this technique, we quantify seasonal variation in the annual diet of a migratory songbird (gray-cheeked thrush, Catharus minimus) and test the hypothesis that migrants change their diet in response to the energetic requirements of different periods of the annual cycle. By measuring δ¹³C and δ¹⁵N values of amino acids from feathers grown on the breeding grounds, blood formed during migration and claw grown on the wintering grounds, we found that migration is associated with greater consumption of fruit, compared to the breeding or wintering periods. This was confirmed by the lower trophic position of blood compared to feather and claw, by a decrease in the δ¹⁵N value of the source amino acid phenylalanine in blood as a function of days of stopover, and by the positive correlation between δ¹⁵N and δ¹³C values of phenylalanine in blood, and not in feather or claw. This study illustrates how isotopic analysis of amino acids can contribute to understand food webs, seasonal dietary changes and metabolic routing of nutrients in migratory animals.

Proximate mechanisms affecting seasonal differences in migration speed of avian species

Faster migration in spring than in autumn seems to be a common pattern in birds. This has been ultimately explained by seasonally different selection pressures. Variation in migration speed is proximately caused by adjusting travel speed (distance covered during flight) and/or stopover duration (times when birds rest and refuel). Yet, it remains unclear whether individual seasonal differences in migration speed match the common pattern and what the precise role of the proximate, behavioural mechanisms for adjusting migration speed is. By reviewing 64 studies of 401 tracks, I show that in waders, gulls, swifts, and songbirds speeds were significantly higher in spring, while the opposite was the case in waterfowl and owls. Thus, the ultimate mechanisms selecting for a faster migration in spring might not consistently act across bird groups. Breeding latitude, migration strategy, migration distance, flight style, body mass, and sex did not explain seasonal differences in speed. The ratio between spring and autumn total stopover duration of 257 bird tracks significantly negatively affected the seasonal migration speed ratio of the same individuals in a comparative analysis accounting for shared ancestry. Seasonal variation in stopover duration appears thus to be the main biological mechanism regulating seasonal differences in migration speed.