Juvenile Barn Swallows Hirundo rustica L. from late broods start autumn migration younger,fuel less effectively and show lower return rates than juveniles from early broods

Long‐distance migratory passerines may exhibit different post‐fledging rearing times between their early and late broods due to limited time and energy resources. This could affect the migratory fuelling behaviour, condition and fitness of juvenile birds. We analysed long‐term (13‐year) biometric data (body mass, fat score) on Barn Swallows ringed at the nest and later recaptured during their initial stages of autumn migration. The return rates of early‐ and late‐born juveniles after overwintering in Africa were further compared to estimate their fitness. We found that juveniles from late broods started migrating south on average 11 days younger compared with juveniles from early broods. Body masses of nestlings did not differ between early and late broods, but juveniles from late broods were in better condition after leaving the nest, possibly due to a longer period of parental care. Having started migration, juveniles from late broods were, however, less efficient in accumulating energy than juveniles from early broods. A younger departure age, together with less efficient fuelling of late brood juveniles, may partly explain the 39% lower return probability and hence lower fitness value of late‐brood juveniles. Our study is the first to show that juveniles from the early and late broods exhibit different fuelling behaviour and departure strategies (in terms of age and timing), which may be reflected in the fitness value of offspring.     

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.     

Sex‐specific patterns of fuelling and pre‐breeding body moult of Little Stints Calidris minuta in South Africa

The timing and duration of each stage of the life of a long‐distance migrant bird are constrained by time and resources. If the parental roles of males and females differ, the timing of other life stages, such as moult or pre‐migratory fuelling, may also differ between the sexes. Little is known about sexual differences for species with weak sexual dimorphism, but DNA‐sexing enables fresh insights. The Little Stint Calidris minuta is a monomorphic long‐distance migrant wader breeding in the Arctic tundra. Males compete for territories and perform elaborate aerial displays. Females produce two clutches a season. Each sex may be a bigamist and incubate one nest a season, each with a different partner. We expect that these differences in breeding behaviour entail different preparations for breeding by males and females, so we aimed to determine whether Little Stints showed any sex differences in their strategies for pre‐breeding moult and pre‐migratory fuelling at their non‐breeding grounds in South Africa. We used body moult records, wing length and body mass of 241 DNA‐sexed Little Stints that we caught and ringed between 27 January and 29 April in 2008–2018 at two neighbouring wetlands in North West Province, South Africa. For each individual we assessed the percentage of breeding plumage on its upperparts and took blood samples for DNA‐sexing. We calculated an adjusted Body Moult Index and an adjusted Wing Coverts Moult Index, then used the Underhill–Zucchini moult model to estimate the start dates and the rate of body moult in males and females. We estimated the changes in the sex ratio of the local population during their stay in South Africa, and also estimated the timing and rate of pre‐migratory fuelling and the potential flight ranges for males and females. The males started body moult on average on 7 February and the females on 12 February, but the sexes did not differ in their timing of wing covert moult, which started on average on 10 February. In January to mid‐February, males constituted c. 57% of the population, but their proportion declined afterwards, indicating an earlier departure than females. We estimated that both sexes began pre‐migratory fuelling on average on 15 March. The sexes did not differ in fuelling rate, but most females stayed at the non‐breeding site longer than the males, and thus accumulated more fuel and had longer potential flight ranges. These patterns of moult and fuelling suggest sex differences in preparations for breeding. We suggest that the males depart from South Africa earlier but with smaller fuel loads than the females to establish breeding territories before the females arrive. We conclude that for each sex the observed trade‐offs between fuelling and moult at the non‐breeding grounds are precursors to different migration strategies, which in turn are adaptations for their different roles in reproductive behaviour.     

Age‐specific variation in relationship between moult and pre‐migratory fuelling in Wood Sandpipers Tringa glareola in southern Africa

Trans‐equatorial avian migrants tend to breed, moult and migrate – the main energy‐requiring events in their lifecycle – at different times. Little is known about the relationship between wing moult and pre‐migratory fuelling in waders on their non‐breeding grounds, where time is less constrained than during their brief high‐latitude breeding season. We determined age‐related strategies of Wood Sandpipers Tringa glareola to balance the energetic demands of primary moult against pre‐migratory fuelling in southern Africa by analysing body mass and primary moult at first capture of 1721 birds mist‐netted in 1972–96 at waterbodies in Zimbabwe. Adults moulted all their primaries in August–December, but immatures underwent a supplemental moult of varying numbers of outer primaries in December–April, close to departure. We used locally weighted linear regression to estimate trends in Wood Sandpiper body mass from 1 July to 1 May. They maintained low mass from arrival in July–September to February–early March. Adults fuelled from 10 February to 1 May at a mean rate of 0.25 g/day (sd = 0.16). Most adults (98%) began fuelling 10–75 days after completing primary moult. Immatures fuelled from 4 March to 13 April at 0.24 g/day (sd = 0.14). They used varying strategies depending on their condition: a brief gap between moult and fuelling; an overlap of these processes near departure, leading to slower fuelling; or skipping fuelling altogether and staying in southern Africa for a ‘gap year’. Immatures moulting three or five outer primaries fuelled more slowly than post‐moult birds. Immatures moulting four outer primaries started fuelling 3 weeks later but at a higher rate than did post‐moult birds of this group. In post‐moult immatures, the later they ended moult, the later and faster they fuelled. The heaviest adults and immatures using all moult patterns accumulated fuel loads of c. 50% of lean body mass, and could potentially cross 2397–4490 km to reach the Great Rift Valley in one non‐stop flight. Immatures were more flexible in the timing and extent of moult and in the timing and rate of fuelling than adults. This flexibility enables inexperienced Wood Sandpipers to cope with inter‐annual differences in feeding conditions at Africa's ephemeral inland waterbodies.     

Fuelling decisions in migratory birds: geomagnetic cues override the seasonal effect

Recent evaluations of both temporal and spatial precision in bird migration have called for external cues in addition to the inherited programme defining the migratory journey in terms of direction, distance and fuelling behaviour along the route. We used juvenile European robins (Erithacus rubecula) to study whether geomagnetic cues affect fuel deposition in a medium-distance migrant by simulating a migratory journey from southeast Sweden to the wintering area in southern Spain. In the late phase of the onset of autumn migration, robins exposed to the magnetic treatment attained a lower fuel load than control birds exposed to the ambient magnetic field of southeast Sweden. In contrast, robins captured in the early phase of the onset of autumn migration all showed low fuel deposition irrespective of experimental treatment. These results are, as expected, the inverse of what we have found in similar studies in a long-distance migrant, the thrush nightingale (Luscinia luscinia), indicating that the reaction in terms of fuelling behaviour to a simulated southward migration varies depending on the relevance for the species. Furthermore, we suggest that information from the geomagnetic field act as an important external cue overriding the seasonal effect on fuelling behaviour in migratory birds.     

Characterization of Aspergillus species based on fatty acid profiles

Cellular fatty acid (FA) composition was utilized as a taxonomic tool to discriminate between different Aspergillus species. Several of the tested species had the same FA composition and different relative FA concentrations. The most important FAs were palmitic acid (C16:0), estearic acid (C18:0), oleic acid (C18:1) and linoleic acid (C18:2), which represented 95% of Aspergillus FAs. Multivariate data analysis demonstrated that FA analysis is a useful tool for differentiating species belonging to genus Aspergillus. All the species analyzed showed significantly FA acid profiles (p < 0.001). Furthermore, it will be possible to distinguish among Aspergillus spp. in the Flavi Section. FA composition can serve as a useful tool for the identification of filamentous fungi.     

Inter-and intraspecific differences in fatty acid composition of freshwater crustacean zooplankton

1. The composition of fatty acids (FA) from C14 to C18 was measured for edible seston and for individual Daphnia galeata , Diaphanosoma brachyurum and Eodiaptomus japonicus from Lake Biwa using a pyrolysis–gas chromatography (Py–GC) method. Based on the relative abundance of the FA, inter-and intraspecific differences in composition were examined.
2. Among the FA, C16 : 0 was the most abundant, both in the three zooplankton species and in edible seston smaller than 20 μm, suggesting that the composition of the zooplankton was roughly reflected by their diet. However, the abundance of C18 : 1 relative to C16 : 1 and C18 : 0 was much higher in each zooplankton species than in the diet.
3. Comparison of the relative abundance of FA among the three zooplankton species revealed that intraspecific differences in FA composition are greater than interspecific differences. These results indicate that variability in FA composition is not necessarily species-specific, and can be obscured by variation in composition between individuals.     

Magnetic cues and time of season affect fuel deposition in migratory thrush nightingales (Luscinia luscinia)

Bird migration requires high energy expenditure, and long-distance migrants accumulate fat for use as fuel during stopovers throughout their journey. Recent studies have shown that long-distance migratory birds, besides accumulating fat for use as fuel, also show adaptive phenotypic flexibility in several organs during migration. The migratory routes of many songbirds include stretches of sea and desert where fuelling is not possible. Large fuel loads increase flight costs and predation risk, therefore extensive fuelling should occur only immediately prior to crossing inhospitable zones. However, despite their crucial importance for the survival of migratory birds, both strategic refuelling decisions and variation in phenotypic flexibility during migration are not well understood. First-year thrush nightingales (Luscinia luscinia) caught in the early phase of the onset of autumn migration in southeast Sweden and exposed to a magnetic treatment simulating a migratory flight to northern Egypt increased more in fuel load than control birds. By contrast, birds trapped during the late phase of the onset of autumn migration accumulated a high fuel load irrespective of magnetic treatment. Furthermore, early birds increased less in flight-muscle size than birds trapped later in autumn. We suggest that the relative importance of endogenous and environmental factors in individual birds is affected by the time of season and by geographical area. When approaching a barrier, environmental cues may act irrespective of the endogenous time programme.     

Geomagnetic information modulates nocturnal migratory restlessness but not fueling in a long distance migratory songbird

Geomagnetic cues have been shown to influence migratory orientation and migratory fuelling in night‐migratory songbird species. Here, we used captive‐bred northern wheatears Oenanthe oenanthe from the southern Norwegian population to show that other aspects of the birds’ migratory program can be influenced by magnetic cues as well. We observed that the amount of migratory restlessness increased strongly with progression of the migratory season when the birds were kept constantly in the magnetic field of northern Germany, but the amount of migratory restlessness decreased when the magnetic field changed along the birds’ natural flyway are simulated. Thus, the Earth's magnetic field can also act as a ‘signpost’ cue for fine‐tuning the spatio‐temporal course of migration.     

Variation in blubber fatty acid composition among marine mammals in the Canadian Arctic

The composition of predator adipose stores can provide important insights into foraging patterns and the ecological relationships among species. We determined the fatty acid (FA) composition of 843 blubber samples from 80 bearded seals (Erignathus barbatus), 33 harbor seals (Phoca vitulina), 239 harp seals (Pagophilus groenlandicus), 32 hooded seals (Cystophora cristata), 281 ringed seals (Phoca hispida), 53 walruses (Odobenus rosmarus rosmarus), 105 beluga whales (Delphinapterus leucas), and 20 narwhals (Monodon monoceros) across the Canadian Arctic to examine patterns of variability among and within species. FA signatures accurately distinguished phocid seals, walruses, and whales. Belugas and narwhals had the most similar FA signatures of any two species, suggesting substantial overlap in their diets, especially in the narwhal‐wintering area off eastern Baffin Island. Among phocid seals, harp and hooded seals had the most similar FA signatures. Bearded seals were most similar to walruses, which was consistent with the benthic feeding habits of both species. Within species, geographic differences in FA signatures were found over both large (>4,000 km) and small (<100 km) spatial scales. Overall, within‐species differences were smaller than among‐species differences. In general, FA signature patterns were consistent with previous studies of the ecology and diets of arctic marine mammals.     

Wind tunnel as a tool in bird migration research

Wind tunnels, in which birds fly against an artificially generated air flow, have since long been used to evaluate aerodynamic properties of steady bird flight. A new generation of wind tunnels has also allowed the many processes associated with migratory flights to be studied in captivity. We review how wind tunnel studies of aerodynamics and migratory performance together have helped advancing our understanding of bird migration. Current migration theory is based on the power‐speed relationship of flight as well as flight range equations, both of which can be evaluated using birds flying in wind tunnels. In addition, and depending on wind tunnel properties, performance during gliding and climbing flight, and effects of air pressure, humidity and turbulence on bird flight has been measured. Long‐distance migrant species have been flown repeatedly for up to 16 h non‐stop, allowing detailed studies of the energy expenditure, fuel composition, protein turnover, water balance, immunocompetence and stress associated with sustained migratory flights. In addition, wind tunnels allow the fuelling periods between migratory flights to be studied from new angles. We end our review by suggesting several important topics for future wind tunnel studies, ranging from on of the key questions remaining, the efficiency at which chemical power in converted to mechanical power, to new useful avenues, such as improving and calibrating the techniques used for tracking of individual birds in the wild.     

Body size shapes inter‐specific migratory behaviour: evidence from individual tracks of long‐distance migratory shorebirds

Migration is a common phenomenon across many animal taxa. Understanding how migration scales with body size across species is fundamental in the development of migration theory and in making size‐related predictions. Although aerodynamic theory and ecophysiological scaling laws have assisted greatly in generating such predictions, their verifications have been limited by a lack of empirical data across a range of body sizes. The recent development of ultra‐light tracking devices and its rapid application to migration now allows us to put theory to the test. We used tracking data of seven closely related migratory sandpiper species (family Scolopacidae) along the East Asian‐Australasian Flyway to compare their migratory behaviour when migrating towards the breeding grounds as a function of size (50–750 g). We found a marked decline in migration speed (migration distance divided by total migration duration, including time at stopover sites and in flight) with size. Departure date from the non‐breeding (i.e. wintering) ground and arrival date at the breeding ground also scaled negatively with size. Total migration duration, migration distance, total staging duration (the number of days staying at stopover sites plus days preparing, i.e. fuelling, prior to initial migration) and step length (distance covered within one migratory leg) were not significantly related with size. Correction for phylogeny showed consistent results for all variables. Besides improving our fundamental understanding of inter‐specific variation in migration behaviours, the finding of a clear scaling with size in migration speed and migration timing highlight differential size related capabilities and constraints of migrants. Migratory birds, including sandpipers, are declining on a global scale and particularly along the East Asian‐Australasian Flyway. This notion of size‐dependency in migratory traits may have a bearing on their vulnerability to specific environmental disturbances along their flyways.     

Population assignment reveals low migratory connectivity in a weakly structured songbird

Understanding migratory connectivity is essential for determining the drivers behind population dynamics and for implementing effective conservation strategies for migratory species. Genetic markers provide a means to describe migratory connectivity; however, they can be uninformative for species with weak population genetic structure, which has limited their application. Here, we demonstrated a genomic approach to describing migratory connectivity in the prothonotary warbler, Protonotaria citrea, a Neotropical songbird of conservation concern. Using 26,189 single nucleotide polymorphisms (SNPs), we revealed regional genetic structure between the Mississippi River Valley and the Atlantic Seaboard with overall weak genetic differentiation among populations (F_ST = 0.0055; 95% CI: 0.0051–0.0059). Genetic variation had a stronger association with geographic rather than environmental factors, with each explaining 14.5% and 8.2% of genetic variation, respectively. By varying the numbers of genomic markers used in population assignment models with individuals of known provenance, we identified a maximum assignment accuracy (89.7% to site, 94.3% to region) using a subset of 600 highly differentiated SNPs. We then assigned samples from nonbreeding sites to breeding region and found low migratory connectivity. Our results highlight the importance of filtering markers for informative loci in models of population assignment. Quantifying migratory connectivity for weakly structured species will be useful for expanding studies to a wider range of migratory species across taxonomic groups and may contribute to a deeper understanding of the evolution of migratory strategies.     

Polyunsaturated fatty acids in zooplankton: variation due to taxonomy and trophic position

1. Food quality has major effects on the transfer of energy and matter in food webs, and essential long‐chained polyunsaturated fatty acids (PUFAs) can affect the quality of phytoplankton as food. In a study of oligotrophic lakes in north‐western Sweden, we investigated the fatty acid composition of four planktonic cladocerans and two calanoid copepods, representing herbivorous and carnivorous species. We also collected seston samples. 2. The proportions of long‐chain PUFAs in the organisms increased with their increasing trophic position. Thus, both their quality as food for other organisms, as well as their requirement for fatty acids (FAs), differed among taxa and depended on their trophic position. 3. We found taxon‐specific differences in the FA composition of zooplankton that were not related to sestonic FA composition. This implies that the variation in zooplankton FA composition is constrained by phylogenetic origin, life history characteristics, or both. 4. The cladoceran taxa contained 12–23% eicosapentaenoic acid (EPA) but only 0.9–2.1% docosahexaenoic acid (DHA) of the total FA content. In contrast, the calanoid copepods contained 7–11% EPA and 14–21% DHA. Thus, our results show that differences in the PUFA content among zooplankton species could have repercussions for both food web structure and function.     

Effects of predation danger on migration strategies of sandpipers

We examine the potential selective importance of predation danger on the evolution of migration strategies of arctic‐breeding calidrid sandpipers. Adult calidrids truncate parental care for reasons not obviously related to levels of food abundance on the breeding areas or at migratory stopover sites, suggesting that a different trade‐off occurs between providing additional care and adult survivorship. The southward migrations of adult western sandpipers precede those of migratory peregrine falcons by almost a month. By moving early and quickly, adults remain ahead of migrant falcons all the way to their non‐breeding areas, where they rapidly moult flight feathers. They complete the moult just as falcons arrive in late September–October. By migrating early, they avoid exposure to falcons when they are unusually vulnerable, due to the requirements for fuelling migratory flight and of wing feather moult. Juvenile western sandpipers migrate south just as falcon numbers start to increase, but do not moult flight feathers in their first winter. Pacific dunlin use an alternative strategy of remaining and moulting in Alaska after falcons depart, and migrating to their overwintering sites after migrants have passed. East of the Rocky Mountains, the southbound migration of falcons begins 4–6 weeks later. Southbound semipalmated sandpipers make extended migratory stopovers, but their lengths of stay shorten prior to falcon migration to the sites in September. Predation danger also may affect the evolution of migration routes. Southbound western sandpipers fly directly from Alaska to southern British Columbia, in contrast to the multi‐stage journey northward along the Alaska panhandle. We estimate that a direct flight would be more economical on northward migration, but may be avoided because it would expose sandpipers to higher mass‐dependent predation danger from migratory falcons, which travel north with sandpipers. By contrast, few raptors are present in Alaska during preparation for the southward flight. A temporal and spatial window of safety may also permit semipalmated sandpipers to become extremely vulnerable while preparing for trans‐Atlantic southward flights. Danger management may account for the these previously enigmatic features of calidrid migration strategies, and aspects of those of other birds.     

Eco-evolutionary drivers of avian migratory connectivity

Migratory connectivity, reflecting the extent by which migrants tend to maintain their reciprocal positions in seasonal ranges, can assist in the conservation and management of mobile species, yet relevant drivers remain unclear. Taking advantage of an exceptionally large (~150,000 individuals, 83 species) and more-than-a-century-long dataset of bird ringing encounters, we investigated eco-evolutionary drivers of migratory connectivity in both short- and long-distance Afro-Palearctic migratory birds. Connectivity was strongly associated with geographical proxies of migration costs and was weakly influenced by biological traits and phylogeny, suggesting the evolutionary lability of migratory behaviour. The large intraspecific variability in avian migration strategies, through which most species geographically split into distinct migratory populations, explained why most of them were significantly connected. By unravelling key determinants of migratory connectivity, our study improves knowledge about the resilience of avian migrants to ecological perturbations, providing a critical tool to inform transboundary conservation and management strategies at the population level.     

Variation in energy intake and basal metabolic rate of a bird migrating in a wind tunnel

1. We studied the changes in body mass, metabolizable energy intake rate (ME) and basal metabolic rate (BMR) of a Thrush Nightingale, Luscinia luscinia , following repeated 12-h migratory flights in a wind tunnel. In total the bird flew for 176 h corresponding to 6300 km. This is the first study where the fuelling phase has been investigated in a bird migrating in captivity.
2. ME was very high, supporting earlier findings that migrating birds have among the highest intake rates known among homeotherms. ME was significantly higher the second day of fuelling, indicating a build-up of the capacity of the digestive tract during the first day of fuelling.
3. Further indications of an increase in size or activity level of metabolically active structures during fuelling come from the short-term variation in BMR, which increased over the 2-day fuelling period with more than 20%, and in almost direct proportion to body mass. However, mass-specific BMR decreased over the season.
4. The patterns of mass change, ME and BMR of our focal bird following two occasions of 12-h fasts were the same as after flights, indicating that fast and flight may involve similar physiological processes.
5. The relatively low ME the first day following a flight may be a contributing factor to the well-known pattern that migrating birds during stopover normally lose mass the first day of fuelling.     

Surface trawl survey for U.S. origin Atlantic salmon Salmo salar

A surface trawl survey (2001-2005) in Penobscot Bay, Maine, U.S.A., and the nearshore Gulf of Maine waters was conducted to investigate early marine dynamics of a hatchery-dependent Atlantic salmon Salmo salar population from a severely modified river system. Data generated were used to evaluate the effect of stocking location and time on migration success and to describe the migratory pathways and environments that post-smolts traverse. Significant differences in early migration success were detected among different stocking groups, but subsequent marine survival was independent of stocking group. While the post-smolt population was primarily composed of hatchery origin smolt-stocked fish, other life-stage stocking strategies (i.e. parr stocking) represented a higher proportion of the population than previously assumed. Catch distribution suggests evidence of an initial marine migratory pathway out of the dynamic Penobscot Bay environment. The hypothesized benefits of a predator refuge based on the co-occurring species complex is considered minimal for emigrating post-smolts given a mismatch in the size overlap among species and low abundance of other co-occurring diadromous populations. These data can be used to modify current management actions to optimize S. salar recovery and inform future research agendas.     

Temporal and spatial distributions of the fish larval assemblages of the Ivinheima River sub-basin (Brazil)

Fish, like other animals, choose environments for reproduction that are favorable for their offspring’s initial development. For example, these environments may be chosen to provide shelter and food. This study examined the fish larvae inhabiting the Ivinheima River sub-basin. The study aimed to (i) characterize the taxonomic composition of the larval fish fauna, (ii) analyze the structure of the larval assemblage, and (iii) verify the relationship between the larval assemblages and environmental variables. The sampling areas included three environments: the Ivinheima River, the Finado Raimundo Lagoon and the Patos Lagoon. Sampling was performed between October 2002 and March 2006 during four reproductive periods (RPs). The data were analyzed with correspondence analysis (CA) and canonical correlation analysis (CCA). A total of 120 619 larvae were collected. These larvae belonged to several species characterized by different reproductive strategies. Our results showed differences between the composition of the Ivinheima River’s assemblages, where there were more larvae of migratory fish, and the lagoons, where larvae of sedentary species were predominant. In addition, differences were found among the RPs evaluated in the Ivinheima River: the first and second RPs were characterized by milder floods and by a greater abundance of migratory piscivore larvae, whereas the third and fourth RPs had more outflow and a greater abundance of larvae of detritivore and omnivore species. We conclude that the Ivinheima River plays a significant role in the maintenance of native migratory fish stocks in the upper Paraná River basin and that the environment shows a segregation of the spatial and temporal composition and abundance of larvae related to running and flooding/standing water.     

What determines probability of surviving predator attacks in bird migration?: the relative importance of vigilance and fuel load

Migrating birds must accumulate fuel during their journeys and this fuel load should incur an increased risk of predation. Migratory fuelling should increase individual mass-dependent predation risk for two reasons. First, acquisition costs are connected to the increased time a bird must spend foraging to accumulate the fuel loads and the reduced predator detection that accompanies foraging. Second, birds with large fuel loads have been shown to suffer from impaired predator evasion which makes them more vulnerable when actually attacked. Here, I investigate the relative importance of these two aspects of mass-dependent predation risk and I have used published data and a hypothetical situation for a foraging bird to investigate how much migratory fuelling in terms of escape performance and natural variation in predator detection contribute to individual risk during foraging. Results suggest that for birds foraging close to protective cover the negative impact of fuel load on flight performance is very small, whereas variation in time to predator detection is of great importance for a bird's survival. However, the importance of flight performance for predation risk increases as the distance to cover increases. Hence, variation in predator detection (and vigilance) probably influences individual survival much more than migratory fuel load and consequently, to understand risk management during migration studies that focus on vigilance and predator detection during fuelling are much needed.