Blood parameter changes during stopover in a long-distance migratory shorebird, the bar-tailed godwit Limosa lapponica taymyrensis

Bar-tailed godwits migrate from West African wintering sites to breeding areas in northern Russia with only one stopover. We compared hematocrit (Hct), blood hemoglobin concentration (Hb), and mean cell hemoglobin concentration (MCHb; a measure of the relative proportion of Hb in the cellular blood fraction) between arriving godwits lured to land 60 km short of the stopover site and godwits during subsequent refueling. The Hct and Hb of arriving godwits was low when compared to that of refueling birds. On the stopover site, Hct and Hb correlated positively with size-corrected body mass. In addition, Hb and MCHb reached peak levels in the last days of stopover. We explored the possibility of regenerative anemia in arriving godwits by comparing the fraction of reticulocytes (young red blood cells) between arriving and refueling birds. No differences were found. Therefore, we suggest that the increase in Hct, Hb, and MCHb during refueling is not in response to a severe anemic state at arrival. Rather, we suggest that the increase in blood parameters may anticipate the increased aerobic requirements of impending migratory flight and possibly satisfy heightened oxygen demands of the larger body mass of fattened birds. The Hct increase on the stopover site may also serve to buffer the red blood cell population against possible red blood cell breakdown during long-distance flight.     

Corticosterone,food intake and refueling in a long-distance migrant

Elevated baseline corticosterone levels function to mobilize energy in predictable life-history stages, such as bird migration. At the same time, baseline corticosterone has a permissive effect on the accumulation of fat stores (fueling) needed for migratory flight. Most migrants alternate flight bouts with stopovers, during which they replenish the fuel used during the preceding flight (refueling). The role of corticosterone in refueling is currently unclear. In a fasting–re-feeding experiment on northern wheatears (Oenanthe oenanthe) in autumn we found that baseline total and free corticosterone levels were negatively related with both food intake and the rate of fuel deposition after fasting. This confirms our earlier findings in wild conspecifics in spring and indicates that corticosterone does not stimulate stopover refueling. Whether the negative relationship between baseline corticosterone level and fuel deposition rate is causal is questionable, because within-individual comparison of corticosterone metabolite levels in droppings did not reveal differences between refueling and control periods. In other words, corticosterone does not appear to be down-regulated during refueling, which would be expected if it directly hampers refueling. We discuss possible correlates of corticosterone level that may explain the negative association between corticosterone and stopover refueling. Additionally, we found that fasting decreases total corticosterone level, which contrasts with previous studies. We propose that the difference is due to the other studies being conducted outside of the migration life-history stage, and provide a possible explanation for the decrease in corticosterone during fasting in migrating birds.     

Corticosterone in migrating songbirds during endurance flight

The specific role of the glucocorticoid hormone corticosterone in regulating the migratory stages of flight and refueling in free-living migrants is as yet poorly studied, because these stages are difficult to identify in the field. Night-migrating songbirds provide an excellent model to investigate how corticosterone correlates with behavior and physiology because they fly during the night and rest and forage during the day. We measured baseline corticosterone and the adrenocortical response to restraint in 9 free-ranging songbird species: 3 night-migrating species, 3 day-migrating species, and 3 day-migrating irruptive species. Baseline corticosterone of night migrants was higher in birds caught out of nocturnal migration than in birds resting and foraging, and on the same level as in day migrants, suggesting that a rise in circulating corticosterone may facilitate the heightened metabolic processes of active flight, in particular protein breakdown. Stress-induced corticosterone levels increased in both actively flying birds and birds resting and foraging. The increase was highest in landing birds, which are possibly most sensitive to stress when arriving at an unfamiliar place. Migratory endurance flight is thus characterized by corticosterone concentrations that are lower than those associated with acute stressful and life-threatening episodes. In addition, the responsiveness to stress increased with decreasing fat score in a night-migrating species. Corticosterone approaches therefore stressful concentrations only when fat depots are nearly depleted, possibly to promote protein catabolism and to trigger a change in behavior, i.e., a switch to landing and searching for food.     

Corticosterone in migrating songbirds during endurance flight

The specific role of the glucocorticoid hormone corticosterone in regulating the migratory stages of flight and refueling in free-living migrants is as yet poorly studied, because these stages are difficult to identify in the field. Night-migrating songbirds provide an excellent model to investigate how corticosterone correlates with behavior and physiology because they fly during the night and rest and forage during the day. We measured baseline corticosterone and the adrenocortical response to restraint in 9 free-ranging songbird species: 3 night-migrating species, 3 day-migrating species, and 3 day-migrating irruptive species. Baseline corticosterone of night migrants was higher in birds caught out of nocturnal migration than in birds resting and foraging, and on the same level as in day migrants, suggesting that a rise in circulating corticosterone may facilitate the heightened metabolic processes of active flight, in particular protein breakdown. Stress-induced corticosterone levels increased in both actively flying birds and birds resting and foraging. The increase was highest in landing birds, which are possibly most sensitive to stress when arriving at an unfamiliar place. Migratory endurance flight is thus characterized by corticosterone concentrations that are lower than those associated with acute stressful and life-threatening episodes. In addition, the responsiveness to stress increased with decreasing fat score in a night-migrating species. Corticosterone approaches therefore stressful concentrations only when fat depots are nearly depleted, possibly to promote protein catabolism and to trigger a change in behavior, i.e., a switch to landing and searching for food.     

Plasma corticosterone increases during migratory restlessness in the captive white-crowned sparrow Zonotrichia leucophrys gambelli

Plasma corticosterone increases during the period of spring migration in a variety of bird species. Long-distance migrants show elevations in corticosterone specifically in association with the stage of flight, suggesting that corticosterone may support flight-related processes, for example, locomotor activity and/or energy mobilization. The pattern of corticosterone secretion as it relates to migratory flight has hitherto not been clearly described in migrants that frequently interrupt flight to refuel, for example, the Gambel's white-crowned sparrow (Zonotrichia leucophrys gambelii). The Gambel's white-crowned sparrow fuels by day and expresses peak migratory activity during the first few hours of night. To determine if plasma corticosterone increases in association with the stage of migratory flight also in this short-bout migrant, we induced captive white-crowned sparrows to enter into the migratory condition by placing photosensitive birds on long days (16L:8D) and then evaluated birds for plasma corticosterone and locomotor activity during four time points of the day. Patterns found in long-day birds were compared to those observed in short-day controls (8L:16D). Differences in energy metabolism as determined from plasma metabolites were also evaluated. We found that locomotor activity and corticosterone were significantly elevated at the onset of the dark period, but only in long-day birds. Plasma beta-hydroxybutyrate (a ketone body) was also elevated. Thus, findings suggest that plasma corticosterone and ketogenesis increase in association with migratory restlessness in a short-bout migrant. In fact, corticosterone may play a regulatory role, because it shows a trend to increase already before night-time activity.     

Breeding plumage honestly signals likelihood of tapeworm infestation in females of a long-distance migrating shorebird, the bar-tailed godwit

The indicator mechanism for sexual selection proposed by Hamilton and Zuk (i.e. that sexually selected ornaments signal parasite resistance) has received rather little observational support, and none in the case of long-distance migrant birds. Here we present a test by examining the association between helminth infestations and breeding plumage quality in bar-tailed godwits Limosa lapponica taymyrensis during their spring staging period in the Wadden Sea, The Netherlands. After a non-stop flight from West Africa, these shorebirds refuel in the Wadden Sea in preparation for a second flight to the central Siberian Arctic breeding grounds. Earlier studies have shown that only relatively heavy and well ornamented birds carry out a "top-up" moult during stopover, in which part of the contour feathers recently grown in West Africa are replaced by even fresher ones. Active body moult was therefore taken as the primary indicator of ornament quality. Of 78 birds collected between 1992 and 1997, 42% carried helminths, including four species of digenean trematodes (flukes), three species of cestodes (tapeworms) and an acanthocephalan (spiny-headed worm). Faecal samples examined for helminth eggs in another 92 birds in 1998 and 2000 showed similar rates of infestation. Actively moulting bar-tailed godwits were confirmed to be heavier and to show more extensive breeding plumage than non-moulting birds. In females, but not in males, active moult was associated with fewer cestodes and acanthocephalans. Also, breeding plumage and presence of cestodes were negatively associated in females. We argue that the quality of the breeding plumage reliably indicates parasite resistance in female godwits. The repeatability of plumage scores of females between years is consistent with such resistance having a heritable component. In contrast, male ornaments may demonstrate other qualities, e.g. an ability to combine adequate fuelling and flight performances with moult during the time-stress of migration.     

Spring migration strategies of two populations of bar-tailed godwits,Limosa lapponica,in the Wadden Sea: time minimizers or energy minimizers?

Birds can optimize their migration either by minimizing time of transport, energy expenditure, or predation risk during migration. For each of these optimization criteria different fattening and stopover strategies are predicted. The first two of these optimization criteria are examined here for the bar-tailed godwit ( Limosa lapponica ). In the European Wadden Sea two populations of bar-tailed godwits stop over during spring migration between their wintering and breeding areas. The European population winters mainly in Great Britain and the western part of the Wadden Sea and breeds in Fennoscandia. The Afro-Siberian population winters in West Africa and breeds in Siberia. The European wintering population migrates to the eastern parts of Wadden Sea in March where it stays until early May. During this time birds gain 1.9 g d⁻¹ in body mass for a 1500–2000-km non-stop flight to the breeding areas. Afro-Siberian birds stay only for one month in May in the Wadden Sea where they gain on average 9.4 g d⁻¹ in mass for a 4000-km non-stop flight. Intake rates in April/May did not differ between the two populations (1.5 kJ min⁻¹ and 1.8 kJ min⁻¹ for Siberian and European migrants, respectively) but total energy intake was higher for the Siberian migrants, since they spend 50% of the day foraging vs 30% in the European birds. In contrast to European migrants, Afro-Siberian birds start to moult into breeding plumage already in their winter quarters. During their stopover in the Wadden Sea thermostatic costs are lower than at times when European birds are present. Thus, the higher energy demands of the Afro-Siberian birds seem to be fulfilled by a combination of physiological adaptations and a high working level. European birds seem to adopt an energy-minimized migration strategy whereas Afro-Siberian birds appear to follow a time-minimized migration.     

Seasonal and diel transitions in physiology and behavior in the migratory dark-eyed junco

Body mass, fat stores, activities of lipogenic and lipolytic enzymes, and plasma corticosterone were measured throughout seasonal and diel transitions from fall through spring encompassing the non-migratory stages of early and mid winter, the prealternate molt, and the spring migratory stage in captive dark-eyed juncos to determine the physiological mechanisms underlying adaptations for migration. On a seasonal basis, lipid enzymes and corticosterone varied little throughout the stages even though the birds underwent dramatic alterations in mass, fat deposition, behavior, and activation of the reproductive axis. By contrast, diel changes were found in lipogenesis, lipolysis, muscle lipoprotein lipase, and plasma corticosterone when comparing birds in the two phases of spring migration--active flight and resting, as during times of stopover. In these two phases of migration, coordination of the lipogenic and lipolytic systems appear to maximize storage of fatty acids during rest and delivery/utilization during flight. Diel patterns of corticosterone revealed fairly consistent peaks during the night time (23:00) throughout the nonmigratory period. The profile of this pattern altered during the migratory period with variation between the flight and resting phases. In sum, the results from these captive studies offer a new approach for studying the regulation of migratory physiology in free-living birds.     

Carry‐over effects and compensation: late arrival on non‐breeding grounds affects wing moult but not plumage or schedules of departing bar‐tailed godwits Limosa lapponica baueri

In the annual cycle of migratory birds, temporal and energetic constraints can lead to carry‐over effects, in which performance in one life history stage affects later stages. Bar‐tailed godwits Limosa lapponica baueri, which achieve remarkably high pre‐migratory fuel loads, undertake the longest non‐stop migratory flights yet recorded, and breed during brief high‐latitude summers, may be particularly vulnerable to persistent effects of disruptions to their rigidly‐timed annual routines. Using three years of non‐breeding data in New Zealand, we asked how arrival timing after a non‐stop flight from Alaska (>11 000 km) affected an individual godwit's performance in subsequent flight feather moult, contour feather moults, and migratory departure. Late arrival led to later wing moult, but godwits partially compensated for delayed moult initiation by increasing moult rate and decreasing the total duration of moult. Delays in arrival and wing moult up to 34–37 d had no apparent effect on an individual's migratory departure or extent of breeding plumage at departure, both of which were extraordinarily consistent between years. Thus, ‘errors’ in timing early in the non‐breeding season were essentially corrected in New Zealand prior to spring migration. Variation in migration timing also had no apparent effect on an individual's likelihood of returning the following season. The bar‐tailed godwits’ rigid maintenance of plumage and spring migration schedules, coupled with high annual survival, imply a surprising degree of flexibility to address unforeseen circumstances in the annual cycle.     

Metabolic profile of long-distance migratory flight and stopover in a shorebird

Migrating birds often complete long non-stop flights during which body energy stores exclusively support energetic demands. The metabolic correlates of such long-distance travel in free-living migrants are as yet poorly studied. Bar-tailed godwits, Limosa lapponica taymyrensis, undertake a 4500 km flight to their single spring stopover site and thus provide an excellent model in which to determine the energy fuels associated with endurance travel. To this end, we evaluated plasma concentrations of six key metabolites in arriving godwits caught immediately upon landing near their stopover site. Initial metabolite levels were compared with levels after 5 h of inactive rest to determine how flight per se affects energy metabolism. Birds refuelling on the stopover site were also examined. Arriving godwits displayed elevated plasma free fatty acids, glycerol and butyrate, confirming the importance of lipid fuel in the support of extended migratory activity. Further-more, elevated plasma triglycerides in these birds suggest that fatty acid provisioning is facilitated through hepatic synthesis and release of neutral lipids, as previously hypothesized for small migrants with high mass-specific metabolic rates. Finally, elevations in plasma uric acid suggest that protein breakdown contributes to the support of long-distance movement, to possibly maintain citric acid cycle intermediates, gluconeogenesis and/or water balance.     

Corticosterone and growth hormone levels in shorebirds during spring and fall migration stopover.

Large numbers of shorebirds stop over at Delaware Bay during spring migration and undergo major mass increases within a two- to three-week period. We studied plasma levels of corticosterone and growth hormone in three species of migrants that use this site, sanderlings, Calidris alba, semipalmated plovers, Charadrius semipalmatus, and semipalmated sandpipers, Calidris pusilla. Semipalmated sandpipers were also studied at a fall migration stopover in Manomet, Massachusetts. These two hormones were chosen because they modulate the physiological processes of lipogenesis/lipolysis and promote increased feeding in birds. The stress response was not suppressed in the shorebirds studied, and plasma levels of corticosterone were elevated compared to other studies. We believe that the high levels of corticosterone relate to the rapid fat deposition that takes place at this stop-over site. There was a significant negative correlation between plasma growth hormone and body mass, indicating the lipolytic effects of the growth hormone. Because the lighter birds are recent arrivals to Delaware Bay they may have elevated plasma growth because of fat breakdown during flight to this stop-over site. High levels of growth hormone may also result in protein synthesis, replenishing tissues broken down during the previous migratory bout. J. Exp. Zool. 284:645-651, 1999.     

Body condition is associated with adrenocortical response in the barn swallow (Hirundo rustica L.) during early stages of autumn migration

Migration is an energy-demanding life-history period and also a significant population-limiting factor of long-distance migratory birds. It is important to understand how corticosterone, the main energy regulating hormone in birds, is associated with behavioural and physiological changes during migration. According to the migration modulation hypothesis (MMH), individual birds may express elevated levels of baseline corticosterone to facilitate fuelling, but down-regulate the adrenocortical response in order to protect skeletal muscles from the catabolic effects of the hormone. We measured the baseline and stress-induced levels of corticosterone in barn swallows (Hirundo rustica L.) during early stages of autumn migration. Here, we show that, while barn swallows clearly responded to the capture and handling stress by increasing the corticosterone level, the strength of this acute response was related to their energetic condition: birds with high body mass responded more rapidly and had lower peak values of corticosterone than lighter birds. Further, the baseline levels of corticosterone correlated negatively with the magnitude of the adrenocortical response. Barn swallows did not show elevated baseline levels of corticosterone in the course of autumn, which suggests that, instead of fuelling, the birds were actively migrating. Our results indicate that MMH also applies to aerial feeders, whose foraging habits differ from model birds of previous studies.     

Highly pathogenic avian influenza virus H5N1 infection in a long-distance migrant shorebird under migratory and non-migratory states

Corticosterone regulates physiological changes preparing wild birds for migration. It also modulates the immune system and may lead to increased susceptibility to infection, with implications for the spread of pathogens, including highly pathogenic avian influenza virus (HPAIV) H5N1. The red knot (Calidris canutus islandica) displays migratory changes in captivity and was used as a model to assess the effect of high plasma concentration of corticosterone on HPAIV H5N1 infection. We inoculated knots during pre-migration (N = 6), fueling (N = 5), migration (N = 9) and post-migration periods (N = 6). Knots from all groups shed similar viral titers for up to 5 days post-inoculation (dpi), peaking at 1 to 3 dpi. Lesions of acute encephalitis, associated with virus replication in neurons, were seen in 1 to 2 knots per group, leading to neurological disease and death at 5 to 11 dpi. Therefore, the risk of HPAIV H5N1 infection in wild birds and of potential transmission between wild birds and poultry may be similar at different times of the year, irrespective of wild birds'' migratory status. However, in knots inoculated during the migration period, viral shedding levels positively correlated with pre-inoculation plasma concentration of corticosterone. Of these, knots that did not become productively infected had lower plasma concentration of corticosterone. Conversely, elevated plasma concentration of corticosterone did not result in an increased probability to develop clinical disease. These results suggest that birds with elevated plasma concentration of corticosterone at the time of migration (ready to migrate) may be more susceptible to acquisition of infection and shed higher viral titers—before the onset of clinical disease—than birds with low concentration of corticosterone (not ready for take-off). Yet, they may not be more prone to the development of clinical disease. Therefore, assuming no effect of sub-clinical infection on the likelihood of migratory take-off, this may favor the spread of HPAIV H5N1 by migratory birds over long distances.     

Migration strategies of the Baltic dunlin: rapid jump migration in the autumn but slower skipping type spring migration

Migration during spring is usually faster than during autumn because of competition for breeding territories. In some cases, however, the costs and benefits associated with the environment can lead to slower spring migration, but examples are quite rare. We compared seasonal migration strategies of the endangered Baltic population of the dunlin Calidris alpina schinzii using light‐level geolocator data from 26 individuals breeding in Finland. Autumn migration was faster, with individuals showing a ‘jump’ and ‘skipping’ migration strategy characterised by fewer stationary periods, shorter total stopping time and faster flight. Spring migration was slower, with individuals using a ‘skipping’ strategy. The duration of migration was longer for early departing birds during spring but not during autumn suggesting that early spring migrants are prevented from arriving to the breeding areas or that fueling conditions are worse on the stopover sites for early arriving individuals. Dunlins showed high migratory connectivity. All individuals had one long staging at the Wadden Sea in the autumn after which half of the individuals flew 4500 km non‐stop to Banc d’Arguin, Mauritania. The other half stopped briefly on the Atlantic coast on their way to Mauritania. One bird wintered on the coast of Portugal. Nine individuals that carried geolocators for two years were site faithful to their final non‐breeding sites. Based on the strategies during the non‐breeding period we identified, Baltic dunlin may be especially vulnerable to rapid environmental changes at the staging and non‐breeding areas. Consequently, the preservation of the identified non‐breeding areas is important for their conservation.     

Basal and stress-induced corticosterone levels of garden warblers,Sylvia borin,during migration

Summary Plasma levels of the metabolically and behaviorally active corticosteroid hormone, corticosterone, were studied in garden warblers in the laboratory and in the field during the autumnal migratory phase. Garden warblers showing nocturnal migratory activity in the laboratory had elevated levels of corticosterone at the end of the dark phase and low levels during daytime. When nocturnal migratory activity was experimentally disrupted by food deprivation and subsequent refeeding or after spontaneous termination of migratory activity this rhythm was absent. Garden warblers stopping over in the Sahara desert during autumnal migration had low levels of corticosterone. Levels were negatively correlated with fat stores and body mass in birds sampled throughout the day. These levels were generally lower than those associated with stress in response to repeated handling and blood sampling. The results suggest (1) the existence of diel changes in adrenocortical hormonal activity that could be involved in regulation of migration, and (2) that garden warblers carrying large fat depots are not stressed by prolonged flight or lack of appropiate feeding areas during migration over the desert.Abbreviations ACTH adrenocorticotrope hormone - cpm counts per minute - EtOH ethanol - RIA radioimmunoassay     

Dark-bellied Brent Geese Branta bernicla bernicla, as recorded by satellite telemetry, do not minimize flight distance during spring migration

Nine Dark-bellied Brent Geese Branta bernicla bernicla were equipped with satellite transmitters during spring staging in the Dutch Wadden Sea in 1998 and 1999. The transmitters (in all cases less than 3% of body mass) were attached to the back by a flexible elastic harness. One juvenile female was tracked to the Yamal peninsula in 1998. Eight adult males were selected from a single catch of 75 to span the range of body mass observed on the date of capture (11 May 1999) and all but the lightest individual completed the first lap of the migratory flight to the White Sea, Russia, according to the time schedule normal for the species. Six birds were successfully tracked to Taymyr for a total distance averaging 5004 km (range 45775164) but judging from later movements none bred (although 1999 was a breeding year). Although the routes chosen during spring migration were closely similar, none of the tagged birds migrated together. On average the geese used 16 flights to reach their summer destinations on Taymyr. The longest uninterrupted flights during the first half of the journey (Wadden Sea to Kanin) covered 1056 km (mean of seven adult males, range 7681331), while the corresponding value for the second half of the migration (KaninTaymyr) was only 555 km (mean of six adult males). Only 7% of total time during spring migration was spent in active flight, as contrasted to c.  80% at long-term stopovers. Overall average travelling speed was 118 km/day (range 97148). Including fattening prior to departure the rate of travel falls to 62 km/day (range 4970), in keeping with theoretical predictions. Routes followed deviated from the great circle route, adding at least 700 km (16%) to the journey from Wadden Sea to Taymyr, and we conclude that the coastal route is chosen to facilitate feeding, drinking and resting en route instead of minimizing total flight distance.     

Role of the low-affinity glucocorticoid receptor in the regulation of behavior and energy metabolism in the migratory red knot Calidris canutus islandica

Plasma corticosterone increases in association with migratory flight in the red knot Calidris canutus islandica, suggesting that corticosterone may promote migratory activity and/or energy mobilization in this species. This hypothesis is supported by general effects of glucocorticoids, which include stimulation of locomotion and the mobilization of energy depots. We experimentally examined the role of elevated corticosterone levels in the migratory red knot by comparing foraging behavior, flight frequency, and plasma metabolites between vehicle-injected controls and birds treated with RU486, an antagonist to the genomic low-affinity glucocorticoid receptor (GR). We predicted that RU486 treatment would interfere with energy mobilization. However, we expected no effects on flight activity because recent studies suggest that glucocorticoids affect locomotion through a nongenomic receptor. Finally, because glucocorticoids exert permissive effects on food intake, we postulated that RU486 treatment in the red knot would interfere with feeding. Results were consistent with the latter prediction, suggesting that the GR participates in the promotion of hyperphagia, the intense feeding state that is characteristic of the migratory condition. RU486 treatment did not affect flight frequency, suggesting that corticosterone may support migratory activity through a receptor other than the GR. Energy metabolism (as determined through plasma metabolites) was also unaffected by RU486, possibly because energetic demands experienced by captive birds were low.     

Migrating songbirds on stopover prepare for,and recover from,oxidative challenges posed by long‐distance flight

Managing oxidative stress is an important physiological function for all aerobic organisms, particularly during periods of prolonged high metabolic activity, such as long‐distance migration across ecological barriers. However, no previous study has investigated the oxidative status of birds at different stages of migration and whether that oxidative status depends on the condition of the birds. In this study, we compared (1) energy stores and circulating oxidative status measures in (a) two species of Neotropical migrants with differing migration strategies that were sampled at an autumn stopover site before an ecological barrier; and (b) a species of trans‐Saharan migrant sampled at a spring stopover site after crossing an ecological barrier; and (2) circulating oxidative measures and indicators of fat metabolism in a trans‐Saharan migrant after stopovers of varying duration (0–8 nights), based on recapture records. We found fat stores to be positively correlated with circulating antioxidant capacity in Blackpoll Warblers and Red‐eyed Vireos preparing for fall migration on Block Island, USA, but uncorrelated in Garden Warblers on the island of Ponza, Italy, after a spring crossing of the Sahara Desert and Mediterranean Sea. In all circumstances, fat stores were positively correlated with circulating lipid oxidation levels. Among Garden Warblers on the island of Ponza, fat anabolism increased with stopover duration while oxidative damage levels decreased. Our study provides evidence that birds build antioxidant capacity as they build fat stores at stopover sites before long flights, but does not support the idea that antioxidant stores remain elevated in birds with high fuel levels after an ecological barrier. Our results further suggest that lipid oxidation may be an inescapable hazard of using fats as the primary fuel for flight. Yet, we also show that birds on stopover are capable of recovering from the oxidative damage they have accrued during migration, as lipid oxidation levels decrease with time on stopover. Thus, the physiological strategy of migrating songbirds may be to build prophylactic antioxidant capacity in concert with fuel stores at stopover sites before a long‐distance flight, and then repair oxidative damage while refueling at stopover sites after long‐distance flight.     

The role of wind in passerine autumn migration between Europe and Africa

Large ecological barriers such as oceans and deserts have considerablyshaped the migratory strategies of birds. The ecological barriersposed by the Alps, the Mediterranean Sea, and the Sahara seemto prevent most long-distance migrants from flying on a directsouthward course from Europe to Africa. Migratory routes towardsouthwest and southeast prevail. These two flyways differ withrespect to topography, refueling possibilities, and wind conditions.Aiming at a better understanding of the evolution of both flywaysin spite of differing conditions, we studied potential survivalof passerine birds on their first autumn migration from northernEurope to tropical Africa by means of a computer simulation.Considering real wind conditions at 850 mb (approximately 1500m above sea level), the survival rates of birds with southeasterly(SE) migratory directions were much higher than those of birdswith southwesterly (SW) directions. With the possibility tochoose the altitude (from four levels) with the most favorablewind, both SE and SW migrants had similar high survival, butonly with refueling opportunities in northwest (NW) Africa forSW migrants. Our results suggest that the southwestern flywaydepends on the selection of days, but especially altitudes,with favorable wind conditions and on refueling opportunitiesin NW Africa. The SE flyway is privileged by the frequent favorablewind conditions for crossing the eastern Mediterranean Sea andthe Egyptian desert, where refueling sites are almost absent.Both autumn migration routes would be unlikely without windassistance.     

Body mass change and diet switch tracked by stable isotopes indicate time spent at a stopover site during autumn migration in dunlins Calidris alpina alpina

Birds may change their diet and foraging habitat during or after migration. Dunlins Calidris alpina alpina breed in the tundra of northern Europe and Russia where they feed exclusively on terrestrial prey. However, up to 80% of the flyway population uses the Wadden Sea as their first important staging site on the way to wintering grounds, feeding exclusively on marine prey. Adult birds migrate earlier than immatures and tend to fly non‐stop, whereas immatures may stage for at least a few days en route, mainly in the Baltic region. There they mostly feed on brackish water prey showing similar isotopic values compared to terrestrial prey. When they arrive in the Wadden Sea, dunlin body reserves are depleted and lower than those of individuals that have already staged for several days. We hypothesized that lighter individuals should retain a strong terrestrial isotopic blood signature, while heavier ones should show a stronger marine signature. We found a significant positive correlation between scaled mass index and carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotopes, reflecting the switch from terrestrial to marine prey during migration. A mixing model revealed differences in isotopic values between heavy and light adults and immatures, respectively, in relation to the isotopic prey signatures. Adults showed stronger marine signals compared with immatures, emphasizing the different modes of migration (i.e. a later departure in immatures) as well as the known spatial segregation of age classes in the Wadden Sea, i.e. adults use tidal flats distant from the shore while immatures use coastal areas influenced by terrestrial carbon sources. The results of this study demonstrate the value of scaled mass index in migratory birds as an indicator of time elapsed after diet switching following migration. Furthermore, this study extents the existing knowledge on the timing of dunlin migration by using an isotopic approach.