Flying, fasting, and feeding in birds during migration: a nutritional and physiological ecology perspective

Unlike exercising mammals, migratory birds fuel very high intensity exercise (e.g., flight) with fatty acids delivered from the adipose tissue to the working muscles by the circulatory system. Given the primary importance of fatty acids for fueling intense exercise, we discuss the likely limiting steps in lipid transport and oxidation for exercising birds and the ecological factors that affect the quality and quantity of fat stored in wild birds. Most stored lipids in migratory birds are comprised of three fatty acids (16:0, 18:1 and 18:2) even though migratory birds have diverse food habits. Diet selection and selective metabolism of lipids play important roles in determining the fatty acid composition of birds which, in turn, affects energetic performance during intense exercise. As such, migratory birds offer an intriguing model for studying the implications of lipid metabolism and obesity on exercise performance. We conclude with a discussion of the energetic costs of migratory flight and stopover in birds, and its implications for bird migration strategies.     

Fatty acid composition of depot fat of shorebirds collected at mid‐continental stopover sites during spring migration

ABSTRACT Despite the critical role of fat in providing energy for large‐scale seasonal movements, little is known about the fatty acid composition of shorebird depot fat. Fatty acid composition is important because it may impact flight performance and seasonal migratory movements. We analyzed the fatty acid composition of depot fat of 12 species of shorebirds collected during spring migration at stopover sites in Kansas from two different subcutaneous fat depots (furcular and saddle depots). Five fatty acids (palmitate [16:0], palmitoleate [16:1], stearate [18:0], oleate [18:1n‐9], and vaccenate [18:1n‐7]) accounted for 70–96% of the total composition of depot fat despite the diverse foraging behaviors of the species sampled. This similarity in fatty acid profiles may be due to a limited availability of high‐lipid food items in the shallow water and mudflat habitats where migratory shorebirds forage in the southern Great Plains. In addition, shorebird depot fat was composed primarily of long‐chain saturated and monounsaturated fatty acids (i.e., 16‐ and 18‐carbon), fatty acids thought to be more easily converted to energy during migration. Depot fat in the furcular depot was similar in composition to fat from the saddle depot, and we found no differences in fatty acid composition of the adipose tissue of males and females. Thus, our results suggest that representative shorebird depot fat fatty acid profiles can be obtained even if investigators are limited to sampling one sex, and that adipose tissue collected from saddle depots provide representative samples so biopsies can be limited to areas of the body with the least impact on flight performance.     

The effects of dietary macronutrients on flight ability,energetics, and fuel metabolism of yellow‐rumped warblers Setophaga coronata

The catabolism of protein from organs and muscles during migratory flight is necessary to produce glucose, key metabolic intermediates, and water, but may have negative effects on flight range and refueling at stopovers. We tested the hypothesis, suggested by previous studies, that birds that eat high‐protein insect diets use more protein for fuel in flight than those that eat high‐carbohydrate fruits. First, we fed migratory yellow‐rumped warblers synthetic fruit or mixed insect/fruit diets, and measured metabolic rates and fuel mixture under basal conditions and during exercise in a hop/hover wheel respirometer. Birds eating the fruit diet had greater plasma triglyceride and non‐esterified fatty acid concentrations, and the higher protein mixed diet increased plasma uric acid only during feeding. Diet did not affect metabolic rates or the fuel mixture under resting or exercise conditions. We then fed yellow‐rumped warblers synthetic diets that differed only in the relative proportion of carbohydrate and protein (60:15 versus 15:60 as % dry mass) and tested them in wind tunnel flights lasting up to six hours. Birds fed the high carbohydrate diet became heavier and fatter than when fed the high protein diet. Plasma uric acid concentration was increased and plasma phospholipid concentration was decreased by the high protein diet in the pre‐flight state (after a 3 h fast), but diet only affected plasma phospholipids during flight (lower in high protein birds). Neither diet nor amount of body fat affected the rate of loss of lean mass or fat during flight. Inter‐individual or seasonal differences in diet do not appear to influence the amount of protein catabolized during endurance flight. However, birds fed the high carbohydrate diet had greater voluntary flight duration, independent of body fatness, suggesting that there may be other performance benefits of high carbohydrate diets for migratory birds.     

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.     

Differential Regulation of Adipokines May Influence Migratory Behavior in the White-Throated Sparrow (Zonotrichia albicollis)

White-throated sparrows increase fat deposits during pre-migratory periods and rely on these fat stores to fuel migration. Adipose tissue produces hormones and signaling factors in a rhythmic fashion and may be controlled by a clock in adipose tissue or driven by a master clock in the brain. The master clock may convey photoperiodic information from the environment to adipose tissue to facilitate pre-migratory fattening, and adipose tissue may, in turn, release adipokines to indicate the extent of fat energy stores. Here, we present evidence that a change in signal from the adipokines adiponectin and visfatin may act to indicate body condition, thereby influencing an individual''s decision to commence migratory flight, or to delay until adequate fat stores are acquired. We quantified plasma adiponectin and visfatin levels across the day in captive birds held under constant photoperiod. The circadian profiles of plasma adiponectin in non-migrating birds were approximately inverse the profiles from migrating birds. Adiponectin levels were positively correlated to body fat, and body fat was inversely related to the appearance of nocturnal migratory restlessness. Visfatin levels were constant across the day and did not correlate with fat deposits; however, a reduction in plasma visfatin concentration occurred during the migratory period. The data suggest that a significant change in the biological control of adipokine expression exists between the two migratory conditions and we propose a role for adiponectin, visfatin and adipose clocks in the regulation of migratory behaviors.     

Energetics and metabolite profiles during early flight in American robins (Turdus Migratorius)

Although birds use fat as the primary fuel for migratory flights, carbohydrate and protein catabolism could be significant in the early stages of flight while pathways of fatty acid transport and oxidation are induced. The fuel mixture of long distance migrant birds can also be affected by the rate of water loss, where birds catabolize more protein to increase endogenous water production under dehydrating flight conditions. Despite many studies investigating flight metabolism, few have focused on the metabolic response to flight during the switchover to fat catabolism in migrants, and none have examined the effect of ambient conditions on fuel selection during early flight. We investigated the effect of water loss on the metabolic response to short duration flight in the American robin (Turdus migratorius). Birds were flown in a climatic wind tunnel and changes in body composition and plasma metabolites were measured. As flight duration increased, there was a gradual switchover from carbohydrate and protein catabolism to fat catabolism. Plasma metabolite profiles indicate that the mobilization of fat occurred within 20 min of initiating flight. Plasma glucose decreased and uric acid increased with flight duration. Ambient humidity did not affect fuel mixture. Thus, it seems that the utilization of fat may be delayed as migrants initiate flight. Short-hop migrants may exploit high rates of endogenous water production resulting from carbohydrate and protein catabolism early in flight to offset high water loss associated with low humidity. Rapid catabolism of lean body components at the start of a flight also reduces mass quickly, and may reduce energy costs.     

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.     

Body fat influences departure from stopover sites in migratory birds: evidence from whole-island telemetry

Migration remains one of the great mysteries of animal life. Small migratory birds rely on refuelling stopovers after crossing ecological barriers such as deserts or seas. Previous studies have suggested that fuel reserves may determine stopover duration but this hypothesis could not be tested because of methodological limitations. Here, we provide evidence that subcutaneous fat stores determine stopover duration by measuring the permanence of migratory garden warblers (Sylvia borin) on a small Mediterranean island during spring migration with telemetry methods. Garden warblers with large amounts of fat stores departed the island significantly sooner than lean birds. All except one fat bird left the island on the same evening after capture, with a mean total stopover estimate of 8.8 hours. In contrast, the mean estimated total stopover duration of lean birds was 41.3 hours. To our knowledge, this is the first study that measures the true minimum stopover duration of a songbird during migration.     

Diet preferences for specific fatty acids and their effect on composition of fat reserves in migratory Red-eyed Vireos (Vireo olivaceous)

Fatty acid composition of body fat in birds often differs between bird species and between seasons, and changes in diet may be responsible for this variation. We tested two related hypotheses using Red-eyed Vireos, a long-distance migratory songbird: (1) birds prefer diets with certain fatty acids, and (2) fatty acid composition of the diet primarily determines the composition of lipid reserves. During paired-choice experiments, vireos preferred semi-synthetic diets with triolein (81% digestive extraction efficiency) over diets with tristearin (54% digestive extraction efficiency) and, in general, ate more when offered diets with unsaturated fats compared to saturated fats. These results demonstrate that vireos can discriminate between diets differing only in fatty acid composition and prefer diets with long-chain unsaturated fatty acids. When vireos were fed one of two diets for 1 month, the primary fatty acids in each diet also predominated in the tissues of birds fed each diet. However, some fatty acids that were absent in the diet occurred in bird tissues (e.g., 22:4, 22:5) suggesting that selective metabolism of fatty acids along with diet composition determine the fatty acid composition of lipid reserves in migratory birds.     

Oxidative stress,circulating antioxidants,and dietary preferences in songbirds

Oxidative stress is an unavoidable consequence of metabolism and increases during intensive exercise. This is especially problematic for migratory birds that metabolize fat to fuel long-distance flight. Birds can mitigate damage by increasing endogenous antioxidants (e.g. uric acid) or by consuming dietary antioxidants (e.g. tocopherol). During flight, birds may increase protein catabolism of lean tissue which may increase circulating uric acid and many birds also consume an antioxidant-rich frugivorous diet during autumn migration. We evaluated three related hypotheses in a migratory passerine: (1) protein consumption is positively related to circulating antioxidants, (2) a dietary oxidative stressor [i.e. polyunsaturated fatty acid (PUFA)] influences antioxidant capacity and oxidative damage, and (3) oxidative stress influences dietary antioxidant preferences. White-throated Sparrows (Zonotrichia albicollis) consuming a high protein diet increased circulating uric acid; however, uric acid, antioxidant capacity, and oxidative stress did not differ between birds consuming a high PUFA versus a low PUFA diet, despite increased oxidative damage in high PUFA birds. Birds did not prefer antioxidant-rich diets even when fed high PUFA, low protein. We conclude that White-throated Sparrows successfully mitigated oxidative damage associated with a high PUFA diet and mounted an endogenous antioxidant response independent of uric acid, other circulating antioxidants, and dietary antioxidants.     

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.     

Three-Phase Fuel Deposition in a Long-Distance Migrant,the Red Knot (Calidris canutus piersmai), before the Flight to High Arctic Breeding Grounds

Refuelling by migratory birds before take-off on long flights is generally considered a two-phase process, with protein accumulation preceding rapid fat deposition. The first phase expresses the demands for a large digestive system for nutrient storage after shrinkage during previous flights, the second phase the demands for fat stores to fuel the subsequent flight. At the last staging site in northward migration, this process may include expression of selection pressures both en route to and after arrival at the breeding grounds, which remains unascertained. Here we investigated changes in body composition during refuelling of High Arctic breeding red knots (Calidris canutus piersmai) in the northern Yellow Sea, before their flight to the tundra. These red knots followed a three-phase fuel deposition pattern, with protein being stored in the first and last phases, and fat being deposited mainly in the second phase. Thus, they did not shrink nutritional organs before take-off, and even showed hypertrophy of the nutritional organs. These suggest the build up of strategic protein stores before departure to cope with a protein shortage upon arrival on the breeding grounds. Further comparative studies are warranted to examine the degree to which the deposition of stores by migrant birds generally reflects a balance between concurrent and upcoming environmental selection pressures.     

Polyunsaturated fats,membrane lipids and animal longevity

Fatty acids are essential for life because they are essential components of cellular membranes. Lower animals can synthesize all four classes of fatty acids from non-lipid sources, but both omega-6 and omega-3 cannot be synthesized de novo by ‘higher’ animals and are therefore essential components of their diet. The relationship between normal variation in diet fatty acid composition and membrane fatty acid composition is little investigated. Studies in the rat show that, with respect to the general classes of fatty acids (saturated, monounsaturated and polyunsaturated) membrane fatty acid composition is homeostatically regulated despite diet variation. This is not the case for fatty acid composition of storage lipids, which responds to diet variation. Polyunsaturated fatty acids are important determinants of physical and chemical properties of membranes. They are the substrates for lipid peroxidation and it is possible to calculate a peroxidation index (PI) for a particular membrane composition. Membrane PI appears to be homeostatically regulated with respect to diet PI. Membrane fatty acid composition varies among species and membrane PI is inversely correlated to longevity in mammals, birds, bivalve molluscs, honeybees and the nematode Caenorhabditis elegans.     

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.     

Size and accumulation of fuel reserves at stopover predict nocturnal restlessness in a migratory bird

Early arrival at the breeding site positively affects the breeding success of migratory birds. During migration, birds spend most of their time at stopovers. Therefore, determining which factors shape stopover duration is essential to our understanding of avian migration. Because the main purpose of stopover is to accumulate fat as fuel for the next flight bout, fuel reserves at arrival and the accumulation of fuel are both expected to affect stopover departure decisions. Here, we determined whether fuel reserves and fuel accumulation predict a bird''s motivation to depart, as quantified by nocturnal migratory restlessness (Zugunruhe), using northern wheatears (Oenanthe oenanthe) that were captured and temporarily contained at spring stopover. We found that fuel reserves at capture were positively correlated with Zugunruhe, and negatively correlated with fuel accumulation. This indicates that fat birds were motivated to depart, whereas lean birds were set on staying and accumulating fuel. Moreover, the change in fuel reserves was positively correlated with the concurrent change in Zugunruhe, providing the first empirical evidence for a direct link between fuel accumulation and Zugunruhe during stopover. Our study indicates that, together with innate rhythms and weather, the size and accumulation of fuel reserves shape stopover duration, and hence overall migration time.     

Empirical evidence for differential organ reductions during trans-oceanic bird flight

Since the early 1960s it has been held that migrating birds deposit and use only fat as fuel during migratory flight, with the non-fat portion of the body remaining homeostatic. Recent evidence from field studies has shown large changes in organ sizes in fuelling birds, and theory on fuel use suggests protein may be a necessary fuel during flight. However, an absence of information on the body condition of migrants before and after a long flight has hampered understanding of the dynamics of organs during sustained flight. We studied body condition in a medium-sized shorebird, the great knot (Calidris tenuirostris), before and after a flight of 5400 km from Australia to China during northward migration. Not only did these birds show the expected large reduction in fat content after migration, there was also a decrease in lean tissue mass, with significant decreases in seven organs. The reduction in functional components is reflected in a lowering of the basal metabolic rate by 42% [corrected]. Recent flight models have tried to separate the 'flexible' part of the body from the constant portion. Our results suggest that apart from brains and lungs no organs are homeostatic during long-distance flight. Such organ reductions may be a crucial adaptation for long-distance flight in birds.     

Role of fatty acid-binding protein in lipid metabolism of insect flight muscle

Since insect flight muscles are among the most active muscles in nature, their extremely high rates of fuel supply and oxidation pose interesting physiological problems. Long-distance flights of species like locusts and hawkmoths are fueled through fatty acid oxidation. The lipid substrate is transported as diacylglycerol in the blood, employing a unique and efficient lipoprotein shuttle system. Following diacylglycerol hydrolysis by a flight muscle lipoprotein lipase, the liberated fatty acids are ultimately oxidized in the mitochondria. Locust flight muscle cytoplasm contains an abundant fatty acid-binding protein (FABP). The flight muscle FABP ofLocusta migratoria is a 15 kDa protein with an isoelectric point of 5.8, binding fatty acids in a 1:1 molar stoichiometric ratio. Binding affinity of the FABP for longchain fatty acids (apparent dissociation constant K_d=5.21±0.16 M) is however markedly lower than that of mammalian FABPs. The NH₂-terminal amino acid sequence shares structural homologies with two insect FABPs recently purified from hawkmoth midgut, as well as with mammalian FABPs. In contrast to all other isolated FABPs, the NH₂ terminus of locust flight muscle FABP appeared not to be acetylated. During development of the insect, a marked increase in fatty acid binding capacity of flight muscle homogenate was measured, along with similar increases in both fatty acid oxidation capacity and citrate synthase activity. Although considerable circumstantial evidence would support a function of locust flight muscle FABP in intracellular uptake and transport of fatty acids, the finding of another extremely well-flying migratory insect, the hawkmothAcherontia atropos, which employs the same lipoprotein shuttle system, however contains relatively very low amounts of FABP in its flight muscles, renders the proposed function of FABP in insect flight muscles questionable.     

Plasma and muscle phospholipids are involved in the metabolic response to long-distance migration in a shorebird

We studied: (1) concentrations and fatty acid compositions of plasma non-esterified fatty acids, neutral lipids, and phospholipids, and (2) fatty acid composition of flight muscle phospholipids in wintering, premigratory, and spring and fall migrating western sandpipers ( Calidris mauri). Plasma neutral lipid and phospholipid levels were elevated in migrants, reflecting high rates of fat deposition. An important role of phospholipids in fattening is suggested by the fact that the amount of fatty acids in plasma phospholipids was similar to, or in spring as much as twice, that of neutral lipids. Changes in the ratio of plasma neutral lipids to phospholipids may indicate seasonal changes in triacylglycerol stores of invertebrate prey. Monounsaturation and total unsaturation of plasma neutral lipids and phospholipids increased during migration. Muscle phospholipids were more monounsaturated in spring and fall, but total unsaturation was reduced in fall. Arachidonic acid [20:4(n-6)] was especially abundant in muscle phospholipids in winter (29%) and declined during migration (19-22%), contributing to a decline in the ratio of n-6 to n-3 fatty acids. The abundance of plasma phospholipids and variability of neutral lipid to phospholipid ratio indicates that measurement of plasma phospholipids will improve methods for assessment of fattening rates of birds. The functional significance of changes in muscle phospholipids is unclear, but may relate to depletion of essential n-6 fatty acids during exercise.     

Short-time infusion of fish oil-based lipid emulsions, approved for parenteral nutrition, reduces monocyte proinflammatory cytokine generation and adhesive interaction with endothelium in humans

Potential impact of omega-3 fatty acids, as contained in fish oil, on immunological function has been suggested because observations of reduced inflammatory diseases in Greenland Inuit were published. A fish oil-based lipid emulsion has recently been approved for parenteral nutrition in many countries. We investigated the influence of a short infusion course of fish oil-based (omega-3) vs conventional (omega-6) lipid emulsion on monocyte function. In a randomized design, twelve healthy volunteers received omega-3 or omega-6 lipid infusion for 48 h, with cross-over repetition of the infusion course after 3 mo. Fatty acid profiles, monocyte cytokine release and adhesive monocyte-endothelium interaction were investigated. Resultant omega-6 lipid emulsion increased plasma-free fatty acids including arachidonic acid, whereas the omega-3/omega-6 fatty acid ratio in monocyte membranes remained largely unchanged. It also caused a tendency toward enhanced monocyte proinflammatory cytokine release and adhesive monocyte-endothelium interaction. In contrast, omega-3 lipid emulsion significantly increased the omega-3/omega-6 fatty acid ratio in the plasma-free fatty acid fraction and in monocyte membrane lipid pool, markedly suppressing monocyte generation of TNF-alpha, IL-1, IL-6, and IL-8 in response to endotoxin. In addition, it also significantly inhibited both monocyte-endothelium adhesion and transendothelial monocyte migration, although monocyte surface expression of relevant adhesive molecules (CD11b, CD18, CD49 days, CCR2) was unchanged. Although isocaloric, omega-3 and omega-6 lipid emulsions exert differential impact on immunological processes in humans. In addition to its nutritional value, fish oil-based omega-3 lipid emulsion significantly suppresses monocyte proinflammatory cytokine generation and features of monocyte recruitment.     

Optimal fat loads and long-distance flights by migrating Knots Calidris canutus, Sanderlings C. alba and Turnstones Arenaria interpres

Arctic waders often build up large fat loads and complete their migratory journeys by a few long-distance flights between traditional staging sites. Optimal fat loads and choices of staging sites differ depending on whether the birds are adapted to minimize energy or time spent on migration. In the latter case, we predict that the birds will depart for the next staging site when the instantaneous speed of migration expected after arrival at the next site, exceeds the corresponding speed at the departure site. The instantaneous migration speed is a function of the rate of fat deposition and the current fat load. As a consequence of this, overloading (birds deposit larger fat loads than needed merely for covering the flight distance to the next destination) and by-passing of possible, but low-quality staging sites, are expected under specific conditions in time-selected migration.
Estimates of fat deposition rates and departure fat loads were obtained by captures of Knots Calidris canutus , Sanderlings C. alba and Turnstones Arenaria interpres in W. Iceland during spring migration. Further fat deposition data referring to spring migration of these species were compiled from the literature. Fat deposition rates at different sites, as measured by the daily gain in mass relative to lean body-mass, range between 1.0 and 3.6%/day, and departure fuel loads (in % of lean body-mass) between 27 and 73%.
Comparison with flight range estimates suggests that overloading may be a regular phenomenon during spring migration of Knots, Sanderlings and Turnstones. Furthermore, fat deposition rates at different staging sites, and the general difference in migration patterns between spring and autumn, indicate that by-passing of possible staging sites may well occur. Hence, it cannot be excluded that the waders' migratory habits primarily serve to maximize the overall speed of migration.