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.     

Postexercise ketosis in night-migrating passerine birds

This study investigated the postexercise metabolism of six species of free-living, night-migrating passerine birds (European robin, pied flycatcher, wheatear, redstart, blackcap, and garden warbler). The birds were caught during autumn migration out of their nocturnal flight, and their metabolism changed from a fasting, highly active state to a fasting, resting state. Concentrations of six plasma metabolites of the fat, carbohydrate, and protein metabolism were measured during up to 10 h of recovery time. The metabolic changes indicated a biphasic pattern: (a) a quick first response to the reduced energy demands during the first 20 min of recovery, suggested by an increase and subsequent decrease of free fatty acid levels, and (b) subsequently, a postexercise ketosis and a reduction of lipolysis and proteolysis, suggested by high beta-hydroxy-butyrate and low free fatty acid, glycerol, triglyceride, and uric acid levels. This metabolic pattern differs from that of humans and rats, in which ketosis starts immediately postexercise or is absent in trained subjects. Since migrating birds are naturally adapted to endurance exercise, it is hypothesized that the high and long-lasting postexercise ketosis does not evoke physiological problems (such as hypoglycemia) but, by contrast, increases the ability of birds to rely on lipids, to a very high extent, during and after flight and decreases the dependence on glucose and glucogenic amino acids. Differences between species in fat stores and metabolic pattern support this hypothesis.     

Effects of corticosterone treatment on growth, development, and the corticosterone response to handling in young Japanese quail (Coturnix coturnix japonica)

Corticosterone, a glucocorticoid secreted during stress responses, has a range of actions that help birds respond to stressors. Although effects of corticosterone treatment have been described in several avian species, the impacts of defined increases in plasma corticosterone on early development and on corticosterone stress responses are little known. These issues were addressed by providing quail with different doses of corticosterone in drinking water from days 8 to 38 post-hatch. The corticosterone dose consumed by each bird during treatment days 15-30 was calculated by measuring water intake. The corticosterone dose was inversely, but weakly, correlated with weights of the bursa, thymus, spleen, liver, testes, oviduct, muscle, and body, and positively correlated with peritoneal fat deposition. When birds were divided into groups based on their corticosterone intake, weights of the spleen, thymus, bursa, muscle, testes, and oviduct were significantly reduced in birds receiving the highest doses; with the exception of muscle, similar reductions were also observed in birds receiving medium doses, and thymic growth was inhibited in birds receiving low doses. The acute corticosterone stress response was measured by handling birds for 15 min. Plasma corticosterone was transiently increased at 15 min in control birds in response to the handling stressor. Some birds consuming low doses of corticosterone had corticosterone responses similar to control birds. Initial corticosterone concentrations were elevated in birds consuming higher doses of corticosterone. Plasma corticosterone in these birds decreased from 0 to 15 min, then increased from 15 to 30 min. The initial decrease could be due to corticosterone clearance, whilst the increase could indicate that the birds had a greater response than control birds to isolation as a stressor. Corticosterone treatment may have reduced the strength of corticosterone negative feedback within the hypothalamo-pituitary-adrenal axis. The results indicate that individuals and organs differ in their sensitivity to corticosterone. Moreover, elevated plasma corticosterone may disrupt the acute corticosterone stress response, and may thus reduce the ability of birds to cope with stressors.     

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.     

Metabolic responses of homing pigeons to flight and subsequent recovery

This study examines metabolic changes occurring during short to endurance flights and during subsequent recovery in free-flying pigeons, in particular the change towards lipid utilization with increasing flight duration, lipid supply to the flight muscles, protein utilization and the time needed to metabolically recover. Eight plasma metabolite concentrations were measured in homing pigeons released from sites 20–200 km from the loft (0.3–4.8 h flight duration) just after landing and after keeping birds fasting at rest for 30 and 60 min, respectively, after their return. Birds kept in the loft fasting at rest were used as controls. Plasma free fatty acid and glycerol concentrations increased rapidly with flight duration and leveled off after about 1.5 h. This indicates a marked change towards a high and stable lipid utilization from adipose tissues within 1–2 h of flight. Plasma triglyceride levels and very-low-density lipoproteins were decreased after short flights, but subsequently regained or surpassed fasting levels at rest. This indicates that re-esterification of free fatty acids and delivery as very-low-density lipoproteins to the flight muscles to circumvent constraints of fatty acid supply, as described previously for small passerines, is not as significant in the pigeon which has a much lower mass-specific energy rate. An initial increase in plasma glucose levels and a transient decrease to fasting levels at rest was observed and may reflect the initial use and subsequent exhaustion of glycogen stores. Contrary to other birds and mammals, -hydroxy-butyrate levels increased markedly with flight duration. This may suggest a more important sparing of carbohydrates and protein as gluconeogenic precursors in the pigeon than in other species. Plasma uric acid levels increased linearly up to about 4 h flight duration. This indicates an accelerated protein breakdown during flight which may primarily serve to deliver amino acids as glucogenic precursors and citrate cycle intermediates. With increasing flight duration, the energy sources change from an initial phase based primarily on carbohydrates to a lipid-based endurance phase. It is discussed whether this metabolic change depends on the level of power output or the performed work (energy spent) since the start of flight. During the first hour of recovery, most metabolites reached or approached fasting levels at rest, indicating a marked reduction in lipolysis and protein breakdown. -hydroxy-butyrate levels remained at flight levels and glucose levels increased slightly, indicating a restoration of glycogen stores.Abbreviations VLDL very-low-density lipoproteins - FFA free fatty acids     

Corticosterone secretion patterns prior to spring and autumn migration differ in free-living barn swallows (Hirundo rustica L.)

Recent studies of long-distance migratory birds show that behavioural and physiological changes associated with predictable or unpredictable challenges during the annual cycle are distinctively regulated by hormones. Corticosterone is the primary energy regulating hormone in birds. Corticosterone levels are elevated during stresses but they are also modulated seasonally according to environmental conditions and life-history demands. We measured the baseline and stress-induced levels of corticosterone in the barn swallow (Hirundo rustica L.) just before spring and autumn migrations in South Africa and Finland, respectively. Barn swallows completing their pre-breeding moult had low body condition (residual body mass) and high baseline corticosterone levels in the wintering grounds. In contrast, baseline corticosterone levels in Finland were low and not related to residual mass. These data contradict the first prediction of the migration modulation hypothesis (MMH) by showing no association with baseline corticosterone levels and pre-migratory fuelling. Yet, the adrenocortical response to the capture and handling stress was notably blunted in South Africa compared to a strong response in Finland. Further, individuals that had started fuelling in Finland showed a reduced response to the handling stress. Taken together, elevated baseline corticosterone levels and high residual mass may blunt the adrenocortical response in long-distance migrants and aerial feeders such as the barn swallow. This observation lends support to the second prediction of the MMH.     

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     

Body mass and clutch size may modulate prolactin and corticosterone levels in eiders

Altered body condition, increased incubation costs, and egg loss are important proximate factors modulating bird parental behavior, since they inform the adult about its remaining chances of survival or about the expected current reproductive success. Hormonal changes should reflect internal or external stimuli, since corticosterone levels (inducing nest abandonment) are known to increase while body condition deteriorates, and prolactin levels (stimulating incubation) decrease following egg predation. However, in a capital incubator that based its investment on available body reserves and naturally lost about half of its body mass during incubation, corticosterone should be maintained at a low threshold to avoid protein mobilization for energy supply. This study focused on the regulation of corticosterone and prolactin release in such birds during incubation, when facing egg manipulation (control, reduced, or increased) or a stressful event. Blood samples were taken before and after clutch manipulation and at hatching. Corticosterone levels were determined before and after 30 min of captivity. Female eiders exhibited a high hypothalamic-pituitary-adrenal sensitivity, plasma concentration of corticosterone being increased by four- to fivefold following 30 min of captivity. The adrenocortical response was not modified by body mass loss but was higher in birds for which clutch size was increased. In the same way, females did not show different prolactin levels among the experimental groups. However, when incubation started, prolactin levels were correlated to body mass, suggesting that nest attendance is programmed in relation to the female initial body condition. Moreover, due to an artifactual impact of bird manipulation, increased baseline corticosterone was associated with a prolactin decrease in the control group. These data suggest that, in eiders, body mass and clutch size modification can modulate prolactin and corticosterone levels, which cross-regulate each other in order to finely control incubation behavior.     

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.     

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.     

Baseline and stress-induced levels of corticosterone during different life cycle substages in a shorebird on the high arctic breeding grounds

After a migratory flight of several thousand kilometers to their high arctic breeding grounds, red knots (Calidris canutus islandica, Scolopacidae) showed high baseline concentrations of plasma corticosterone (58 ng/mL). Such high baseline corticosterone levels may be conditional for the right behavioral and metabolic adjustments to environmental and social stresses that shorebirds experience on arrival in an unpredictable tundra breeding environment. Despite the high baseline levels of corticosterone, red knots still showed a marked stress response during the postarrival period, with corticosterone concentrations increasing significantly during a 60-min period of confinement. Baseline levels of corticosterone declined as the breeding season progressed. Red knots with brood patches, that is, birds that had completed egg laying and commenced incubation, had a reduced adrenocortical response to the stress of confinement compared with red knots with no, or with half-developed, brood patches. This is consistent with the idea that birds breeding in extreme environments with short breeding seasons may exhibit a decreased adrenocortical response to stressful events to prevent high corticosterone concentrations from inducing interruptions of reproductive behavior.     

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.     

During stopover, migrating blackcaps adjust behavior and intake of food depending on the content of protein in their diets

During migration, birds undergo alternating periods of fasting and re-feeding that are associated with dynamic changes in body mass (m(b)) and in organ size, including that of the digestive tract. After arrival at a migratory stopover site, following a long flight, a bird must restore the tissues of its digestive tract before it can refuel. In the present study we examined how the availability of dietary protein influences refueling of migrating blackcaps (Sylvia atricapilla) during a migratory stopover. We tested the following predictions in blackcaps deprived of food and water for 1-2 days to induce stopover behavior: (1) birds provided with a low-protein diet will gain m(b), lean mass and fat mass, and increase in pectoral muscle size slower than do birds fed a high-protein diet; (2) since stopover time is shorter in spring, birds will gain m(b) and build up fat tissue and lean tissue faster than in autumn; and (3) if low dietary protein limits a bird's ability to gain m(b) and fat reserves, then birds that do not obtain enough protein will initiate migratory restlessness (Zugunruhe) earlier than will birds with adequate dietary protein. These predictions were tested by providing captured migrating blackcaps with semisynthetic isocaloric diets differing only in their protein content. Each day, we measured m(b), and food intake; also lean mass and fat mass were measured using dual energy X-ray absorptiometry. In addition, we monitored nocturnal activity with a video recording system. In both spring and autumn, birds fed diets containing either 3 or 20% protein increased in m(b), lean mass and fat mass at similar rates during the experiment. However, the group receiving 3% protein ate more than did the group receiving 20% protein. In support of our predictions, m(b), lean mass, fat mass, and intake of food all were higher in spring than in autumn. We also found that in spring all birds had higher levels of migratory restlessness, but birds fed 3% protein were less active at night than were birds fed 20% protein, possibly an adaptation conserving energy and protein. We conclude that protein requirements of migrating blackcaps during stopover are lower than expected, and that birds can compensate for low dietary protein by behavioral responses, i.e. hyperphagia and decreased migratory restlessness, that ensure rapid refueling.     

Corticosterone, locomotor performance, and metabolism in side-blotched lizards (Uta stansburiana)

Elevated levels of circulating corticosterone commonly occur in response to stressors in wild vertebrates. A rise in corticosterone, usually in animals of subordinate rank, results in a variety of effects on behavior and physiology. Behavioral and physiological responses to short-term increases in corticosterone are well studied. In contrast, the effects of chronic elevated levels of corticosterone are poorly understood, particularly in lizards. Here, we examined the long-term effects of exogenous corticosterone on locomotor performance, resting and active metabolic rate, and hematocrit in male side-blotched lizards Uta stansburiana. Corticosterone implantation resulted in higher levels of stamina relative to sham-surgery controls. In addition, lizards with elevated corticosterone exhibited lower resting metabolic rates relative to controls. Corticosterone had no effect on peak activity metabolism but did result in faster recovery times following exhaustive exercise. We suggest that elevated levels of corticosterone in response to dominance interactions promote enhanced locomotor abilities, perhaps as a flight response to avoid agonistic interactions. Furthermore, stressed lizards are characterized by lower resting metabolic rates, which may serve as strategy to conserve energy stores and enhance survival.     

Effect of endurance flight on haematocrit in migrating birds

The effects of an endurance flight on the haematocrit, the percentage of packed red blood cells per blood volume, were examined within the framework of six possible factors explaining possible changes in the haematocrit. Two approaches were adopted: (1) the haematocrit was studied in four species of passerine birds which landed on an Italian island after having crossed the Mediterranean Sea on their spring migration in a non-stop flight; (2) the haematocrit was evaluated in six individual red knots after a flight of 1, 2, 4 and 10 h in a wind tunnel and the data thus obtained compared with data on resting birds with or without food. In the four passerine species, the haematocrit decreased from 51% in fat birds to 48% in lean birds. In lean birds, the haematocrit dropped from 48% in birds with well-developed breast muscles to 36% in birds with emaciated breast muscles. In the red knots, the haematocrit was dependent on body mass in flying and resting birds. The haematocrit decreased from about 51% pre-flight to about 49% within 1 h of flight and remained at this level for up to 10 h of flight. Taking the results from the passerines and the red knots together, it seems that the haematocrit drops by a few percentage points within 1 h after the onset of flight, decreases very slowly with decreasing body mass and decreases more steeply in very lean birds having entered stage III of fasting. This indicates that dehydration is not an underlying factor in decreased haematocrit because if this were the case we would expect an increase with endurance flight. We found no effect of the presence of blood parasites on haematocrit. With the onset of flight, haemodilution may be adaptive, because it reduces blood viscosity and, thereby, energy expenditure by the heart, or it may be a sign of water conservation as an insurance against the risk of dehydration during long non-stop flights. During endurance flight, a reduction in the haematocrit may be adaptive, in that oxygen delivery capacity is adjusted to the decreased oxygen needs as body mass decreases. A decreasing haematocrit would also allow birds to reduce heart beat frequency and/or heart size, because blood viscosity decreases disproportionally with decreasing haematocrit. However, when energy stores are about to come to an end and birds increase protein breakdown, the haematocrit decreases even further, and birds probably become anaemic due to a reduced erythropoiesis.     

Focus: The Science of Stress: The Effect of a Combined Fast and Chronic Stress on Body Mass,Blood Metabolites,Corticosterone, and Behavior in House Sparrows (Passer domesticus)

One aspect of the Reactive Scope Model is wear-and-tear, which describes a decrease in an animal’s ability to cope with a stressor, typically because of a period of chronic or repeated stressors. We investigated whether wear-and-tear due to chronic stress would accelerate a transition from phase II to phase III of fasting. We exposed house sparrows (Passer domesticus) to three weeks of daily fasts combined with daily intermittent repeated acute stressors to create chronic stress, followed by two weeks of daily fasts without stressors. We measured circulating glucose, β-hydroxybutyrate (a ketone), and uric acid in both fasted and fed states. We expected birds to be in phase II (high fat breakdown) in a fasted state, but if wear-and-tear accumulated sufficiently, we hypothesized a shift to phase III (high protein breakdown). Throughout the experiment, the birds exhibited elevated β-hydroxybutyrate when fasting but no changes in circulating uric acid, indicating that a transition to phase III did not occur. In both a fasted and fed state, the birds increased glucose mobilization throughout the experiment, suggesting wear-and-tear occurred, but was not sufficient to induce a shift to phase III. Additionally, the birds exhibited a significant decrease in weight, no change in corticosterone, and a transient decrease in neophobia with chronic stress. In conclusion, the birds appear to have experienced wear-and-tear, but our protocol did not accelerate the transition from phase II to phase III of fasting.     

Baseline and stress-induced plasma corticosterone during long-distance migration in the bar-tailed godwit, Limosa lapponica

The specific roles of corticosterone in promotion of avian migration remain unclear even though this glucocorticosteroid is elevated in many migrating bird species. In general, glucocorticosteroids promote metabolic homeostasis and may elicit effects on feeding and locomotion. Because the migratory stages of refueling and flight are characterized by distinct behaviors and physiology, the determination of corticosterone levels during each stage should help identify potential processes in which corticosterone is involved. We measured baseline levels of corticosterone in bar-tailed godwits (Limosa lapponica) during two distinct stages of migration: (1) immediately after arrival at a false stopover site just short of the Wadden Sea and (2) throughout the subsequent 4-wk refueling period on the Wadden Sea. Plasma corticosterone was higher in arriving than in refueling birds. In addition, corticosterone increased with size-corrected body mass during the refueling phase, suggesting that corticosterone rises as birds prepare to reinitiate flight. Therefore, elevated corticosterone appears associated with migratory flight and may participate in processes characterizing this stage. We also performed a capture stress protocol in all birds and found that corticosterone increased in both arriving and refueling godwits. Therefore, the normal course of migration may be typified by corticosterone concentrations that are lower than those associated with stressful and life-threatening episodes.     

A biochemical study of fasting, subfeeding, and recovery processes in yellow-legged gulls

An investigation of the effects of fasting, subfeeding, and refeeding on plasma biochemistry was carried out on 22 captive yellow-legged gulls Larus cachinnans Pallas. These birds showed the same fasting endurance model described in other species, but with an important decrease in glucose plasma concentration and very great differences between individuals when reaching the deterioration limit, suggesting a moderate physiological adaptation to long periods of fasting. A different model was proposed in subfed gulls in relation to fasted gulls, based on lipid and protein use, which could be reflected by changes in nitrogen wastes and triglyceride levels in this experiment. Thus, the subfed gulls might use protein directly from the diet as an energy source, thereby reducing the use of fat stores. The gulls quickly recovered body mass during the refeeding period, but while some plasma substances quickly reached their initial values, others showed many changes before the end of the experiment, which could reflect a process of metabolic restabilization. These results contribute to a better knowledge of fasting, subfeeding, and refeeding processes in birds and can be added to a recent study about fasting in gulls.