The autumnal fattening of the long-distance migratory garden warbler (Sylvia borin) is stimulated by intermittent fasting

To investigate the proximate influence of a changing food availability on the seasonal fattening of migratory birds, garden warblers (Sylvia borin) following postnuptial moult were food restricted once a week. Body mass, food intake, plasma hormone and metabolite levels were measured and compared to birds which always had ad libitum food access. The food-restricted birds increased their body mass significantly earlier than the controls. The accelerated fattening was initially not accompanied by hyperphagia and may be due to either an increased food utilisation efficiency or a reduced metabolic rate. An increase of basal glucagon and corticosterone and a decrease of insulin levels prior to fattening were not significant, however, they resulted in a significant decrease of the insulin:glucagon ratio. This ratio was also lower in food-restricted birds than in control birds and may account for the difference in the fattening progress. We conclude that seasonal fattening may be stimulated by a catabolic impulse which could be imposed in free-living birds by a decrease of food availability and/or by an increase of energy expenditure. A negative energy balance is hypothesised to be a common proximate factor affecting migratory as well as winter fattening. Accepted: 24 August 2000     

The role of corticosterone in supporting migratory lipogenesis in the dark-eyed Junco, Junco hyemalis: a model for central and peripheral regulation

The functional role of corticosterone (CORT) in regulating migratory hyperphagia and lipogenesis was investigated in an annual migrant, the dark-eyed junco (Junco hyemalis). Intraperitoneal injections of either dexamethasone (9 microg DXM/500 microL of 5% EtOH in saline, n=10) to inhibit an increase in baseline CORT or saline (5% EtOH, n=9) were given every 48 h for 15 d after transfer from short (10.5L:13.5D) to long (15.5L:8.5D) days. Food intake, body mass, furcular fat deposition scores, and nocturnal migratory activity were recorded for 29 d after photostimulation. Both groups showed the same increase in daily food intake over the study period (DXM=52%, control=41%). Controls began to increase baseline CORT and mass about 2 wk after photostimulation. DXM-treated birds maintained low CORT and did not increase mass or CORT until injections ceased, at which time they gained mass at the same rate shown earlier by controls. DXM-treated birds did not show greater levels of migratory activity despite experiencing an increase in energy intake during the CORT-inhibited period. Collectively, the results support the migration modulation hypothesis, illustrating how an increase in baseline CORT is needed to support the development of migratory condition. We address the apparent conflict with earlier studies on CORT and migratory food intake and propose a model in which migratory hyperphagia is supported by changes in centrally regulated responses to CORT that can occur even if CORT remains low and lipogenesis is regulated predominantly by peripheral mechanisms that require an increase in baseline CORT.     

迁徙鸟类中途停歇期的生理生态学研究

大多数候鸟的迁徙活动由迁徙飞行和中途停歇两个部分组成。在迁徙过程中,鸟类要多次交替经历消耗能量的飞行阶段和积累能量的中途停歇阶段。从鸟类在中途停歇时期的能量积累速度、体重变化模式以及迁徙飞行中的禁食或食物限制、食物种类的改变、中途停歇的能量快速积累过程对消化器官的影响等方面,对目前迁徙鸟类的生理生态学研究成果进行回顾,并提出有待解决的问题及今后的研究方向。     

Digestive tract function in the long-distance migratory garden warbler,Sylvia borin

Digestive tract morphology and function of captive garden warblers (Sylvia borin) were measured during four stages of their endogenous circannual rhythm: before, during and after their autumn fattening prior to migration to wintering grounds in Africa, and after a partially simulated migratory flight. Food intake increased by 33% during fattening, utilization efficiency of dry matter tended to increase, and that of energy increased significantly (P0.01). This was because digestive tract capacity (measured as dry tissue mass) increased, so that mean retention time of food remained constant before, during and after fattening (80–84 min). After a 48-h period of starvation of fattened birds to partially simulate a migratory flight, food intake was lower on the first day of refeeding than on the next 4 days, and utilization efficiency was higher on that day, at least partly because of a longer mean retention time (111 min versus 78 min on the third day). Digestive tract dry tissue mass fell by 50% during starvation, and that of the small intestine by 63%. It is concluded that the garden warbler adapts to long-distance migration without feeding by rapidly reducing the size of its digestive tract, an expensive tissue to maintain, during migration in order to save weight and energy, and possibly also to supply part of the fuel and protein required for the flight. The cost of this strategy appears to be the time taken to rebuild the gut at stopover sites with food, but the low probability of finding such a site in the Sahara Desert means that this strategy is probably optimal for garden warblers.     

Annual changes in body mass in captive Svalbard ptarmigan: role of changes in locomotor activity and food intake

The influence of changes in the amount of locomotor activity on the annual body mass cycle was investigated in captive Svalbard ptarmigan kept indoors at thermoneutrality and exposed to seasonal changes in daylength or continuous light from the summer onwards. In both groups there was a close correlation between locomotor activity and metabolic rate. Only birds exposed to changes in daylength showed an annual cycle in locomotor activity, with low activity in autumn and mid-winter and a peak in spring. The birds permanently exposed to continuous light had a relatively low activity throughout the year with no systematical changes. Body mass began to increase in both groups in early autumn and the food intake was elevated during most of the following fattening period. It is concluded that elevated food intake is the prime factor involved in autumnal fattening in captive Svalbard ptarmigan. Body mass increased significantly faster under decreasing daylength compared with continuous light, associated with a lower activity as well as a higher food intake. The birds exposed to continuous light maintained a high body mass and a relatively low activity level during spring. In birds exposed to changing daylength, body mass fell from late February onwards, which is about 3 months later than in outdoor caged or free-living Svalbard ptarmigan. In the birds exposed to increasing daylength a fourfold increase in the amount of locomotor activity occurred from February to April. This increased activity was correlated with a negative energy balance and may be casually associated with the fall in body mass in these birds. Under outdoor conditions, elevated locomotor activity in spring may be responsible for a continuation of the decline in body mass commencing in November, despite a slight tendency for an increased food intake towards the end of this period.Abbreviations BM body mass - CE _f caloric equivalent of food - EAE energy assimilation efficiency - EE energy expenditure - FI food intake - LA locomotor activity - LL continuous light - LD simulated annual changes in daylength - MEI metabolizable energy intake - MR metabolic rate - RQ respiratory quotient     

Metabolic consequences of overlapping food restriction and cell‐mediated immune response in a long‐distance migratory shorebird,the little ringed plover Charadrius dubius

Investment in immunity is commonly viewed as an energetically costly activity in birds. Although several studies have focused on the energy cost of mounting an immune response and its concomitant physiological trade‐offs, nothing is known about the metabolic adjustments experienced by immunochallenged birds under resource limitation, or about the basal metabolism cost of mounting cell‐mediated immune (CMI) responses in bird species other than non‐migratory passerines. Here we measured the basal metabolic rate (BMR), inflammatory response, and body mass in ad libitum fed and food‐restricted little ringed plovers Charadrius dubius challenged with phytohemagglutinin (PHA) in order to assess the energy cost, the strength, and the time course of the CMI response in a long‐distance migratory bird in different nutritional states. We found that ad libitum birds injected with PHA significantly increased both mass‐independent BMR and inflammatory response, whereas birds with an induced food restriction‐immune response overlap experienced a mass‐independent BMR downregulation and decreased inflammatory response relative to ad libitum birds. We suggest that both the BMR downregulation and the diminished inflammatory response observed in birds facing such an overlap could be energy‐saving mechanisms to maintain the body mass above a critical level and maximize fitness.     

Diet quality and food limitation affect the dynamics of body composition and digestive organs in a migratory songbird (Zonotrichia albicollis)

Migrating songbirds interrupt their feeding to fly between stopover sites that may vary appreciably in diet quality. We studied the effects of fasting and food restriction on body composition and digestive organs in a migratory songbird and how these effects interacted with diet quality to influence the rate of recovery of nutrient reserves. Food limitation caused white-throated sparrows to reduce both lean and fat reserves, with about 20% of the decline in lean mass represented by a decline in stomach, small intestine, and liver. During refeeding on diets similar in nutrient composition to either grain or fruit, food-limited grain-fed birds ate 40% more than did control birds, and they regained body mass, with on average 60% of the increase in body mass composed of lean mass including digestive organs. In contrast, food-limited fruit-fed birds did not eat more than did control birds and did not regain body mass, suggesting that a digestive constraint limited their food intake. The interacting effects of food limitation and diet quality on the dynamics of body composition and digestive organs in sparrows suggest that the adequacy of the diet at stopover sites can directly influence the rate of recovery of body reserves in migrating songbirds and hence the pace of their migration.     

Thermogenic side effects to migratory predisposition in shorebirds

In the calidrine sandpiper red knot (Calidris canutus), the weeks preceding takeoff for long-distance migration are characterized by a rapid increase in body mass, largely made up of fat but also including a significant proportion of lean tissue. Before takeoff, the pectoral muscles are known to hypertrophy in preparation for endurance flight without any specific training. Because birds facing cold environments counterbalance heat loss through shivering thermogenesis, and since pectoral muscles represent a large proportion of avian body mass, we asked the question whether muscle hypertrophy in preparation for long-distance endurance flight would induce improvements in thermogenic capacity. We acclimated red knots to different controlled thermal environments: 26 degrees C, 5 degrees C, and variable conditions tracking outdoor temperatures. We then studied within-individual variations in body mass, pectoral muscle size (measured by ultrasound), and metabolic parameters [basal metabolic rate (BMR) and summit metabolic rate (M(sum))] throughout a 3-mo period enclosing the migratory gain and loss of mass. The gain in body mass during the fattening period was associated with increases in pectoral muscle thickness and thermogenic capacity independent of thermal acclimation. Regardless of their thermal treatment, birds showing the largest increases in body mass also exhibited the largest increases in M(sum). We conclude that migratory fattening is accompanied by thermoregulatory side effects. The gain of body mass and muscle hypertrophy improve thermogenic capacity independent of thermal acclimation in this species. Whether this represents an ecological advantage depends on the ambient temperature at the time of fattening.     

Availability of water affects renewal of tissues in migratory blackcaps during stopover

Migrating blackcaps (Sylvia atricapilla) were used to test the predictions that (1) the rebuilding of the digestive tract, as reflected by mass-specific consumption of food on the first 2-3 days of a stopover, is faster in birds with access to drinking water than in birds without, and (2) that adipose tissue and pectoral muscles grow faster and to a greater extent in birds with unlimited access to water. We simulated migratory stopover in two experiments. In Experiment I, each of 31 birds was randomly assigned to one of three experimental groups for 6 days. Along with mealworms (～64% water) ad libitum, Group 1 received drinking water ad libitum; Group 2 had 0.5 h/day access to water; and Group 3 had no access to water. In Experiment II, 30 birds were offered a mixed diet for insectivorous birds (～33% water) ad libitum for 6 days, while randomly assigned to two groups: (1) Water ad libitum-control; and (2) 30 min access to water twice a day. We measured lean mass and fat mass using dual energy X-ray absorptiometry, as well as body mass (m(b)), pectoral muscle index (PMI), and daily intake of food and water. Mean daily water intake was significantly different among the groups in both experiments. However, the availability of drinking water positively affected the rates of gain of lean and fat mass only in birds fed with the mixed, relatively dry diet. Furthermore, mass-specific daily food intake was affected by the availability of drinking water only in the mixed diet experiment, in which birds with unlimited access to drinking water reached an asymptote, 1 day earlier than birds in the water-restricted group. We suggest that in birds consuming diets with low water content, the lack of sufficient drinking water may result in slower rebuilding of the digestive tract, or may influence biochemical processes in the gut that result in slower growth of tissue. Although blackcaps obtained sufficient water from preformed and metabolic water to renew lost tissues when eating mealworms, given access to water, the birds drank prodigiously. Our results also suggest that if drinking water is unavailable to migrating blackcaps, their choices are restricted to water-rich foods, which may constrain their rate of feeding and thus the rate at which they deposit fat. Consequently, drinking water may have an important influence on birds' migratory strategies with respect to habitat selection, use of energy, and the saving of time.     

Phenotypic flexibility in digestive system structure and function in migratory birds and its ecological significance

Birds during migration must satisfy the high energy and nutrient demands associated with repeated, intensive flight while often experiencing unpredictable variation in food supply and food quality. Solutions to such different challenges may often be physiologically incompatible. For example, increased food intake and gut size are primarily responsible for satisfying the high energy and nutrient demands associated with migration in birds. However, short-term fasting or food restriction during flight may cause partial atrophy of the gut that may limit utilization of ingested food energy and nutrients. We review the evidence available on the effects of long- and short-term changes in food quality and quantity on digestive performance in migratory birds, and the importance of digestive constraints in limiting the tempo of migration in birds. Another important physiological consequence of feeding in birds is the effect of diet on body composition dynamics during migration. Recent evidence suggests that birds utilize and replenish both protein and fat reserves during migration, and diet quality influences the rate of replenishment of both these reserves. We conclude that diet and phenotypic flexibility in both body composition and the digestive system of migratory birds are important in allowing birds to successfully overcome the often-conflicting physiological challenges of migration.     

Energy metabolism, thermogenesis and body mass regulation in tree shrew (Tupaia belangeri) during subsequent cold and warm acclimation

Environmental cues play important roles in the regulation of an animal's physiology and behavior. The purpose of the present study was to test the hypothesis that ambient temperature is a cue to induce adjustments in body mass, energy intake and thermogenic capacity, associated with changes in serum leptin levels in tree shrews (Tupaia belangeri). We found that tree shrews increased basal metabolic rate (BMR), energy intake and subsequently showed a significant decrease in body mass after being returned to warm ambient temperature. Uncoupling protein 1 (UCP1) content in brown adipose tissue (BAT) increased during cold acclimation and reversed after rewarming. The trend of energy intake increased during cold acclimation and decreased after rewarming; the trend of energy intake during cold acclimation was contrary to the trend of energy intake during rewarming. Further, serum leptin levels were negatively correlated with body mass. Together, these data supported our hypothesis that ambient temperature was a cue to induce changes in body mass and metabolic capacity. Serum leptin, as a starvation signal in the cold and satiety signal in rewarming, was involved in the processes of thermogenesis and body mass regulation in tree shrews.     

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

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

Testosterone and opioids interact to regulate feeding in a male migratory songbird.

A male migratory songbird (dark-eyed junco, Junco hyemalis) was used as a model for studies on the influence of testosterone (T) on feeding, and on interactive effects on this behavior between T and the opioid antagonist naloxone hydrochloride (Nal). Administered chronically to birds exposed to nonstimulating photoperiods, T increased food intake by 30-58% without altering the body mass, the fat index, or the standard metabolic rate. An intramuscular injection of Nal decreased feeding temporarily in a dose-related manner. T-treated juncos exhibited a decreased sensitivity to the anorexic influence of Nal administration, demonstrating that T interacts with opioids to control food consumption. Neuroendocrine mechanisms that potentially account for this interaction are discussed.     

Metabolic effects of low-energy diet on steller sea lions, Eumetopias jubatus

Diets of six Steller sea lions (Eumetopias jubatus) were switched between a high (herring) and a low (squid) energy density food for 14 d to determine the effects on ingested prey mass, body mass, resting metabolic rate, and the heat increment of feeding. Body mass was measured daily, and resting metabolism was measured weekly by gas respirometry. Ingested food mass did not differ significantly between the squid diet and the control or the recovery herring diet periods. As a result of differences in energy density, gross energy intake was significantly lower during the squid diet phase than during either the control or recovery periods. As a result, sea lions lost an average of 1.1 kg/d, totaling 12.2% of their initial body mass by the end of the experimental period. The heat increment of feeding for a 4-kg squid meal was significantly lower than for a similarly sized meal of herring. Decreases in both absolute (24.0 to 18.0 MJ/d, -24%) and mass-corrected (903 to 697 kJ/d/kg0.67, -20%) metabolism were observed by the end of the squid feedings. This study suggests that sea lions can depress their resting metabolism in response to decreases in energy intake or body mass, regardless of satiation level.     

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.     

随机限食与重喂食小鼠的能量代谢和行为的可塑性

分别测定了随机限食和重喂食驯化的雌性KM小鼠的体重、摄食量、基础代谢率(BMR)、行为活动、身体脂肪和性腺重量.随机限食使摄食量增加、BMR和活动行为降低,生长发育迟缓,但对身体脂肪无显著影响.重喂食后上述指标均恢复到对照组水平,表现出显著的可塑性变化.结果表明,动物通过能量摄入和支出的权衡策略适应难以预测的食物资源变化,能量代谢和活动行为的可塑性调节在能量代谢的权衡策略中发挥重要作用.     

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.     

单宁酸对布氏田鼠能量代谢的影响

为了解单宁酸对成年布氏田鼠(Lasiopodomys bandtii**)能量代谢和产热的影响,本文采用含0、3.3%和6.6%单宁酸浓度的食物饲喂布氏田鼠21 d,对其体重、基础代谢率、非颤抖性产热和能量收支等进行了测定。代谢率采用封闭式流体压力呼吸计测定;非颤抖性产热用皮下注射去甲肾上腺素诱导;能量摄人采用食物平衡法测定。结果发现:(1) 单宁酸食物对布氏田鼠的体重没有明显影响;(2)取食含6.6%单宁酸食物的动物的基础代谢率于第10 d高于对照组。20 d时,3组动物的基础代谢率没有显著差异;(3) 单宁酸食物对非颤抖性产热没有显著影响;(4) 食用含单宁酸食物的动物的摄人能和消化能于第10 d显著低于对照组,但第20 d时则差异不显著。这些结果表明:布氏田鼠的基础代谢率和能量摄入对单宁酸的反应具有时段性,短期内能量消耗增加,随着动物对食物的适应,生理功能恢复到正常水平。     

Energy metabolism, testosterone and corticosterone in white-crowned sparrows

The influence of the steroid hormones testosterone and corticosterone on energy metabolism and activity of birds is largely enigmatic. We measured resting metabolic rate during night and day in 12 long-term castrated and 12 intact male white-crowned sparrows (Zonotrichia leucophrys gambelii) under short-day (8:16 SD), long-day (20:4 LD), LD+testosterone implant and LD−testosterone implant conditions. Each male was sequentially measured under all four conditions. Photostimulation increased testosterone, resting metabolic rate, food intake, hopping activity and body mass in castrates and intact males. Surprisingly, testosterone levels and metabolic rates did not differ between intact and castrated males. Testosterone implantation increased activity and food intake, but decreased body mass and resting metabolic rate in both groups. Removing testosterone implants reversed the effects on resting metabolic rate, activity and food intake. Corticosterone levels, measured immediately at the end of metabolism measurements, showed birds were not stressed. Corticosterone had no apparent relationship with resting metabolic rate and there was no interaction between corticosterone and testosterone. Overall, positive changes in testosterone levels resulted in a decrease of resting metabolic rate. We speculate that testosterone increases activity, and birds compensate for increased activity metabolism by reducing resting metabolic rate. Accepted: 18 July 1999     

Protein utilization during starvation in fat and lean Svalbard ptarmigan (Lagopus mutus hyperboreus)

Summary Body protein sparing during starvation has been examined in fat and lean Svalbard ptarmigan. Protein utilization was determined from daily N excretion and from the rate of decrease in body mass. Changes in plasma concentrations of -hydroxybutyrate, free fatty acids, glucose, and uric acid were also recorded. When fat birds were starved for 15 days protein catabolism initially fell (phase I) and was thereafter kept low (phase II). This was evident from the temporal pattern in both N excretion and body mass loss. In two birds, N excretion eventually increased, revealing enhanced protein catabolism and thus a third phase of starvation. Changes in protein utilization were paralleled by changes in plasma uric acid. Approximately 9% of the energy demand was covered by breakdown of body protein during phase II. The importance of fat catabolism in providing energy was indicated by markedly elevated plasma levels of -hydroxybutyrate and free fatty acids. When lean birds were starved for 5 days there appeared to be no phase II. The temporal pattern of body mass loss indicated phase I and III but that of N excretion only phase III. The relative contribution of body protein to energy demand increased from 22% at day 2 to 41% at the end of starvation and was paralleled by increased plasma uric acid. When data from lean and fat birds were pooled, the changes in uric acid and N excretion were highly correlated (r=0.92, P<0.001), indicating that plasma uric acid is a reliable index of protein breakdown in starving Svalbard ptarmigan. In conclusion, starving fat Svalbard ptarmigan have a much greater capacity to spare body protein than lean birds. Fat birds effectively reduce protein catabolism and maintain this at a low level whereas starving lean birds increase protein catabolism.Abbreviations -OHB -hydroxybutyrate - BM body mass - BMR basal metabolic rate; dne daily nitrogen excretion - FFA free fatty acids - MR metabolic rate