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Migratory birds deposit fat and protein before passing ecological barriers and must economize these during such crossings. Birds crossing the western Sahara during autumn face a trade-off between cold and humid air along with head winds at high altitudes versus warm and dry air along with tail winds at low altitudes. Since water loss rate increases with temperature, migrants should avoid warm and dry air to save water and hence fly at high altitudes. By quantifying nocturnal songbird migration across the western Sahara with radar, we found that more than 60% of the songbirds migrated below 1000m above ground level. Thus, the majority of songbirds performed sustained migratory flights in much warmer and drier conditions than predicted (weighted means: 30°C; relative humidity: 27%; water vapour density: 7.8g/m3). Based on the metabolically available water from fat and protein catabolism, we estimated the maximum possible overall water loss rate of a flying model bird, the garden warbler, Sylvia borin, for the entire Sahara crossing at 0.29g/h. This is considerably lower than water loss rates for the same model bird passing our study site, 0.62g/h at 30°C, based on applied calculations of physiological studies. Our results clearly show that migrating songbirds can fly at much higher temperatures, and have considerably lower water loss rates, than predicted. This new insight based on observations under natural conditions will have substantial impact on the development of new physiological models for birds and other animals with restricted access to water. [Copyright &y& Elsevier]

Abstract: Long-distance migrants face the challenge of a short window for reproduction that requires optimal timing and full functional gonads. Male garden warblers (Sylvia borin) meet these demands by initiating testicular recrudescence during spring migration, enabling them to reproduce immediately after arrival at the breeding grounds. In a combined field and laboratory study, we investigated testicular size, plasma levels of luteinizing hormone (LH), androstenedione (AE), 5α-dihydrotestosterone (DHT), testosterone and nocturnal migratory restlessness (Zugunruhe) under different stopover conditions. We manipulated food availability, the duration of stopover and simulated migration by food deprivation. Garden warblers showed significantly retarded testicular development after nine days of stopover under limited food conditions compared to birds that had ad libitum access to food. However, there was no significant difference in Zugunruhe between the two groups. Thus, the degree of Zugunruhe was unaffected by the quality of the stopover site and migration continued independent of the developmental state of the testis. We suggest that male garden warblers face the necessity to either compensate for slowed testicular recrudescence during the subsequent leg of migration and delay arrival at the breeding grounds, or arrive with less developed testes. Either of these may reduce annual reproductive success. [Copyright &y& Elsevier]

The article studies the role of melatonin in bird migration and seeks a method to manipulate melatonin levels on a short-term basis. It investigates whether skin application of melatonin results in physiologically changes of plasma melatonin levels in two small passerine songbird species, the European stonechat and the garden warbler. The results of the study demonstrate that skin application of melatonin increases circulating levels of the hormone with a period of 30 to 60 min.

Large numbers of passerine migrants cross the Sahara desert every year on their way to-and-from wintering areas in tropical Africa. In the desert, hardly any fuelling opportunities exist and most migrants have to prepare in advance. A central question is how inexperienced birds know where to fuel. Inexperienced garden warblers Sylvia borin were studied in Greece just before the desert crossing in autumn. Body mass data collected at two sites indicate that most birds do not fuel for the desert crossing further north. For the first time, detailed information about stopover duration close to the Sahara desert was studied by using light weight radio-transmitters. Results from Crete show that most first-year garden warblers arrive with relatively small fuel loads in relation to lean body mass (<30%), stay for 13–20 d and depart with an average fuel load of about 100%. Radio-tagged birds performed small scale movements initially and took advantage of fig fruits. Birds trapped at fig trees were heavier than birds trapped with tape lures, showing that tape lures can bias the sample of migrants trapped. The precise fuelling pattern found indicates that first-year migrants must also include external spatial cues to make the preparation for crossing the desert in the right area. [ABSTRACT FROM AUTHOR]

Background: Foraging behaviour is an essential ecological process linking different trophic levels. A central assumption of foraging theory is that food selection maximises the fitness of the consumer. It remains unknown, however, whether animals use innate or learned behaviour to discriminate food rewards. While many studies demonstrated that previous experience is a strong determinant of complex food choices such as diet mixing, the response to simple nutritional stimuli, such as sugar concentrations, is often believed to be innate. Results: Here we show that previous experience determines the ability to track changes in sugar composition in same-aged individuals of a short-lived migratory songbird, the garden warbler (Sylvia borin). Although birds received identical foods for seven months prior to the experiment, wild-caught birds achieved higher sugar intake rates than hand-raised birds when confronted with alternative, differently coloured, novel food types. Hand-raised and wild birds did not differ in their initial colour selection or overall food intake, but wild birds were quicker to adjust food choice to varying sugar intake. Conclusion: Over a period of at least seven months, broader previous experience translates into a higher plasticity of food choice leading to higher nutrient intake. Our results thus highlight the need to address previous long-term experience in foraging experiments. Furthermore, they show that hand-raised animals are often poor surrogates for testing the foraging behaviour of wild animals. [ABSTRACT FROM AUTHOR]

Cluster N is a cluster of forebrain regions found in night-migratory songbirds that shows high activation of activity-dependent gene expression during night-time vision. We have suggested that Cluster N may function as a specialized night-vision area in night-migratory birds and that it may be involved in processing light-mediated magnetic compass information. Here, we investigated these ideas. We found a significant lateralized dominance of Cluster N activation in the right hemisphere of European robins ( Erithacus rubecula). Activation predominantly originated from the contralateral (left) eye. Garden warblers ( Sylvia borin) tested under different magnetic field conditions and under monochromatic red light did not show significant differences in Cluster N activation. In the fairly sedentary Sardinian warbler ( Sylvia melanocephala), which belongs to the same phyolgenetic clade, Cluster N showed prominent activation levels, similar to that observed in garden warblers and European robins. Thus, it seems that Cluster N activation occurs at night in all species within predominantly migratory groups of birds, probably because such birds have the capability of switching between migratory and sedentary life styles. The activation studies suggest that although Cluster N is lateralized, as is the dependence on magnetic compass orientation, either Cluster N is not involved in magnetic processing or the magnetic modulations of the primary visual signal, forming the basis for the currently supported light-dependent magnetic compass mechanism, are relatively small such that activity-dependent gene expression changes are not sensitive enough to pick them up. [ABSTRACT FROM AUTHOR]

Abstract: We quantified in the garden warbler (Sylvia borin) and the barn swallow (Hirundo rustica), two long-distance migratory songbirds, the early oxidative damage (ROMs) and plasma anti-oxidant capacity (OXY) variation of individuals caught at a stop-over site after a sustained flight across the sea, during spring migration. Our main goal was to quantify the oxidative damage and anti-oxidant capacity variation in these two migratory species in relation to fat and muscle stores. The birds were sampled in Ponza, a small island along the migratory route of these species. The levels of ROMs and OXY did not show any differences between the two species and in general were higher in individuals with higher fat and protein stores. Nevertheless, the balance between ROMs and OXY was better in individuals in good condition. These patterns were similar in both species. No sex differences emerged for both ROMs and OXY in the barn swallow, the only species that could be sexed. Both markers of oxidative stress did not show any significant variation across a 30-min restrained experiment. These data are the first of this kind in wild birds in a migratory context and suggest that individuals in better condition are exposed to lower oxidative stress, providing an indirect evidence of the oxidative cost caused by prolonged flights. [Copyright &y& Elsevier]

When confined to a cage, migratory songbirds exhibit nocturnal migratory restlessness (also called Zugunruhe) during the spring and autumn migratory periods, even though these birds are exclusively diurnal during the remainder of the year. Zugunruhe, which has been demonstrated to be under the direct control of a circannual timer, is characterized by a stereotypic "wing-whirring" behavior while the bird is perched. To elucidate the role played by the circadian system in the regulation of Zugunruhe, the authors studied the activity of garden warblers (Sylvia borin), long-distance nocturnal migrants, under skeleton photoperiods of different lengths and under constant dim light. In 11.5D:1L:10.5D:1L skeleton photoperiods, the authors found that Zugunruhe free-ran in a substantial proportion of birds, while their normal daily activities (e.g., feeding and preening) remained synchronized to 24 h. Some birds expressing Zugunruhe under constant dim light continued to show 2 distinct bouts of activity: one corresponding to daily activities, the other to wing-whirring. In some cases, these 2 bouts crossed while free-running with different periods. Birds expressing Zugunruhe also had significantly longer free-running periods than birds that did not. The study data suggest that the seasonal appearance of Zugunruhe is the result of the interactions of at least 2 circadian oscillators and that it is the phase relationship of these 2 oscillators that determines when nocturnal migratory restlessness is expressed. Furthermore, these data are consistent with the previously proposed internal coincidence hypothesis as a model for the ontogeny of circannual rhythms. [ABSTRACT FROM AUTHOR]

The Garden Warbler is a classic subject for the study of Palaearctic–African bird migration strategies. Most studies have considered the situation close to the breeding areas, while the African and especially the sub-Saharan part of the species’ migration have received comparatively little attention. Here we use autumn and spring ringing data from Nigeria and The Gambia to study the movements and energetics of the species in West Africa during the non-breeding season. The first Garden Warblers arrive south of the desert around the beginning of September, roughly at the same time as the median date for their passage through the Baltic Sea region and c. 3 weeks before their median passage date through southern Italy. In the Nigerian Sahel savannahs, where, owing to the rainy season and its associated increase in food availability, many more Garden Warblers stop over in autumn than in the dry spring, the median date of passage is 1 October. The body mass on arrival south of the desert is normally only a few grams more than the lean body mass (LBM; 15 g) – with a mean of 16.6 g (sd = ±1.8 g) in The Gambia and 17.4 g (sd = ±1.8 g) in the Nigerian Sahel. After resting and refuelling in the Sahel, Sudan and Guinea-type savannahs the Garden Warblers depart during November–December for wintering areas further south. Before leaving, they again increase their body mass, with an average fuel load of c. 20%, and often more than 50% relative to LBM. Some of the birds passing through Nigeria probably spend midwinter around the Congo Basin. During spring they return northwards to the Guinea savannah zone in April and fuel-up there for the trans-Sahara passage. At this time they normally increase their body reserves to around 50% of the LBM, but c. 10% of the birds gain 100%, thus doubling their mass. The passage there peaks around 20 April and continues well into May. That the main take-off northwards is directly from the Guinea savannahs is indicated by the very low numbers trapped in the Sahel during spring. [ABSTRACT FROM AUTHOR]

Animals are often confronted with environmental challenges and the way in which they cope with these challenges can have important fitness consequences. There is increasing evidence that individuals differ consistently in their reaction to the environment (personality traits). However, little is known about whether different life-styles (e.g., resident or migratory) influence personality traits and if so, in what manner. We compared neophobic and exploratory behaviours, both of which play an important role in personality traits, between two closely related species, the resident Sardinian warblers and the migratory garden warblers, at two different times during the year. Neophobia was tested by placing a novel object, a mop, beside the feeding dish and measuring the latency to feed (neophobia score). Exploration was tested by offering another novel object, a tube, attached to a perch at a neutral location and measuring latency to approach and investigate the tube (exploration score). Both tests were carried out at the end of the breeding season and repeated ten months later in spring. The Sardinian warblers showed consistent behavioural reactions over time. Furthermore, neophobia and exploration scores were negatively related. The garden warblers neither behaved consistently over time nor was there a correlation between neophobia and exploration. Overall, Sardinian warblers were less neophobic and more explorative than garden warblers. The different reactivity may be due to a different frequency distribution of the individuals of the two species along a reactivity axis. It can be concluded that the Sardinian warblers have personality traits. The situation is less clear in the garden warblers. Possibly, different life-styles require different organisation of behaviours. [ABSTRACT FROM AUTHOR]

Mass changes of skeletal muscles occur in a variety of species during the migratory period. Phenotypic flexibility of flight muscle mass is considered to represent adaptations of the flight muscle to changing power requirements associated with changes in body mass. We analyzed the relationship between muscle masses and body mass for garden warblers (Sylvia borin) sampled during spring migration in Tanzania, Ethiopia, and Egypt, and during autumn migration in Turkey. Flight muscle mass was positively related to body mass of warblers at only one of the four sites, in Egypt where warblers had just arrived after a long migratory flight. Analysis of covariance (ANCOVA) revealed a significant interaction term between sampling site and body mass (P < .001), indicating that flight muscle and body mass differed from site to site. We therefore question the idea that changing power requirements associated with changes in body mass cause mass changes of the flight muscle. We further suggest that different migration strategies across different landscapes shape the relationship between flight muscle and body mass. Flights across major ecological barriers may cause substantial catabolism of flight muscle protein until the limit necessary for flight, while migration across "common landscape," which enables a bird to land and to feed and/or drink, may occur without the need to catabolize flight muscle protein. However, a differential relationship between flight muscle mass and body mass described here for a long-distance migrant seems as well to occur in a short-distance migrant and is therefore unlikely to be part of an adaptive syndrome typical for long-distance migration. [ABSTRACT FROM AUTHOR]

Abstract: The energetic demands of long-distance migratory birds change drastically, depending on the stage of their life cycle. Changing demands are reflected in the up and down regulation of adipose tissue and organ mass. This paper presents new data on organ size changes during different stages of spring migration of garden warblers (Sylvia borin). Phenotypic mass changes were quantified in 13 organs of birds caught in Tanzania, Ethiopia and Egypt. We also sampled birds after a simulated stopover in Egypt. Some organs increased in mass up to about 1.5-fold during migration from Tanzania to Ethiopia, while some remained unchanged or even decreased in mass. During flight across the Sahara, nearly all organ masses including heart and flight muscles were reduced. Exceptionally large reductions (∼50%) were observed for liver, bile, spleen, kidney and digestive tract organs. The only exceptions were the testes, which increased 4-fold in mass. During the simulated stopover in Egypt, a significant recovery was observed for kidney, liver, heart, proventriculus, and small intestine. The testes continued to increase in mass. Flexible remodeling of organ size in the course of spring migration thus comprises significant changes for all quantified organs, with a variety of organ-specific patterns. Individual organ patterns are differentially shaped by functional aspects according to the different organ requirements in the alternation of flight and stopover phases, energetics, future demands, and protein requirements. Anticipatory mechanisms account for the size change of the testes, and we suggest the same for the kidney and the gall bladder. [Copyright &y& Elsevier]

Animals need to be able to identify other species, which is crucial in competition for ecological resources, for using other species as a cue in habitat selection, and for the establishment and maintenance of parasite-host species associations in brood-parasitic species. The ability to discriminate between conspecifics and heterospecifics may be crucially important also in refinement of mating preferences during speciation and for premating isolation of sympatric species. It has long been hypothesized that species identification ability is based on learned associations between multiple features that distinguish species. Here I test this hypothesis using dual-choice song playback experiments with interspecifically territorial male blackcaps (Sylvia atricapilla) that defend territories against male garden warblers (Sylvia borin). I show that male blackcaps can associate species-specific songs with species-specific plumage and also that they retain the memory of this association for an 8-month period without contact with heterospecific rivals. Apparently, yearling male blackcaps achieve this ability several months prior to their first breeding attempt. This is the first time a long-term memory of associations between species-specific signals from two different sensory modalities (visual and auditory) has been shown to be important for distinguishing conspecifics from heterospecifics. [ABSTRACT FROM AUTHOR]

Night-migratory songbirds are known to use a magnetic compass , but how do they detect the reference direction provided by the geomagnetic field, and where is the sensory organ located? The most prominent characteristic of geomagnetic sensory input, whether based on visual patterns or magnetite-mediated forces , is the predicted symmetry around the north-south or east-west magnetic axis. Here, we show that caged migratory garden warblers perform head-scanning behavior well suited to detect this magnetic symmetry plane. In the natural geomagnetic field, birds move toward their migratory direction after head scanning. In a zero-magnetic field , where no symmetry plane exists, the birds almost triple their head-scanning frequency, and the movement direction after a head scan becomes random. Thus, the magnetic sensory organ is located in the bird''s head, and head scans are used to locate the reference direction provided by the geomagnetic field. [Copyright &y& Elsevier]

Migratory birds stay in a particular area for only short periods and must rapidly acquire appropriate information about stopover sites. In contrast, resident birds can update their knowledge whenever necessary. In a novel environment migrants are thus predicted to assess relevant information faster than nonmigrants. We tested this prediction with two closely related species, the migratory garden warbler, Sylvia borin, and the resident Sardinian warbler, S. melanocephala momus. Each bird was allowed to explore two unfamiliar boxes, one of which contained food. Garden warblers found food significantly faster than Sardinian warblers. Garden warblers may have a wide-area exploration strategy to assess a novel room and Sardinian warblers may have a local-area exploration strategy. We conclude that a wide-area exploration strategy might be required for migrants to assess environmental information as rapidly as possible. [Copyright &y& Elsevier]

We address the question of whether physiological flexibility in relation to climate is a general feature of the metabolic properties of birds. We tested this hypothesis in hand-raised Garden Warblers (Sylvia borin), long-distance migrants, which normally do not experience great temperature differences between summer and winter. We maintained two groups of birds under cold and warm conditions for 5 months, during which their body mass and food intake were monitored. When relatedness (siblings vs. non-siblings) of the experimental birds was taken into account, body mass in cold-acclimated birds was higher than in warm-acclimated birds. BMR, measured at the end of the 5-month temperature treatment, was also higher in the cold- than the warm-acclimated group. Migrant birds thus seem to be capable of the same metabolic cold-acclimation response as has been reported in resident birds. The data support the hypothesis that physiological flexibility is a basic trait of the metabolic properties of birds. [Copyright &y& Elsevier]

There is currently a great deal of interest in using stable-isotope methods to investigate diet and migratory connections in wild passerines. To apply these methods successfully, it is important to understand how stable isotopes discriminate or change between diet and the tissue of interest and what the element-turnover rates are in metabolically active tissues. Of particular use are studies that sample birds non-destructively through the use of blood and feathers. We investigated patterns of isotopic discrimination between diet and blood and feathers of Garden Warblers (Sylvia borin) raised on an isotopically homogeneous diet (48% C, 5% N) and then switched to one of two experimental diets, mealworms (56.8% C, 8.3% N) and elderberries, Sambucus niger (47.4% C, 1.5% N). We established that the discrimination factors between diet and blood appropriate for stable carbon (δ13C) and nitrogen (δ15N) isotopes are +1.7‰ and +2.4‰, respectively. For feathers, these values were +2.7‰ and +4‰, respectively. Turnover of elemental nitrogen in whole blood was best approximated by an exponential-decay model with a half-life of 11.0 ± 0.8 days (mean ± SD). Corresponding turnover of carbon was estimated to range from 5.0 ± 0.7 to 5.7 ± 0.8 days. We conclude that this decoupling of nitrogen- and carbon-turnover rates can be explained by differences in metabolic routing of dietary macromolecules. Our results suggest that tracking frugivory in migratory passerines that switch diets between insects and fruits may be complicated if only a trophic-level estimate is made using δ15N measurements. [ABSTRACT FROM AUTHOR]

In migrating birds, the success of migration is determined by stopover duration, the most important factor determining overall speed of migration, and fuel deposition rate. However, very little is known about stopover durations of small migrant birds, because appropriate methods for data analysis were lacking until recently. We used a new capture-recapture analysis to estimate stopover durations of 1st-year reed warblers Acrocephalus scirpaceus, sedge warblers A. schoenobaenus and garden warblers Sylvia borin at 17 stopover sites in Europe and Africa during autumn. Average stopover duration of non-moulting reed warblers was 9.5 days while moulting conspecifics stayed about twice as long. Average stopover duration of sedge warblers was 9.1 days and, in contrast to the other two species, differed between years at several sites. Garden warblers stayed 7.7 days on average. The long stopover duration of the reed warbler, resulting in slow overall migration speed, is related to its low fuel deposition rate. It can be explained by low, but predictable, food resources and an early departure during moult. Compared to the reed warbler, the stopover duration of the sedge warbler varies more between sites and probably also between years, as the supply of its preferred diet (reed aphids) is spatially and temporally unpredictable but can be superabundant. The short stopover duration of the garden warbler, leading to high overall migration speed, can be related to high fuel deposition rates, probably brought about by a change to an abundant, predictable and long-lasting fruit diet. Within species, stopover duration did not change significantly along the migration route. Hence, an increase of migration speed along the migration route, as suggested in the literature, may be caused by longer flight bouts in the south. However, it remains largely unknown which environmental and possibly endogenous factors regulate stopover duration. [ABSTRACT FROM AUTHOR]

Samples of flight and leg muscle tissue were taken from migratory garden warblers at three different stages of migration: (1) pre-flight: when birds face an extended flight phase within the next few days, (2) post-flight: when they have just completed an extended flight phase, and (3) recovery: when they are at the end of a stop-over period following an extended flight phase. The changes in body mass are closely related to the changes in flight (P<0.001) and leg muscle mass (P<0.001), suggesting that the skeletal muscles are involved in the protein metabolism associated with migratory flight. From pre- to post-flight, the flight and the leg muscle masses decrease by about 22%, but are restored to about 12% above the pre-flight masses during the recovery period. Biochemical analyses show that following flight a selective reduction occurred in the myofibrillar (contractile) component of the flight muscle (P<0.01). As this selective reduction accounts only for a minor part of the muscle mass changes, sarcoplasmic (non-contractile) and myofibrillar proteins of both the flight and leg muscle act as a protein source during long-distance migration. As a loss of leg muscle mass is additionally observed besides the loss in flight muscle mass, mass change seems not to be strictly associated with the mechanical power output requirements during flight. Whereas the specific content of sarcoplasmic proteins in the flight muscle is nearly twice as high as that in the leg muscle (P<0.001), the specific content of myofibrillar proteins differs only slightly (P<0.05), being comparably low in both muscles. The ratio of non-contractile to contractile proteins in the flight muscle is one of the highest observed in muscles of a vertebrate. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Presents information on a study which examined the responses of garden warblers under monochromatic light of various wavelengths. Methodology of the study; Results and discussion on the study.

Presents an analysis of the role of the direction of celestial rotation in the development of stellar orientation in young migratory birds, focusing on garden warblers, Sylvla borin. Study methodology; Results of study; Importance of the direction of celestial rotation; Discussion of the findings.

Focuses on the phenotypic organ flexibility in Garden Warblers during long-distance migration. Comparison of biological changes of migrating birds before and after desert crossings; Production of protein from the muscles of the breast, leg and digestive tract; Rate of organ reduction after migration flights; Role of the protein metabolites as intermediates in the citric cycle for the oxidation of fat during long flights.

Investigates the relative importance of response to changes in local food availability and time-place learning of garden warblers, Sylvia borin. Experiment setups; Interaction between Circadian timing and food availability; Hunger and timing.

Most migratory birds breeding in the northern hemisphere show regular changes in the direction of their migratory flights from roughly south in autumn to roughly north in spring. To test the hypothesis that these changes are primarily controlled by endogenous physiological changes, we carried out experiments with the garden warbler, Sylvia borin. 2. Eleven birds were hand-raised and subsequently kept under a constant 12-h photoperiod and constant temperature conditions throughout their first fall and spring migratory seasons. At regular intervals, the birds were tested in circular orientation cages for directional preferences of their nocturnal migratory restlessness. During the tests, the birds were exposed to the local magnetic field of the earth, but had no view of the sky. 3. Birds showed a significant preference for a southerly direction in autumn and for a northerly direction in spring (Figs. 2 and 3). These results support the hypothesis that the reversal of the migratory direction results from spontaneous changes in the preferred direction relative to external orienting cues that are controlled by an endogenous circannual clock. [ABSTRACT FROM AUTHOR]

Garden warblers ( Sylvia borin) were subjected to starvation trials during their autumnal migratory phase in order to simulate a period of non-stop migration. Before, during and after this treatment the energy expenditure, activity, food intake and body mass of the subjects were monitored. Assimilation efficiency was constant throughout the experiments. The catabolized (during starvation) and deposited body tissue (during recovery) consisted of 73% fat. Basal metabolic rate was decreased during the starvation period and tended to a gradual increase during the recovery period. The reduced basal metabolic rate can possibly be attributed to a reduced size/function of the digestive system, which is consistent with the sub-maximal food intake immediately after resuming the supply of food to the experimental birds. The observed reductions in basal metabolic rate during starvation and activity during recovery can be viewed as adaptations contributing to a higher economization of energy supplies. The experimental birds were unable to eat large quantities of food directly after a period of starvation leading to a comparatively low, or no increase in body mass. Such a slow mass increase is in agreement with observations of migratory birds on arrival at stop-over sites. [ABSTRACT FROM AUTHOR]

The present study describes the effects of food-deprivation on migratory restlessness and diurnal activity in the garden warbler, a long-distance migrating passerine. Fourteen first-year birds were held from August through March in individual registration cages and exposed to daylength changes experienced by birds that winter at the equator. With the beginning of the fall migratory season the birds increased their body weights due to fat deposition and showed nocturnal migratory restlessness characteristic for this species. After the birds had deposited more than about 4 g of fat they were food-deprived until their body mass had declined to a value about 1 g above fat free mass. Subsequently they were re-fed. This procedure was repeated through fall and winter, beyond the termination of spontaneous fall migratory restlessness. - Food deprivation consistently caused an increase in duration and intensity of nocturnal activity. After the end of spontaneous migratory restlessness and before the onset of winter molt it led to a reinduction of nocturnal activity. When nocturnal activity increased, the duration of diurnal activity decreased, whereas its intensity increased. With regard to the performance of free-living migrants these results suggest (1) that nocturnal migratory activity is enhanced in birds travelling across areas with unprofitable resting places; (2) that nocturnal migratory activity may be reactivated in birds that have settled for the winter in an area with deteriorating food supply; and (3) that the pattern of diurnal activity may change conspicuously when nocturnal migratory activity is enhanced or resumed. [ABSTRACT FROM AUTHOR]

The article presents a summary of studies conducted on the migratory strategies of trans-Saharan migrant passerines particularly the Garden Warbler Sylvia borin. It cites Ottosson et al who concluded that Garden Warblers stop over in the Sahel and Guinea-savannah regions of West Africa in autumn although none of these regions regularly hosts wintering birds. The findings illustrates the problems of using the average mass of birds at stopover sites for estimating departure mass/fuel loads but a focus on the highest masses poses possible solution.