Hungry or angry? Experimental evidence for the effects of food availability on two measures of stress in developing wild raptor nestlings.

Food shortage challenges the development of nestlings; yet, to cope with this stressor, nestlings can induce stress responses to adjust metabolism or behaviour. Food shortage also enhances the antagonism between siblings, but it remains unclear whether the stress response induced by food shortage operates via the individual nutritional state or via the social environment experienced. In addition, the understanding of these processes is hindered by the fact that effects of food availability often co-vary with other environmental factors. We used a food supplementation experiment to test the effect of food availability on two complementary stress measures, feather corticosterone (CORTf) and heterophil/lymphocyte ratio (H/L) in developing red kite (Milvus milvus) nestlings, a species with competitive brood hierarchy. By statistically controlling for the effect of food supplementation on the nestlings' body condition, we disentangled the effects of food and ambient temperature on nestlings during development. Experimental food supplementation increased body condition, and both CORTf and H/L were reduced in nestlings of high body condition. Additionally, CORTf decreased with age in non-supplemented nestlings. H/L decreased with age in all nestlings and was lower in supplemented last-hatched nestlings compared with non-supplemented ones. Ambient temperature showed a negative effect on H/L. Our results indicate that food shortage increases the nestlings' stress levels through a reduced food intake affecting both their nutritional state and their social environment. Thus, food availability in conjunction with ambient temperature shapes between- and within-nest differences in stress load, which may have carry-over effects on behaviour and performance in further life-history stages.

Variation in feather corticosterone levels in Alpine swift nestlings provides support for the hypo-responsive hypothesis.

In birds, feather corticosterone values (CORTf) are increasingly used as a retrospective and integrative proxy of an individual's physiological state during the period of feather growth. Relatively high CORTf values are usually interpreted as an indicator of exposure to energy-demanding or stressful conditions during feather growth. However, in nestlings this interpretation might not always hold true. The reasons are that, firstly nestlings (especially altricial ones) still develop their hypothalamo-pituitary-adrenal (HPA) reactivity during the growth of their feathers. Hence, at a young age, nestlings might be unable to mount a substantial adrenocortical stress response. Secondly, some species are able to down-regulate their metabolism during food scarcity and therewith probably also their CORT release. Consequently, CORTf values may not unambiguously reflect whether nestlings have suffered from energy-demanding or stress situations. Relatively high CORTf values might indicate either energy-demanding or stressful conditions ('stress responsive hypothesis'), or - conversely - favourable conditions during the period of feather growth ('hypo-responsive hypothesis'). In the altricial Alpine swift (Tachymarptis melba), we tested which factors help to distinguish between the two hypotheses by considering factors which affect CORT release (brood size, weather) and factors which are affected by high CORT levels (nestling size and condition). We measured CORTf in 205 nestlings over 7 years and collected data on brood size, body size, body condition and prevailing weather. Nestling CORTf values were positively correlated with body condition and negatively with adverse weather, supporting the hypo-responsive hypothesis. Results from the Alpine swift study, supplemented with a survey of the literature, show that relatively easily collected parameters on brood size, nestling size and condition, and environmental factors can help to distinguish between the two hypotheses. A meaningful interpretation of nestling CORTf should only be made in the context of species-specific traits.

Factors modulating the behavioral and physiological stress responses: Do they modify the relationship between flight initiation distance and corticosterone reactivity?

Understanding how vulnerable species are to new stressors, such as anthropogenic changes, is crucial for mitigating their potential negative consequences. Many studies have investigated species sensitivity to human disturbance by focusing on single behavioral or physiological parameters, such as flight initiation distance and glucocorticoid levels. However, little is known about the differential effect that modulating factors might have on behavioral versus physiological stress responses across species. This lack of knowledge make difficult to understand the relationship between both types of reactions, and thus to assess to what extent a behavioral reaction is representative of an internal physiological stress response or vice versa. We collected published data on bird flight initiation distances (FID) and corticosterone (CORT) responses, the two most frequently used indicators of stress reaction. We then investigated how spatio-temporal factors or species-specific characteristics relate to these behavioral and physiological stress responses, and potentially modify the relationship between them. Additionally, we evaluated the strength of the correlation between the two stress responses (behavioral and physiological). Our findings showed that FID and CORT responses were poorly correlated across species, and the lack of correlation was attributable to modulating factors (e.g. latitude and body mass) which influence behavior and physiology differently. These modulating factors, therefore, should be taken into consideration to better interpret FID and CORT responses in the context of species vulnerability to stress.

The relationship between daily behavior, hormones, and a color dimorphism in a seabird under natural continuous light.

The predictable oscillation between the light of day and the dark of night across the diel cycle is a powerful selective force that has resulted in anticipatory mechanisms in nearly all taxa. At polar latitude, however, this oscillation becomes highly attenuated during the continuous light of polar day during summer. A general understanding of how animals keep time under these conditions is poorly understood. We tested the hypothesis that the common murre (a seabird, Uria aalge) can use melatonin and corticosterone, hormones associated with timekeeping, to track the diel cycle despite continuous light. We also tested the assumption that common murres breeding during polar summer schedule their colony attendance by time of day and sex, as they do at subpolar latitude. In the Atlantic population, common murres have a plumage color dimorphism associated with fitness-related traits, and we investigated the relationship of this dimorphism with colony attendance, melatonin, and corticosterone. The common murres did not schedule their attendance behavior by time of day or sex, yet they had higher concentrations of melatonin and, to a more limited extent, corticosterone during "night" than "day". Melatonin also linked to behavioral state. The two color morphs tended to have different colony-attendance behavior and melatonin concentrations, lending support for balancing selection maintaining the plumage dimorphism. In common murres, melatonin can signal time of day despite continuous light, and the limited diel variation of corticosterone contributes to the mounting evidence that polar-adapted birds and mammals require little or no diel variation in circulating glucocorticoids during polar day.

Seasonal changes in yolk hormone concentrations carry-over to offspring traits.

Yolk hormones are substances which transmit non-genetic factors from the mother to the next generation. The systematic changes of yolk hormone concentrations within asynchronously hatching clutches have been interpreted as a means to adaptively shape the offspring's phenotype. However, in synchronously hatching clutches the role of yolk hormones is less understood. We investigated whether seasonal changes between eggs in the yolk hormones testosterone (Testo), progesterone (Prog) and corticosterone (Cort) also occur in the grey partridge, a synchronously hatching precocial species without direct food competition between siblings. Specifically we asked whether yolk hormone concentrations systematically vary with season and whether they affect the offspring's hatching mass, mass gain, circulating baseline and stress-induced Cort. Additionally, we investigated the effect of genetic background and food availability on yolk hormone concentrations by subjecting grey partridge hens of two strains (wild and domesticated) to two different feeding regimes (predictable vs. unpredictable feeding) during egg laying. We hypothesized that egg hormone concentrations change over the season, but breeding in captivity over many generations and ad libitum food access could have resulted in domestication effects which abolished potential seasonal effects. Results showed that progressing season had a strong positive effect on yolk Prog and yolk Testo, but not on yolk Cort. Feeding regimes and strain had no effect on yolk hormones. Offspring mass and mass gain increased and baseline Cort decreased with progressing season. In addition, yolk Testo correlated positively with offspring mass gain and negatively with baseline Cort, while yolk Prog had a positive correlation with baseline Cort. Strain and feeding regimes of the mother had no effect on offspring traits. In conclusion, grey partridge chicks hatching late in the season might benefit from the increased concentrations of the growth-stimulating yolk Testo and by this catch-up in development. Hence, yolk hormone concentration could adaptively shape the offspring phenotype in a precocial species.

Melatonin and corticosterone profiles under polar day in a seabird with sexually opposite activity-rhythms.

The 24 h geophysical light-dark cycle is the main organizer of daily rhythms, scheduling physiology and behavior. This cycle attenuates greatly during the continuous light of summer at polar latitudes, resulting in species-specific and even individual-specific patterns of behavioral rhythmicity, but the physiological mechanisms underlying this variation are poorly understood. To address this knowledge gap and to better understand the roles of the hormones melatonin and corticosterone in rhythmic behavior during this 'polar day', we exploited the behavior of thick-billed murres (Uria lomvia), a charadriiform seabird with sexually opposite ('antiphase') activity-rhythms that have a duration of 24 h. Melatonin concentration in the plasma of inactive males was unexpectedly high around midday and subsequently fell during a sudden decrease in light intensity as the colony became shaded. Corticosterone concentration in plasma did not vary with time of day or activity in either sex. While the reasons for these unusual patterns remain unclear, we propose that a flexible melatonin response and little diel variation of corticosterone may be adaptive in thick-billed murres, and perhaps other polar birds and mammals, by stabilizing glucocorticoids' role of modulating energy storage and mobilization across the diel cycle and facilitating the appropriate reaction to unexpected stimuli experienced across the diel cycle while attending the colony.

Energy metabolism during endurance flight and the post-flight recovery phase.

Migrating birds are known to fly non-stop for thousands of kilometres without food or water intake and at a high metabolic rate thereby relying on energy stores which were built up preceding a flight bout. Hence, from a physiological point of view the metabolism of a migrant has to switch between an active fasting phase during flight and a fuelling phase during stopover. To meet the energetic and water requirements of endurance flight, migratory birds have to store an optimal fuel composition and they have to be able to quickly mobilize and deliver sufficient energy to the working flight muscles. After flight, birds have to recover from a strenuous exercise and sleeplessness, but, at the same time, they have to be alert to escape from predators and to prepare the next flight bout. In this overview, metabolic adaptations of free-ranging migrants to both phases will be presented and compared with results from windtunnel studies. The questions whether migratory strategy (long distance versus short distance) and diet composition influence the metabolic pathways will be discussed.

Why Do Migratory Birds Sing on Their Tropical Wintering Grounds?

Many long-distance migratory birds sing extensively on their tropical African wintering grounds, but the function of this costly behavior remains unknown. In this study, we carry out a first empirical test of three competing hypotheses, combining a field study of great reed warblers (Acrocephalus arundinaceus) wintering in Africa with a comparative analysis across Palearctic-African migratory songbird species. We asked whether winter song (i) functions to defend nonbreeding territories, (ii) functions as practice to improve complex songs for subsequent breeding, or (iii) is a nonadaptive consequence of elevated testosterone carryover. We found support for neither the long-assumed territory-defense hypothesis (great reed warblers had widely overlapping home ranges and showed no conspecific aggression) nor the testosterone-carryover hypothesis (winter singing in great reed warblers was unrelated to plasma testosterone concentration). Instead, we found strongest support for the song-improvement hypothesis, since great reed warblers sang a mate attraction song type rather than a territorial song type in Africa, and species that sing most intensely in Africa were those in which sexual selection acts most strongly on song characteristics; they had more complex songs and were more likely to be sexually monochromatic. This study underlines how sexual selection can have far-reaching effects on animal ecology throughout the annual cycle.

Seasonal rainfall at long-term migratory staging sites is associated with altered carry-over effects in a Palearctic-African migratory bird.

BACKGROUND: An understanding of year-round habitat use is essential for determining how carry-over effects shape population dynamics in long-distance migratory songbirds. The recent discovery of long-term migratory staging sites in many species, prior to arrival at final wintering sites, adds complexity to efforts to decipher non-breeding habitat use and connections between sites. We investigated whether habitat conditions during migratory staging carry over to influence great reed warbler (Acrocephalus arundinaceus) body condition at final wintering sites in Zambia. We asked whether the presence/absence and strength of such carry-over effects were modified by contrasting rainfall conditions during 2 years. RESULTS: First, we found that individuals staging in a dry year had higher corticosterone (CORTf) and stable nitrogen isotope values (suggesting higher aridity) than birds staging in a wet year, indicating that regional weather affected staging conditions. Second, we found that carry-over effects from staging habitat conditions (measured via carbon and nitrogen isotopes) to final winter site body condition (measured via scaled mass index and ß-hydroxybutyrate) were only present in a dry year, suggesting that environmental factors have consequences for the strength of carry-over effects. Our results also suggest that wet conditions at final winter sites may buffer the effects of poor staging conditions, at least in the short term, since individuals that staged in a dry year had higher scaled mass indices in Zambia than individuals that staged in a wet year. CONCLUSIONS: This study provides a first insight into the connections between long-term migratory staging sites and final wintering sites, and suggests that local environmental factors can modify the strength of carry-over effects for long-distance migratory birds.

Disturbance of wildlife by outdoor winter recreation: allostatic stress response and altered activity-energy budgets.

Anthropogenic disturbance of wildlife is of growing conservation concern, but we lack comprehensive approaches of its multiple negative effects. We investigated several effects of disturbance by winter outdoor sports on free-ranging alpine Black Grouse by simultaneously measuring their physiological and behavioral responses. We experimentally flushed radio-tagged Black Grouse from their snow burrows, once a day, during several successive days, and quantified their stress hormone levels (corticosterone metabolites in feces [FCM] collected. from individual snow burrows). We also measured feeding time allocation (activity budgets reconstructed from radio-emitted signals) in response to anthropogenic disturbance. Finally, we estimated the related extra energy expenditure that may be incurred: based on activity budgets, energy expenditure was modeled from measures of metabolism obtained from captive birds subjected to different ambient temperatures. The pattern of FCM excretion indicated the existence of a funneling effect as predicted by the allostatic theory of stress: initial stress hormone concentrations showed a wide inter-individual variation, which decreased during experimental flushing. Individuals with low initial pre-flushing FCM values augmented their concentration, while individuals with high initial FCM values lowered it. Experimental disturbance resulted in an extension of feeding duration during the following evening foraging bout, confirming the prediction that Black Grouse must compensate for the extra energy expenditure elicited by human disturbance. Birds with low initial baseline FCM concentrations were those that spent more time foraging. These FCM excretion and foraging patterns suggest that birds with high initial FCM concentrations might have been experiencing a situation of allostatic overload. The energetic model provides quantitative estimates of extra energy expenditure. A longer exposure to ambient temperatures outside the shelter of snow burrows, following disturbance, could increase the daily energy expenditure by > 10%, depending principally on ambient temperature and duration of exposure. This study confirms the predictions of allostatic theory and, to the best of our knowledge, constitutes the first demonstration of a funneling effect. It further establishes that winter recreation activities incur costly allostatic behavioral and energetic adjustments, which call for the creation of winter refuge areas together with the implementation of visitor-steering measures for sensitive wildlife.

Distinct responses of baseline and stress-induced corticosterone levels to genetic and environmental factors.

Glucocorticoid (GC) hormones, i.e. corticosterone (CORT) in birds, support physiological homeostasis and facilitate adaptations to stressful situations. However, maintaining high GC levels are energetically costly and interfere with other physiological processes. To keep the balance of costs and benefits of GC hormones, various mechanisms act to adapt GC levels to environmental conditions on different timescales, i.e. over generations, between parents and their offspring and within the life-time of a single individual. We elucidated whether two strains (domesticated and wild) of grey partridges (Perdix perdix) differed in the developmental trajectories of baseline and stress response CORT throughout the first 80 days of life. We also explored the potential of prenatal and postnatal factors, e.g. parental origin, predictable vs. unpredictable food treatments, individual and social factors to modify these trajectories. Baseline CORT was similar between strains and unaffected by perinatal food treatments. It was negatively related to body size and body condition. Conversely, the CORT stress response was not markedly affected by physiological condition. It was stronger in wild than in domesticated birds and it increased with age. Birds subjected to prenatal unpredictable food supply exhibited an accelerated development of the CORT stress response which could reflect an adaptive maternal effect. We conclude that the vital role of baseline CORT may allow little adaptive scope since changes can quickly become detrimental. In contrast, the CORT stress response may show considerable adaptive potential which might ultimately support homeostasis in a changing environment.

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.

Energy supply during nocturnal endurance flight of migrant birds: effect of energy stores and flight behaviour.

BACKGROUND: Migrating birds fly non-stop for hours or even for days. They rely mainly on fat as fuel complemented by a certain amount of protein. Studies on homing pigeons and birds flying in a wind-tunnel suggest that the shares of fat and protein on total energy expenditure vary with flight duration and body fat stores. Also, flight behaviour, such as descending flight, is expected to affect metabolism. However, studies on free flying migrant birds under natural conditions are lacking. METHODS: On a Swiss Alpine pass, we caught three species of nocturnal migrant passerines out of their natural migratory flight. Since most night migrants start soon after dusk, we used time since dusk as a measure of flight duration. We used plasma concentrations of metabolites of the fat, protein, and carbohydrate metabolism as indicators of relative fuel use. We used flight altitudes of birds tracked with radar and with atmospheric pressure loggers to characterize flight behaviour. RESULTS: The indicators of fat catabolism (triglycerides, very low-density lipoproteins, glycerol) were positively correlated with body energy stores, supporting earlier findings that birds with high fat stores have a higher fat catabolism. As expected, plasma levels of triglycerides, very low-density lipoproteins, glycerol and ß-hydroxy-butyrate increased at the beginning of the night, indicating that nocturnal migrants increased their fat metabolism directly after take-off. Surprisingly, fat catabolism as well as glucose levels decreased in the second half of the night. Data from radar observations showed that the number of birds aloft, their mean height above ground and vertical flight speed decreased after midnight. Together with the findings from atmospheric pressure-loggers put on three species, this shows that nocturnal migrants migrating over continental Europe descend slowly during about 1.5 h before final landfall at night, which results in 11-30% energy savings according to current flight models. CONCLUSIONS: We suggest that this slow descent reduces energy demands to an extent which is noticeable in the plasma concentration of lipid, protein, and carbohydrate metabolites. The slow descent may facilitate the search for a suitable resting habitat and serve to refill glycogen stores needed for foraging and predator escape when landed.

Modulation of the adrenocortical response to acute stress with respect to brood value, reproductive success and survival in the Eurasian hoopoe.

Reproducing parents face the difficult challenge of trading-off investment in current reproduction against presumed future survival and reproduction. Glucocorticoids are supposed to mediate this trade-off because the adrenocortical response to stress disrupts normal reproductive behaviour in favour of self-maintenance and own survival. According to the brood-value hypothesis, individuals with a low survival probability until the next reproductive season have to invest in current reproduction, a process driven by a down-regulation of their adrenocortical response. If the adrenocortical response to stress effectively mediates the trade-off between current reproduction versus future survival and reproduction, we expect a negative relationship with reproductive success and a positive correlation of the adrenocortical stress response with survival. We studied the relationship between corticosterone secretion in parents and their current brood value, reproductive success and survival in a short-lived multi-brooded bird, the Eurasian hoopoe Upupa epops. The adrenocortical response to acute handling stress was correlated with the brood value within the individual (first and second broods of the year) and between individuals. Birds breeding late in the season mounted a lower total corticosterone response to acute stress than birds breeding earlier, while females showed lower levels than males. We observed a negative relationship between the adrenocortical stress response and rearing success or fledging success in females, as predicted by the brood-value hypothesis. However, we could not evidence a clear link between the adrenocortical stress response and survival. Future research testing the brood-value hypothesis and trade-offs between current reproduction and future survival should also measure free corticosterone and carefully differentiate between cross-sectional (i.e. between-individual) and individual-based experimental studies.

Sudden weather deterioration but not brood size affects baseline corticosterone levels in nestling Alpine swifts.

While evidence is accumulating that stress-induced glucocorticoid responses help organisms to quickly adjust their physiology and behaviour to life-threatening environmental perturbations, the function and the ecological factors inducing variation in baseline glucocorticoid levels remain poorly understood. In this study we investigated the effects of brood size by experimentally manipulating the number of nestlings per brood and the effect of weather condition on baseline corticosterone levels of nestling Alpine swifts (Apus melba). We also examined the potential negative consequences of an elevation of baseline corticosterone on nestling immunity by correlating corticosterone levels with ectoparasite intensity and the antibody production towards a vaccine. Although nestlings reared in enlarged broods were in poorer condition than nestlings reared in reduced broods, they showed similar baseline corticosterone levels. In contrast, nestling baseline corticosterone levels were higher immediately after cold and rainy episodes with strong winds. Neither nestling infestation rate by ectoparastic flies nor nestling antibody production against a vaccine was correlated with baseline corticosterone levels. Thus, our results suggest that altricial Alpine swift nestlings can quickly modulate baseline corticosterone levels in response to unpredictable variations in meteorological perturbation but not to brood size which may be associated with the degree of sibling competition. Apparently, short-term elevations of baseline corticosterone have no negative effects on nestling immunocompetence.

Nutritional correlates and mate acquisition role of multiple sexual traits in male collared flycatchers.

The information content of a sexual signal may predict its importance in a multiple signal system. Many studies have correlated sexual signal expression with the absolute levels of nutrient reserves. In contrast, the changes of nutrient reserves associated with signal expression are largely unknown in the wild due to technical limitations although they are important determinants of signal information content. We compared two visual and eight acoustic sexual traits in male collared flycatchers to see whether the nutritional correlates of expression predict the role of the signal in sexual selection. We used single point assays of plasma lipid metabolites to estimate short-term changes in nutritional state in relation to sexual trait expression during courtship. As a measure of sexual selection, we estimated the relationship with pairing latency after arrival in a 4-year dataset. Males which found a mate rapidly were characterized by large wing and forehead patches, but small song strophe complexity and small figure repertoire size. Traits more strongly related to pairing latency were also more closely related to changes in nutrient reserves. This indicates a link between signal role and information content. Small wing patches and, surprisingly, complex songs seemed to indicate poor phenotypic quality and were apparently disfavoured at mate acquisition in our population. Future studies of the information content of sexual traits, especially dynamic traits such as song, may benefit from the use of plasma metabolite profiles as non-invasive indicators of short-term changes in body condition.

Development of the adrenocortical response to stress in Eurasian kestrel nestlings: defence ability, age, brood hierarchy and condition.

The developmental hypothesis proposes that the adrenocortical response to stress during postnatal development in birds should not develop when the benefits of elevated corticosterone do not outweigh the deleterious effects on growth and development. We tested three predictions developed from this hypothesis in free-living, semi-altricial Eurasian kestrel Falco tinnunculus nestlings. We measured baseline and handling-induced corticosterone levels and the binding capacity of corticosteroid binding globulins CBG on day 10 and 21 of age and related these to age, the development of the defence behaviour, hatching asynchrony and fat stores (a measure of body condition). First, the adrenocortical response to handling (total plasma corticosterone) increased with age and thus during the time when nestlings developed the ability to defend themselves, but free corticosterone did not, because of a concomitant increase of CBG with age. Second, nestlings with adequate fat stores mounted a stronger adrenocortical stress response to an acute stressor, while nestlings with low fat stores avoided additional energy expenses. While baseline corticosterone levels were negatively related to fat stores, increase in corticosterone to handling was positively related. Third, both baseline corticosterone levels and the adrenocortical response to handling were not related to hatching order, but predominantly determined by body condition. The pattern of decreasing corticosterone levels with hatching order found in the lab seems to be neutralized by opposite effects of varying body condition on corticosterone levels in free-living birds. We argue that the postnatal adrenocortical response to stress is adaptively modulated by both variations in the release of corticosterone and in CBG, which is particularly important because elevated corticosterone may adversely affect the phenotype.

Pheomelanin-based coloration and the ability to cope with variation in food supply and parasitism.

Although gene by environment interactions may play a key role in the maintenance of genetic polymorphisms, little is known about the ecological factors involved in these interactions. We investigated whether food supply and parasites can mediate covariation between the degree of adult pheomelanin-based coloration, a heritable trait, and offspring body mass in the tawny owl (Strix aluco). We swapped clutches between nests to allocate genotypes randomly among environments. Three weeks after hatching, we challenged the immune system of 80 unrelated nestlings with either a phytohemagglutinin (PHA) or a lipopolysaccharide, surrogates of alternative parasites, and then fed them ad lib. or food-restricted them during the following 6 days in the laboratory. Whatever the immune challenge, nestlings fed ad lib. converted food more efficiently into body mass when their biological mother was dark pheomelanic. In contrast, food-restricted nestlings challenged with PHA lost less body mass when their biological mother was pale pheomelanic. Nestling tawny owls born from differently melanic mothers thus show differing reaction norms relative to food availability and parasitism. This suggests that dark and pale pheomelanic owls reflect alternative adaptations to food availability and parasites, factors known to vary in space and time.

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.

Information from the geomagnetic field triggers a reduced adrenocortical response in a migratory bird.

Long-distance migrants regularly pass ecological barriers, like the Sahara desert, where extensive fuel loads are necessary for a successful crossing. A central question is how inexperienced migrants know when to put on extensive fuel loads. Beside the endogenous rhythm, external cues have been suggested to be important. Geomagnetic information has been shown to trigger changes in foraging behaviour and fuel deposition rate in migratory birds. The underlying mechanism for these adjustments, however, is not well understood. As the glucocorticoid hormone corticosterone is known to correlate with behaviour and physiology related to energy regulation in birds, we here investigated the effect of geomagnetic cues on circulating corticosterone levels in a long-distance migrant. Just as in earlier studies, juvenile thrush nightingales (Luscinia luscinia) caught during autumn migration and exposed to the simulated geomagnetic field of northern Egypt increased food intake and attained higher fuel loads than control birds experiencing the ambient magnetic field of southeast Sweden. Our results further show that experimental birds faced a reduced adrenocortical response compared with control birds, thus for the first time implying that geomagnetic cues trigger changes in hormonal secretion enabling appropriate behaviour along the migratory route.