Blood chemistry in relation to nutrition and ectoparasite load in Wilson’s storm-petrels<Emphasis Type="Italic"> Oceanites oceanicus</Emphasis>

Haematology and plasma biochemistry values are useful tools for ecological research, providing information on the physiological state and adaptation of individuals to their habitat, changes in nutritional state of birds, body condition, the level of parasite infestation, etc. We studied the effect of stress factors on haematological and plasma biochemistry values in adult and nestling Wilsons storm petrels Oceanites oceanicus (Aves, Procellariiformes). We measured packed cell volume, triglyceride levels, plasma protein levels, plasma hue and plasma corticosterone of nestlings and breeding adults at King George Island, South Shetland Islands. We used a snow storm as a natural experiment to test what effect starvation has on the stress response of nestlings. In particular, we predicted that: (1) plasma protein concentrations and plasma hue reflect ectoparasite load, (2) triglycerides and hue reflect the nutritional state, and (3) corticosterone levels increase with ectoparasite load and starvation. In line with our predictions, plasma triglycerides were higher in nestlings than adults, indicating a resorptive nutritional state in nestlings, during which dietary fat is deposited in adipose tissues. In adults, plasma triglycerides were positively correlated with body mass. Corticosterone levels increased in response to handling in adults, while we did not find a stress response in nestlings in good condition. However, nestlings sampled after their nests had been blocked by a snow storm showed a stress response in excess of that of adults. In nestlings sampled after the snow storm, corticosterone peak levels were positively correlated with the infestation intensity of the ectoparasite Philoceanus robertsi (Phthiraptera: Ischnocera), suggesting that the stress response is increased when more stressors act at the same time. In adults, plasma hue and proteins decreased with increasing ectoparasite load.     

Natural variation in stress response is related to post-stress parental effort in male house sparrows

The central life-history trade-off between current and future reproductive effort seems to be mediated by corticosterone in birds. However, still little is known about how naturally occurring corticosterone levels during an acute stress may influence subsequent parental behavior. In this study we observed the parental behavior of free-living male house sparrows (Passer domesticus) both before and after they were subjected to a standard capture–handling stress. We investigated the relationships between corticosterone levels, pre- and post-stress parental behavior, while we statistically controlled for a number of other variables using a multivariate regression method, the path analysis. We found that males' baseline feeding rate predicted the body mass of the nestlings, indicating that male parental care is directly linked to fitness. Corticosterone levels were not explained by baseline feeding rate, but both baseline and stress-induced corticosterone levels had a negative influence on the males' post-stress feeding behavior. Moreover, males with large bib size had a stronger stress response and lower post-stress feeding rate than small bibbed males. These results indicate that naturally occurring variation in baseline and stress-induced corticosterone levels may influence subsequent parental decisions: individuals mounting a robust stress response are likely to reduce their parental commitment. Parental effort may be regulated in a complex manner, with corticosterone mediating the life-history trade-off between current reproduction and survival. However, different resolutions of this trade-off were apparent only following the stress, therefore the ability to modulate the stress response and maintain parental care in stressful situations may be important in life-history evolution.     

Age and food deprivation affects expression of the glucocorticosteroid stress response in Magellanic penguin (Spheniscus magellanicus) chicks

We examined how the glucocortical stress response in free-living Magellanic penguin (Spheniscus magellanicus) chicks changes with age and whether adrenocortical function of chicks within a brood varies in relation to food provisioned by adults. Chicks showed little corticosterone response to capture stress shortly after hatching, an intermediate response around 45-d posthatch, and a robust stress response near fledging. However, in response to an adrenocorticotropic hormone (ACTH) challenge, hatchlings were capable of secreting corticosterone at adult-like levels. The larger sibling in broods of two showed a similar gradual stress-response development pattern. In contrast, by day 45, when differences in body condition were well established between siblings, the smaller, food-deprived chicks significantly increased baseline levels of corticosterone but showed normal stress-induced levels. Near fledging, baseline levels had returned to normal, but stress-induced levels were lower than expected. Similar to altricial species, normally developing semialtricial Magellanic penguin chicks do not express a robust corticosterone stress response until near fledging. Chronic stressors such as food deprivation cause corticosterone use to be up-regulated earlier than expected. However, in cases of extended chronic stress, down-regulation may ensue, thus avoiding the negative effects of chronically elevated levels of corticosterone.     

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.     

Harsh conditions during early development influence telomere length in an altricial passerine: Links with oxidative stress and corticosteroids

Stress during early development can induce substantial long‐term effects in organisms. In the case of birds, despite growth compensations, nestlings reared under harsh conditions typically show reduced survival chances in adulthood. It has been proposed that environmental early‐life stressors could affect longevity via effects on telomere length, possibly mediated through oxidative stress. However, the link between these processes is not clear. In this study, we experimentally manipulated brood size in spotless starlings (Sturnus unicolor) to test the causal relationship between early stress, oxidative and corticosterone‐mediated stress and telomere shortening. Our results show that experimentally enlarged brood sizes led to a reduction in morphometric development on nestlings, the effect being stronger for females than males. Additionally, basal corticosterone levels increased with increasing brood size in female nestlings. Neither plasma antioxidant status nor malondialdehyde levels (a marker of lipid peroxidation) were affected by experimental brood size, although the levels of a key intracellular antioxidant (glutathione) decreased with increasing brood size. We found that the treatment showed a quadratic effect on nestling telomere lengths: these were shortened either by increases or by decreases in the original brood size. Our study provides experimental evidence for a link between developmental stress and telomere length, but does not support a direct causal link of this reduction with corticosterone or oxidative stress. We suggest that future studies should focus on how telomere length responds to additional markers of allostatic load.     

Signaling stress? An analysis of phaeomelanin-based plumage color and individual corticosterone levels at two temporal scales in North American barn swallows, Hirundo rustica erythrogaster

Sexually selected traits confer greater reproductive benefits to individuals with more elaborate forms of the signal. However, whether these signals convey reliable information about the physiology underlying trait development remains unknown in many species. The steroid hormone corticosterone (CORT) mediates important physiological and behavioral processes during the vertebrate stress response, and CORT secretion itself can be modulated by melanocortins. Thus, sexually selected melanin-based plumage coloration could function as an honest signal of an individual's ability to respond to stressors. This hypothesis was tested in North American barn swallows, Hirundo rustica erythrogaster, where males with darker ventral plumage color exhibit higher phaeomelanin content and are more successful at reproduction. Because reproductive behavior occurs months after plumage signals are developed, we also addressed the potential temporal disconnect of physiological state during trait development and trait advertisement by analyzing three different measurements of CORT levels in adult males during the breeding season (trait advertisement) and in nestling males while they were growing their feathers (trait development). Variation in adult plumage color did not predict baseline or stress-induced CORT, or stress responsiveness. Likewise, there was no relationship between nestling plumage color and any of the CORT measurements, but heavier nestlings had significantly lower baseline CORT. Our finding that a predominantly phaeomelanin-based trait is unrelated to circulating CORT suggests that phaeomelanin and eumelanin signals may convey different physiological information, and highlights the need for further study on the biochemical links between the hypothalamic–pituitary–adrenal (HPA) axis and the production of different melanin-based pigments.     

Repeatability of baseline and stress-induced corticosterone levels across early life stages in the Florida scrub-jay (Aphelocoma coerulescens)

Recent studies have posited that the pattern of glucocorticoid secretion within an individual represents a stable, fixed physiological trait. To test this hypothesis, we assessed the repeatability of baseline and stress-induced corticosterone (CORT) secretion across developmental stages and years in Florida scrub-jays. We sampled individuals from multiple cohorts repeatedly from the age of 11 days post-hatch up to 4 years of age. We found a significant degree of repeatability within individuals in stress-induced corticosterone levels, i.e., the amount of hormone secreted during a standardized stress protocol (corrected integrated corticosterone). However, baseline corticosterone levels were not statistically repeatable, although there was some indication that nestling levels predicted levels at 1 year of age. The results of this study indicate that stress-induced CORT levels are consistent within individual scrub-jays, and the degree to which a young jay mounts an acute stress response appears to be somewhat “set” by the age of nutritional independence. Thus stress-induced corticosterone secretion appears to be a stable, repeatable trait within individuals and as such may be subject to natural selection.     

Adrenocortical responses in zebra finches (Taeniopygia guttata): individual variation, repeatability, and relationship to phenotypic quality

Although individual variation is a key requirement for natural selection, little is known about the magnitude and patterns of individual variation in endocrine systems or the functional significance of that variation. Here we describe (1) the extent and repeatability of inter-individual variation in adrenocortical responses and (2) its relationship to sex-specific phenotypic quality, such as song duration and frequency and timing of egg laying. We measured adrenocortical responses to a standardized stressor in zebra finches (Taeniopygia guttata) at two life history stages: approximately day 16 (nestlings) and 3 months of age (sexually mature adults). Subsequently, we assessed phenotypic (reproductive) quality of all individuals as adults. Marked inter-individual variation in the adrenocortical response was seen in both sexes and ages, e.g., stress-induced corticosterone ranged from 2.2 to 62.5 ng/mL in nestlings and 5.0-64.0 ng/mL in adults. We found sex differences in (a) inter-individual variation in the adrenocortical response, (b) repeatability, and (c) relationships between corticosterone levels and phenotypic quality. In males, variation in nestling corticosterone was weakly but positively correlated with brood size and negatively correlated with nestling mass (though this relationship was dependent on one individual). There was no significant correlation of adrenocortical responses between two stages in males and adult phenotypic quality was significantly correlated only with adult corticosterone levels. In contrast, in females there was no relationship between nestling corticosterone and brood size or mass but adrenocortical response was repeatable between two stages (r2=0.413). Phenotypic quality of adult females was correlated with nestling baseline and adrenocortical response.     

Selection for fast and slow exploration affects baseline and stress-induced corticosterone excretion in Great tit nestlings, Parus major

In nestlings, glucocorticoid (GC) secretion has short-term and long-term f itness consequences. For example, short-time elevations trigger begging activity, whereas chronically elevated GC levels impair body condition, growth and cognitive abilities. Despite a growing body of literature on personality traits, the effects of selection for fast and slow exploration on GC secretion have received little attention. We compared baseline and stress-induced hypothalamic–pituitary–adrenal (HPA) axis activity of hand-reared great tit nestlings of lines selected for fast and slow exploration. Nestling droppings were collected under three conditions: control, test (following handling stress, day 14 after hatching) and the following day. The concentrations of excreted immunoreactive corticosterone metabolites (CM) were determined via an enzyme immunoassay. We also observed nestlings' begging behaviour. CM differed significantly between the lines. Nestlings of the fast line excreted lower CM than slow-line birds. In response to handling stress, nestlings excreted significantly higher concentrations of CM than during the control and on the day after handling. Sex and begging activity were not related to CM levels. Under the control condition, but not after handling, males begged significantly more often than females. In both lines, adults excreted significantly less CM compared to nestlings. Both nestlings and adults of the slow line produced higher baseline CM values than fast-line birds. Fast-line nestlings excreted lower baseline CM than nestlings of a wild population not selected for fast or slow exploration. Slow-line nestlings did not. Our results show that selection on the basis of exploratory behaviour affected HPA axis reactivity.     

Fluctuating feather asymmetry in relation to corticosterone levels is sex-dependent in Eurasian treecreeper (Certhia familiaris) nestlings

Fluctuating asymmetry (FA) has been widely used as a stress-related phenotypic marker of developmental instability. However, previous studies relating FA to various stressful conditions have produced inconsistent results and we still lack quantitative individual-level evidence that high FA is related to stress in wild vertebrate species. We studied how baseline plasma levels of corticosterone predicted FA of wing and tail feathers in free-living Eurasian treecreeper (Certhia familiaris) nestlings. We found a sex-specific association between corticosterone levels and FA: high corticosterone levels were related to an increased FA in male but not in female nestlings. These results suggest that in treecreepers, FA may correlate with individual stress hormone levels, male developmental trajectory being potentially more sensitive to stress than that of the female.     

Corticosterone manipulations alter morph-specific nestling provisioning behavior in male white-throated sparrows, Zonotrichia albicollis

In the polymorphic white-throated sparrow (Zonotrichia albicollis), tan-striped males provision nestlings at higher rates than do white-striped males. In a previous study, we found that tan-striped males had lower baseline corticosterone levels than white-striped males during the nestling stage. To determine if this variation in corticosterone influences morph-specific differences in nestling provisioning behavior, we used intraperitoneal osmotic pumps to increase baseline corticosterone levels in tan-striped males (TS CORT) and administer RU486, a glucocorticoid receptor antagonist, in white-striped males (WS RU486). These manipulations essentially reversed morph-specific nestling provisioning behavior in males. TS CORT males fed nestlings at lower rates than TS controls (vehicle-only implant), and at similar rates to WS controls (vehicle-only implant), while WS RU486 males fed nestlings at higher rates than WS controls, and at similar rates to TS controls. These results demonstrate that (1) increases in baseline corticosterone (i.e., below concentrations associated with the adrenocortical response to stress) can directly or indirectly inhibit nestling provisioning behavior, and (2) corticosterone influences morph-specific variation in parental behavior in male white-throated sparrows. This study contributes to the growing evidence that modulating baseline CORT mediates parental care and self-maintenance activities in birds, and thus may serve as a mechanism for balancing current reproductive success with survival.     

Parental Alarm Calls of the White‐Crowned Sparrow Fail to Stimulate Corticosterone Production in Nest‐Bound Offspring

Alarm calling by parents is widespread among animals and has strong implications for parent and offspring fitness, yet it is virtually unknown whether parental alarm calls can initiate a corticosterone response in offspring. We investigated whether parental alarm calls of the white‐crowned sparrow, Zonotrichia leucophrys, activated the corticosterone response of their nest‐bound young, as such a response might prepare older nestlings for premature fledging and increase their survival when contacted by a predator at the nest. We conducted an experiment in which nestlings were either exposed to parent alarm calls (treatment) or experienced a period without parental alarm calls (control) immediately prior to blood sampling. We then sampled nestlings to measure corticosterone levels within 4 min of first contact (baseline corticosterone) and 60 min later (handling‐induced corticosterone). Young nestlings (i.e. 3–4 d post‐hatch) did not exhibit a corticosterone response to parental alarm calls or to handling, as mean corticosterone levels were similar in the control and treatment groups for both baseline and 60‐min post‐baseline samples. Against our predictions, there was no difference in mean levels of baseline corticosterone between control and treatment groups in older nestlings (i.e. 7?8 d post‐hatch) that were capable of surviving out of the nest. However, we did find a significant increase in mean levels of corticosterone after handling in both groups, which indicated that older nestlings were able to mount a functional corticosterone response when confronted with a potential predator. Why older nestlings did not initiate a corticosterone response after exposure to parental alarm calls is unclear but may have occurred because the costs of mounting such a response outweighed the benefits, perhaps because of growth or developmental costs.     

Effects of repeated investigator handling of Leach's Storm‐Petrel chicks on growth rates and the acute stress response

Disturbance during development may have lasting effects on the growth rates and stress physiology of birds. Although repeated handling by researchers is often necessary, the possible effects of such handling on the development of semi‐altricial young are unclear. We examined the effect of daily handling on growth rates and plasma corticosterone levels of Leach's Storm‐Petrel (Oceanodroma leucorhoa) chicks on Kent Island in the Bay of Fundy, New Brunswick, Canada, during the 2011 nesting season. From post‐hatch day 7 to post‐hatch days 14–36, birds in the experimental group were extracted from burrows and measured (wing, tarsus, and mass) for ～3 min every day, whereas birds in the control group were left undisturbed. After the treatment period, blood was collected from birds in both groups within 3 min of initially reaching into burrows (baseline) and after a 30‐min restraint stress test to assess the effect of early life disturbance on programming of the hypothalamic‐pituitary‐adrenal (HPA) axis. A second acute restraint stress test was conducted three weeks after the end of the treatment period to investigate possible longer term effects of early life disturbance. Growth rates of wings and tarsi were similar for handled chicks (N = 18) and non‐handled control chicks (N = 21), as were baseline and 30‐min acute restraint stress‐induced corticosterone levels. As also reported in previous studies of other altricial and semi‐altricial species, older chicks (42–64 d old) had higher plasma corticosterone levels than younger chicks (21–43 d old) after acute restraint stress tests, reflecting delayed development of the HPA axis. The age‐related increase in HPA axis sensitivity observed prior to fledging could facilitate foraging and predator avoidance behaviors while minimizing exposure to high levels of corticosterone earlier in development. Overall, we found no evidence that repeated disturbance influenced either growth rates or HPA axis programming of Leach's Storm‐Petrel chicks.     

Acute stress hyporesponsive period in nestling Thin-billed prions <Emphasis Type="Italic">Pachyptila belcheri</Emphasis>

When confronted with acute stressors, vertebrates show a highly conserved evolved sequence of physiological, hormonal and behavioural responses, including the activation of the hypothalamic–pituitary–adrenal axis. Many young vertebrates show a stress hyporesponsive period, where they exhibit a reduced glucocorticoid response. Here, we analyzed the stress response of nestling Thin-billed prions Pachyptila belcheri and compared chicks with different previous experience with capture and handling. We found that chicks had a stress response, but baseline and peak levels were below those measured in adults. The stress response of the chicks was rapid and followed by fast recovery, such that the total amount of corticosterone released in response to handling was very much lower in chicks than adults. These results indicate that nestling Thin-billed prions exhibit a stress hyporesponsive period. This was not due to habituation, as CORT measurements at baseline and elevated levels were similar in chicks handled daily and naïve chicks. The comparison with other published studies showed that the stress response of chicks usually peaks earlier and lower than in adults, and researchers should take care to measure stress-induced levels at an appropriate sampling time.     

Age-Related Changes in Nestling Diet of the Cooperatively Breeding Green Woodhoopoe

In many socially monogamous bird species, parents of altricial young respond to the increasing demands of growing nestlings by increasing their feeding rate and the size of prey items delivered and by altering the types of prey provided. In some cooperatively breeding species, similar changes in feeding rate and prey size have been documented. However, potential changes in the types of prey delivered, both as nestlings age and by different group members, remain largely unexplored. Moreover, studies rarely compare the diet fed to nestlings with that eaten by the provisioning adults themselves. Here, I show that green woodhoopoe ( Phoeniculus purpureus ) nestlings receive a smaller proportion of spiders and larger proportions of caterpillars and centipedes as they grow older. Both male and female adults delivered a higher proportion of spiders to young nestlings than they ate while self-feeding, probably in response to particular nutritional requirements of the chicks. However, only males altered the proportions of caterpillars and centipedes delivered, providing smaller proportions to young nestlings than eaten themselves. These prey items may be too large for young nestlings to handle, and males may make a greater adjustment in provisioning diet than females because they collect more caterpillars and centipedes than do females. Although there were sex differences in provisioning diet, there were no differences between same-sex breeders and helpers in terms of the overall proportions of prey delivered or the changes with nestling age. Hence, individuals of different reproductive status may be following the same provisioning rules, at least in terms of prey type.     

Stress during adolescence enhances locomotor sensitization to nicotine in adulthood in female, but not male, rats

A wide body of research has indicated that perinatal exposure to stressors alters the organism, notably by programming behavioral and neuroendocrine responses and sensitivity to drugs of abuse in adulthood. Recent evidence suggests that adolescence also may represent a sensitive period of brain development, and yet there has been little research on the long-lasting effects of stressors during this period. We investigated the effects of pubertal social stress (PS; daily 1-h isolation followed by pairing with a new cage mate on postnatal days 33-48) on locomotor sensitization to injections of nicotine and corticosterone response to restraint stress when the rats were adults (approximately 3 weeks after PS). There were no differences among the groups in locomotor activity to injections of saline. However, PS females had enhanced locomotor sensitization to repeated doses of nicotine compared to control (non-stressed; NS) females, whereas PS males and NS males did not differ. PS enhanced the corticosterone response to restraint in male rats previously sensitized to nicotine and decreased the corticosterone response in nonsensitized male rats. In contrast, PS females and NS females did not differ in plasma corticosterone levels in response to restraint stress, but NS females showed enhanced corticosterone release to restraint after sensitization to nicotine. Thus, during adolescence, social stressors can have long-lasting effects, and the effects appear to differ for males and females.     

Selection for fast and slow exploration affects baseline and stress-induced corticosterone excretion in Great tit nestlings,Parus major

In nestlings, glucocorticoid (GC) secretion has short-term and long-term f itness consequences. For example, short-time elevations trigger begging activity, whereas chronically elevated GC levels impair body condition, growth and cognitive abilities. Despite a growing body of literature on personality traits, the effects of selection for fast and slow exploration on GC secretion have received little attention. We compared baseline and stress-induced hypothalamic–pituitary–adrenal (HPA) axis activity of hand-reared great tit nestlings of lines selected for fast and slow exploration. Nestling droppings were collected under three conditions: control, test (following handling stress, day 14 after hatching) and the following day. The concentrations of excreted immunoreactive corticosterone metabolites (CM) were determined via an enzyme immunoassay. We also observed nestlings' begging behaviour. CM differed significantly between the lines. Nestlings of the fast line excreted lower CM than slow-line birds. In response to handling stress, nestlings excreted significantly higher concentrations of CM than during the control and on the day after handling. Sex and begging activity were not related to CM levels. Under the control condition, but not after handling, males begged significantly more often than females. In both lines, adults excreted significantly less CM compared to nestlings. Both nestlings and adults of the slow line produced higher baseline CM values than fast-line birds. Fast-line nestlings excreted lower baseline CM than nestlings of a wild population not selected for fast or slow exploration. Slow-line nestlings did not. Our results show that selection on the basis of exploratory behaviour affected HPA axis reactivity.     

Removal of a nest-mate elicits an age-dependent increase in plasma corticosterone of nestling Black-legged Kittiwakes

ABSTRACT.   Plasma corticosterone concentrations in birds often increase about 3 min after exposure to a stressor such as capture and handling. When measuring adrenal responsiveness of nestlings in broods with more than one nestling, standardizing capture protocols to equalize the stressor among nestlings if they simultaneously perceive the presence and activity of a researcher as a stressor is logistically difficult. The objective of our study was to determine if nestling Black-legged Kittiwakes ( Rissa tridactyla ) in broods of two mount a corticosterone response when the first nestling is removed from the nest or, alternatively, if each initiates a corticosterone response only at the time it is handled. We obtained blood samples from one nestling within 3 min of initial disturbance of the nest, and then removed and sampled its sibling 10 min later. For younger nestlings, we found no difference in corticosterone levels between those sampled at 3 min and their sibling sampled at 10 min. In contrast, older nestlings sampled at 10 min after initial nest disturbance had elevated corticosterone levels compared to those sampled within 3 min. In addition, nestlings sampled within 3 min of capture had elevated corticosterone when exposed to protracted periods of investigator disturbance at nearby nests. Our results suggest that it is necessary to treat initial disturbance of the nest as the onset of the stress response for all nestlings in multi-nestling broods when handling older nestlings or nestlings of unknown age. In addition, for species that nest in dense colonies, the presence of an investigator at one nest may be a stressor for nestlings in adjacent nests.     

Increased sibling competition does not increase testosterone or corticosterone levels in nestlings of the spotless starling (Sturnus unicolor)

Nestling begging in passerine birds is a complex behaviour that is shaped by a multitude of ecological factors and could be physiologically mediated by varying levels of steroid hormones. Previous research has shown links between sibling competition and testosterone and corticosterone in several bird species. The spotless starling (Sturnus unicolor) is a medium sized passerine in which nestlings compete intensively for resources, often resulting in marked size hierarchies that can have profound effects on their fitness. We tested the hypothesis that an increase in sibling competition levels would result in increases in testosterone and corticosterone in this species. To this end we conducted a brood size manipulation, creating small, medium and large broods. This manipulation had the expected effect on morphology: nestling size and mass decreased with increasing brood size. Androgen levels varied in response to brood size manipulation but, contrary to expectations, the largest concentrations were found in reduced brood sizes. Corticosterone levels increased with increasing brood size, but this effect disappeared when we corrected for the time taken to process nestlings. Cell-mediated immune response was found to decrease with increasing brood size and testosterone levels. The results suggest that the proposed link between testosterone and corticosterone and sibling competition does not hold in this species, and underlines the diversity of species-specific responsiveness to steroids.     

Experimentally delayed hatching triggers a magnified stress response in a long-lived bird

In birds, the timing of breeding is a key life-history trait with crucial fitness consequences. We predicted that parents may value a brood less if it hatched later than expected, thereby decreasing their parental effort. In addition, breeding effort would be further modulated by the age-specific decline of future breeding opportunities. We experimentally investigated whether snow petrels, Pagodroma nivea, were less committed to care for a chick that hatched later than expected. The timing of hatching was manipulated by swapping eggs between early and late known-age pairs (7-44 years old), and investigations on hormonal and behavioral adjustments were conducted. As a hormonal gauge of parental commitment to the brood, we measured the corticosterone stress response of guarding adults. Indeed, an acute stress response mediates energy allocation towards survival at the expense of current reproduction and is magnified when the current brood value is low, as it is expected to be in young and/or delayed parents. As predicted, egg desertion and the magnitude of the stress response was stronger in delayed pairs compared to control ones. However, the treatment did not decrease the length of the guarding period, chick condition and chick survival. In addition, old parents resisted stress better (lower stress-induced corticosterone levels) than young ones. Our study provides evidence that snow petrels, as prudent parents, may value a brood less if it hatched later than expected. Thus, in long-lived birds, the responsiveness to stressors appeared to be adjusted according to the individual prospect of future breeding opportunities (age) and to the current brood value (timing of breeding).