Developmental stress increases reproductive success in male zebra finches

There is increasing evidence that exposure to stress during development can have sustained effects on animal phenotype and performance across life-history stages. For example, developmental stress has been shown to decrease the quality of sexually selected traits (e.g. bird song), and therefore is thought to decrease reproductive success. However, animals exposed to developmental stress may compensate for poor quality sexually selected traits by pursuing alternative reproductive tactics. Here, we examine the effects of developmental stress on adult male reproductive investment and success in the zebra finch (Taeniopygia guttata). We tested the hypothesis that males exposed to developmental stress sire fewer offspring through extra-pair copulations (EPCs), but invest more in parental care. To test this hypothesis, we fed nestlings corticosterone (CORT; the dominant avian stress hormone) during the nestling period and measured their adult reproductive success using common garden breeding experiments. We found that nestlings reared by CORT-fed fathers received more parental care compared with nestlings reared by control fathers. Consequently, males fed CORT during development reared nestlings in better condition compared with control males. Contrary to the prediction that developmental stress decreases male reproductive success, we found that CORT-fed males also sired more offspring and were less likely to rear non-genetic offspring compared with control males, and thus had greater overall reproductive success. These data are the first to demonstrate that developmental stress can have a positive effect on fitness via changes in reproductive success and provide support for an adaptive role of developmental stress in shaping animal phenotype.     

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.     

Stressful Dieting: Nutritional Conditions but Not Compensatory Growth Elevate Corticosterone Levels in Zebra Finch Nestlings and Fledglings

Unfavourable conditions throughout the period of parental care can severely affect growth, reproductive performance, and survival. Yet, individuals may be affected differently, depending on the developmental period during which constraints are experienced. Here we tested whether the nestling phase compared to the fledgling phase is more susceptible to nutritional stress by considering biometry, physiology, sexually selected male ornaments and survival using zebra finches (Taeniopygia guttata) as a model species. As nestlings (day 0–17) or fledglings (day 17–35), subjects were raised either on low or high quality food. A low quality diet resulted in significantly elevated baseline corticosterone titres in both nestlings and fledglings. Subjects showed substantial compensatory growth after they had experienced low quality food as nestlings but catch-up growth did neither lead to elevated baseline corticosterone titres nor did we detect long term effects on biometry, male cheek patch, or survival. The compensation for temporally unfavourable environmental conditions reflects substantial phenotypic plasticity and the results show that costs of catch-up growth were not mediated via corticosterone as a physiological correlate of allostatic load. These findings provide new insights into the mechanisms and plasticity with which animals respond to periods of constraints during development as they may occur in a mistiming of breeding.     

Corticosterone levels in host and parasite nestlings: Is brood parasitism a hormonal stressor?

Parasite chicks from non-evictor species usually try to monopolize host parental care, thereby increasing considerably the level of food competition in the nest. Here, we propose that brood parasitism is an important stressor for host and parasite nestlings and explore this hypothesis in the non-evictor great spotted cuckoo (Clamator glandarius) and its main hosts, the same-sized black-billed magpie (Pica pica) and the larger carrion crow (Corvus corone). We experimentally created 3-nestling broods of different brood compositions (only cuckoo chicks, only host chicks, or cuckoo and host chicks together) and measured baseline corticosterone levels of nestlings along their developmental period (early, middle and late). We found that brood parasitism increased corticosterone levels in magpie nestlings in the mid and late nestling period compared to those raised in unparasitized nests. Interestingly, carrion crow nestlings from parasitized nests only increased their corticosterone levels in the mid nestling period, when the competition for food with the cuckoo nestling was highest. Our results suggest that brood parasitism could be a potential physiological stressor for host nestlings, especially during the developmental stages where food requirements are highest. Conversely, cuckoo nestlings could be physiologically adapted to high competition levels since they did not show significant differences in corticosterone levels in relation to brood composition.     

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.     

Development of stress response in nestling pied flycatchers

Birds respond to unpredictable events by secreting corticosterone, which induces various responses to cope with stressful situations. However, the evidence is still elusive whether altricial nestlings perceive and respond to external stressors. We investigated the development of adrenocortical stress response to handling-related stressor in nestlings of a small passerine bird, the pied flycatcher (Ficedula hypoleuca). Nestlings were held in isolation from their parents during the experiment to ensure that they indeed respond to handling, not to parental alarm calls. We found that both 9- and 13-day-old nestlings were able to elicit hormonal stress response. Although baseline as well as stress-induced corticosterone levels rose slightly with age, the magnitude of difference between the control and stress-induced levels remained similar in both age groups. However, comparison with adults showed that the stress response of nestlings prior to fledging was still incomplete and significantly lower than in adults. Overall, our results indicate that altricial nestlings do respond to acute stressors, but on the contrary to previous predictions the development of corticosterone stress response during growth period is not gradual and varies remarkably between different passerine species.     

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.     

Fluctuating asymmetry as a biomarker of habitat fragmentation in an area-sensitive passerine,the Eurasian treecreeper (Certhia familiaris)

The research investigating fluctuating asymmetry (FA), a phenotypic proxy of developmental instability, as a potential early-warning biomarker of anthropogenic stress like habitat fragmentation has produced controversial results. We examined the influence of habitat fragmentation at the landscape-scale, divided into habitat amount and configuration, on feather length FA in the Eurasian treecreeper (Certhia familiaris), an area-sensitive old-growth forest passerine breeding in boreal forests that are currently under intense management. Our study included one tail and wing feather, measured in both sexes of the 14-day-old nestlings and adults. Habitat amount was measured as the proportions of suitable forest habitats and open unsuitable areas, mean patch isolation and nesting patch size whereas habitat configuration was measured as mean patch density, size and shape and edge density. We found only weak sex- and age-related associations between feather length FA and habitat fragmentation that explained just 4.9% of variance in FA. Habitat loss was associated with higher FA in males only while the habitats with low degree of configuration tended to be related to lower FA in adults only. From the biomarker perspective, FA may not thus hold a great promise as a strong and general indicator of habitat fragmentation.     

Elevated stress hormone diminishes the strength of female preferences for acoustic signals in the green treefrog

Mate selection can be stressful; time spent searching for mates can increase predation risk and/or decrease food consumption, resulting in elevated stress hormone levels. Both high predation risk and low food availability are often associated with increased variation in mate choice by females, but it is not clear whether stress hormone levels contribute to such variation in female behavior. We examined how the stress hormone corticosterone (CORT) affects female preferences for acoustic signals in the green treefrog, Hyla cinerea. Specifically, we assessed whether CORT administration affects female preferences for call rate — an acoustic feature that is typically under directional selection via mate choice by females in most anurans and other species that communicate using acoustic signals. Using a dual speaker playback paradigm, we show that females that were administered higher doses of CORT were less likely to choose male advertisement calls broadcast at high rates. Neither CORT dose nor level was related to the latency of female phonotactic responses, suggesting that elevated CORT does not influence the motivation to mate. Results were also not related to circulating sex steroids (i.e., progesterone, androgens or estradiol) that have traditionally been the focus of studies examining the hormonal basis for variation in female mate choice. Our results thus indicate that elevated CORT levels decrease the strength of female preferences for acoustic signals.     

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.     

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.     

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.     

Corticosterone excretion patterns and affiliative behavior over development in ravens (Corvus corax)

Averse effects of social stress may be buffered by the presence of social allies, which mainly has been demonstrated in mammals and recently also in birds. However, effects of socio-positive behavior prior to fledging in relation to corticosterone excretion in altricial birds have not been investigated yet. We here monitored corticosterone excretion patterns in three groups of hand raised juvenile ravens (n=5, 6 and 11) in the nest, post-fledging (May-July) and when ravens would be independent from their parents (September-November). We related these corticosterone excretion patterns to socio-positive behavior. Behavioral data were collected via focal sampling in each developmental period considered. We analyzed amounts of excreted immunoreactive corticosterone metabolites (CM) using enzyme immuno assays. We collected fecal samples in each developmental period considered and evaluated the most appropriate assay via an isolation stress experiment. Basal CM was significantly higher during the nestling period than post-fledging or when birds were independent. The time nestlings spent allopreening correlated negatively with mean CM. Post-fledging, individuals with higher CM levels sat close to (distance <50 cm) conspecifics more frequently and tended to preen them longer. When birds were independent and a stable rank hierarchy was established, dominant individuals were preened significantly longer than subordinates. These patterns observed in ravens parallel those described for primates, which could indicate that animal species living in a complex social environment may deal with social problems in a similar way that is not restricted to mammals or primates.     

Forest management is associated with physiological stress in an old-growth forest passerine

We investigated how physiological stress in an area-sensitive old-growth forest passerine, the Eurasian treecreeper (Certhia familiaris), is associated with forest fragmentation and forest structure. We found evidence that the concentrations of plasma corticosterone in chicks were higher under poor food supply in dense, young forests than in sparse, old forests. In addition, nestlings in large forest patches had lower corticosterone levels and a better body condition than in small forest patches. In general, corticosterone levels were negatively related to body condition and survival. We also found a decrease in corticosterone levels within the breeding season, which may have been a result of an increase in food supply from the first to the second broods. Our results suggest that forest fragmentation may decrease the fitness of free-living individual treecreepers.     

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.     

Developmental stress and telomere dynamics in a genetically polymorphic species

A central objective of evolutionary biology is understanding variation in life‐history trajectories and the rate of aging, or senescence. Senescence can be affected by trade‐offs and behavioural strategies in adults but may also be affected by developmental stress. Developmental stress can accelerate telomere degradation, with long‐term longevity and fitness consequences. Little is known regarding whether variation in developmental stress and telomere dynamics contributes to patterns of senescence during adulthood. We investigated this question in the dimorphic white‐throated sparrow (Zonotrichia albicollis), a species in which adults of the two morphs exhibit established differences in behavioural strategy and patterns of senescence, and also evaluated the relationship between oxidative stress and telomere length. Tan morph females, which exhibit high levels of unassisted parental care, display faster reproductive senescence than white females, and faster actuarial senescence than all of the other morph–sex classes. We hypothesized that high oxidative stress and telomere attrition in tan female nestlings could contribute to this pattern, since tan females are small and potentially at a competitive disadvantage even as nestlings. Nestlings that were smaller than nest mates had higher oxidative stress, and nestlings with high oxidative stress and fast growth rates displayed shorter telomeres. However, we found no consistent morph–sex differences in oxidative stress or telomere length. Results suggest that oxidative stress and fast growth contribute to developmental telomere attrition, with potential ramifications for adults, but that developmental oxidative stress and telomere dynamics do not account for morph–sex differences in senescence during adulthood.     

Food Supplementation Fails to Reveal a Trade-Off between Incubation and Self-Maintenance in Female House Wrens

Incubating birds must allocate their time and energy between maintaining egg temperature and obtaining enough food to meet their own metabolic demands. We tested the hypothesis that female house wrens (Troglodytes aedon) face a trade-off between incubation and self-maintenance by providing females with supplemental food during incubation. We predicted that food supplementation would increase the amount of time females devoted to incubating their eggs, lower their baseline plasma corticosterone levels (a measure of chronic stress), and increase their body mass, haematocrit (a measure of anaemia), and reproductive success relative to control females. As predicted, food-supplemented females spent a greater proportion of time incubating their eggs than control females. Contrary to expectation, however, there was no evidence that food supplementation significantly influenced female baseline plasma corticosterone levels, body mass, haematocrit, or reproductive success. However, females with high levels of corticosterone at the beginning of incubation were more likely to abandon their nesting attempt after capture than females with low levels. Corticosterone significantly increased between the early incubation and early nestling stages of the breeding cycle in all females. These results suggest that although food supplementation results in a modest increase in incubation effort, it does not lead to significantly lower levels of chronic stress as reflected in lower baseline corticosterone levels. We conclude that female house wrens that begin the incubation period with low levels of plasma corticosterone can easily meet their own nutritional needs while incubating their eggs, and that any trade-off between incubation and self-feeding does not influence female reproductive success under the conditions at the time of our study.     

Relationship between fluctuating asymmetry and fitness within and between stressed and unstressed populations of the wolf spider Pirata piraticus

Although developmental instability, measured as fluctuating asymmetry (FA), is expected to be positively related to stress and negatively to fitness, empirical evidence is often lacking or contradictory when patterns are compared at the population level. We demonstrate that two important properties of stressed populations may mask such relationships: (i) a stronger relationship between FA and fitness, resulting in stronger selection against low quality (i.e. developmental unstable) individuals and (ii) the evolution of adaptive responses to environmental stress. In an earlier study, we found female wolf spiders Pirata piraticus from metal exposed populations to be characterized by both reduced clutch masses and increased egg sizes, the latter indicating an adaptive response to stress. By studying the relationship between these two fitness related traits and levels of FA at individual level, we here show a significant negative correlation between FA and clutch mass in metal stressed populations but not in unstressed reference populations. As a result, levels of population FA may be biased downward under stressful conditions because of the selective removal of developmentally unstable (low quality) individuals. We further show that females that produced larger eggs in stressed populations exhibited lower individual FA levels. Such interaction between individual FA and fitness with stress may confound the effect of metal stress on FA, resulting in an absence of relationships between FA, fitness and stress at the population level.     

Behavioural and hormonal stress responses during chick rearing do not predict brood desertion by female in a small Arctic seabird

We examined behavioural and hormonal stress responses in a small seabird (little auk, Alle alle), which exhibits a transition from biparental to male-only care towards the end of the nesting period, in order to understand the mechanisms underlying this parental strategy. We hypothesized that the male staying with the chick should be less sensitive to stressors. As such the male might offer the offspring more efficient protection during the fledging period than the female. We tested this hypothesis by observing male and female behaviour in a neophobia test. We also measured the birds' baseline and stress-induced levels of corticosterone and prolactin using the standardized capture-and-restraint protocol. Both sexes respond rapidly to foreign objects, delaying the entry time to the nest with food, consuming the food load, and/or temporarily abandoning feeding. However, we did not find any differences between the sexes in the frequency of each behaviour or in the time of the first reaction to the experimental treatment. Level of both corticosterone and prolactin increased after the experimental treatment. However, we did not find sex differences in baseline and stress-induced hormone levels. The results indicate that the males are as much sensitive to the stress situation as the females. Thus, the pattern of male and female behavioural and hormonal responses to stress does not predict their behaviour at the final breeding stage.     

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.