The reproductive season is energetically costly as revealed by elevated glucocorticoid concentrations, constrained immune functions and an increased risk of infections. Social allies and affiliative interactions may buffer physiological stress responses and thereby alleviate associated effects. In the present study, we investigated the seasonal differences of immune reactive corticosterone metabolite concentrations, endoparasite burden (nematode eggs and coccidian oocysts) and affiliative interactions in northern bald ibis (Geronticus eremita), a critically endangered bird. In total, 43 individually marked focal animals from a free-ranging colony were investigated. The analyses included a description of initiated and received affiliative interactions, pair bond status as well as seasonal patterns of hormone and endoparasite levels. During the reproductive season, droppings contained parasite eggs more often and corticosterone metabolite levels were higher as compared to the period after reproduction. The excretion rate of endoparasite products was lower in paired individuals than in unpaired ones, but paired animals exhibited higher corticosterone metabolite concentrations than unpaired individuals. Furthermore, paired individuals initiated affiliative behaviour more frequently than unpaired ones. This suggests that the reproductive season influences the excretion patterns of endoparasite products and corticosterone metabolites and that affiliative interactions between pair partners may positively affect endoparasite burden during periods of elevated glucocorticoid levels. Being embedded in a pair bond may have a positive impact on individual immune system and parasite resistance.

Most migrating birds make stopovers to replenish fuel stores. The decision to resume migration from stopover to a large extent shapes the temporal organization of migration. This decision is known to be shaped by a suite of intrinsic and extrinsic factors such as the bird's fuel stores and current weather conditions. However, how departures from stopover are physiologically regulated is largely unknown. We here present data that strongly indicate that corticosterone, a hormone with a stimulatory effect on locomotion, acts as a mediator between fuel stores and departure from stopover. In migrating northern wheatears (Oenanthe oenanthe) temporarily caged at stopover, we observed a positive relationship between the change in fuel stores and the concurrent change in glucocorticoid metabolite (GCM) levels measured in the birds' droppings. We also found a positive relationship between the change in GCM levels and the change in the intensity of nocturnal migratory restlessness. As in northern wheatears nocturnal migratory restlessness is an accurate proxy for stopover departure likelihood, our results indicate that corticosterone mediates between fuel stores and the decision to resume migration. Our unique longitudinal study represents a considerable advance in our understanding of the endocrine regulation of avian migration.

AbstractSocial life is profitable, but it facilitates conflicts over resources and creates interdependence between individuals. Separating highly social animals triggers intense reactions aimed at re‐establishing lost connections. Less is known, however, about behavioural and physiological responses to separation in socially facultative species, where individuals temporarily form groups and may subsequently leave them. Non‐breeding common ravens (Corvus corax) gather in large numbers at feeding and roosting sites, but otherwise spend time seemingly solitary or in small subgroups. We here studied how ravens cope with being socially isolated, and investigated the life characteristics that might explain potential individual differences. For this, we individually separated captive subadult ravens (n = 25) and housed them in physical and visual isolation from their group members across 4 d. During the separation period, we collected behavioural data and measured the amount of immunoreactive corticosterone metabolites from bird droppings to assess the ravens’ physiological stress response. We found behavioural indicators of stress at the start of the separation, when ravens showed higher levels of tension than of comfort – a pattern that reversed at the end of the separation. Furthermore, we found that the upbringing of ravens affected their behaviour during separation. Hand‐raised birds produced more vocalisations in the beginning of the separation, and were less active at the end, while the reverse pattern occurred with parent‐raised ravens. Contrary to our predictions, we did not find differences in hormonal responses between the beginning and end of the separation period or any link between hormonal responses and behaviours. Ravens’ behavioural responses to social separation stress seem to be dependent on their arousal states, although possible links with hormonal reactions remain unclear. Our results show that behavioural reactions are not always linked with hormonal responses to stress, and further emphasise the importance of investigating effects of early‐life experiences.

The expression of key traits like parental provisioning and offspring begging is confounded by the parent–offspring conflict in species with parental care, with offspring seeking greater parental investment than parents are willing to provide. Given the reciprocal interplay of these traits, selection has likely favored specific parent–offspring trait combinations, and it has been a longstanding question which party benefits most from this linkage. This will become apparent in a mismatch situation, which we here experimentally created by reciprocally cross-fostering blue tit broods. We hypothesized that offspring fledgling mass and their excreted corticosterone metabolite (CM) levels (reflecting stress) should vary with the rate of (foster) parental care (if provisioning is under full parental control), with offspring begging (if offspring is fully in control), or an interaction of both traits reflecting stable end points along a power continuum. We found a significant interaction effect, that is, highly demanding broods reached lowest fledgling mass when raised by low providing parents (due to partly unrewarded costly begging) but highest fledgling mass when raised by high providers. This pattern, however, was not reflected in offspring CM levels. Parental provisioning of the foster parents affected CM levels again in interaction with the offspring’s begging level, but this pattern may rather reflect intensity and frequency of sibling competition. Taken together, our results suggest that an adjustment of the offspring’s phenotype to the post-hatching social environment is primarily beneficial for highly demanding offspring and that parents have the upper hand (but probably not full control) over provisioning.

Non-breeding common ravens (Corvus corax) live in complex social groups with a high degree of fission–fusion dynamics. They form valuable relationships and alliances with some conspecifics, while taking coordinated action against others. In ravens, affiliates reconcile their conflicts, console each other after conflicts with a third party, and provide each other with social support — all behaviors that presumably reduce corticosterone levels and alleviate stress. However, how well an individual is socially integrated in a (sub)group might vary substantially. This raises the question whether the social integration of a raven affects its stress responses to fission–fusion dynamics. The present study aims to investigate this effect experimentally by separating single ravens (n=16) individually from their group for four days and subsequently reintroducing them. To determine stress response patterns in the separated individuals we measured the amounts of immunoreactive corticosterone metabolites (CM) in droppings. We compared two enzyme immunoassays, which we validated by conducting an ACTH challenge, and finally decided to apply an 11-oxoetiocholanolone enzyme immunoassay. Additionally, we determined levels of social integration using focal observations. Our findings suggest that a strong social integration is related to low CM levels when the individuals are within the group and high levels during separations, implying that separation leads to stress in these birds. In contrast, poorly socially integrated ravens seem to exhibit the opposite pattern, indicating that to them group living is more stressful than being temporarily separated. We, therefore, conclude that the birds' adrenocortical activity is modulated by their social integration.

We manipulated dietary lead (Pb) levels of nestlings in wild populations of the great tit (Parus major L) to find out if environmentally relevant Pb levels would affect some physiological biomarkers (haematocrit [HT], fecal corticosterone metabolites [CORT], heat shock proteins [HSPs], erythrocyte delta-aminolevulinic acid dehydratase activity [ALAd]), growth (body mass, wing length), phenotype (plumage coloration) or survival of nestlings. The responses to three experimental manipulation (control, low and high: 0, 1 and 4 μg/g body mass/day) are compared with those in a P. major population breeding in the vicinity of a heavy metal source, a copper smelter. Our Pb supplementation was successful in raising the fecal concentrations to the levels found in polluted environments (high: 8.0 μg/g d.w.). Despite relatively high range of exposure levels we found only few effects on growth rates or physiology. The lack of blood ALAd inhibition suggests that the circulating Pb levels were generally below the toxic level despite that marked accumulation of Pb in femur (high: 27.8 μg/g d.w.) was observed. Instead, birds in the metal polluted environment around the smelter showed decreased growth rates, lower HT, higher CORT, less colorful plumage and lower survival probabilities than any of the Pb treated groups. These effects are likely related to decreased food quality/quantity for these insectivorous birds at the smelter site. In general, the responses of nestlings to metal exposure and/or associated resource limitation were not gender specific. One of the stress proteins (HSP60), however, was more strongly induced in Pb exposed males and further studies are needed to explore if this was due to higher accumulation of Pb or higher sensitivity of males. In all, our results emphasize the importance of secondary pollution effects (e.g. via food chain disruption) on reproductive output of birds.

The decision made by migrating birds to stop refueling and to depart from stopover depends on cues from innate rhythms, intrinsic factors such as fuel reserves, and extrinsic factors such as weather conditions. The physiological mechanism behind this decision, however, is largely unexplored. The transition from refueling to flight involves an increase in both locomotion and energetic demands. Because, at baseline levels, corticosterone stimulates locomotion and is involved in the mobilization of energy, this hormone could encourage departure of migrants. We collected field data on baseline corticosterone, migratory restlessness, and actual departure in northern wheatears at stopover. Additionally, in refueling long-term captive conspecifics, we measured migratory restlessness while simultaneously collecting droppings to determine glucocorticoid metabolite (GCM) levels. We found that migratory restlessness at stopover was positively correlated with corticosterone level. Similarly, in refueling long-term captive birds, migratory restlessness was positively correlated with concurrently measured GCM levels in droppings. To our best knowledge, our study is the first to simultaneously measure a hormonal signal and migratory restlessness. In conclusion, our results are consistent with the hypothesis that, by increasing locomotor activity, baseline corticosterone is involved in the regulation of departure of migrants at stopover. Future studies could reveal how corticosterone is up-regulated in migrants that are ready to depart.

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.

The welfare and productivity of South American camelids may be affected by stressful events. The purpose of this study was to validate a non-invasive method for stress monitoring using faecal samples and to apply it to evaluate a stressful event, such as confinement. For physiological validation, nine alpacas (Vicugna pacos) and six llamas (Lama glama) were subjected to pharmacological stimulation of their adrenal cortex. Serial faecal samples were collected during 48 h before and after stimulation. During confinement, faecal samples from six llamas were collected twice per day during six consecutive days. Faeces belonging to 18 vicuñas (Vicugna vicugna) were collected before and one day after their capture for confinement (Chacu). Faecal cortisol metabolites (FCM) were extracted from each sample

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.

The presence of social allies may buffer adverse consequences of social stress. This has mainly been demonstrated in mammals and recently also in birds. The behaviour of social allies might crucially influence to which extent social context may buffer the behavioural and hormonal response to stress. We here examined the influence of social context on the hormonal response to handling stress in great tits (Parus major) selected for fast and slow exploration. We tested 16 male-female pairs (8 fast-fast pairs, 8 slow-slow pairs) after the breeding season. We subjected females to handling stress and thereafter observed their behaviour and collected droppings for the following 21/2h with their mate being either absent or present when the females came back into her home cage. As control the same females were not handled prior to observation and their mate was present. In addition, we tested 7 fast and 7 slow unpaired females in the conditions control and mate absent. We measured immunoreactive corticosterone metabolites (CM) in droppings using an enzyme immunoassay. Fast females excreted significantly higher CM mean values when they were alone after handling stress (mate absent) than in the control condition and in condition mate present. Slow females tended to show a similar pattern. While fast females increased their locomotory activity, slow females sat close to their mates longer after handling stress compared to control days. Pair mates resting and feeding synchronously excreted lower CM than asynchronous pairs, irrespective of their behavioural phenotype. Paired and unpaired females did neither differ in behavioural nor in hormonal stress response, indicating that observed differences between condition mate absent and mate present in paired females were not due to an accumulation of stressors (mate absent plus handling) in condition mate absent. We here show for the first time, that depending on behavioural phenotype birds increased social proximity after a stressful event and that pair synchrony may modulate corticosterone excretion.

Individuals consistently differ in behavioural phenotypes. Here we examine the interaction between behavioural phenotype and response to social context during novel object exploration in a neophobic corvid species, the raven (Corvus corax). The presence of conspecifics tends to encourage object exploration and learning but may also delay or even inhibit exploratory behaviour. Factors such as individual differences in response to social context may determine whether the presence of a conspecific facilitates or inhibits approach to novel objects. We confronted eleven six-month-old hand-raised ravens with novel objects, both individually and in dyadic combinations. We defined individuals as "fast'' and "slow'' explorers on the basis of their approach latency to novel objects when tested individually. The presence of a conspecific delayed the approach of fast birds to novel objects. Slow birds, in contrast, approached the novel objects with lower latencies and spent more time close to them when in dyads with fast siblings than when alone. The individuals' approach behaviour seemed to determine whether social context facilitated or delayed exploratory behaviour. This may contribute to explaining ambiguous results concerning the effects of social context in previous studies.

Ravens scavenging on carcasses devote much of their time to carrying off loads of food for temporary storage in caches. Ways of reducing conspecific pilfering of these caches include camouflaging caches with debris and positioning caches at a distance from and/or out of sight of potential competitors. We examined the acquisition and improvement of these caching skills in young captive ravens. We identified a hierarchy of elements in the caching behaviour, ranging from simply pressing the caching items towards solid structures to the appropriate covering of them with surrounding material. The development of these elements was closely linked with the birds' acquisition of Piagetian Stages of object permanence. Furthermore, we distinguished between the young ravens' initial caching of edible and inedible items. We tracked the individuals' temporal development of placing their caches at a distance from and out of sight of conspecifics and their social interactions over caches. Behaviours aimed at preventing pilfering were associated with competitive interactions and were manifested only with edible items. We conclude that the skills for camoflaging caches are primarily affected by maturation of representational capacities for hidden (i.e. not visible) items, whereas the skills for the positioning of caches are strongly affected by experience gained through social interactions.

Neophobia may constrain explorative behaviour, learning and innovation, while social context may facilitate approach to novel objects and acceptance of novel food. We examined the effects of neophobia on the exploration of novel objects in relation to social context in ravens (Corvus corax). Ravens are suitable subjects for studying effects of social context, as they are highly neophobic scavengers that recruit conspecifics to food. We tested two groups of six and 11 hand-raised birds in three conditions: single-bird, in dyadic combinations and sibling subgroups of three to six birds. Contrary to expectation, individuals of both groups were quicker to approach novel objects when tested alone than when tested with conspecifics. However, they spent more time close to and manipulating the novel objects in the social conditions (dyadic and group) than when being alone. We discuss the possibility that the higher latencies of dyads and groups to approach novel objects may reflect a 'negotiation' process in a 'war of attrition' between the individuals over risk-taking.

Social context has been shown to encourage, or to delay object exploration and learning. This ambiguity might be due to factors such as social relationships and personality of the individuals involved. Here, we investigated in ravens (Corvus corax) individuals' consistency in response to novel objects over development and across contexts: alone versus social. In the social setting we focussed on the effects of social relationships on social facilitation during the approach to novel objects. We tested 11 hand-raised ravens with novel objects individually at three and six months of age and in dyadic combinations at six months of age. Individuals were consistent over development and contexts in their response to different novel objects. Birds joined siblings faster to approach novel objects than non-siblings. They also spent more time sitting close to siblings than to non-siblings. In male-male dyads but not in female-female dyads, subordinates approached the novel objects significantly faster than dominant birds. In contrast, dominant males were the first to approach the novel objects in mixed-sex combinations. Hence, the effect of social context seems to depend on the social relationships towards the companions and on the combination of the sexes.

The ability to follow gaze (i.e. head and eye direction) has recently been shown for social mammals, particularly primates. In most studies, individuals could use gaze direction as a behavioural cue without understanding that the view of others may be different from their own. Here, we show that hand-raised ravens not only visually co-orient with the look-ups of a human experimenter but also reposition themselves to follow the experimenter's gaze around a visual barrier. Birds were capable of visual co-orientation already as fledglings but consistently tracked gaze direction behind obstacles not before six months of age. These results raise the possibility that sub-adult and adult ravens can project a line of sight for the other person into the distance. To what extent ravens may attribute mental significance to the visual behaviour of others is discussed.