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Observing interactions between others can provide important information to individuals. Male songbirds often engage in singing contests where they vary the type and timing of signals and provide eavesdropping individuals with information about their competitiveness. How this information is used and its effect on subsequent spatial behaviour and reproductive decisions of eavesdroppers is not well understood. Here we tested whether great tits use information gathered by eavesdropping on male singing interactions to assess rivals and (potential) mates. We used interactive playback experiments to engage territorial males in song contests with either a more (song overlapping and more persistent singing) or less challenging (song alternating and less persistent singing) intruder. We followed male and female movements by automated radiotracking, determined paternity using microsatellite analysis and maternal investment by quantifying egg weights and provisioning behaviour. We expected that mates of males exposed to the challenging treatment would subsequently foray more often off territory to assess other males and potential extrapair mates and invest less in their broods. Moreover, we expected that neighbours would adjust their foraying behaviour according to information gained by eavesdropping. Females, however, did not alter their foraying behaviour or brood investment and neither female nor male neighbours changed their visiting behaviour to playback territories. Our results provide no evidence that females used information gathered by eavesdropping on asymmetric song interactions in reproductive decisions or that song interactions affected movements across territories in the neighbourhood. Overlapping or singing for a longer time on an intruded upon territory may not always be perceived as a higher level of threat, and reproductive decisions and assessment of familiar individuals are likely to be based on multiple sources of information rather than on a single interaction. • First study to radiotrack mate and neighbour responses to male birdsong playback. • Female reproductive decisions were not affected by a male singing interaction. • Females did not foray more into neighbouring territories after territorial challenge. • Forays by neighbours did not change after playback. • Female mate assessment appears robust against short-term information changes. [ABSTRACT FROM AUTHOR]

Gathering information about the environment, such as the location and quality of food, is crucial for an animal's survival, particularly in a changing environment. An animal can collect 'personal information' by interacting with the environment itself, or it can collect 'social information' by observing the behaviour of others. The use of these two types of information varies across different situations and between individuals. Personality is a concept that captures consistent interindividual differences in behaviour and could be one of the factors driving interindividual variation in information use. We tested this by conducting behavioural experiments based on a colour association task in captive great tits, Parus major , originating from lines bidirectionally selected for high and low exploratory behaviour. We quantified personal information use by measuring to what extent a bird relied on previously rewarded options instead of novel options. Social information use was measured by recording how birds chose according to social information provided by video playbacks of a conspecific. Here, we demonstrate that variation in the use of both personal and social information is indeed personality related. In their decision making, slow explorers relied more on prior knowledge, from both personal and social origins, whereas fast explorers tended to ignore the available information and chose more randomly. The differences between the personality types imply different costs or constraints in acquiring and/or applying the two types of information, possibly due to variation in, for example, cognitive styles. In conclusion, we demonstrate that personality types have different strategies to cope with environmental uncertainty. • Both social and personal information use are personality related. • Faster explorers tend to sample novel options instead of using personal information. • The use of social information decreases with exploratory tendency. • Slow explorers base their behaviour more on prior knowledge than fast explorers. [ABSTRACT FROM AUTHOR]

In evolutionary endocrinology, there is a growing interest in the extent and basis of individual variation in endocrine traits, especially circulating concentrations of hormones. This is important because if targeted by selection, such individual differences present the opportunity for an evolutionary response to selection. It is therefore necessary to examine whether hormone traits are repeatable in natural populations. However, research in this area is complicated by the fact that different hormone traits can be correlated. The nature of these trait correlations (i.e., phenotypic, within-, or among-individual) is critically relevant in terms of the evolutionary implications, and these in turn, depend on the repeatability of each hormone trait. By decomposing phenotypic correlations between hormone traits into their within- and among-individual components it is possible to describe the multivariate nature of endocrine traits and generate inferences about their evolution. In the present study, we repeatedly captured individual great tits ( Parus major ) from a wild population and measured plasma concentrations of corticosterone. Using a mixed-modeling approach, we estimated repeatabilities in both initial (cf. baseline; CORT0) and stress-induced concentrations (CORT30) and the correlations between those traits among- and within-individuals. We found a lack of repeatability in both CORT0 and CORT30. Moreover, we found a strong phenotypic correlation between CORT0 and CORT30, and due to the lack of repeatability for both traits, there was no among-individual correlation between these two traits—i.e., an individual’s average concentration of CORT0 was not correlated with its average concentration of CORT30. Instead, the phenotypic correlation was the result of a strong within-individual correlation, which implies that an underlying environmental factor co-modulates changes in initial and stress-induced concentrations within the same individual over time. These results demonstrate that (i) a phenotypic correlation between two hormone traits does not imply that the traits are correlated among individuals; (ii) the importance of repeated sampling to partition within- and among-individual variances and correlations among labile physiological traits; and (iii) that environmental factors explain a considerable fraction of the variation and co-variation in hormone concentrations. [ABSTRACT FROM AUTHOR]

Highlights: [•] Wild great tits exhibit stable individual differences in exploration behavior. [•] We tested for a link between this complex phenotypic trait and HPA reactivity. [•] Slower explorers had an earlier elevation of stress-induced corticosterone (<3min). [•] Slower explorers also had higher concentrations 90min after stressor onset. [•] Results suggest a link between animal personality and HPA reactivity. [Copyright &y& Elsevier]

Anthropogenic noise can have serious implications for animals, especially when they communicate acoustically. Yet, the impacts of noise may depend not only on noise characteristics but also on an individual's coping style or personality. We tested whether noise is more disturbing if it masks communication signals, and whether characteristics of both the noise and the individual affect its impact. Using a unique population of personality-typed great tits, Parus major, we tested whether the kind of noise and parental personality affect parental nestbox visits and nestling begging. Nestboxes were exposed to automated noise playbacks, differing in spectral composition (noise masking begging calls, nonmasking noise or no noise). Parental nestbox visits were recorded using RFID transponders. Video and audio recordings were used to quantify nestling begging. Nestlings mainly begged in silence and in the presence of parents. Parents reduced nestbox visits during noise treatments regardless of the kind of noise and initially reacted more strongly to nonmasking noise. Moreover, slower explorers took longer to enter the nestbox during noise than faster explorers. Total visit rates during noise depended on parental sex and personality. In females, bolder individuals, but in males shyer individuals, reduced total visits during noise. These results extend previous findings in showing experimentally that the disturbance effects of noise do not depend on whether or not the noise directly interferes with information exchange by masking signals. Moreover, personality- and sex-specific responses to noise indicate that anthropogenic disturbance can differentially affect individuals within populations, which will influence mitigation strategies. [ABSTRACT FROM AUTHOR]

Animals constantly need to cope with changes in their environment. Coping with changes in cues that are associated with the location and abundance of food is essential for being able to adjust behaviourally to a variable environment. The use of cues in decision making requires appropriate levels of attention and learning ability, which may be affected by the personality of an individual. The relationship between personality, attention and learning as essential mechanisms for behavioural adaptation, however, is not well understood. We studied the relationship between attention to environmental cues, behavioural flexibility in learning and exploratory behaviour, a proxy for personality, in great tits, Parus major. We used a dimensional shift learning paradigm; a learning task involving several stages differing in complexity and requiring attention to changes in relevant cues. The results show personality differences in performance in learning flexibility in only the apparently most difficult stage, yet in opposite directions for males and females. Fast-exploring males showed more flexible learning abilities than slow males, whereas in females slow explorers outperformed fast explorers. These context-dependent and sex-specific personality effects reveal behavioural and cognitive mechanisms that may underlie observed sex- and personality-dependent fitness differences in natural populations. [Copyright &y& Elsevier]

Individuals within species differ in their behavioural reactions to the environment. Consistent individual differences in these behaviours (personality traits) are often correlated and known to be under natural selection. These differences are frequently associated with variation in physiological traits, such as endocrine profiles. For example, variation in circulating testosterone levels is associated with variation in several personality traits and has been hypothesized to be a marker for personality in humans and rodents. The importance of testosterone in controlling both behavioural strategies and individual physiological differences suggests that direct selection on personality traits might cause pleiotropic selection on the physiological mechanisms underlying these traits. To test this hypothesis, we quantified levels of plasma testosterone levels and measured phytohaemagglutinin (PHA)-induced immune responses of male great tits, Parus major, in lines artificially selected for diverging levels of avian personality (‘fast’ and ‘slow’ exploratory behaviour). We found that testosterone levels were highly repeatable within individuals and fluctuated predictably over the season. Contrary to our expectations, ‘slow’ explorers had consistently higher levels of baseline testosterone and higher immune responses than ‘fast’ explorers. These results show that phenotypic selection for variation in personality traits corresponds to consistent differences in hormone profile and immune function, but that higher aggression levels do not need to be associated with higher baseline testosterone levels. Our results confirm that personality traits have evolved as a result of selection on both the underlying controlling physiological mechanisms and the phenotypic traits. [ABSTRACT FROM AUTHOR]

Animals differ in their behaviour comparable to how humans differ in personality: individuals consistently differ in suites of correlated traits. Relationships between ‘personality traits’ and fitness imply that personality traits can evolve by means of natural selection. We studied whether animal personality is also involved in sexual selection. We investigated whether exploratory behaviour (an aspect of animal personality, ranging from ‘slow’ to ‘fast’) correlated with the occurrence of extrapair paternity (EPP) in broods of wild great tits. We expected that EPP rates should be highest for females mated with social partners of the same personality type (i.e. for slow–slow or fast–fast pairs, but not other pair combinations). We found that the likelihood of EPP was highest for these pairs. Disassortative extrapair mating with respect to personality can be the consequence of several non-mutually exclusive processes. It might be caused by adaptive mate choice, which allows assortatively paired females to produce offspring with either more variable or more intermediate phenotypes, but it could also be the consequence of behavioural incompatibility between extreme behavioural phenotypes. Our findings indicate that personality differences play a role in the mechanism behind extrapair behaviours and we therefore conclude that it is now plausible that partner preference is based not only on morphological characteristics, but also on consistent behavioural traits or personality. [Copyright &y& Elsevier]

Trees release Herbivore-Induced Plant Volatiles (HIPVs) into the air in response to damage inflicted by insects. It is known that songbirds use those compounds to locate their prey, but more recently the idea emerged that songbirds could also use those odours as cues in their reproductive decisions, as early spring HIPVs may contain information about the seasonal timing and abundance of insects. We exposed pre-breeding great tits (Parus major) to the odours of caterpillar-infested trees under controlled conditions, and monitored reproduction (timing of egg laying, number of eggs, egg size) and two of its main hormonal drivers (testosterone and 17β-estradiol in males and females, respectively). We found that females exposed to HIPVs did not advance their laying dates, nor laid larger clutches, or larger eggs compared to control females. 17β-estradiol concentrations in females were also similar between experimental and control birds. However, males exposed to HIPVs had higher testosterone concentrations during the egg-laying period. Our study supports the hypothesis that insectivorous songbirds are able to detect minute amounts of plant odours. The sole manipulation of plant scents was not sufficient to lure females into a higher reproductive investment, but males increased their reproductive effort in response to a novel source of information for seasonal breeding birds. • Insectivorous songbirds are able to detect minute amounts of plant odours (HIPVs). • Female great tits expose to HIPVs do no adjust reproductive investment. • HIPVs do not trigger increase of 17β-estradiol concentrations in females. • Males expose to HIPVs increase testosterone levels during the egg-laying period. [ABSTRACT FROM AUTHOR]

Adapting to environmental change is a major challenge faced by animals and the role of individual behavioural differences in facilitating this process is currently the focus of much research. Innovation, the generation of a novel behaviour or use of a known behaviour in a novel context, is one form of behaviour that enables animals to respond to change. By deciphering the mechanisms underlying innovativeness, especially those that explain consistent differences between individuals, we can further understand the consequences of this behavioural variation. We tested whether motivation, experience, inhibitory control and personality were linked to different stages of sequential innovative problem-solving performance among great tits, Parus major , and of their overall innovativeness across tasks. We gave animals originating from lines bidirectionally selected for fast or slow early exploratory behaviour, a multiaccess problem-solving device. Diverse motor skills and behavioural flexibility were required to solve all three different access points sequentially over trials. Food-deprived, highly motivated birds had shorter latency to touch the device, were more likely to solve an access point within a trial, and solved a greater diversity of them, than their less motivated counterparts. Solving success increased with accuracy when interacting with the device (proportion of touches to functional components of the device compared to all touches to the device per trial), and with previous experience. Personality selection lines and inhibitory control had little effect. Repeatability analysis showed that between-individual differences in problem-solving performance were explained by: (1) pseudorepeatable effects (upward bias) linked to hunger-induced motivation, (2) repeatable differences in accuracy when interacting with devices, and (3) a feedback loop caused by experience gained over successive trials. Our results highlight the challenges of characterizing consistent individual differences in behaviour generally and support the idea that complex sources of variation play an important role in problem-solving performance. • We decomposed sequential innovative problem-solving success in a multiaccess task. • Individuals showed consistent differences in problem-solving performance. • Exploratory behaviour and inhibitory control played no role. • Differences were mostly explained by motivation and accuracy interacting with devices. • Positive feedback through experience also played a role. [ABSTRACT FROM AUTHOR]

Quantifying the direction and strength of mate preference is essential to improve our understanding of sexual selection. Experimental designs, however, often do not consider how individuals evaluate and compare the available options, which may affect the results significantly. Preferences are often assumed to be absolute, with individuals assigning a fixed, absolute value to a cue or potential partner they encounter. However, individuals may instead also compare the available options, in which case the social context plays an essential role in the preference for each potential partner. Here we investigated the importance of considering the choosers' evaluation process in mate preference tests. Using a mate preference study on wild great tit, Parus major , heterozygosity, breast stripe size and yellowness as a case study, we tested whether individuals use absolute or comparative mate preferences. We analysed how the perceived average attractiveness and the variation in attractiveness of the group of potential mates affected the measured preference functions. We found that the average attractiveness of the stimulus groups affected the total time individuals spent visiting all stimulus birds, and that the variation in attractiveness within groups affected the measured preference slopes. This indicates that the social context will affect the measured responses to stimulus groups, and that great tits may use both absolute and comparative evaluation. Considering how a study species encounters and evaluates potential mates and how the social environment may affect preferences is essential when choosing an appropriate experimental design to obtain reliable measurements of mate preferences. We therefore strongly advise future studies to consider not only the absolute stimulus trait values, but also the context in which they are presented. The ability to quantify preferences accurately will increase our understanding of mate preferences, mate choice and ultimately sexual selection. • The choice for a mate is not only made on absolute quality. • The social context affects the choice for a mate. • Individuals can use comparisons between options for their choice. • Birds used a combination of absolute and comparative evaluation of stimulus groups. • How species evaluate mates is crucial for the choice of experimental design. [ABSTRACT FROM AUTHOR]

Personality in animal behaviour describes the observation that behavioural differences between individuals are consistent over time and context. Studies of group-living animals show that movement order among individuals is also consistent over time and context, suggesting that some individuals lead and others follow. However, the relationship between leadership and personality traits is poorly studied. We measured several personality traits and leadership of individual barnacle geese, Branta leucopsis. We measured body size and scored the dominance of individuals living in a stable group situation before subjecting them to an open-field test, an activity test, a novel-object test, and a leadership test in which the order of the movement of individuals in pairs towards a feeding patch was scored. We found high repeatability for activity and novel-object scores over time. Leadership was strongly correlated with novel-object score but not with dominance rank, activity or exploration in an open field. These results provide evidence that leadership is closely related to some aspects of personality. Interestingly, an individual''s arrival at the food patch was affected not only by the novel-object score of the focal individual, but also by the novel-object score of the companion individual, indicating that movement patterns of individuals living in groups are affected by the personality traits of other group members and suggesting that movement patterns of a group may be shaped by the mix of personality types present in the group. [Copyright &y& Elsevier]

In resource defence mating systems, males monopolize a resource that is of primary importance for breeding females. For secondary cavity nesters, the availability of suitable nesting sites is important in determining the strength of intrasexual competition, whereby phenotypic and behavioural traits will be favoured that enable individuals to gain access to these sites. The traits that are important in male competition may additionally affect mate choice decisions and a female''s investment in the current brood. In a field study on blue tits, Cyanistes caeruleus, we increased intrasexual competition by experimentally limiting nest sites in experimental plots and compared these plots to control plots. Birds breeding in experimental plots did not differ phenotypically from birds in control plots. However, females that bred in the nest site-limited plots fed their offspring at a higher rate than control females. This result indicates that increased competition for limited resources led to more investment in current reproduction, either because successful females were of higher intrinsic quality or because they adjusted their investment in relation to superior territory or male characteristics. [Copyright &y& Elsevier]

Variation in the reactivity of the endocrine stress axis is thought to underlie aspects of persistent individual differences in behavior (i.e. animal personality). Previous studies, however, have focused largely on estimating baseline or peak levels of glucocorticoids (CORT), often in captive animals. In contrast, the temporal dynamics of the HPA axis—how quickly it turns on and off, for example—may better indicate how an individual copes with stressors. Moreover, these HPA components might be correlated, thereby representing endocrine suites. Using wild-caught great tits ( Parus major ) we tested birds for exploratory behavior using a standardized novel environment assay that serves as a validated proxy for personality. We then re-captured a subset of these birds ( n = 85) and characterized four components of HPA physiology: baseline, endogenous stress response, a dexamethasone (DEX) challenge to estimate the strength of negative feedback, and an adrenocorticotropic hormone (ACTH) challenge to estimate adrenal capacity. We predicted that these four HPA responses would be positively correlated and that less exploratory birds would have a more rapid onset of the stress response (a CORT elevation during the baseline bleed) and weaker negative feedback (higher CORT after DEX). We found support for the first two predictions but not the third. All four components were positively correlated with each other and less exploratory birds exhibited an elevation in CORT during the baseline bleed (<3 min from capture). Less exploratory birds, however, did not exhibit weaker negative feedback following the DEX challenge, but did exhibit weaker adrenal capacity. Together, our findings provide partial support for the hypothesis that the temporal reactivity of the HPA axis is linked with consistent individual differences in behavior, with more cautious (slower exploring) individuals exhibiting a faster CORT response. [ABSTRACT FROM AUTHOR]

Stress physiology is thought to contribute to individual differences in behaviour. In part this reflects the fact that canonical personality measures consist of responses to challenges, including novel objects and environments. Exposure to novelty is typically assumed to induce a moderate increase in glucocorticoids (CORT), although this has rarely been tested. We tested this assumption using great tits, Parus major , selected for divergent personalities (bold-fast and shy-slow explorers), predicting that the shy birds would exhibit higher CORT following exposure to a novel object. We also scored behavioural responses to the novel object, predicting that bold birds would more frequently approach the novel object and exhibit more abnormal repetitive behaviours. We found that the presence of a novel object did induce a moderate CORT response, but selection lines did not differ in the magnitude of this response. Furthermore, although both selection lines showed a robust CORT elevation to a subsequent restraint stressor, the CORT response was stronger in bold birds and this effect was specific to novel object exposure. Shy birds showed a strong positive phenotypic correlation between CORT concentrations following the novel object exposure and the subsequent restraint stress. Behaviourally, the selection lines differed in their response during novel object exposure: as predicted, bold birds more frequently approached the novel object and shy birds more strongly decreased overall locomotion during the novel object trial, but birds from both selection lines showed significant and similar frequencies of abnormal repetitive behaviours during novel object exposure. Our findings support the hypothesis that personality emerges as a result of correlated selection on behaviour and underlying endocrine mechanisms and suggest that the relationship between endocrine stress physiology and personality is context dependent. [ABSTRACT FROM AUTHOR]

Hormonal pleiotropy—the simultaneous influence of a single hormone on multiple traits—has been hypothesized as an important mechanism underlying personality, and circulating glucocorticoids are central to this idea. A major gap in our understanding is the neural basis for this link. Here we examine the stability and structure of behavioral, endocrine and neuroendocrine traits in a population of songbirds ( Parus major ). Upon identifying stable and covarying behavioral and endocrine traits, we test the hypothesis that risk-averse personalities exhibit a neuroendocrine stress axis that is systemically potentiated—characterized by stronger glucocorticoid reactivity and weaker negative feedback. We show high among-individual variation and covariation (i.e. personality) in risk-taking behaviors and demonstrate that four aspects of glucocorticoid physiology (baseline, stress response, negative feedback strength and adrenal sensitivity) are also repeatable and covary. Further, we establish that high expression of mineralocorticoid and low expression of glucocorticoid receptor in the brain are linked with systemically elevated plasma glucocorticoid levels and more risk-averse personalities. Our findings support the hypothesis that steroid hormones can exert pleiotropic effects that organize behavioral phenotypes and provide novel evidence that neuroendocrine factors robustly explain a large fraction of endocrine and personality variation. [ABSTRACT FROM AUTHOR]

In many seasonally breeding birds, female and male testosterone (T) levels peak at the start of the breeding season, coinciding with pair bonding and nesting activities. Shortly after the onset of egg laying, T levels slowly decline to baseline levels in both sexes, but more rapidly so in females. During this period, T in males may still function to facilitate territorial behaviour, mate guarding and extra pair copulations, either via short lasting peaks or elevated basal levels of the hormone. In some species, however, males become insensitive to increased T after the onset of egg laying. It has been postulated that in these species bi-parental care is essential for offspring survival, as T is known to inhibit paternal care. However, only very few studies have analysed this for females. As females are heavily involved in parental care, they too might become insensitive to T after egg laying. Alternatively, because territorial defence, mate guarding and extra pair copulations are expected to be less important for females than for males, they may not have had the need to evolve a mechanism to become insensitive to T during the period of maternal care, because their natural T levels are never elevated during this part of the breeding season anyway. We tested these alternative hypotheses in female great tits ( Parus major ). Male great tits have previously been shown to be insensitive to T after egg laying with regard to nestling feeding behaviour (but not song rate). When females had started nest building, we experimentally elevated their T levels up to the nestling feeding phase, and measured incubation behaviour (only females incubate) and reproductive success. T did not significantly affect nest building or egg laying behaviour, although egg laying tended to be delayed in T females. Females with experimentally enhanced T maintained lower temperature during incubation but did not spend less time incubating. This might explain the reduced hatching success of their eggs, smaller brood size and lower number of fledglings we found in this study. As in this species T-dependent behaviour by females during the phase of parental care is not needed, the results support the hypothesis that in this species the need for selection in favour of T-insensitivity did not occur. [ABSTRACT FROM AUTHOR]

Territorial animals settle territory disputes and discourage conspecific intrusion via close-range confrontations as well as nonconfrontational long-range signalling. Since individuals often differ consistently in general aggression and risk taking, the relative use of either close- or long-range territorial defence behaviour is likely to vary with the personality of the territory owner. Here we quantified the relationship between dawn song, a well-studied long-range signal, and responses to a close-range confrontation as well as how individuals in a territorial population vary in this relationship. For this we recorded dawn song and experimentally simulated territory intrusions via playbacks in wild personality-typed male great tits, Parus major . We show that males that sang at a higher rate at dawn also showed stronger vocal responses towards a simulated intruder, but spent less time in proximity to the intruder. Moreover, males with a higher exploration score, an established proxy for personality traits, showed the strongest vocal and spatial responses during the confrontation, yet exploration behaviour did not predict the dawn song rate. These findings highlight the importance of both confrontational and nonconfrontational territorial behaviours as well as personality for the social and territorial dynamics of animal populations. [ABSTRACT FROM AUTHOR]

In various animal species individuals differ consistently in their behaviour, often referred to as personality. In several species these personality differences also correlate with differences in social behaviour. This is important as the social environment is a key selection pressure in many animal populations, mediated, for example, via competition or access to social information. Using social network analysis, recent studies have furthered our understanding of the role of personality in the social environment, usually by focusing on swarming or flocking populations. However, social associations in such populations are fundamentally different from those in territorial populations, where individuals meet less frequently and where the costs and benefits of spatial associations differ from those for swarming or flocking species. In this study we therefore tested whether social network position is related to individual differences in exploration behaviour, an established measure of an avian personality trait, using a wild, territorial, personality-typed great tit, Parus major , population. By means of novel, large-scale, automated tracking (Encounternet) we show, while controlling for average territory distance, that slower exploring males had less central social network positions. Yet, slower explorers overall did not travel shorter distances than faster explorers, indicating that a less central social network position was not merely a consequence of lower activity. Finally, males with less central social network positions did not have reduced breeding success compared to males with more central positions. Our results suggest that territorial individuals influence the structuring of their own social environment in relation to their personality. This is relevant, because the establishment of social relations and familiarity with possible competitors is predicted to be important in many territorial populations. [ABSTRACT FROM AUTHOR]

Abstract: Animal ‘personality’ describes consistent individual differences in suites of behaviors, a phenomenon exhibited in diverse animal taxa and shown to be under natural and sexual selection. It has been suggested that variation in personality reflects underlying physiological variation; however there is limited empirical evidence to test this hypothesis in wild animals. The hypothalamic–pituitary–adrenal axis is hypothesized to play a central role in personality variation. Here we tested whether in great tits Parus major variation in personality traits is related to plasma concentrations of corticosterone (CORT). Using a capture-restraint protocol we examined baseline and stress-induced CORT levels in two captive experimental groups: (1) birds selected for divergent personalities (‘fast-bold’ and ‘slow-shy’ explorers); and (2) non-selected offspring of wild parents. We first tested for differences in CORT between selection lines, and second examined the relationship between responses in a canonical personality test and CORT concentrations in non-selected birds. We found support for our prediction that the slow-shy line would exhibit a higher acute stress response than the fast-bold line, indicating a genetic correlation between exploratory behavior and stress physiology. We did not, however, find that continuous variation in exploratory behavior co-varies with CORT concentrations in non-selected birds. While our results provide support for the idea that personality emerges as a result of correlated selection on behavior and underlying physiological mechanisms, they also indicate that this link may be particularly evident when composite personality traits are the target of selection. [Copyright &y& Elsevier]

We investigated whether individual great tits, Parus major, vary consistently in their exploratory behaviour in a novel environment and measured the repeatability and heritability of this trait. Wild birds were caught in their natural habitat, tested in the laboratory in an open field test on the following morning, then released at the capture site. We measured individual consistency of exploratory behaviour for recaptured individuals (repeatability) and estimated the heritability with parent–offspring regressions and sibling analyses. Measures of exploratory behaviour of individuals at repeated captures were consistent in both sexes and study areas (repeatabilities ranged from 0.27 to 0.48). Exploration scores did not differ between the sexes, and were unrelated to age, condition at fledging or condition during measurement. Heritability estimates were 0.22–0.41 (parent–offspring regressions) and 0.37–0.40 (sibling analyses). We conclude that (1) consistent individual variation in open field behaviour exists in individuals from the wild, and (2) this behavioural variation is heritable. This is one of the first studies showing heritable variation in a behavioural trait in animals from the wild, and poses the question of how this variation is maintained under natural conditions. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved. [Copyright &y& Elsevier]