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The gut microbiota have important consequences for host biological processes and there is some evidence that they also affect fitness. However, the complex, interactive nature of ecological factors that influence the gut microbiota has scarcely been investigated in natural populations. We sampled the gut microbiota of wild great tits (Parus major) at different life stages allowing us to evaluate how microbiota varied with respect to a diverse range of key ecological factors of two broad types: (1) host state, namely age and sex, and the life history variables, timing of breeding, fecundity and reproductive success; and (2) the environment, including habitat type, the distance of the nest to the woodland edge, and the general nest and woodland site environments. The gut microbiota varied with life history and the environment in many ways that were largely dependent on age. Nestlings were far more sensitive to environmental variation than adults, pointing to a high degree of flexibility at an important time in development. As nestlings developed their microbiota from one to two weeks of life, they retained consistent (i.e., repeatable) among‐individual differences. However these apparent individual differences were driven entirely by the effect of sharing the same nest. Our findings point to important early windows during development in which the gut microbiota are most sensitive to a variety of environmental drivers at multiple scales, and suggest reproductive timing, and hence potentially parental quality or food availability, are linked with the microbiota. Identifying and explicating the various ecological sources that shape an individual's gut bacteria is of vital importance for understanding the gut microbiota's role in animal fitness. [ABSTRACT FROM AUTHOR]

Feathers have a diversity of functions in birds and are costly to produce, so their growth rate and mass can be reliable indicators of nutritional condition at the time of production. Despite the potential for feather metrics to advance our understanding of foraging, they are underused in avian ecology. One reason for this is the difficulty of interpreting whether individual variation is driven by ontogenetic, genetic or environmental effects, which is exacerbated by the fact that most analyses have been done on cross‐sectional data. We addressed this deficit using a longitudinal dataset of tail feathers collected from great tits Parus major to test for ontogenetic and genetic effects on growth rate, mass and length, while controlling for body/feather size differences and other confounding factors. First, we found that the type of moult episode and experimentally‐induced replacement differentially affected the length, mass and growth of feathers, providing evidence of an ontogenetic effect that should be considered when comparing these feather traits across individuals as a measure of condition. Second, we detected moderate to high repeatability and heritability values from parent–offspring regression for these three feather traits, which are suggestive of an underlying genetic component of variation. Third, we used a mean centring within‐individual approach to test whether feather growth rate and feather mass (length‐corrected) are indeed positively correlated with each other as overlapping indicators of body condition in birds, and found that this association, although positive, is weak and only significant between individuals. This suggests that both metrics are not so intimately linked as originally thought, and probably have different sensitivities to variation in foraging performance and ecological conditions. Together with the higher plasticity of feather growth rate compared to feather mass, our results support the idea that feather growth rate is better suited for examining short‐term responses to environmental variation. [ABSTRACT FROM AUTHOR]

Inhibitory control is one of several cognitive mechanisms required for self-regulation, decision making and attention towards tasks. Inhibitory control is expected to influence behavioural plasticity in animals, for example in the context of foraging, social interaction or responses to sudden changes in the environment. One widely used inhibitory control assay is the 'detour task' where subjects must avoid impulsively touching transparent barriers positioned in front of food, and instead access the food by an alternative but known route. However, because the detour task has been reported to measure factors unrelated to inhibitory control, including motivation, previous experience and persistence, the task may be unreliable for making cross-species comparisons, estimating individual differences and linking performance with socioecological traits. To address these concerns, we designed a variant of the detour task for wild great tits, Parus major , and deployed it at the nesting site across two spring seasons. We compared task performance of the same individuals in the wild across 2 years, and with their performance in captivity when tested using the classical cylinder detour task during the nonbreeding season. Potential confounds of motivation, previous experience, body size, sex, age and personality did not significantly predict performance, and temporal and contextual repeatability were low but significant. These results support the hypothesis that our assays captured intrinsic differences in inhibitory control. Instead of dismissing detour tasks and 'throwing the baby out with the bathwater', we suggest confounds are likely system and experimental-design specific, and that assays for this potentially fundamental but largely overlooked source of behavioural plasticity in animal populations, should be validated and refined for each study system. • The detour task is a widely used measure of inhibitory control in animal cognition. • Individual consistency across wild and captive contexts has never been explored. • We compared a traditional detour task with one that was deployed at bird nestboxes. • Performance was consistent across wild and captive contexts and across years. • Individual performance was not sensitive to bias from extraneous influences. [ABSTRACT FROM AUTHOR]

Behavioural flexibility allows animals to adjust to changes in their environment. Although the cognitive processes that explain flexibility have been relatively well studied in psychology, this is less true for animals in the wild. Here we use data collected automatically during self-administered discrimination-learning trials for two passerine species, and during four phases (habituation, initial learning, first reversal and second reversal) in order to decompose sources of consistent among-individual differences in reversal learning, a commonly used measure for cognitive flexibility. First, we found that, as expected, proactive interference was significantly repeatable and had a negative effect on reversal learning, confirming that individuals with poor ability to inhibit returning to a previously rewarded feeder were also slower to reversal learn. Second, to our knowledge for the first time in a natural population, we examined how sampling of non-rewarding options post-learning affected reversal-learning performance. Sampling quantity was moderately repeatable in blue tits but not great tits; sampling bias, the variance in the proportion of visits to each non-rewarded feeder, was not repeatable for either species. Sampling behaviour did not predict variation in reversal-learning speed to any significant extent. Finally, the repeatability of reversal learning was explained almost entirely by proactive interference for blue tits; in great tits, the effects of proactive interference and sampling bias on the repeatability of reversal learning were indistinguishable. Our results highlight the value of proactive interference as a more direct measurement of cognitive flexibility and shed light on how animals respond to changes in their environment. [ABSTRACT FROM AUTHOR]

Organisms are constantly under selection to respond effectively to diverse, sometimes rapid, changes in their environment, but not all individuals are equally plastic in their behaviour. Although cognitive processes and personality are expected to influence individual behavioural plasticity, the effects reported are highly inconsistent, which we hypothesise is because ecological context is usually not considered.We explored how one type of behavioural plasticity, foraging flexibility, was associated with inhibitory control (assayed using a detour‐reaching task) and exploration behaviour in a novel environment (a trait closely linked to the fast–slow personality axis). We investigated how these effects varied across two experimentally manipulated ecological contexts—food value and predation risk.In the first phase of the experiment, we trained great tits Parus major to retrieve high value (preferred) food that was hidden in sand so that this became the familiar food source. In the second phase, we offered them the same familiar hidden food at the same time as a new alternative option that was visible on the surface, which was either high or low value, and under either high or low perceived predation risk. Foraging flexibility was defined as the proportion of choices made during 4‐min trials that were for the new alternative food source.Our assays captured consistent differences among individuals in foraging flexibility. Inhibitory control was associated with foraging flexibility—birds with high inhibitory control were more flexible when the alternative food was of high value, suggesting they inhibited the urge to select the familiar food and instead selected the new food option. Exploration behaviour also predicted flexibility—fast explorers were more flexible, supporting the information‐gathering hypothesis. This tendency was especially strong under high predation risk, suggesting risk aversion also influenced the observed flexibility because fast explorers are risk prone and the new unfamiliar food was perceived to be the risky option. Thus, both behaviours predicted flexibility, and these links were at least partly dependent on ecological conditions.Our results demonstrate that an executive cognitive function (inhibitory control) and a behavioural assay of a well‐known personality axis are both associated with individual variation in the plasticity of a key functional behaviour. That their effects on foraging flexibility were primarily observed as interactions with food value or predation risk treatments also suggest that the population‐level consequences of some behavioural mechanisms may only be revealed across key ecological conditions. [ABSTRACT FROM AUTHOR]

The producer–scrounger game is a key element of foraging ecology in many systems. Producing and scrounging typically covary negatively, but partitioning this covariance into contributions of individual plasticity and consistent between individual differences is key to understanding population‐level consequences of foraging strategies. Furthermore, little is known about the role cognition plays in the producer–scrounger game.We investigated the role of cognition in these alternative foraging tactics in wild mixed‐species flocks of great tits and blue tits, using a production learning task in which we measured individuals' speed of learning to visit the single feeder in an array that would provide them with a food reward. We also quantified the proportion of individuals' feeds that were scrounges ('proportion scrounged'); scrounging was possible if individuals visited immediately after a previous rewarded visitor. Three learning experiments—initial and two reversal learning—enabled us to estimate the repeatability and covariance of each foraging behaviour.First, we examined whether individuals learned to improve their scrounging success (i.e. whether they obtained food by scrounging when there was an opportunity to do so). Second, we quantified the repeatability of proportion scrounged, and asked whether proportion scrounged affected production learning speed among individuals. Third, we used multivariate analyses to partition within‐ and among‐individual components of covariance between proportion scrounged and production learning speed.Individuals improved their scrounging success over time. Birds with a greater proportion scrounged took longer to learn their own rewarding feeder. Although multivariate analyses showed that covariance between proportion scrounged and learning speed was driven primarily by within‐individual variation, that is, by behavioural plasticity, among‐individual differences also played a role for blue tits.This is the first demonstration of a cognitive trait influencing producing and scrounging in the same wild system, highlighting the importance of cognition in the use of alternative resource acquisition tactics. The results of our covariance analyses suggest the potential for genetic differences in allocation to alternative foraging tactics, which are likely species‐ and system‐dependent. They also point to the need to control for different foraging tactics when studying individual cognition in the wild. [ABSTRACT FROM AUTHOR]

Natal body mass is a key predictor of viability and fitness in many animals. While variation in body mass and therefore juvenile viability may be explained by genetic and environmental factors, emerging evidence points to the gut microbiota as an important factor influencing host health. The gut microbiota is known to change during development, but it remains unclear whether the microbiome predicts fitness, and if it does, at which developmental stage it affects fitness traits.We collected data on two traits associated with fitness in wild nestling great tits Parus major: weight and survival to fledging. We characterised the gut microbiome using 16S rRNA sequencing from nestling faeces and investigated temporal associations between the gut microbiome and fitness traits across development at Day‐8 (D8) and Day‐15 (D15) post‐hatching. We also explored whether particular microbial taxa were 'indicator species' that reflected whether nestlings survived or not.There was no link between mass and microbial diversity on D8 or D15. However, we detected a time‐lagged relationship where weight at D15 was negatively associated with the microbial diversity at D8, controlling for weight at D8, therefore reflecting relative weight gain over the intervening period.Indicator species analysis revealed that specificity values were high and fidelity values were low, suggesting that indicator taxa were primarily detected within either the survived or not survived groups, but not always detected in birds that either survived or died. Therefore these indicator taxa may be sufficient, but not necessary for determining either survival or mortality, perhaps owing to functional overlap in microbiota.We highlight that measuring microbiome‐fitness relationships at just one time point may be misleading, especially early in life. Instead, microbial‐host fitness effects may be best investigated longitudinally to detect critical development windows for key microbiota and host traits associated with neonatal weight. Our findings should inform future hypothesis testing to pinpoint which features of the gut microbial community impact on host fitness, and when during development this occurs. Such confirmatory research will shed light on population level processes and could have the potential to support conservation. [ABSTRACT FROM AUTHOR]