The interconnecting links between individuals in an animal social network are often defined by discrete, directed behaviours, but where these are difficult to observe, a network link (edge) may instead be defined by individuals sharing a space at the same time, which can then be used to infer a social association. The method by which these associations are defined should be informed by the biological significance of edges, and therefore often vary between studies. Identifying an appropriate measure of association remains a challenge to behavioural ecologists. Here, we use automatically recorded feeder visit data from four bird systems to compare three methods to identify a social association: (1) strict time-window, (2) co-occurrence in a group, and (3) arrival-time. We tested the similarity of the resulting networks by comparing the repeatability and sensitivity of individuals’ social traits (network degree, strength, betweenness). We found that networks constructed using different methods but applying similar, ecologically relevant definitions of associations based on individuals’ spatio-temporal co-occurrence, showed similar characteristics. Our findings suggest that the different methods to construct animal social networks are comparable, but result in subtle differences driven by species biology and feeder design. We urge researchers to carefully evaluate the ecological context of their study systems when making methodological decisions. Specifically, researchers in ecology and evolution should carefully consider the biological relevance of an edge in animal social networks, and the implications of adopting different definitions.

Although variation in effect sizes and predicted values among studies of similar phenomena is inevitable, such variation far exceeds what might be produced by sampling error alone. One possible explanation for variation among results is differences among researchers in the decisions they make regarding statistical analyses. A growing array of studies has explored this analytical variability in different fields and has found substantial variability among results despite analysts having the same data and research question. Many of these studies have been in the social sciences, but one small “many analyst” study found similar variability in ecology. We expanded the scope of this prior work by implementing a large-scale empirical exploration of the variation in effect sizes and model predictions generated by the analytical decisions of different researchers in ecology and evolutionary biology. We used two unpublished datasets, one from evolutionary ecology (blue tit, Cyanistes caeruleus, to compare sibling number and nestling growth) and one from conservation ecology (Eucalyptus, to compare grass cover and tree seedling recruitment). The project leaders recruited 174 analyst teams, comprising 246 analysts, to investigate the answers to prespecified research questions. Analyses conducted by these teams yielded 141 usable effects (compatible with our meta-analyses and with all necessary information provided) for the blue tit dataset, and 85 usable effects for the Eucalyptus dataset. We found substantial heterogeneity among results for both datasets, although the patterns of variation differed between them. For the blue tit analyses, the average effect was convincingly negative, with less growth for nestlings living with more siblings, but there was near continuous variation in effect size from large negative effects to effects near zero, and even effects crossing the traditional threshold of statistical significance in the opposite direction. In contrast, the average relationship between grass cover and Eucalyptus seedling number was only slightly negative and not convincingly different from zero, and most effects ranged from weakly negative to weakly positive, with about a third of effects crossing the traditional threshold of significance in one direction or the other. However, there were also several striking outliers in the Eucalyptus dataset, with effects far from zero. For both datasets, we found substantial variation in the variable selection and random effects structures among analyses, as well as in the ratings of the analytical methods by peer reviewers, but we found no strong relationship between any of these and deviation from the meta-analytic mean. In other words, analyses with results that were far from the mean were no more or less likely to have dissimilar variable sets, use random effects in their models, or receive poor peer reviews than those analyses that found results that were close to the mean. The existence of substantial variability among analysis outcomes raises important questions about how ecologists and evolutionary biologists should interpret published results, and how they should conduct analyses in the future.

The interconnecting links (edges) between individuals (nodes) in an animal social network are often defined by discrete, directed behaviours (interactions). However, where interactions are difficult to observe, a network edge is instead defined as individuals sharing space or overlapping in time (an association). Despite an increasingly accessible toolkit to assemble and analyse animal social networks, defining associations remains a challenge in behavioural ecology. While different study systems have used different ways to validate the definition of an association, an empirical comparison of how these different methods compare is lacking. Here, we apply three methods to define social associations, by 1) strict time-window, 2) co-occurrence in a group, and 3) arrival-time, in four bird systems. We first test the ability of each method to detect individually repeatable social traits. Then we describe the structure of each network using Jaccard similarity and Mantel tests, and finally, we test the sensitivity of network structure to changing parameters within the three definitions. We found that the network structure was largely robust to changing how associations were defined, with subtle differences. We suggest that these differences are the result of an inappropriate definition of association in the context of experimental design and system ecology. Researchers in ecology and evolution should carefully consider the biological relevance of association definition prior to starting research into animal social behaviour.

To survive, prey animals must correctly assess and respond to predation, by vigilantly scanning their environment for threats, assessing predation risk through gaze aversion (responding fearfully to predator gaze), and escaping efficiently. As these anti-predatory behaviours are integrated through the nervous and motor systems, they could be disrupted by neurotoxic contaminants, such as lead (Pb), which is ubiquitous in the Anthropocene. Here, we examined the effects of Pb on anti-predatory behaviours of free-living house sparrows (Passer domesticus), in the mining city of Broken Hill, Australia, where birds have been exposed to elevated Pb levels for many generations. We found that sparrows in higher-Pb contaminated areas spent more time scanning their surroundings and were more reluctant to approach a feeder under direct, experimentally introduced human gaze, than sparrows in lower-Pb contaminated areas. Higher-Pb birds had slightly worse (though not statistically different) escape flight performance than their lower-Pb counterparts. Our results suggest that greater exposure to Pb is associated with increased fear, which may or may not be linked to Pb-compromised escape performance. We highlight the importance of considering multi-faceted, integrated effects of environmental pollution on the behaviours of urban wildlife.

Humans become more selective with whom they spend their time, and as a result, the social networks of older humans are smaller than those of younger ones. In non-human animals, processes such as competition and opportunity can result in patterns of declining sociality with age. While there is support for declining sociality with age in mammals, evidence from wild bird populations is lacking. Here, we test whether sociality declines with age in a wild, insular bird population, where we know the exact ages of individuals. Using 6 years of sociality data, we find that as birds aged, their degree and betweenness decreased. The number of same-age birds still alive also decreased with age. Our results suggest that a longitudinal change in sociality with age may be, in part, an emergent effect of natural changes in demography. This highlights the need to investigate the changing costs and benefits of sociality across a lifetime. This article is part of the discussion meeting issue ‘Understanding age and society using natural populations’.

Telomeres are a popular biomarker of senescence, as telomere dynamics are linked with survival and lifespan. However, the evolutionary potential of telomere dynamics, and the selection pattern that gives rise to senescence, are not well known. To better understand this, it is necessary to quantify genetic variation in telomere length, and how such variation changes with age. Here, we analysed a longitudinal dataset (2,156 samples from 1,267 individuals across 15 years) from a wild, insular house sparrow (Passer domesticus) population with complete life-history and genetic relatedness data. Using a series of ‘animal’ models, we confirmed that telomere length (TL): (1) changes with age, reflecting senescence in this population. We then showed that (2) TL is repeatable (15.7%, 95% CrI: 10.2–21.0%) and heritable (14.0%, 95% CrI: 8.7–19.5%); and, for the first time in the wild, (3) that TL shows a genotype-by-age interaction, meaning that genotypes differ in their rate of TL change, where additive genetic variance increases at older ages. Our findings provide empirical evidence from a wild population that supports hypotheses explaining the evolution of senescence, and highlights the importance of telomere dynamics as a key biomarker of body physiology for the evolution of senescence.

Explaining variation in individual fitness is a key goal in evolutionary biology. Recently, telomeres, repeating DNA sequences capping the ends of chromosomes, have gained attention as a biomarker for body state, individual quality, and ageing. However, existing research has provided mixed evidence for whether telomere length correlates with fitness components, including survival and reproductive output. Moreover, few studies have examined how telomere shortening correlates with fitness in wild populations. Here, we intensively monitored an insular population of house sparrows on Lundy Island, UK, and collected longitudinal telomere and life history data spanning 16 years from 1,225 individuals. We tested whether telomere length and/or shortening predict fitness measures, namely survival, lifespan, as well as annual and lifetime reproductive success. Telomere length positively predicted immediate survival up to one year after measurement, independent of age, but did not predict lifespan, suggesting either a diminishing telomere length – survival correlation with age, or other extrinsic factors of mortality. The positive effect of telomere length on survival translated to reproductive benefits, as birds with longer telomeres produced more genetic recruits over their lifetime, but not annually, suggesting variation in individual quality. The rate of telomere shortening, however, correlated with neither lifespan nor lifetime reproductive success. Our results provided further evidence that telomere length correlates with fitness, and they contributed to our understanding of how telomere dynamics link with individual quality.

Animal sociality, an individual's propensity to associate with others, has fitness consequences through mate choice, for example, directly, by increasing the pool of prospective partners, and indirectly through increased survival, and individuals benefit from both. Annually, fitness consequences are realized through increased mating success and subsequent fecundity. However, it remains unknown whether these consequences translate to lifetime fitness. Here, we quantified social associations and their link to fitness annually and over lifetime, using a multi-generational, genetic pedigree. We used social network analysis to calculate variables representing different aspects of an individual's sociality. Sociality showed high within-individual repeatability. We found that birds with more opposite-sex associates had higher annual fitness than those with fewer, but this did not translate to lifetime fitness. Instead, for lifetime fitness, we found evidence for stabilizing selection on opposite-sex sociality, and sociality in general, suggesting that reported benefits are only short-lived in a wild population, and that selection favors an average sociality.

The question of why females engage in extra-pair behaviors is long-standing in evolutionary biology. One suggestion is that these behaviors are maintained through pleiotropic effects on male extra-pair behaviors (genes controlling extra-pair reproduction are shared between sexes, but only beneficial to one sex, in this case, males). However, for this to evolve extra-pair reproduction must be both heritable and positively genetically correlated between sexes. Previous studies have suggested low heritability with no evidence for between-sex genetic correlations in extra-pair reproduction. However, these have not considered indirect genetic effects (derived from the behavior of others, IGEs) from the social partner, the influence of the social partner's genotype on the phenotype of an individual, despite the potential of IGEs to uncover hidden heritable variation. Using data from a closed-house sparrow population with a genetic pedigree spanning two decades, we tested the influence of social partner IGEs on heritable variation and genetic correlation estimates of extra-pair reproduction. We found that the inclusion of IGEs resulted in larger heritable genetic variance for both male and female extra-pair heritability. While IGEs did not change between-sex genetic correlations, we found they reduced uncertainty in those estimates. Future studies should consider the effect of IGEs on the mechanisms of sex-specific extra-pair reproduction.

Telomere dynamics are linked with both cellular and organismal senescence, and life history, individual quality and health. Telomere dynamics, particularly telomere length, have therefore garnered much research interest in evolutionary biology. To examine the evolution of telomere length, it is important to quantify its heritability, the proportion of total variation explained by additive genetic effects. Many studies have quantified telomere length heritability, but estimates are varied, and no general conclusion has been drawn. Additionally, it is unclear whether biological and methodological factors influence telomere length heritability estimates. We present the first meta‐analysis of telomere length heritability, using 104 estimates from 43 studies over 18 vertebrate species. We calculated an overall mean heritability and examined how estimates varied by study, phylogeny, species‐specific ecology, environmental setting, age at sampling, laboratory methods, statistical methods, sex and repeated measurements. Overall heritability was moderate (44.9%, 95% CI: 25.2–64.7%), and there was considerable heterogeneity in heritability estimates, in particular among studies and estimates. Laboratory method influenced heritability estimates, with in‐gel hybridization TRF yielding higher heritabilities than qPCR and Southern blot TRF. There was also an effect from statistical method, with twin‐based and SNP‐based estimates lower than correlation‐based or pedigree‐based estimates. Our results highlight an overall heritable basis of telomere length, and we recommend future research on a wider range of taxa, and the use of variance‐partitioning methods with relatedness or SNP data over correlation methods to minimize heritability estimation bias.

Fitness is at the core of evolutionary theory, but it is difficult to measure accurately. One way to measure long-term fitness is by calculating the individual's reproductive value, which represents the expected number of allele copies an individual passes on to distant future generations. However, this metric of fitness is scarcely used because the estimation of individual's reproductive value requires long-term pedigree data, which is rarely available in wild populations where following individuals from birth to death is often impossible. Wild study systems therefore use short-term fitness metrics as proxies, such as the number of offspring produced. This study compared two frequently used short-term metrics for fitness obtained at different offspring life stages (eggs, hatchlings, fledglings and recruits), and compared their ability to predict reproductive values derived from the genetic pedigree of a wild passerine bird population. We used twenty years of precise field observations and a near-complete genetic pedigree to calculate reproductive success, individual growth rate and de-lifed fitness as lifetime fitness measures, and as annual de-lifed fitness. We compared the power of these metrics to predict reproductive values and lineage survival to the end of the study period. The three short-term fitness proxies predict the reproductive values and lineage survival only when measured at the recruit stage. There were no significant differences between the different fitness proxies at the same offspring stages in predicting the reproductive values and lineage survival. Annual fitness at one year old predicted reproductive values equally well as lifetime de-lifed fitness. However, none of the short-term fitness proxies were strongly associated with the reproductive values. The commonly used short-term fitness proxies best predict long-term fitness when measured at recruitment stage. Thus, because lifetime fitness measured at recruit stage and annual fitness in the first year of life were the best proxies of long-term fitness in short-lived birds, we encourage their future use.

In the plant-insect-insectivorous bird food chain, directional changes in climate can result in mismatched phenology, potentially affecting selection pressures. Phenotypic plasticity in the timing of breeding, characterized by reaction norm slopes, can help maximize fitness when faced with earlier prey emergence. In temperate passerines, the timing of tree budburst influences food availability for chicks through caterpillar phenology and the resulting food abundance patterns. Thus, the timing of tree budburst might serve as a more direct proxy for the cue to time egg-laying. The evolutionary potential of breeding plasticity relies on heritable variation, which is based upon individual variation, yet studies on individual variation in plasticity are few. Here, we tested for the laying date-budburst date and the clutch size-laying date reaction norms, and examined 1) the among-individual variance in reaction norm intercepts and slopes; and 2) the selection differentials and gradients on these intercepts and slopes. Using long-term data of oak (genus Quercus) budburst and blue tit (Cyanistes caeruleus) reproduction, we applied within-subject centering to detect reaction norms, followed by bivariate random regression to quantify among-individual variance in reaction norm properties and their covariance with fitness. Individuals significantly differed in intercepts and slopes of both laying date-budburst date and clutch size-laying date reaction norms, and directional selection was present for an earlier laying date and a larger clutch size (intercepts), but not on plasticity (slopes). We found that individuals have their own regimes for adjusting egg-laying and clutch size. This study provides further support of individual variation of phenotypic plasticity in birds.

The question of why females engage in extra-pair behaviours is long-standing in evolutionary biology. One suggestion is that these behaviors are maintained through pleiotropic effects on male extra-pair behaviors and lifetime reproductive success (genes controlling extra-pair behaviours are shared between sexes, but only beneficial to one, in this case, males). However, for this to occur extra-pair behaviour must be heritable and positively genetically correlated between sexes. Although previous studies have suggested low heritability with no evidence for between-sex genetic correlations in extra-pair behaviours, indirect genetic effects (those derived from the behaviour of others, IGEs) from the social partner, the influence of the social partner’s genotype on the phenotype of an individual, have not been considered, despite the potential to uncover hidden heritability. Using data from a closed house sparrow population with a genetic pedigree spanning two decades, we tested the influence of IGEs on heritability and genetic correlation estimates of extra-pair behaviour. We found that the inclusion of IGEs improved model fit for both male and female extra-pair heritability. While IGEs did not change between-sex genetic correlations, we found a reduction in uncertainty in our estimates. Future studies should consider the effect of IGEs on the mechanisms of sex specific extra-pair behaviour.

Animal sociality, an individuals propensity to association with others, has consequences for fitness, and particularly mate choice. For example, directly, by increasing the pool of prospective partners, and indirectly through increased survival. Individuals benefit from both over the short-term as these benefits are associated with mating status and subsequent fecundity, but whether animal sociality also translates into fitness is unknown. Here, we quantified social associations and their link with annual and lifetime fitness, measured as the number of recruits and in de-lifed fitness. We measured this in birds visiting a feeding station over two non-breeding periods, using social network analysis and a multi-generational genetic pedigree. We find high individual repeatability in sociality. We found that individuals with an average sociality had the highest fitness, and that birds with more opposite-sex associates had higher fitness, but this did not translate to improved lifetime fitness. For lifetime fitness, we found evidence for stabilizing selection on between sex sociality measures, suggesting that such benefits are only short-lived in a wild population.

Dataset contains blue tit (Cyanistes caeruleus) breeding and oak (Quercus) budburst phenology obtained from a wild population in Silwood Park, UK, 2002-2019. Nest boxes were examined every year in the breeding season and data recorded include laying date, clutch size, female ID, and number of 7-day-old chicks in the brood. Oak budburst dates associated with a brood record were obtained from trees surrounding a nest box by summarizing the mean dates at which trees reach budburst stages 1(budburst) to 6(fully tanninized). All dates were recorded as April Days (0 = 1st Apr). Z-scores represent annually standardized values.R script details statistical analyses to estimate among-individual variance in laying date-budburst reaction norm, and clutch size-laying date reaction norm, as well as selection pressures on these reaction norms.

In the plant-insect-insectivorous-bird system, changing climates can result in mis-timing in bird reproduction, potentially impacting chick survival. To adapt to earlier prey emergence, birds can make use of phenotypic plasticity, which can be characterized by reaction norms. Despite gaining focus in research, studies on avian reproductive reaction norms as traits are scarce, particularly on laying-date-budburst-date and clutch-size-laying-date reaction norms. Here we examined the possibility of evolution of these two reaction norms from a quantitative genetics viewpoint, and tested whether 1) there is among-individual variance in reaction norms properties (intercept and slope); 2) there is selection on these reaction norm properties; and 3) variances and selection pressures differ between the two reaction norms. Data of oak (genus Quercus) budburst and blue tit (Cyanistes caeruleus) reproduction were collected from a wild population for 18 years. We used bivariate random regression models with a Bayesian approach to test for among-individual variance in reaction norm properties and their covariance with fitness. Individuals significantly differed in intercepts and slopes of both laying-date-budburst-date and clutch-size-laying-date reaction norms, and directional selection was present for an earlier laying date and a larger clutch size, but not on either plasticity. Results suggested that variation in reaction norm properties can be attributed to genetic and environmental effects, and that stabilizing selection on plasticity could be tested as a next step. This study compliments previous research on the evolution of reaction norms and helps shed light on their genetic structure, the context of their selection, and their mediation in birds.