Variation in age structure influences population dynamics, yet we have limited understanding of the spatial scale at which its fluctuations are synchronised between populations. Using 32 great tit populations, spanning 4° W-33° E and 35°-65° N involving > 130,000 birds across 67 years, we quantify spatial synchrony in breeding demographic structure (subadult vs. adult breeders) and its drivers. We show that larger clutch sizes, colder winters, and larger beech crops lead to younger populations. We report distance-dependent synchrony of demographic structure, maintained at approximately 650 km. Despite covariation with demographic structure, we do not find evidence for environmental variables influencing the scale of synchrony, except for beech masting. We suggest that local ecological and density-dependent dynamics impact how environmental variation interacts with demographic structure, influencing estimates of the environment's effect on synchrony. Our analyses demonstrate the operation of synchrony in demographic structure over large scales, with implications for age-dependent demography in populations.

Evolutionary theorists have emphasized for over half a century that population sampling must be conducted at the intergenerational boundary if the distinct effects of selection and inheritance are to be reliably quantified, with individuals recognized at the point of conception and lifetime reproductive success (LRS) defined as the total number of zygotic offspring produced per zygote. However, in those species whose ecology is otherwise well-suited to individual-level population studies, the prenatal part of an individual's life is often difficult to observe. While uncertainty has long surrounded the fertilization status of unhatched bird eggs-hatching failure can arise through fertilization failure or prenatal mortality-2 recent studies show fertilization failure to be extremely rare within 2 of the most popular avian study species. As such, unhatched eggs are highly reliable indicators of prenatal mortality. Although the generality of these results remains unclear, they demonstrate that prenatality can be incorporated into the observable lifespan of free-living animals. This allows zygotic LRS to be retrospectively quantified using historical nest observations and facilitates a more complete characterization of the evolutionary dynamics of wild populations.

Spatio-temporal variation in age structure influences population dynamics, yet we have limited understanding of the spatial scale at which its fluctuations are synchronised between populations. Using 32 great tit populations, spanning 3200km and > 130,000 birds across 67 years, we quantify spatial synchrony in breeding age structure and its drivers. We show that larger clutch sizes, colder winters and summers, and larger beech crops lead to younger populations. We report distant-dependent spatial synchrony of age structure, which is maintained at approximately 650km. Despite covariation with age structure, reproductive and environmental variables do not influence the scale of synchrony, except for a moderate effect of beech masting. We suggest that local ecological and density-dependent dynamics impact how environmental variation interacts with age structure, influencing estimates of the environment’s effect on spatial synchrony. Our analyses demonstrate the operation of synchrony in age structure over large scales, with implications for age-dependent demography in populations. ### Competing Interest Statement The authors have declared no competing interest.

Abstract Abstract Animal migrations represent the regular movements of trillions of individuals. The scale of these movements has inspired human intrigue for millennia and has been intensively studied by biologists. This research has highlighted the diversity of migratory strategies seen across and within migratory taxa: while some migrants temporarily express phenotypes dedicated to travel, others show little or no phenotypic flexibility in association with migration. However, a vocabulary for describing these contrasting solutions to the performance trade‐offs inherent to the highly dynamic lifestyle of migrants (and strategies intermediate between these two extremes) is currently missing. We propose a taxon‐independent organising framework based on energetics, distinguishing between migrants that forage as they travel (income migrants) and those that fuel migration using energy acquired before departure (capital migrants). Not only does our capital:income continuum of migratory energetics account for the variable extent of phenotypic flexibility within and across migrant populations, but it also aligns with theoreticians' treatment of migration and clarifies how migration impacts other phases of the life cycle. As such, it provides a unifying scale and common vacabulary for comparing the migratory strategies of divergent taxa.

The rate of adaptive evolution, the contribution of selection to genetic changes that increase mean fitness, is determined by the additive genetic variance in individual relative fitness. To date, there are few robust estimates of this parameter for natural populations, and it is therefore unclear whether adaptive evolution can play a meaningful role in short-term population dynamics. We developed and applied quantitative genetic methods to long-term datasets from 19 wild bird and mammal populations and found that, while estimates vary between populations, additive genetic variance in relative fitness is often substantial and, on average, twice that of previous estimates. We show that these rates of contemporary adaptive evolution can affect population dynamics and hence that natural selection has the potential to partly mitigate effects of current environmental change.

Simon Evans struck a chord when he asked if others worry asking seminar questions. Simon Evans struck a chord when he asked if others worry asking seminar questions.

Within-population variation in the traits underpinning reproductive output has long been of central interest to biologists. Since they are strongly linked to lifetime reproductive success, these traits are expected to be subject to strong selection and, if heritable, to evolve. Despite the formation of durable pair bonds in many animal taxa, reproductive traits are often regarded as female-specific, and estimates of quantitative genetic variation seldom consider a potential role for heritable male effects. Yet reliable estimates of such social genetic effects are important since they influence the amount of heritable variation available to selection. Based on a 52-year study of a nestbox-breeding great tit (Parus major) population, we apply "extended" bivariate animal models in which the heritable effects of both sexes are modeled to assess the extent to which males contribute to heritable variation in seasonal reproductive timing (egg laying date) and clutch size, while accommodating the covariance between the two traits. Our analyses show that reproductive timing is a jointly expressed trait in this species, with (positively covarying) heritable variation for laydate being expressed in both members of a breeding pair, such that the total heritable variance is 50% larger than estimated by traditional models. This result was robust to explicit consideration of a potential male-biased environmental confound arising through sexually dimorphic dispersal. In contrast to laydate, males' contribution to heritable variation in clutch size was limited. Our study thus highlights the contrasting extent of social determination for two major components of annual reproductive success, and emphasizes the need to consider the social context of what are often considered individual-level traits.

Prenatal mortality is typically overlooked in population studies, which biases evolutionary inference by confounding selection and inheritance. Birds represent an opportunity to include this 'invisible fraction' if each egg contains a zygote, but whether hatching failure is caused by fertilization failure versus prenatal mortality is largely unknown. We quantified fertilization failure rates in two bird species that are popular systems for studying evolutionary dynamics and found that overwhelming majorities (99.9%) of laid eggs were fertilized. These systems thus present opportunities to eliminate the invisible fraction from life-history data.

Temporal variation in natural selection is predicted to strongly impact the evolution and demography of natural populations, with consequences for the rate of adaptation, evolution of plasticity, and extinction risk. Most of the theory underlying these predictions assumes a moving optimum phenotype, with predictions expressed in terms of the temporal variance and auto-correlation of this optimum. However, empirical studies seldom estimate patterns of fluctuations of an optimum phenotype, precluding further progress in connecting theory with observations. To bridge this gap, we assess the evidence for temporal variation in selection on breeding date by modeling a fitness function with a fluctuating optimum, across 39 populations of 21 wild animals, one of the largest compilations of long-term datasets with individual measurements of trait and fitness components. We find compelling evidence for fluctuations in the fitness function, causing temporal variation in the magnitude, but not the direction of selection. However, fluctuations of the optimum phenotype need not directly translate into variation in selection gradients, because their impact can be buffered by partial tracking of the optimum by the mean phenotype. Analyzing individuals that reproduce in consecutive years, we find that plastic changes track movements of the optimum phenotype across years, especially in bird species, reducing temporal variation in directional selection. This suggests that phenological plasticity has evolved to cope with fluctuations in the optimum, despite their currently modest contribution to variation in selection.

Access to analytical code is essential for transparent and reproducible research. We review the state of code availability in ecology using a random sample of 346 nonmolecular articles published between 2015 and 2019 under mandatory or encouraged code-sharing policies. Our results call for urgent action to increase code availability: only 27% of eligible articles were accompanied by code. In contrast, data were available for 79% of eligible articles, highlighting that code availability is an important limiting factor for computational reproducibility in ecology. Although the percentage of ecological journals with mandatory or encouraged code-sharing policies has increased considerably, from 15% in 2015 to 75% in 2020, our results show that code-sharing policies are not adhered to by most authors. We hope these results will encourage journals, institutions, funding agencies, and researchers to address this alarming situation.

Understanding species coexistence has long been a major goal of ecology. Coexistence theory for two competing species posits that intraspecific density dependence should be stronger than interspecific density dependence. Great tits and blue tits are two bird species that compete for food resources and nesting cavities. On the basis of long-term monitoring of these two competing species at sites across Europe, combining observational and manipulative approaches, we show that the strength of density regulation is similar for both species, and that individuals have contrasting abilities to compete depending on their age. For great tits, density regulation is driven mainly by intraspecific competition. In contrast, for blue tits, interspecific competition contributes as much as intraspecific competition, consistent with asymmetric competition between the two species. In addition, including age-specific effects of intra- and interspecific competition in density-dependence models improves predictions of fluctuations in population size by up to three times.

Sexual reproduction is inherently interactive, especially in animal species such as humans that exhibit extended pair bonding. Yet we have little knowledge of the role of male characteristics and their evolutionary impact on reproductive behavioural phenotypes, to the extent that biologists typically consider component traits (e.g. reproductive timing) as female-specific. Based on extensive genealogical data detailing the life histories of 6435 human mothers born across four centuries of modern history, we use an animal modelling approach to estimate the indirect genetic effect of men on the reproductive phenotype of their partners. These analyses show that a woman's reproductive timing (age at first birth) is influenced by her partner's genotype. This indirect genetic effect is positively correlated with the direct genetic effect expressed in women, such that total heritable variance in this trait is doubled when heritable partner effects are considered. Our study thus suggests that much of the heritable variation in women's reproductive timing is mediated via partner effects, and that the evolutionary potential of this trait is far greater than previously appreciated.

Secondary sexual traits have high heritabilities and are exposed to strong, environmentally sensitive selection, and so are expected to evolve rapidly in response to sustained environmental change. We examine the eco-evolutionary dynamics of ornament expression in a long-term study population of collared flycatchers, Ficedula albicollis, in which forehead patch size, which positively influences male reproductive success, declined markedly over 34 years. Annual fitness selection on forehead patch size switched from positive to negative during the study, a reversal that is accounted for by rising spring temperatures at the breeding site: highly ornamented males were selectively favoured following cold breeding seasons but selected against following warm breeding seasons. An 'individual animal model' describes a decline in the genetic values of breeding males during the study, which simulations showed was unlikely to result from drift alone. These results are thus consistent with adaptive evolution of a sexually selected trait in response to climate change.

It was recently proposed that long-term population studies be exempted from the expectation that authors publicly archive the primary data underlying published articles. Such studies are valuable to many areas of ecological and evolutionary biological research, and multiple risks to their viability were anticipated as a result of public data archiving (PDA), ultimately all stemming from independent reuse of archived data. However, empirical assessment was missing, making it difficult to determine whether such fears are realistic. I addressed this by surveying data packages from long-term population studies archived in the Dryad Digital Repository. I found no evidence that PDA results in reuse of data by independent parties, suggesting the purported costs of PDA for long-term population studies have been overstated.

Carotenoid-based coloration plays an important role in signaling, is often sexually dimorphic, and is potentially subject to directional and/or sex-specific selection. To understand the evolutionary dynamics of such color traits, it is essential to quantify patterns of inheritance, yet nonautosomal sources of genetic variation are easily overlooked by classical heritability analyses. Carotenoid metabolism has recently been linked to mitochondria, highlighting the potential for color variation to be explained by cytoplasmically inherited factors. In this study, we used quantitative genetic animal models to estimate the importance of mitochondrial and sex chromosome-linked sources of genetic variation in coloration in two songbird populations in which dietary carotenoids are either unmodified (great tit plumage) or metabolized into alternative color forms (zebra finch beak). We found no significant Z-linked genetic variance in great tit plumage coloration, while zebra finch beak coloration exhibited significant W linkage and cytoplasmic inheritance. Our results support cytoplasmic inheritance of color in the zebra finch, a trait based on endogenously metabolized carotenoids, and demonstrate the potential for nonautosomal sources to account for a considerable share of genetic variation in coloration. Although often overlooked, such nonautosomal genetic variation exhibits sex-dependent patterns of inheritance and potentially influences the evolution of sexual dichromatism.

Summary Carotenoid‐based colours are model traits for research on animal signalling and sexual selection but, whereas the consequences of variable expression have been extensively studied, its causes are rarely quantified. This issue is complicated by the composite nature of carotenoid‐based colour patches, which combine pigments and a reflective background. Ultimately, the evolution of such colours will be determined by the processes that govern variable expression of these mechanisms. We present a novel approach to assessing the quantitative genetics of colour expression, in which reflectance spectra are analysed directly, thereby avoiding the data loss and inherent subjectivity of summary colour variables. Further, the influence of the component mechanisms can be distinguished in spectral analyses due to their contrasting wavelength‐dependencies. Using data from a 6‐year study of carotenoid‐based plumage reflectance in wild great tits (Parus major), we employ a multi‐parallel ‘animal modelling’ approach to estimate sources of variance for narrow (2 nm) wavebands across the visible spectrum. Moderate heritability estimates were limited to the violet‐blue region of the spectrum, diagnostic of the carotenoid content of plumage being heritable. The natal environment effect was limited entirely to the violet‐blue, again indicating that it relates to variation in carotenoid content of feathers. Other wavelengths were sensitive to annual and permanent environmental variation but only marginally influenced by additive genetic variation. Hence, background reflectance is the component that is more sensitive to the environment. Analysing reflectance spectra directly provided an objective perspective of the dynamics of colour expression that is not apparent when relying on summary colour scores. In this case, our results suggest that carotenoid deposition may be an effective target of selection and hence could explain the important role carotenoids frequently play in intraspecific signalling.

Within-population colour variation is widespread in animals, yet the determinants of variable coloration have been relatively neglected by ecologists. Age-dependent expression of conspicuous coloration is prevalent, particularly in birds. Such patterns can be generated by multiple combinations of demographic heterogeneity or within-individual change; longitudinal analyses are necessary to establish the importance of these processes. Further, although pigment-based colours are composite traits, produced by multiple component mechanisms (e.g. feather microstructure and carotenoid pigmentation), the contributions of these mechanisms to components of age dependence are rarely considered, even though doing so may yield information about the ecological causes for age-dependent coloration. We used a large-scale, longitudinal study of carotenoid-based plumage coloration in great tits (Parus major) to show age dependence of plumage coloration is driven almost exclusively by within-individual effects in the first 2years of life. Using wavelength-specific analyses, we show that feather microstructure, while sensitive to annual variation, is independent of age, with increased carotenoid deposition driving changes in coloration. However, estimates of local carotenoid availability did not explain the change in coloration within individuals, suggesting that pigment availability may not be limiting. We thus show that it is individual-level changes in the pigment component of carotenoid-based coloration that determines age-dependent colour expression in great tits. More generally, our study highlights the utility of wavelength-specific analyses in determining the mechanisms underlying changes in expression of composite colour traits.