Avian bill size is a morphological trait with evolutionary and ecological importance. Obtaining large-scale observer-independent measures of bill length and bill depth has proven to be challenging. We developed a device, the Bill Phenotyping Box, that allows taking standardized still images from wild small passerine birds in the field. We combine this with dedicated software that, based on a single human action, determines both bill length (tip to nostril) and depth (at 2/3 of the distance from the tip). We tested for consistency of observations by correlated measurements from two independently taken still images, which was high (r=0.85). We showed that the measurement depends on the angle of the bill position relative to the camera of the device but this error is relatively small compared to the between-observer variation in hand-based measures. We show that these hand-based measures are strongly observer-dependent and that calibrating observers to each other may take at least 50 measurements, which is not feasible in field studies with dozens of observers. We thus have developed a new method that allows large scale observer-independent bill morphological measurement on wild passerines. Both the specifications of the device and of the software is openly available

Anthropogenic environmental change is a major driver of global bird declines, affecting species across continents, ecosystems, and life-history strategies. As such, it has drawn much attention in both primary research studies and meta-analyses. Because meta-analyses influence scientific consensus and conservation policy, it is essential to evaluate the representativeness and transparency of this evidence. However, despite the growing number of meta-analyses , these aspects have never been assessed, creating a clear need for a comprehensive global evaluation of meta-analyses on anthropogenic impacts of birds. Here, we present the first global synthesis, including 149 meta-analyses of anthropogenic influences on birds. We analyzed their thematic, taxonomic, ecological, and geographic coverage, evaluated adherence to reporting and methodological standards, and assessed research production, collaboration, and societal visibility using bibliometric and altmetric approaches. Meta-analyses addressed a wide range of anthropogenic pressures and birds’ responses, however with uneven attention to different topics. Habitat loss and fragmentation, agriculture, and urbanisation were overrepresented across studies, while light and noise pollution, invasive species, and hunting were largely neglected. Responses focused mainly on species abundance, diversity, and reproduction, with limited attention to behaviour, movement, migration, or phenology. Taxonomic coverage was biased towards Passeriformes, and geographic coverage skewed toward North America and Europe. Reporting standards were not widely followed, and almost half of the meta-analyses would not be possible to repeat or update. Almost none of the meta-analyses were preregistered or estimated risk of bias in primary studies, though most controlled for non-independence, and tested for publication bias. Bibliometric and altmetric analyses revealed high collaboration but geographic imbalance among authors. Overall, meta-analytical research on anthropogenic impacts on birds is extensive - but thematically, taxonomically, ecologically, and geographically uneven, with suboptimal transparency. Addressing these limitations is crucial to improve the reliability, comparability, and policy relevance, ultimately supporting more effective conservation strategies for birds.

Given its sensitivity to environmental variation and its central role in physiological signalling, the endocrine system has strong potential to mediate the responses of wild animals to ongoing climate change. Evidence has accumulated, showing that breeding females can transfer maternal hormones to their offspring, thereby modifying offspring phenotype, constituting a potential avenue of transgenerational plasticity to adaptively shape offspring phenotype according to the anticipated environment. This hypothesis requires rigorous testing of several important prerequisites, including whether maternal hormone transfer is itself environmentally responsive. In this study, using a widely distributed bird species – the great tit (Parus major) – as our model system, we conducted a large-scale study across 12 European populations to examine if the amount of maternally transferred yolk thyroid hormones (THs) covaries with environmental variables such as latitude, environmental iodine, and ambient temperature, as well as laying dates as an integrative measure of early spring conditions. Across the 12 sampled populations, we found very little among-population variation in the amount of maternally transferred yolk THs. Within populations, however, yolk triiodothyronine (T3), the active form of THs, showed significant negative relationships with laying date and the average temperature during the period of egg formation, whereas thyroxine (T4), the other main TH, directly produced by the thyroid gland, only showed a less pronounced relationship with average temperature. Given the well-established role of T3 in thermoregulation, our results suggested that the temperature-dependent pattern in maternal yolk T3 reflects the thermoregulatory requirement in the maternal system during egg formation, a hypothesis that warrants further experimental validation. Whether this pattern facilitates or constrains offspring phenotypic plasticity in response to climate change represents an important avenue for further research.

Anthropogenic light affects the presence and behaviour of many species. Bats are generally repelled by light, but some fast-flying and agile species opportunistically forage on insects attracted to light sources. As insect activity is dependent on ambient temperature, prey abundance around light sources likely varies between cold and warm nights. Little is known on how the interacting effects of ambient temperature and anthropogenic light influence bat activity. We recorded activity of bats foraging around experimental light posts emitting different light spectra (white, green and red light) during 6 years for 2726 full nights to test how ambient temperature affects foraging activity of the abundant common pipistrelle (Pipistrellus pipistrellus). We found a strong interaction between light treatment and ambient temperature on the amount and timing of foraging activity. In warm nights, the total foraging activity increased near light posts, and the centre of foraging activity shifted towards the middle of the night. The interactive effect of ambient temperature on common pipistrelle bats foraging around anthropogenic light may contribute to shifts in food web interactions in warmer environments. This suggests that the ecological impact of anthropogenic light on bat biodiversity could be greater in warmer urban areas and warming climates.

The impacts on ecosystems of global and local shifts in the environment due to e.g. climate change, land use change, and urbanisation present us with complex and interacting societal challenges. Solving a single problem often aggravates another. Scientific insights needed to face these challenges are based on interdisciplinary multi-scale research. It requires that we work together across (sub)disciplinary boundaries and combine our insights and knowledge to understand the complexities and interrelatedness within ecosystems. Making this combination is currently hampered by the availability and scatteredness of datasets and the lack of methods and tools to combine knowledge, data and models. Developing digital twins of ecosystems could provide a paradigm to achieve this combination of knowledge, data and models, and generate the insight and knowledge that we require to face the challenges of global and local change. A digital twin of an ecosystem is a digital representation of an ecosystem tailored to the user’s research objectives and perspective of that ecosystem. They bring together (long-term) data collected of multiple ecosystem facets and across multiple scales, process-based models, and data science and computational modelling tools, making them a versatile instrument. Digital twins can be used to understand ecosystem functioning, by aiding the discovery of intricate ecological relationships not yet understood or measured. Furthermore, they are ideal for exploring the effects of in situ management strategies, or for the detection of early-warning signals of global change impacts on ecosystem dynamics. LTER-LIFE is a scientific infrastructure, embedded in LTER-NL (Long-Term Ecosystem Research Netherlands) and LifeWatch ERIC, aimed at supporting shared and integrated ecological research, providing scientists the platform and tools to build digital twins of ecosystems. We do this by supporting the sharing of data and models and bringing these together into a site-focused virtual lab where people can work together. LTER sites provide an ideal opportunity for this as they often come with a long history of extensive data collection and research across multiple spheres. Drawing on experience of building digital twins in the Veluwe LTSER platform, this keynote will take you through the journey of digital twinning parts of an ecosystem with a long legacy of ecological and environmental research and data collection. As these data exist in many different formats, we built data mobilisation and FAIRification pipelines to augment the focal experimental sites in the Veluwe with relevant historical, privately-owned or national datasets. Subsequently, we explored the potential of the Notebook as a Virtual Research Environment (NaaVRE), which provided us with a virtual lab in which we developed workflows for processing and linking FAIRified data with process-based and data-driven models. Ultimately, we show that this way of working can bring together researchers to generate insights on the impacts of climate change, land use change, and urbanisation on the functioning of the ecosystem, providing scientific support for management and policy decisions.

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.

To forecast how fast populations can adapt to climate change, it is essential to determine the evolutionary potential of different life-cycle stages under selection. In birds, timing of gonadal development and moult are primarily regulated by photoperiod, while laying date is highly phenotypically plastic to temperature. We tested whether geographic variation in phenology of these life-cycle events between populations of great tits (Parus major) has a genetic basis, indicating that contemporary genetic adaptation is possible. We carried out a common garden experiment in which we bred first- and second-generation pairs in captivity originating from eggs from Gotland (Sweden) and Hoge Veluwe (The Netherlands), two populations that showed different temperature sensitivity of laying date in a recent meta-analysis. We recorded the phenology of egg-laying, moult and gonadal size in early spring. We found no significant differences in laying date between the populations, but they did differ in moult timing and testis size. This implies that under climate change the timing of gonadal development and moult, which are mainly regulated by photoperiod, will not respond to increased temperature but can respond by genetic adaptation in response to selection, while the opposite holds for laying date, perhaps indicating that plasticity is constraining genetic adaptation.

Artificial illumination at night is an anthropogenic disturbance which severely impacts our societies and ecosystems. Nocturnal insects are crucial ecosystem service providers and their fitness can be greatly affected by anthropogenic light at night, among others via positive phototaxis. The rising popularity and efficiency of light emission diode (LED) lighting technology accentuate the need to understand how both spectrum and intensity affect insect behaviour, especially when building spectrally tuneable lighting systems to mitigate the impact of light pollution. We measured the phototactic responses of insects of different taxa to artificial light at night using LED traps emitting light at four narrowband wavelengths of three intensities over 11 nights at two locations in natural forested areas in the Netherlands. We found that different light colours elicit order‐specific phototactic responses. For example, Diptera were strongly attracted towards green and blue light, while moth species displayed phototactic responses mostly towards ultraviolet emissions. In addition, increased intensity levels positively affected the strength of attraction, irrespective of light colour. Finally, our approach enabled us to calculate spectral sensitivity curves to compare spectral phototaxis responses between insect orders. Although there is no ‘one size fits all’ solution, we conclude that mitigation of light pollution affecting insect attraction can be best achieved by using longer wavelengths of low light intensity.

Animals living in cities are smaller than their conspecifics from rural areas but whether such differences are caused by genetic differences or food constraints remains untested. We performed a multi-generation common garden study where we raised great tits (Parus major), originating from eggs collected from multiple Dutch cities and forests under the same conditions for two generations. Offspring from city birds had a smaller tarsus than forest birds in both generations, demonstrating that these morphological differences are genetic. Next, we tested whether size differences are an adaptation to the low food abundance when offspring are raised in the city. Third-generation birds of both origins were given food amounts mimicking being raised in forests or cities during the second part of their nestling development. While the treatment resulted in birds in the lower feeding frequency treatment to be smaller, city and forest birds responded the same way, suggesting that city birds do not cope better with reduced food availability. Our study shows that the smaller size of urban birds has a genetic basis and is not only caused by a plastic response to restricted resources in the urban environment. Our experiment does not provide evidence that these genetic differences have evolved as an adaptive response to a reduced food availability in cities.

Artificial light at night can act as a barrier and cause habitat fragmentation, especially for bat species that are generally considered to be light-averse. Bats use linear structures to commute from their roost to their foraging areas. Trawling bats such as the pond bat (Myotis dasycneme) forage predominantly above water bodies and use waterways as commuting routes. Artificial light along these potentially leads to interruptions of commuting routes, or changes in flight behaviour of trawling bats, but impact of light may vary with light spectrum and intensity. Here, we tested whether pond bats change their flight speed and straightness in response to four light spectra at two light intensities by placing an experimental lamp post at bridges over waterways that are used by pond bats as commuting routes. We used a microphone array to precisely reconstruct the flight path of each passing bat and calculate flight parameters. Flight speed of commuting pond bats was unaffected by the presence of light, regardless of the light spectrum. Pond bats only fly straighter when exposed to white light (3000 K). The short presence of a lamp post with realistic light intensity on a bridge may therefore not act as a barrier. However, other direct effects cannot be excluded and the long-term presence of a similar light installation may still have impact.

Whether avian migrants can adapt to their changing world depends on the relative importance of genetic and environmental variation for the timing and direction of migration. In the classic series of field experiments on avian migration, A. C. Perdeck discovered that translocated juveniles failed to reach goal areas, whereas translocated adults performed ‘true-goal navigation’. His translocations of > 14 000 common starlings ( Sturnus vulgaris ) suggested that genetic mechanisms guide juveniles into a population-specific direction, i.e. ‘vector navigation’. However, alternative explanations involving social learning after release in juveniles could not be excluded. By adding historical data from translocation sites, data that was unavailable in Perdeck's days, and by integrated analyses including the original data, we could not explain juvenile migrations from possible social information upon release. Despite their highly social behaviour, our findings are consistent with the idea that juvenile starlings follow inherited information and independently reach their winter quarters. Similar to more solitarily migrating songbirds, starlings would require genetic change to adjust the migration route in response to global change.

Artificial light at night (ALAN) widely affects wildlife by blurring light-dark differences, including transitions such as sunrise and sunset, thereby affecting regulation of diel rhythms. As a result, activity onsets in many wild diurnal songbirds advance under ALAN. From chronobiological studies, it is known that the direction and strength of the response to light depends on when during the night exposure takes place. However, these experiments are mostly done under continuous light conditions, when animals have free-running rhythms. It remains unclear whether phase-dependence also holds in entrained, wild songbirds; i.e., does the effect of ALAN on activity patterns differ between exposure in the morning compared to the evening? This information is essential to assess the effects of mitigation measures by limiting ALAN to selected times of the night. We exposed incubating great tits (Parus major) inside the nest-box to 4 h of dim light, of which 1 h overlapped with dawn before sunrise or dusk after sunset. We found a small advancing effect of morning-light on activity onset and of evening-light on offset compared to dark controls but not vice versa. Breeding success and chick condition were unaffected by the light treatments. However, light-treated females had lower weights 9-18 days after the end of the treatment compared to the controls, independent of whether ALAN occurred in the morning or the evening, indicating possible costs of ALAN. Despite the weak behavioral response, ALAN might have affected the females' circadian clock or physiology resulting in lower body condition.

The recognition that climate change is occurring at an unprecedented rate means that there is increased urgency in understanding how organisms can adapt to a changing environment. Wild great tit (Parus major) populations represent an attractive ecological model system to understand the genomics of climate adaptation. They are widely distributed across Eurasia and they have been documented to respond to climate change. We performed a Bayesian genome-environment analysis, by combining local climate data with single nucleotide polymorphisms genotype data from 20 European populations (broadly spanning the species’ continental range). We found 36 genes putatively linked to adaptation to climate. Following an enrichment analysis of biological process Gene Ontology (GO) terms, we identified over-represented terms and pathways among the candidate genes. Because many different genes and GO terms are associated with climate variables, it seems likely that climate adaptation is polygenic and genetically complex. Our findings also suggest that geographical climate adaptation has been occurring since great tits left their Southern European refugia at the end of the last ice age. Finally, we show that substantial climate-associated genetic variation remains, which will be essential for adaptation to future changes.

A major aim of evolutionary biology is to understand why patterns of genomic diversity vary within taxa and space. Large-scale genomic studies of widespread species are useful for studying how environment and demography shape patterns of genomic divergence. Here, we describe one of the most geographically comprehensive surveys of genomic variation in a wild vertebrate to date; the great tit (Parus major) HapMap project. We screened ca 500,000 SNP markers across 647 individuals from 29 populations, spanning similar to 30 degrees of latitude and 40 degrees of longitude - almost the entire geographical range of the European subspecies. Genome-wide variation was consistent with a recent colonisation across Europe from a South-East European refugium, with bottlenecks and reduced genetic diversity in island populations. Differentiation across the genome was highly heterogeneous, with clear 'islands of differentiation', even among populations with very low levels of genome-wide differentiation. Low local recombination rates were a strong predictor of high local genomic differentiation (FST), especially in island and peripheral mainland populations, suggesting that the interplay between genetic drift and recombination causes highly heterogeneous differentiation landscapes. We also detected genomic outlier regions that were confined to one or more peripheral great tit populations, probably as a result of recent directional selection at the species' range edges. Haplotype-based measures of selection were related to recombination rate, albeit less strongly, and highlighted population-specific sweeps that likely resulted from positive selection. Our study highlights how comprehensive screens of genomic variation in wild organisms can provide unique insights into spatio-temporal evolutionary dynamics.

To understand to what extent evolution can contribute to bending the curve of ongoing biodiversity losses, we urgently need to characterize what determines the adaptive potential of populations. We argue that capitalising on existing examples of genetic adaptation to climate change provides the opportunities to fill this major knowledge gap. We performed a systematic literature review and obtained 40 empirical examples of species with direct evidence of wild populations undergoing genetic adaptation in response to climate change selection. Only two of these examples (crustacean Daphnia magna and plant Brassica rapa) presented robust evidence for genetic adaptation driven by climate change, using strong inference methods to show that (1) a phenotypic change over time occurred, (2) the phenotype has a genetic basis, (3) the fitness of the phenotype depends on a climatic variable, (4) climate change-induced selection occurred across generations, and (5) it was assessed to what extent the genetic change involved a response to selection compared to the contribution of other evolutionary processes. There thus are ample opportunities to strengthen the evidence base for these existing examples such that they can contribute to understanding when and how genetic adaptation to climate change can take place. Moreover, improving the spatial and temporal replication of these existing studies is highly needed to identify general principles across species and populations. Especially genomics studies using high-resolution temporal sampling provide important information about the process and rate of evolution, but the field currently lacks such high-resolution temporal genomics studies. We urge the field to capitalize on and strengthen these existing examples of genetic adaptation so that we can identify which drivers and constraints determine the likelihood and rate of evolutionary responses to climate change.

Anthropogenic climate change has led to globally increasing temperatures at an unprecedented pace and, to persist, wild species have to adapt to their changing world. We, however, often fail to derive reliable predictions of species' adaptive potential. Genomic selection represents a powerful tool to investigate the adaptive potential of a species, but constitutes a 'blind process' with regard to the underlying genomic architecture of the relevant phenotypes. Here, we used great tit (Parus major) females from a genomic selection experiment for avian lay date to zoom into this blind process. We aimed to identify the genetic variants that responded to genomic selection and epigenetic variants that accompanied this response and, this way, might reflect heritable genetic variation at the epigenetic level. We applied whole genome bisulfite sequencing to blood samples of individual great tit females from the third generation of bidirectional genomic selection lines for early and late lay date. Genomic selection resulted in differences at both the genetic and epigenetic level. Genetic variants that showed signatures of selection were located within genes mostly linked to brain development and functioning, including LOC107203824 (SOX3-like). SOX3 is a transcription factor that is required for normal hypothalamo-pituitary axis development and functioning, an essential part of the reproductive axis. As for epigenetic differentiation, the early selection line showed hypomethylation relative to the late selection line. Sites with differential DNA methylation were located in genes important for various biological processes, including gonadal functioning (e.g., MSTN and PIK3CB). Overall, genomic selection for avian lay date provided insights into where within the genome the heritable genetic variation for lay date, on which selection can operate, resides and indicates that some of this variation might be reflected by epigenetic variants.

Diel rhythms are driven by genetic and environmental components. These rhythms are mediated by the circadian clock, and entail rhythmicity of various physiological and behavioural traits. Although individuals show to some extent repeatable timing (i.e., chronotype), there is ample variation of diel timing observed within and between individuals of the same species. Here, we investigated various environmental factors, including timing of the social partner, that could explain day-to-day variation within individuals. Synchronisation with the social partner during provisioning timing could increase breeding success, and decrease extrapair paternity opportunities for females during the fertile period in case of consistent timing across the breeding stages. Therefore, we also investigated fitness consequences of between-individual variation. We first assessed the magnitude of between- and within-individual variation in the timing of nest visits by great tits (Parus major). We monitored nest visits of males and females in 37 broods during chick provisioning in 2020 and 2021. Next, we explored the responsiveness of the diel timing to environmental variables, specifically comparing abiotic and social factors. The onset of nest visits varied significantly with day within the breeding season, rainfall and the diel timing of the breeding partner but not with night temperature. In response to the partner's onset, females responded stronger compared to males. By contrast, offset was generally more variable within individuals and less variation was explained by the environmental variables. Both males and females delayed their activity offset with the progressing season and females also had a later onset with more daytime rainfall. Further, the reproductive output and extrapair paternity were independent of parental chronotype and their synchronisation within pairs. It is possible that consistency of chronotypes is less important for reproductive success than the ability to plastically respond to changing environmental conditions. Thus, the next step could be to investigate potential individual differences in plasticity which could be even linked to specific chronotypes. This information might be crucial to predict how species can cope with unpredictable environmental conditions. ### Competing Interest Statement The authors have declared no competing interest.