Blood parasites are widespread pathogens of wild birds, yet their reproductive consequences remain poorly understood. We investigated associations between blood parasite infections and reproductive performance in three passerine species breeding sympatrically on Gotland, Sweden: the resident great tit (Parus major) and blue tit (Cyanistes caeruleus) and the migratory collared flycatcher (Ficedula albicollis). The study was based on data from 181 nests and 1201 nestling observations. Adult birds were screened for Plasmodium, Haemoproteus, and Trypanosoma infections using next-generation sequencing, and infection intensity of malaria parasites (Plasmodium/Haemoproteus) and Trypanosoma was quantified using quantitative PCR. We tested whether parental infection status and parasite intensity were associated with reproductive traits measured at the nest level (hatching date, clutch size, and brood size) and nestling level (body mass, growth, and condition). Associations between infections and reproduction differed among parasite groups, parental sexes, and host species, with some of the strongest and most consistent patterns involving Haemoproteus infections. Most significant relationships were detected for nestling traits rather than breeding parameters, and a substantial proportion of associations remained significant after false discovery rate (FDR) correction. Both positive and negative associations were observed, and significant interactions between female and male infection status indicated that offspring performance depended on both parents’ infection status. Moreover, analyses based on infection status and parasite intensity revealed partly different patterns of association with reproductive traits. Overall, these results demonstrate that the reproductive consequences of blood parasite infections are parasite-specific and context-dependent.

Artificial light at night (ALAN) disrupts biological rhythms across taxa, yet behavioural mechanisms linking exposure to population consequences remain poorly understood. Long-distance migrants encounter ALAN across their annual cycle but remain understudied despite experiencing stronger performance impacts than sedentary species. We experimentally introduced ALAN into nest-boxes of a migratory passerine, the collared flycatcher (Ficedula albicollis), breeding in Gotland, Sweden. Nestlings were exposed to ALAN from 2 days post-hatching until fledging, and we video-recorded parental and nestling activity over 24 h on day 8 post-hatching. Across a high-resolution behavioural dataset (32 100 nestling and 3709 parental events), ALAN-exposed nestlings begged more frequently and for longer at night compared with dark controls, revealing disrupted circadian activity. These effects cascaded to parents: both females and males began feeding earlier and ceased later, while reducing hourly feeding rates relative to controls. Consequently, ALAN nestlings fledged at older ages, consistent with delayed development, though reproductive success (number of fledged offspring) was unaffected. Our study provides clear empirical support for socially mediated behavioural mechanisms whereby both parents respond to ALAN, disrupting family level coordination, developmental trajectories and parental investment. This temporal destabilization of social synchrony uncovers a novel ecological pathway through which anthropogenic light alters behavioural timekeeping and life-history trajectories in the wild.

Stochastic influences arising from demographic and environmental stochasticity strongly affect eco-evolutionary dynamics. Demographic stochasticity causes reduction in the long-run population growth rates at small population sizes and generates genetic drift. The effects of stochastic environmental fluctuations on fitness are more difficult to assess because they may introduce stochasticity in the selection processes. Here, we examined stochastic variation in the response to selection on 3 breeding parameters in a collared flycatcher (Ficedula albicollis) population using density-dependent and frequency-dependent environmental covariances among individuals with different phenotypes. Environmental fluctuations induced considerable variation among individual fitness dependent on their phenotype and population size. These stochastic effects on fitness were strongest for individuals with large clutches or few fledglings, and in years with few conspecifics. Contrastingly, we found no evidence that the stochastic environmental effects on individual fitness varied with laying date. Furthermore, we found that phenotype-specific effects of environmental fluctuations were less correlated for fledgling number than for laying date and clutch size, resulting in less correlation in fitness between pairs with similar fledgling numbers. Our analyses showed that the stochastic component in the response to selection on clutch size and laying date caused by environmental stochasticity at the carrying capacity was of the same order as the component due to genetic drift. This means that stochasticity can affect phenotypic evolution even in large populations via stochastic selection generated by environmental fluctuations.

Here, we present an analysis based on a 40-year dataset from a nest-box population of the collared flycatcher (Ficedula albicollis). Our objective is to examine the impact of weather conditions during the incubation and nestling period on key indicators of individual fitness, including offspring production and local annual recruitment rate. Our findings provide compelling evidence that climatic conditions experienced during both incubation and nestling periods significantly impact the number of fledglings and recruits. Specifically, we observed that higher precipitation during the nestling period negatively affects the number of fledglings and increases brood failure. Interestingly, higher precipitation during the incubation period is linked to increased recruitment numbers. Moreover, we found that warmer weather during both the incubation and nestling periods decreases brood failure, and more importantly, higher temperatures during the nestling period are positively associated with the number of recruits. These results underscore the complex interplay between weather patterns and avian reproductive strategies, highlighting the importance of long-term ecological studies in understanding the impacts of climate change on bird populations. By addressing the variability of climatic influences across different life stages, future research can help develop more comprehensive models for predicting the resilience of avian species in the face of ongoing climate challenges.

Artificial light at night (ALAN), a growing consequence of global urbanisation, disrupts natural light cues that regulate biological rhythms across taxa. However, the behavioural pathways linking ALAN exposure to broader ecological impacts remain poorly understood. Here, we experimentally introduced ALAN into nestboxes of a long-distance migratory bird, the collared flycatcher ( Ficedula albicollis ), breeding in Gotland, Sweden, to assess ALAN effects on development and reproductive ecology. Nestlings were exposed to ALAN from two days post-hatching until fledging, and we video-recorded parental and nestling activity over 24 hours on day 8 post-hatching. From a high-resolution behavioural dataset (32,100 nestling and 3,709 parental events), we found that ALAN-exposed nestlings begged more frequently and for longer at night compared to dark controls, revealing disrupted nocturnal behaviour. These effects cascaded to parental care: both females and males in ALAN nests extended their daily activity, initiating feeding earlier and terminating later, but traded off this extension by reducing their hourly feeding counts compared to parents in dark conditions. Consequently, nestlings under ALAN fledged at an older age and had a lower fledging rate after day eight post-hatching, despite no difference in total fledging numbers compared to controls. Our findings provide the first comprehensive experimental evidence that ALAN alters the behaviour of both parents alongside their offspring and reduces reproductive success in a long-distance migrant, a species group that increasingly encounters light pollution during nocturnal migration. ALAN, though directly affecting mainly nestlings and females in nestboxes, triggered socially-mediated responses that altered the circadian behaviour of entire bird families. This study underscores the need to consider behavioural disruption as a critical mechanism underlying the ecological impacts of ALAN in natural populations. ![Figure][1] Summary figure: Cascading effects of artificial light at night, an aspect of human-induced light pollution, on the behaviour and reproductive ecology of collared flycatchers ### Competing Interest Statement The authors have declared no competing interest. National Science Center, 2019/35/D/NZ8/00889 [1]: pending:yes

Understanding how weather conditions during early development influence reproductive success is essential for predicting avian responses to climate change. We used a 40-year dataset from a nest-box population of collared flycatchers (Ficedula albicollis) to examine how temperature and precipitation during the incubation and nestling periods affected three key components of reproductive performance: hatchling number, fledgling production, and local offspring recruitment. We found that higher ambient temperatures during the nestling period were associated with a decreased probability of brood failure and a higher number of recruits. In contrast, a higher sum of precipitation during the nestling stage was associated with an increased likelihood of brood failure. Interestingly, weather conditions during incubation had no statistically detectable influence on reproductive performance. Moreover, our results indicated that the within-season variation in climatic conditions, rather than the between-season variation, was primarily associated with reproductive outcomes. These findings underscore the importance of considering both overall climate conditions across seasons and short-term weather parameters within breeding seasons when evaluating reproductive success and predicting the ecological consequences of climate change.

Using a 37-year dataset of Ficedula albicollis (Collared Flycatcher) we examined how incubation duration is influenced by environmental conditions and female characteristics. In our study, we clearly demonstrated that the length of the incubation period exhibits variability and may be subject to selection. Our findings indicate that the duration of the incubation period tended to affect hatching success but played a more substantial role in recruitment, which is a key indicator of fitness. We found that incubation duration is significantly affected by laying date, clutch size, female body condition, and ambient temperature, including its variability. Interestingly, both low and high ambient temperatures are linked to shorter incubation periods. In turn, an increase in temperature variability is associated with shortening incubation periods. Further research is needed to explore the long-term impacts of these incubation strategies on population viability, particularly in the face of increasing climate variability. center dot Using a 37-year dataset of Ficedula albicollis (Collared Flycatcher), we examined how environmental conditions and female characteristics influence incubation duration and its consequences for hatching success and recruitment of the offspring.center dot Our study indicates that the duration of the incubation period has the potential to affect hatching success, but it apparently plays a crucial role in offspring recruitment, which is a key indicator of fitness.center dot We found that clutch size, female body condition, and ambient temperature, including its variability, significantly affect incubation duration.center dot Low and high temperatures are linked to shorter incubation periods, while an increase in temperature variability is associated with shortening incubation periods.center dot These findings provide insights into how environmental conditions may influence incubation as a key avian reproductive trait, with potential implications for bird responses to climate change. W oparciu o 37-letni zbi & oacute;r danych z populacji Ficedula albicollis przeanalizowano, w jaki spos & oacute;b warunki & sacute;rodowiskowe oraz cechy fenotypowe samic wp & lstrok;ywaj & aogon; na d & lstrok;ugo & sacute;& cacute; okresu inkubacji. Wyniki jednoznacznie wskazuj & aogon;, & zdot;e czas trwania inkubacji wykazuje znaczn & aogon; zmienno & sacute;& cacute; i mo & zdot;e podlega & cacute; presji selekcyjnej. Zaobserwowano, & zdot;e czas trwania inkubacji wp & lstrok;ywa zar & oacute;wno na sukces wyklucia, jak i na rekrutacj & eogon; potomstwa, b & eogon;d & aogon;c & aogon; kluczowym komponentem dostosowania. Czas trwania inkubacji istotnie zale & zdot;a & lstrok; od daty zniesienia pierwszego jaja, wielko & sacute;ci l & eogon;gu, kondycji cia & lstrok;a samicy oraz & sacute;redniej temperatury otoczenia i jej zmienno & sacute;ci. Zar & oacute;wno niskie, jak i wysokie warto & sacute;ci temperatury wi & aogon;za & lstrok;y si & eogon; ze skr & oacute;ceniem okresu inkubacji, podobnie jak zwi & eogon;kszona zmienno & sacute;& cacute; temperatury. Dalsze badania s & aogon; niezb & eogon;dne, aby oceni & cacute; d & lstrok;ugoterminowe konsekwencje tych wzorc & oacute;w dla stabilno & sacute;ci populacji w obliczu szybko post & eogon;puj & aogon;cych zmian klimatu.

Avian blood parasites play a crucial role in wildlife health and ecosystem dynamics, exhibiting heterogeneous spatial distribution influenced by various factors. Although factors underlying heterogeneity in infection with blood parasites have been explored in many avian hosts, their importance in the context of host species and the parasite taxon remains poorly understood, particularly in cohabiting host species. Using next-generation sequencing for parasite screening, we investigate the association between Haemoproteus, Plasmodium and Trypanosoma infections in relation to individual parameters, host densities and landscape features in 3 cavity-nesting passerines: great tit (Parus major), blue tit (Cyanistes caeruleus) and collared flycatcher (Ficedula albicollis) in a highly fragmented forest habitat. Overall, Haemoproteus infections predominated, followed by Plasmodium and Trypanosoma, with great tits most and collared flycatchers least parasitized. There were no common patterns across host species in the probability of infection with locally transmitted parasites from each genus. Specifically, in all cases, the effect of particular parameters, if present, was observed only in 1 host species. Body condition influenced Haemoproteus and Plasmodium infections differently in tits. Host density, whether own species or all pooled, explained Haemoproteus infections in great tits and collared flycatchers, and Plasmodium in great tits. Landscape metrics, such as moisture index and distance to coast edge and pastures, affected infection probability in specific host–parasite combinations. Relative risk maps revealed infection risk gradients, but spatial variation repeatability over time was low. Our study highlights the complex dynamics of avian blood parasites in multi-host systems, shedding light on host–parasite interactions in natural ecosystems.

The gut microbiome (hereafter, GM) varies across individuals of the same species and this pattern has been observed in multiple wild species. Evidence shows that the GM connects to individual health and survival especially in captive species, but more research is needed to understand how the GM connects to host fitness in wild species. We used long-term monitoring data to investigate whether the GM of collared flycatchers Ficedula albicollis associates with annual and lifetime reproductive success (LRS), and survival to the following breeding season. This is the first study that 1) characterized the collared flycatcher GM, and 2) investigated how variation in the GM related to LRS in wild birds. Our results showed that higher GM diversity was associated with a higher annual and lifetime reproductive success in especially male collared flycatchers. We also found that the compositional variation in collared flycatcher GMs was explained by sex, age, and breeding habitat, but not by annual or lifetime reproductive success. Individuals that died before the next breeding season had higher abundances of ASVs belonging to the pathogenic families Enterobacteriaceae and Parachlamydiaceae, and the genera Corynebacteria and Sphingomonas. Our results show that the GM associates with different aspects of host fitness in a wild bird population. More research is needed to evaluate if there is a causal relationship between the GM and individual fitness. These findings also contribute to our understanding of the GMs role in evolution by elucidating the connection between the GM (trait) and reproductive success. ### Competing Interest Statement The authors have declared no competing interest.

In birds, extrapair paternity (EPP) constitutes an alternative mating strategy, with potentially important fitness consequences for both males and females and their offspring. Several factors have been identified that can influence the occurrence of EPP, but the role of environmental variability has so far received relatively little attention. Using long-term data set from a wild population of the blue tit (Cyanistes cearuleus), we assess the importance of ambient temperature in modulating the levels of extrapair paternity. Here, we showed that the variability of local thermal conditions affects the occurrence of EPP. Specifically, we found that the probability of EPP increased with rising variability in ambient temperature experienced by females prior to egg laying. This pattern is consistent with an idea of plastic female responses to unpredictable environments. Our results suggest that extrapair mating may represent an adaptive behavioral strategy to compensate for the potential negative effects of unstable environmental conditions. In our new paper, we present evidence that temperature variability plays a crucial role in shaping the dynamics of avian mating relationships, specifically in terms of extrapair paternity. Extrapair paternity, or mating outside of the established pair bonds, has long intrigued scientists studying bird behavior. This unexpected behavior, as suggested by the study, may represent an adaptive strategy employed by birds to navigate the challenges posed by increasingly adverse environmental conditions.

Breeding responses of organisms to environmental changes may profoundly depend on an individual’s age, as an age-environment interaction may be expected to affect the expression of reproductive traits. However, little is known about how this interaction affects short-lived species that experience various environmental conditions in adulthood. Here, we used a 32-year dataset from the collared flycatcher, Ficedula albicollis, population to test whether and how the environment interacts with age to shape female age-specific reproduction. To characterise environmental variation, we applied the remotely sensed normalised difference vegetation index (NDVI), estimating vegetation productivity, and used it as a surrogate for habitat quality. Then, we analysed how within-individual age and NDVI determine patterns in laying date, clutch size, offspring production, and recruitment. We found that young and old females, but not middle-aged females, breeding under low NDVI started to lay eggs later and produced smaller clutches than females of the same age breeding under higher NDVI. No such effects were observed for offspring production or recruitment. Our study provides evidence that both an individual’s age and the environmental variation experienced during adulthood may be crucial for shaping reproductive patterns in short-lived avian species, as has been found in long-lived birds.

Abstract Background Relatively few studies have examined the interactive effects of ecological factors on physiological responses in wild animals. Nearly all of them have been short-term investigations that did not include experimental manipulations, limiting our ability to understand how climate change will affect natural populations. Using a 10-year brood size manipulation experiment in wild blue tits (Cyanistes caeruleus), we quantified the impact of weather conditions and brood competition on the body mass and structural size (tarsus length) of nestlings just prior to leaving the nest. Results We found that variation in nestling body mass on day 14 after hatching was explained by an interactive effect between average ambient temperature experienced during nestling period and brood size treatment. Specifically, in control broods nestling body mass was correlated with temperature in a non-linear manner (concave) with the vertex point (maximum body mass) at ca. 13 °C. In contrast, in enlarged broods nestling body mass permanently increased (also non-linearly) as temperature advanced. Conclusions Our results highlight the importance of considering the effects of brood rearing conditions alongside other environmental factors experienced during growth while investigating early-life environmental effects on body condition.

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.

Microbiome constitutes an important axis of individual variation that, together with genes and the environment, influences an individual's physiology and fitness. Microbiomes are dependent not only on an individual's body condition but also on external factors, such as diet or stress levels, and as such can be involved into feedbacks between the external ecological factors and internal physiology. In our study, we used a wild population of blue tits Cyanistes caeruleus to investigate the impact of external habitat composition on the microbiome of adult birds. We hypothesized that – through differences in plant composition, potentially affecting diet complexity – habitat type may impact the diversity and structure of the gut microbiome. Blue tits breeding in dense deciduous forests tended to have more diverse microbiomes and be significantly different in terms of microbiome composition from birds breeding in open, sparsely forested hay meadows. Distinct study plots also tended to differ in a number of parameters describing microbiome diversity. We observed no microbiome differentiation according to individual characteristics such as sex or age. The study emphasizes that external environment is one of the important modulators of microbiome diversity and calls for more such studies in wild animal populations.

There is growing evidence that engaging in extra‐pair copulations may be a strategy by which females can modify their initial mate choice if they are constrained by primary choice of the social mate. Several factors such as genetic similarity and adult phenotypic traits can affect extra‐pair paternity (EPP) patterns, but the relative importance of these factors may vary among species. Moreover, interactive effects of male and female characteristics have rarely been considered. Here, we aimed to study how multiple parameters characterizing a breeding pair (i.e. genetic similarity between mates, partners’ age and laying date) predict the occurrence of EPP at the brood level. The study uses 4 years of data from a wild population of Blue Tits Cyanistes caeruleus . Contrary to predictions of the inbreeding avoidance hypothesis, we did not find a positive relationship between the occurrence of EPP and the relatedness of social mates. We also found that the probability of EPP was unrelated to laying date. However, EPP was predicted by an interaction of social partners' ages. Specifically, EPP was less likely when old females were paired with old males in comparison to old females paired to young males. Our study suggests that the occurrence of EPP may be the result of behavioural interactions in which both male and female age are important for determining the outcome. Our results confirm the importance of considering the interactions of male and female characteristics in studies investigating EPP patterns.

Abstract Achromatic patches are a common element of plumage patterns in many bird species and there is growing body of evidence that in many avian taxa they can play a signaling role in mate choice. Although the blue tit Cyanistes caeruleus is a well-established model species in the studies on coloration, its white wing patch has never been examined in the context of sex-specific trait expression. In this exploratory study, we examined sexual size dimorphism and dichromatism of greater covert’s dots creating white wing patch and analyzed its correlations with current body condition and crown coloration—a trait with established role in sexual selection. Further, we qualitatively analyzed microstructural barb morphology underlying covert’s coloration. We found significant sexual dimorphism in the dot size independent of covert size and sexual dichromatism in both white dot and blue outer covert’s vane spectral characteristics. Internal structure of covert barbs within the white dot was similar to the one found in barbs from the blue part that is, with a medullary area consisting of dead keratinocytes containing channel-type ß-keratin spongy nanostructure and centrally located air cavities. However, it lacked melanosomes which was the main observed difference. Importantly, UV chroma of covert’s blue vane was positively correlated with crown UV chroma and current condition (the latter only in males), which should be a premise for further research on the signal function of the wing stripe.

Adaptive topography is a central concept in evolutionary biology, describing how the mean fitness of a population changes with gene frequencies or mean phenotypes. We use expected population size as a quantity to be maximized by natural selection to show that selection on pairwise combinations of reproductive traits of collared flycatchers caused by fluctuations in population size generated an adaptive topography with distinct peaks often located at intermediate phenotypes. This occurred because r- and K-selection made phenotypes favored at small densities different from those with higher fitness at population sizes close to the carrying capacity K. Fitness decreased rapidly with a delay in the timing of egg laying, with a density-dependent effect especially occurring among early-laying females. The number of fledglings maximizing fitness was larger at small population sizes than when close to K. Finally, there was directional selection for large fledglings independent of population size. We suggest that these patterns can be explained by increased competition for some limiting resources or access to favorable nest sites at high population densities. Thus, r- and K-selection based on expected population size as an evolutionary maximization criterion may influence life-history evolution and constrain the selective responses to changes in the environment.

Ambient temperature experienced by an animal during development or subsequently as an adult can affect many aspects of its behaviour and life-history traits. In birds, egg incubation is a vital component of reproduction and parental care. Several studies have suggested that environmental factors (such as nest microclimate) can influence the ability of incubating parents to maintain suitable conditions for embryo development. Here, we manipulated the developmental conditions of embryos through a modification of nest box thermal microclimate to investigate female incubation behaviour and its impact on offspring fitness-related traits in a wild population of the Collared Flycatcher ( Ficedula albicollis ). The temperature in experimental nests was increased using a heat-pack placed under the roof of a nest box, resulting in an average temperature increase of 2.5 ºC, which corresponds to projected climate change scenarios. We demonstrated that females from nests with elevated temperature spent less time in the nest box during egg incubation and had more off-bouts than females from control nests. Moreover, we found that offspring from the experimentally heated nests had larger body mass at fledging in comparison to the control ones. Our study indicates that nest microclimate during the incubation period affects female incubation behaviour and offspring quality, indicating that environmental variation in nest temperature early in ontogeny can have important and long-lasting fitness consequences.

Due to its central importance to many aspects of evolutionary biology and population genetics, the long-term effective population size (Ne ) has been estimated for numerous species and populations. However, estimating contemporary Ne is difficult and in practice this parameter is often unknown. In principle, contemporary Ne can be estimated using either analyses of temporal changes in allele frequencies, or the extent of linkage disequilibrium (LD) between unlinked markers. We applied these approaches to estimate contemporary Ne of a relatively recently founded island population of collared flycatchers (Ficedula albicollis). We sequenced the genomes of 85 birds sampled in 1993 and 2015, and applied several temporal methods to estimate Ne at a few thousand (4000-7000). The approach based on LD provided higher estimates of Ne (20,000-32,000) and was associated with high variance, often resulting in infinite Ne . We conclude that whole-genome sequencing data offers new possibilities to estimate high (>1000) contemporary Ne , but also note that such estimates remain challenging, in particular for LD-based methods for contemporary Ne estimation.