While the direct toxicological effects of metal exposure on animals, including birds, are well documented, indirect mechanisms remain poorly understood. This applies to effects on microbiota, despite the emerging evidence of its crucial role in host's physiological functions. We investigated the metal exposure, growth, and fledging of great tits (Parus major) in six rural vs. industrial/urban area (IUA) comparisons across Europe to identify associations with bird gut microbiota. To capture a range of pollution profiles, IUAs included both city settings and industrial sites such as a copper-nickel smelter, a metallurgical plant, a pulp mill, and a lead mine. Fecal samples from 191 broods were analyzed for bacterial 16S rRNA and concentrations of 18 elements, of which nine common contaminants were selected for further analyses. Nestlings in IUAs showed higher metal exposure than those in rural sites, except near a pulp mill, with opposing results. An index describing impervious land cover was a weak predictor for most microbiota metrics. Instead, the rural vs. IUA comparison potentially caught the environmental characteristics better, showing effects on the fledgling number, body mass, microbial composition, and abundances of several taxa, though these patterns were location dependent and may reflect secondary effects of pollution, like changes in habitat quality and diet. Taxa with links to both metal levels and nestling performance were identified. Despite reduced emissions in Europe, wild birds remain exposed to metal pollution, particularly near industrial areas. Overall, our findings suggest that anthropogenic influence associates with wildlife microbiomes and health in a context-dependent manner.

Air pollutants, such as particulate matter, are known to contribute to disease in humans, but their impact on wildlife remains understudied. A contributing factor to this health impact is the fraction of inhaled particles depositing in the respiratory system. Compared to humans, birds are equipped with a very different respiratory system, which may render them more or less susceptible to air pollution. Although evidence for adverse pollution effects in birds has emerged, little is known about particle dynamics inside the avian lungs. To elucidate this, we exposed zebra finches (Taeniopygia guttata) to ambient-like ultrafine model particles under controlled conditions and quantified deposition in the lungs and, in a smaller subset, also the heart, liver and red blood cells. Birds were exposed to 50 or 100 nm particles at air temperatures of 5 or 25 °C, all of which fall within environmentally relevant exposure conditions. Lung deposition was highest for the smallest particles, in line with the size dependence of particle diffusion rate. While deposited fraction in the lung remained unaffected, deposited dose rate (DR) increased at the lower temperature, indicating that the effect was driven solely by elevated minute ventilation (oxygen consumption). Particle clearance measurements 2 weeks after exposure revealed that 44% of the initially deposited 100 nm particles remained in the avian lungs. We found no evidence that particles translocated to the heart, liver or red blood cells in the analyzed subsets. These findings provide an ecologically relevant foundation for understanding particle exposure dynamics in an avian model and support future work assessing health effects of particulate pollution in birds.

Telomere length is an integrative marker of cumulative physiological stress, yet the metabolic mechanisms linking environmental conditions to telomere dynamics in wild birds remain largely unknown. Here, we tested whether plasma metabolomic profiles predict relative telomere length (RTL) in free-living great tits (Parus major), and whether these associations are modulated by age class and urban versus forest habitat. We profiled 85 targeted metabolites using liquid chromatography-mass spectrometry in 378 adults sampled across replicated urban and forest sites in southern Sweden during both breeding and non-breeding seasons. Using pathway-level principal component analyses, we tested associations between seven metabolic pathways (methionine and betaine metabolism, amino acid catabolism and the urea cycle, fatty acid and carnitine metabolism, purine and pyrimidine metabolism, and histidine metabolism) and RTL measured by qPCR. During the breeding season, RTL was significantly associated with metabolic variation across all seven pathways, but these associations were consistently reversed between first-year (2 yr) and older (3+ yr) birds. In older individuals, higher levels of amino acid catabolism end-products (arginine, urate, glutamate), methionine cycle intermediates, and purine degradation products were associated with shorter telomeres, suggesting an elevated oxidative burden and declining cellular maintenance capacity with reproductive experience. In younger birds, these same metabolites were positively associated with RTL, possibly reflecting ample antioxidant reserves during a first breeding attempt. Habitat type did not modulate any metabolite–RTL relationship, and no significant associations were detected outside the breeding season. These findings reveal age-structured metabolic correlates of telomere maintenance, specifically during reproduction, advancing our understanding of how life-history trade-offs are expressed at the biochemical level in a wild songbird.

Avian haemosporidian parasites are globally widespread with a broad repertoire of hosts. When infected, the host can either reduce (resistance) and/or limit the severity of parasitaemia (tolerance). Oxidative stress plays a pivotal role in the host's resistance and tolerance, as well as its detrimental endpoints. The rationale behind this paradox lies in the dual role of reactive oxygen species (ROS): they are both beneficial and detrimental for the host, while being harmful to the parasite. Thus, it is in the parasite's interest to maintain a reduced environment within the host's cell, whereas the host needs a fine‐tuned balance between generating ROS to eliminate the parasites and maintaining sufficient antioxidant levels to protect itself. This dynamic we refer to as the host–parasite oxidative arms race. Here, Eurasian siskins Spinus spinus were experimentally infected with Plasmodium ashfordi to investigate how the fundamental antioxidant system – the glutathione system – responds to infection over time compared to control birds. By combining physiological and gene expression data from both the parasite and the host at different time points, we provide evidence for this oxidative arms race. The gene expression data show that the parasite aims to eliminate ROS through its high expression of superoxide dismutase (SOD), glutathione reductase (GR), and glutathione synthetase. In contrast, the host upregulates glutathione S‐transferases (GSTs) and glutathione peroxidases (GPX), which may result in the reduced physiological levels of glutathione seen at the end of the experiment. Although the parasite seems to win the race in terms of the oxidative state of the cell, the marked decrease in parasitaemia from day 21 (44%) to day 31 (15%) suggests that the host's strategy is sufficient to defeat the parasite. Future studies should include measures of oxidative damage to reveal whether there are any long‐term costs related to the host's strategy at different time points of infection.

The capacity for birds to adjust their breeding time to variation in spring temperature via plasticity is crucial for insectivorous passerines in temperate areas, particularly in a context of climate disruptions. Recent studies suggested that phenological plasticity varies in response to environmental change via urbanization. We investigated the effects of urbanization on laying date, its phenotypic plasticity in response to spring temperature, and the between-individual variation in laying date, using data from five long-term studies of European great tits, Parus major, in forest and urban areas. First, we compared laying phenology and its plasticity in response to spring temperature between urban and forest populations. We confirmed that birds lay eggs earlier in urban environments in four populations and revealed reduced phenological plasticity in more urbanized environments in two populations. Additionally, we demonstrated greater between-individual variation in laying date in two urban areas. Second, we focused on urban populations only, and showed that the proportion of impervious surface area had little effect on the laying date. Overall, urbanization was associated with earlier breeding and less plasticity, although the strength of these associations varied among cities, likely owing to variation in the intensity of urbanization, landscape connectivity and habitat composition.

Bird embryos develop inside eggs, which contain maternal substances that can shape offspring phenotype and fitness. Yolk fatty acids are a key energy source for the developing embryo, with omega‐6 (ω‐6) and omega‐3 (ω‐3) polyunsaturated fatty acids (PUFAs) obtained exclusively from the diet. Therefore, food availability during the breeding season is expected to influence yolk fatty acid composition and, consequently, embryo development and growth. However, the effects of dietary variation on yolk fatty acids remain unexplored in wild birds. We investigated interannual variation in yolk fatty acid composition of free‐living great tits Parus major in relation to fluctuations in beech Fagus sylvatica fructification – their preferred food – across two years differing strongly in seed abundance. We hypothesized that differences in beech seed availability would alter yolk fatty acid composition between years of high and low fructification, with corresponding correlations to the fatty acid profiles of available seeds. Hence, we analyzed fatty acids from seeds of beech and two conifers (Picea abies and Pinus sylvestris), which account for most of the tree cover in the study area, and from 112 eggs collected from 107 nests over two years. Beech seeds contained higher proportions of saturated (SFA), monounsaturated (MUFA), and ω‐3 PUFAs, and lower proportions of ω‐6 PUFAs than conifer seeds. Correspondingly, egg yolks had more SFA, MUFA, and ω‐3 PUFAs in the year of high beech abundance, and more ω‐6 PUFAs in the year of low abundance. Notably, the proportion of the conifer‐associated ω‐6 PUFA pinolenic acid was 64.25 times higher in years of low compared to high beech fructification, suggesting marked interannual variation in yolk fatty acid composition, likely reflecting shifts in maternal diet. Future experiments need to establish causal links between diet and yolk composition and assess the fitness consequences of different seed resources for breeding females and their offspring.

Cross-system fluxes of aquatic insects rich in omega-3 long-chain polyunsaturated fatty acids (ω-3 LC-PUFAs), specifically eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), may subsidise insectivorous vertebrates that are suffering due to terrestrial insect declines. The benefits of high ω-3 LC-PUFA diets, such as improved growth and immunocompetence, have previously been demonstrated for riparian insectivores. Understanding the potential for aquatic insects to benefit a range of species is necessary for informing land management practices. Using the spotted flycatcher (Muscicapa striata), a habitat-generalist, insectivorous bird as a model, we tested how blood plasma ω-3 LC-PUFA concentrations and the ratio of ω-6:ω-3 fatty acids were related to the body condition of 14 adults and 84 chicks in the wild. We measured how variation in ω-3 LC-PUFAs and ω-6:ω-3 was related to river proximity, to test whether they were related to aquatic insect availability. We assessed how these relationships may extend beyond the individual level by monitoring flying insect availability throughout two breeding seasons and testing its ability to predict reproductive success. EPA was positively correlated with body condition in chicks and adults and declined with distance from a river. Conversely, ω-6:ω-3 was negatively correlated with body condition and increased with distance from a river. Breeding success was positively correlated with aquatic insect availability, suggesting that access to a high ω-3 LC-PUFA diet has the potential to mitigate declines in generalist insectivores. Our results highlight how the maintenance or creation of high-quality freshwater habitats may be promoted in a conservation context for non-riparian species.

The human influence on Earth's ecosystems is omnipresent. Artificial light at night (ALAN), anthropogenic noise, and air pollution are inherent features of human activities and infrastructure and pose novel environmental challenges to urban‐dwelling wildlife. So far, most of the studies investigating the impacts of exposure to urban pollutants on animals have either investigated the effects of urban environments per se or of single pollutants. However, urban pollutants co‐occur, and interactive effects may arise when acting in combination, but we lack a deeper understanding of the effects of combined exposures. Here, we experimentally exposed captive zebra finches Taeniopygia guttata in a full‐factorial design to increased levels of ALAN, anthropogenic noise and/or soot and measured oxidative stress status in blood before and after a 5‐day exposure. We found that the combined exposure to ALAN and noise led to a positive synergistic effect (higher levels than the sum of individual effects) on the antioxidant glutathione and a negative synergistic effect (lower levels than the sum of individual effects) on the levels of oxidative damage, measured as the concentration of reactive oxygen metabolites. Soot had no effect on the avian oxidative stress status in the blood immediately after the exposure, neither singly nor in combination with other pollutants. To conclude, our results indicate that a combination of stressors can have complex non‐additive interactive effects on oxidative stress status after a short‐term exposure. Surprisingly, a combined exposure to ALAN and anthropogenic noise leads to a stronger antioxidant response that seems to prevent oxidative damage than exposure to only one of the stressors. Whether the increased antioxidant defence entails any long‐term costs remains to be determined in future studies. Read the free Plain Language Summary for this article on the Journal blog.

The loss of habitat heterogeneity due to agricultural intensification has led to a global decline in farmland birds. Among them is the Eurasian Kestrel Falco tinnunculus, which occupies high trophic levels and may be adversely affected by reduced food quantity or quality and consequent health impacts. In this study, we investigate the effects of habitat heterogeneity, individual heterozygosity and diet diversity on five different health indices (integument coloration, dietary antioxidants, haematocrit, body condition and parasite infection). The study was conducted in farmland areas of western Finland during a year of exceptionally low vole abundance. We found no obvious relationship between diet diversity and habitat heterogeneity. An interaction between diet diversity and individual heterozygosity in females suggested that diet specialists were able to maintain more intensely coloured integuments only if they had higher genetic diversity. In addition, more heterozygous females were less likely to be infected with Haemoproteus than females with lower individual genetic diversity. Finally, specialist males with lower diet diversity had higher body condition than males with a more generalist diet. Our results suggest that variation in individual quality and foraging ecology should be considered in conjunction with spatial variation in habitat heterogeneity to understand sex-specific variation in kestrel health. These findings add to a better understanding of the mechanisms linking land-use change to health indices in a common avian predator, which can be used as a health sentinel in European agroecosystems.

While organisms have evolved to cope with predictable changes in the environment, the rapid rate of current global change presents numerous novel and unpredictable stressors to which organisms have had less time to adapt. To persist in the urban environment, organisms must modify their physiology, morphology and behaviour accordingly. Metabolomics offers great potential for characterising organismal responses to natural and anthropogenic stressors at the systems level and can be applied to any species, even without genomic knowledge. Using metabolomic profiling of blood, we investigated how two closely related species of passerine bird respond to the urban environment. Great tits Parus major and blue tits Cyanistes caeruleus residing in urban and forest habitats were sampled during the breeding (spring) and non-breeding (winter) seasons across replicated sites in southern Sweden. During breeding, differences in the plasma metabolome between urban and forest birds were characterised by higher levels of amino acids in urban-dwelling tits and higher levels of fatty acyls in forest-dwelling tits. The suggested higher rates of fatty acid oxidation in forest tits could be driven by habitat-associated differences in diet and could explain the higher reproductive investment and success of forest tits. High levels of amino acids in breeding urban tits could reflect the lack of lipid-rich caterpillars in the urban environment and a dietary switch to protein-rich spiders, which could be of benefit for tackling inflammation and oxidative stress associated with pollution. In winter, metabolomic profiles indicated lower overall levels of amino acids and fatty acyls in urban tits, which could reflect relaxed energetic demands in the urban environment. Our metabolomic profiling of two urban-adapted species suggests that their metabolism is modified by urban living, though whether these changes represent adaptative or non-adaptive mechanisms to cope with anthropogenic challenges remains to be determined.

Anthropogenic changes to the environment expose wildlife to many pollutants. Among these, tropospheric ozone is of global concern and a highly potent pro-oxidant. In addition, human activities include several other implications for wildlife, e.g., changed food availability and changed distribution of pathogens in cities. These co-occurring habitat changes may interact, thereby modulating the physiological responses and costs related to anthropogenic change. For instance, many food items associated with humans (e.g., food waste and feeders for wild birds) contain relatively more ω6-than ω3-polyunsaturated fatty acids (PUFAs). Metabolites derived from ω6-PUFAs can enhance inflammation and oxidative stress towards a stimulus, whereas the opposite response is linked to ω3-derived metabolites. Hence, we hypothesized that differential intake of ω6-and ω3-PUFAs modulates the oxidative stress state of birds and thereby affects the responses towards pro-oxidants. To test this, we manipulated dietary ω6:ω3 ratios and ozone levels in a full-factorial experiment using captive zebra finches (Taeniopygia guttata). Additionally, we simulated an infection, thereby also triggering the immune system’s adaptive pro-oxidant release (i.e., oxidative burst), by injecting lipopolysaccharide. Under normal air conditions, the ω3-diet birds had a lower antioxidant ratio (GSH/GSSG ratio) compared to the ω6-diet birds. When exposed to ozone, however, the diet effect disappeared. Instead, ozone exposure overall reduced the total concentration of the key antioxidant glutathione (tGSH). Moreover, the birds on the ω6-rich diet had an overall higher antioxidant capacity (OXY) compared to birds fed a ω3-rich diet. Interestingly, only the immune challenge increased oxidative damage, suggesting the oxidative burst of the immune system overrides the other pro-oxidative processes, including diet. Taken together, our results show that ozone, dietary PUFAs, and infection all affect the redox-system, but in different ways, suggesting that the underlying responses are decoupled despite that they all increase pro-oxidant exposure or generation. Despite lack of apparent cumulative effect in the independent biomarkers, the combined single effects could together reduce overall cellular functioning and efficiency over time in wild birds exposed to pathogens, ozone, and anthropogenic food sources.

Identifying key molecular pathways and genes involved in the response to urban pollutants is an important step in furthering our understanding of the impact of urbanisation on wildlife. The expansion of urban habitats and the associated human-introduced environmental changes are considered a global threat to the health and persistence of humans and wildlife. The present study experimentally investigates how short-term exposure to three urban-related pollutants -soot, artificial light at night (ALAN) and traffic noise-affects transcriptome-wide gene expression in livers from captive female zebra finches (Taeniopygia guttata). Compared to unexposed controls, 17, 52, and 28 genes were differentially expressed in soot, ALAN and noise-exposed birds, respectively. In soot-exposed birds, the enriched gene ontology (GO) terms were associated with a suppressed immune system such as interferon regulating genes (IRGs) and responses to external stimuli. For ALAN-exposed birds, enriched GO terms were instead based on downregulated genes associated with detoxification, redox, hormonal-, and metabolic processes. Noise exposure resulted in downregulation of genes associated with the GO terms: cellular responses to substances, catabolic and cytokine responses. Among the individually differentially expressed genes (DEGs), soot led to an increased expression of genes related to tumour progression. Likewise, ALAN revealed an upregulation of multiple genes linked to different cancer types. Both sensory pollutants (ALAN and noise) led to increased expression of genes linked to neuronal function. Interestingly, noise caused upregulation of genes associated with serotonin regulation and function (SLC6A4 and HTR7), which previous studies have shown to be under selection in urban birds. These outcomes indicate that short-term exposure to the three urban pollutants perturbate the liver transcriptome, but most often in different ways, which highlights future studies of multiple-stress exposure and their interactive effects, along with their long-term impacts for urban-dwelling wildlife.

Abstract Children's interactions with nature are important mediators of health benefits and future relationships with nature and conservation. However, there are growing concerns that children are becoming disconnected from nature as societal changes make natural environments and their affordances less accessible. There is now a need to explore ways to reconnect children to nature. We evaluated bird feeding as an environmental education intervention on species knowledge, attitudes towards birds and well‐being of children (aged 10–11 years) in 14 urban and rural schools across three city regions. We used surveys to investigate the role of urbanisation and socio‐economic factors in children's relationships with nature and possible modulating effects on the intervention response. Local nature, close to the children's homes, was associated with better well‐being and modulated the effect of the intervention on species knowledge: children from areas with less local nature gained more from the project. Urbanisation per se had no direct impacts but did correlate with the types of affordances associated with nature by the children, suggesting differences in how urban and rural children utilise nature. We found that socio‐economic factors impacted pre‐existing species knowledge and attitudes towards nature. Higher education was associated with better species knowledge. Higher income correlated with children participating in more outdoor activities, which subsequently correlated with more positive attitudes towards birds. Species knowledge also correlated with positive attitudes. We found large differences in the intervention impact between schools, likely explained by a pivotal role of teachers facilitating nature interactions in school‐based projects. Our findings show that bird feeding has potential as an intervention to connect children with nature, but ecological and social contexts moderate its efficacy. Local greenspace and socio‐economics influenced children's relationships with nature in both urban and rural areas, indicating that local conditions rather than urbanisation levels govern connection to nature. Role models appeared crucial, suggesting that specialised environmental education organisations have significant potential to enhance nature connection through interventions, particularly in areas with low socio‐economic levels and sparse nature. Read the free Plain Language Summary for this article on the Journal blog.

Urbanisation is one of the biggest environmental challenges of our time, yet we still lack an integrative understanding of how cities affect behaviour, physiology and parasite susceptibility of free-living organisms. In this study, we focus on carotenoids, strictly dietary micronutrients that can either be used as yellow-red pigments, for integument colouration (signalling function), or as antioxidants, to strengthen the immune system (physiological function) in an urban predator, the Eurasian kestrel (Falco tinnunculus). Kestrels are specialised vole hunters but shift to avian prey in cities where diurnal rodents are not sufficiently available. This different foraging strategy might determine the quantity of carotenoids available. We measured integument colouration, circulating carotenoids in the blood and ectoparasite burden in kestrels along an urban gradient. Our results showed that nestlings that were raised in more urbanised areas displayed, unrelated to their ectoparasite burden, a paler integument colouration. Paler colours were furthermore associated with a lower concentration of circulating carotenoids. These findings support the hypothesis that the entire urban food web is carotenoid deprived and only prey of low quality with low carotenoid content is available (e.g. fewer carotenoids in urban trees, insects, small birds and finally kestrels). The alternative hypothesis that nestlings allocate carotenoids to reduce physiological stress and/or to cope with parasites rather than invest into colouration could not be supported. Our study adds to existing evidence that urban stressors negatively affect carotenoid production in urban areas, a deficiency that dissipate into higher trophic levels.

Climate change and urbanisation are among the most pervasive and rapidly growing threats to biodiversity worldwide. However, their impacts are usually considered in isolation, and interactions are rarely examined. Predicting species' responses to the combined effects of climate change and urbanisation, therefore, represents a pressing challenge in global change biology. Birds are important model taxa for exploring the impacts of both climate change and urbanisation, and their behaviour and physiology have been well studied in urban and non-urban systems. This understanding should allow interactive effects of rising temperatures and urbanisation to be inferred, yet considerations of these interactions are almost entirely lacking from empirical research. Here, we synthesise our current understanding of the potential mechanisms that could affect how species respond to the combined effects of rising temperatures and urbanisation, with a focus on avian taxa. We discuss potential interactive effects to motivate future in-depth research on this critically important, yet overlooked, aspect of global change biology. Increased temperatures are a pronounced consequence of both urbanisation (through the urban heat island effect) and climate change. The biological impact of this warming in urban and non-urban systems will likely differ in magnitude and direction when interacting with other factors that typically vary between these habitats, such as resource availability (e.g. water, food and microsites) and pollution levels. Furthermore, the nature of such interactions may differ for cities situated in different climate types, for example, tropical, arid, temperate, continental and polar. Within this article, we highlight the potential for interactive effects of climate and urban drivers on the mechanistic responses of birds, identify knowledge gaps and propose promising future research avenues. A deeper understanding of the behavioural and physiological mechanisms mediating species' responses to urbanisation and rising temperatures will provide novel insights into ecology and evolution under global change and may help better predict future population responses.