Great Tit Nestlings Research Articles

Early-life diet composition affects phenotypic variation of correlated animal personality traits.

Behavioural traits are under both genetic and environmental influence during early life stages. Early environmental conditions related to the amount and type of food have been found to alter behaviour in many organisms. However, how early life diet affects the variation in and the correlation between behavioural traits is largely unknown. Using a multivariate approach, we investigated how variation in parental prey selection is related to three repeatable nestling personality traits, and explored the within and between-individual covariation between these behaviours in a wild passerine, the great tit (Parus major). Our results confirm that breath rate, docility and handling aggression (HA) in great tit nestlings are repeatable traits. Contrary to our expectation, the three nestling personality traits did not form a behavioural 'syndrome' on the phenotypic level in the study population, but we found two of three expected phenotypic correlations, mostly at the within-individual level. Moreover, we found that breath rate significantly decreased with a higher number of spiders in the diet, and docility and handling aggression were significantly and inversely related to higher numbers of noctuids and tortricids in the diets of individuals within broods. Thus, our findings suggest that provisioning quantity and quality during the early life, affects variation in behavioural phenotypes, which occurs mainly at the within-individual level.

Early developmental carry-over effects on exploratory behaviour and DNA methylation in wild great tits (Parus major).

Adverse, postnatal conditions experienced during development are known to induce lingering effects on morphology, behaviour, reproduction and survival. Despite the importance of early developmental stress for shaping the adult phenotype, it is largely unknown which molecular mechanisms allow for the induction and maintenance of such phenotypic effects once the early environmental conditions are released. Here we aimed to investigate whether lasting early developmental phenotypic changes are associated with post-developmental DNA methylation changes. We used a cross-foster and brood size experiment in great tit (Parus major) nestlings, which induced post-fledging effects on biometric measures and exploratory behaviour, a validated personality trait. We investigated whether these post-fledging effects are associated with DNA methylation levels of CpG sites in erythrocyte DNA. Individuals raised in enlarged broods caught up on their developmental delay after reaching independence and became more explorative as days since fledging passed, while the exploratory scores of individuals that were raised in reduced broods remained stable. Although we previously found that brood enlargement hardly affected the pre-fledging methylation levels, we found 420 CpG sites that were differentially methylated between fledged individuals that were raised in small versus large sized broods. A considerable number of the affected CpG sites were located in or near genes involved in metabolism, growth, behaviour and cognition. Since the biological functions of these genes line up with the observed post-fledging phenotypic effects of brood size, our results suggest that DNA methylation provides organisms the opportunity to modulate their condition once the environmental conditions allow it. In conclusion, this study shows that nutritional stress imposed by enlarged brood size during early development associates with variation in DNA methylation later in life. We propose that treatment-associated DNA methylation differences may arise in relation to pre- or post-fledging phenotypic changes, rather than that they are directly induced by the environment during early development.

Plumage coloration differs between offspring raised in natural cavities and nestboxes

Abstract Most of our knowledge of secondary hole-nesting birds comes from populations breeding in human-provided nestboxes, yet these might differ from natural cavities in a number of parameters, including internal dimensions or microclimate, leading to differences in reproductive ecology. Here, we demonstrate differences in plumage coloration, an important visual signal of individual quality, in Blue Tit (Cyanistes caeruleus) and Great Tit (Parus major) nestlings raised in natural cavities and nestboxes. For this study, we collected feather samples over two breeding seasons and applied reflectance spectrophotometry to calculate color metrics. Carotenoid chroma was higher in nestbox-reared Blue Tits, whereas brightness was higher in nestbox-reared Great Tits (with a similar tendency for Blue Tits). The latter result might be explained by the better nutritional condition of Great Tit nestlings raised in nestboxes. Furthermore, we found no evidence for preference of adults expressing more elaborate plumage towards a specific cavity type in either species. Consequently, we assume that differences in nestling plumage reported here are driven by rearing conditions (nestboxes vs. natural cavities) and not by differences in plumage-based parental quality. Our study adds to the growing body of evidence confirming that anthropogenic environmental modifiers, such as nestboxes, might influence avian physiology and the resulting phenotype.

The mercury flow through a terrestrial songbird food chain in subtropical pine forest: Elucidated by Bayesian isotope mixing model and stable mercury isotopes

In order to comprehend the transfer of inorganic mercury (IHg) and methylmercury (MeHg) within food chains in terrestrial pine forests, we collected samples of Great Tit nestlings, common invertebrates, plants, and soil in a subtropical pine forest and used Bayesian isotope mixing model analysis, Hg daily intake, and stable Hg isotopes to elucidate the flow of MeHg and IHg in these food chains. Results indicate that caterpillars and cockroaches are the predominant prey items for nestlings, accounting for a combined contribution of 81.5%. Furthermore, caterpillars, cockroaches, and spiders were found to contribute the most (∼80%) of both IHg and MeHg that dietary accumulated in nestlings. The provisoned invertebrates tend to supply more IHg and diluting the proportion of MeHg as total Hg (MeHg%). Notably, nestling feathers displayed the highest Δ199Hg values but a relatively lower MeHg%, suggesting an imbalanced incorporation of Hg from maternal transfer and dietary accumulation during the nestling stage. This study highlights the efficacy of nestlings as indicators for identifying Hg sources and transfers in avian species and food chains. However, caution must be exercised when using Hg isotope compositions in growing feathers, and the contribution of maternally transferred Hg should not be ignored.

Developmental stress does not induce genome-wide DNA methylation changes in wild great tit (Parus major) nestlings.

The environment experienced during early life is a crucial factor in the life of many organisms. This early life environment has been shown to have profound effects on morphology, physiology and fitness. However, the molecular mechanisms that mediate these effects are largely unknown, even though they are essential for our understanding of the processes that induce phenotypic variation in natural populations. DNA methylation is an epigenetic mechanism that has been suggested to explain such environmentally induced phenotypic changes early in life. To investigate whether DNA methylation changes are associated with experimentally induced early developmental effects, we cross-fostered great tit (Parus major) nestlings and manipulated their brood sizes in a natural study population. We assessed experimental brood size effects on pre-fledging biometry and behaviour. We linked this to genome-wide DNA methylation levels of CpG sites in erythrocyte DNA, using 122 individuals and an improved epiGBS2 laboratory protocol. Brood enlargement caused developmental stress and negatively affected nestling condition, predominantly during the second half of the breeding season, when conditions are harsher. Brood enlargement, however, affected nestling DNA methylation in only one CpG site and only if the hatch date was taken into account. In conclusion, this study shows that nutritional stress in enlarged broods does not associate with direct effects on genome-wide DNA methylation. Future studies should assess whether genome-wide DNA methylation variation may arise later in life as a consequence of phenotypic changes during early development.

Microsatellite Instability Assay as a Potential Approach to Evaluate Genotoxicity: Lead Exposure in a Nestling Passerine Bird at the Stage of Intensive Erythropoiesis.

Although many avian studies have investigated the toxic effects of lead on important biochemical and physiological processes, organ and system function, and behavior, studies evaluating the specific genotoxic effects of exposure to lead are scarce. Nowadays, rapid technological advances can provide new molecular techniques in this regard. In this study, as a novel approach in bird studies, we used a panel of ten microsatellite loci to investigate the microsatellite instability (MSI) in response to experimental lead intoxication in a common hole-nesting species, the great tit Parus major. For this purpose, an experiment based on an intentional single supplementation of a lead (II) acetate trihydrate compound was conducted, with the use of two different doses, applied to randomly chosen great tit nestlings from randomly selected broods, being at the stage of intensive erythropoiesis. Although this preliminary study did not find any MSI in the seven microsatellite markers retained for the final comparison, it contributes to the examination of this molecular technique in field conditions as being potentially applicable in ecotoxicological bird studies. We believe that certain issues should be considered in finding an explanation for our result. First, the single doses of lead used in this study may have been too weak to induce genetic instability. Second, the panel of microsatellite markers studied may have been unsusceptible to lead genotoxicity in general. Third, the relatively short time interval (5 days) between the experimental procedure (lead exposure) and the sampling of post-exposure material (blood) for genetic analyses could have limited the effect of lead genotoxicity. Further analyzes are needed to verify these findings and to evaluate the scope of application of the MSI analysis in wild bird population studies.

No evidence for associations between brood size, gut microbiome diversity and survival in great tit (Parus major) nestlings

BackgroundThe gut microbiome forms at an early stage, yet data on the environmental factors influencing the development of wild avian microbiomes is limited. As the gut microbiome is a vital part of organismal health, it is important to understand how it may connect to host performance. The early studies with wild gut microbiome have shown that the rearing environment may be of importance in gut microbiome formation, yet the results vary across taxa, and the effects of specific environmental factors have not been characterized. Here, wild great tit (Parus major) broods were manipulated to either reduce or enlarge the original brood soon after hatching. We investigated if brood size was associated with nestling bacterial gut microbiome, and whether gut microbiome diversity predicted survival. Fecal samples were collected at mid-nestling stage and sequenced with the 16S rRNA gene amplicon sequencing, and nestling growth and survival were measured.ResultsGut microbiome diversity showed high variation between individuals, but this variation was not significantly explained by brood size or body mass. Additionally, we did not find a significant effect of brood size on body mass or gut microbiome composition. We also demonstrated that early handling had no impact on nestling performance or gut microbiome. Furthermore, we found no significant association between gut microbiome diversity and short-term (survival to fledging) or mid-term (apparent juvenile) survival.ConclusionsWe found no clear association between early-life environment, offspring condition and gut microbiome. This suggests that brood size is not a significantly contributing factor to great tit nestling condition, and that other environmental and genetic factors may be more strongly linked to offspring condition and gut microbiome. Future studies should expand into other early-life environmental factors e.g., diet composition and quality, and parental influences.

The effect of daylight exposure on the immune response and body condition of Great Tit nestlings

Natural light plays an important role in many aspects of birds’ biology, ecology and behaviour. Wild birds experience various light conditions, both in terms of its intensity and spectral properties, which vary within the nesting habitat of avian species. Recent study has shown that some bird species prefer to build their nests in bright nest sites. Light is one of the major factors affecting early development and condition of nestlings, but to date, this effect has not been tested in wild conditions. This study aims to answer whether birds could benefit from nesting in brighter light environment. Potential benefits may occur in nestlings’ condition, growth rate and breeding output. The research was carried out in the population of Great Tits (Parus major) breeding in two types nest boxes which differed with internal illumination. In “dark” boxes, the only source of light was the entrance hole. “Bright” boxes have additional side semi-transparent windows. We conducted two experiments in which light environment in nest box was set after (Experiment I) or before (Experiment II) birds settle in. Our study showed that light environment did not significantly affect nestlings body condition and growth. However, females in their first breeding attempt showed strong preference to the brightened nest boxes. In this group, the offspring were heavier and showed stronger immune responses than nestlings reared in dark nest boxes. Nestlings in bright nest boxes fledged about one day earlier. Nesting in brightened nest boxes may provide potential benefit for adult females in their first breeding attempt. The interaction between maternal effect and light environment is being discussed.

Tree Species Diversity and Forest Edge Density Jointly Shape the Gut Microbiota Composition in Juvenile Great Tits (Parus major).

Despite the microbiome’s key role in health and fitness, little is known about the environmental factors shaping the gut microbiome of wild birds. With habitat fragmentation being recognised as a major threat to biological diversity, we here determined how forest structure influences the bacterial species richness and diversity of wild great tit nestlings (Parus major). Using an Illumina metabarcoding approach which amplifies the 16S bacterial ribosomal RNA gene, we measured gut microbiota diversity and composition from 49 great tit nestlings, originating from 23 different nests that were located in 22 different study plots across a gradient of forest fragmentation and tree species diversity. Per nest, an average microbiome was determined on which the influence of tree species (composition and richness) and forest fragmentation (fragment area and edge density) was examined and whether this was linked to host characteristics (body condition and fledging success). We found an interaction effect of edge density with tree species richness or composition on both the microbial richness (alpha diversity: Chao1 and Shannon) and community structure (beta diversity: weighted and unweighted UniFrac). No significant short-term impact was observed of the overall faecal microbiome on host characteristics, but rather an adverse effect of specific bacterial genera on fledging success. These results highlight the influence of environmental factors on the microbial richness as well as the phylogenetic diversity during a life stage where the birds’ microbiota is shaped, which could lead to long-term consequences for host fitness.

Biomarkers of oxidative stress, metabolic processes, and lysosomal activity in the muscle tissue of the great tit (Parus major) living in sodium industry and agricultural areas in Inowrocław region (central part of northern Poland)

An region of ecological hazard (EHR) is an area where, as a result of intensive human economic activity, degradation of components of the natural environment has taken place, leading to the deterioration of the ecological balance. EHR management is a globalised ever-increasing challenge. To eliminate the hazardous effect of these pollutants, research has been accelerated worldwide. The current study analyzed the specific biomarkers of the lipid and protein oxidation, total antioxidative status, activities of antioxidant enzymes (superoxide dismutase, catalase, glutathione reductase and peroxidase), lysosomal enzymes (alanyl aminopeptidase, leucyl aminopeptidase, acid phosphatase), and biomarkers of aerobic and anaerobic metabolic pathways (activities of alanine and aspartate aminotransferases, succinate dehydrogenase, lactate dehydrogenase, lactate and pyruvate levels) in skeletal muscle tissue of wild great tit nestlings in environments with different levels of anthropogenic pressure such as sodium industry and agriculture in the central part of northern Poland (Inowrocław EHR). Control samples were collected from Tuchola Forest and Borkowo village, where no direct sources of contamination were found. The relevance of the study was to assess the changes in lysosomal functioning caused by pollution-induced oxidative stress that may indicate multidirectional adaptative mechanisms of metabolic processes occurring in the wild birds to compensate for the negative effects of contamination. It was shown that the initiation of oxidative stress caused by anthropogenic pollution shifted the balance of the normal functioning of lysosomal enzymes towards their increased activity. A general tendency towards an increase in the intensity of lipid peroxidation processes with an increasing level of oxidatively modified proteins (aldehydic and ketonic derivatives) and a simultaneous reduction in the TAS was observed in the muscle tissue of great tits living in the anthropogenically modified areas. The intensity of lipid peroxidation and protein damage caused changes and reorganization of the energy-related metabolic pathways in the muscle tissue of wild great tits living in the sodium industry and agricultural areas of the EHR.

Great tits feed their nestlings with more but smaller prey items and fewer caterpillars in cities than in forests

Rapidly increasing urbanisation is one of the most significant anthropogenic environmental changes which can affect demographic traits of animal populations, for example resulting in reduced reproductive success. The food limitation hypothesis suggests that the shortage of high-quality nestling food in cities is a major factor responsible for the reduced reproductive performance in insectivorous birds. To study this explanation, we collected data on the parental provisioning behaviour of urban and forest great tits (Parus major) in three years that varied both in caterpillar availability (the main food of great tit nestlings) and in reproductive success of the birds. In all years, urban parents provisioned caterpillars in a smaller proportion to their nestlings, but the total amount of food per nestling (estimated by the volumes of all prey items) did not differ between habitats. In the two years with much lower reproductive success in urban than forest habitats, urban parents had higher provisioning rates, but provided more non-arthropod food and brought smaller prey items than forest parents. In the year with reduced habitat difference in reproductive success, urban parents were able to compensate for the scarcity of caterpillars by provisioning other arthropods rather than non-arthropod food, and by delivering larger preys than in the other years. Specifically, in this latter year, caterpillars provisioned by urban pairs were cc. twice as large as in the other two years, and were similar in size to caterpillars provisioned in the forest broods. These results show that although urban great tit parents can provide the same quantity of food per nestling as forest parents by reducing their brood size and increasing the per capita feeding rates for nestlings, they cannot compensate fully for the scarcity of high-quality preys (caterpillars) in poor years. In some years, however, favourable conditions for urban caterpillar development can greatly reduce food limitation in cities, allowing urban birds to achieve higher reproductive success. We suggest that urban green areas designed and managed in a way to facilitate conditions for phytophagous arthropods could improve habitat quality for urban birds.

A comparison of the diet of urban and forest great tits in a Mediterranean habitat

The low breeding performance and body condition of nestling passerine birds in urban environments has been attributed to the poor quality and low abundance of food in these settings. However, detailed data on prey provided by parents to their chicks in the urban habitat is scarce. Here we used video cameras set in nest boxes to compare the diet of urban and forest great tits Parus major when provisioning their chicks in a Mediterranean area. We additionally analysed brood size and fledgling success. Breeding success of urban great tits was lower than that of forest birds. Urban parents displayed a lower average hourly feeding rate per nestling than forest parents. Among the three prey item categories, the percentage of spiders did not vary according to habitat. However, the percentage of caterpillars delivered to the nest by great tit parents was higher in the forest than in the urban habitat while the percentage of ‘other’ prey showed a reverse pattern. ‘Other’ prey were mainly adult butterflies and wasps in the urban habitat. Our paper adds to the view that the low feeding rates and scarcity of caterpillars in urban environments may be the underlying cause constraining the growth of great tit nestlings in these areas.

Exposure to artificial light at night alters innate immune response in wild great tit nestlings.

ABSTRACTThe large-scale impact of urbanization on wildlife is rather well documented; however, the mechanisms underlying the effects of urban environments on animal physiology and behaviour are still poorly understood. Here, we focused on one major urban pollutant – artificial light at night (ALAN) – and its effects on the capacity to mount an innate immune response in wild great tit (Parus major) nestlings. Exposure to ALAN alters circadian rhythms of physiological processes, by disrupting the nocturnal production of the hormone melatonin. Nestlings were exposed to a light source emitting 3 lx for seven consecutive nights. Subsequently, nestlings were immune challenged with a lipopolysaccharide injection, and we measured haptoglobin and nitric oxide levels pre- and post-injection. Both haptoglobin and nitric oxide are important markers for innate immune function. We found that ALAN exposure altered the innate immune response, with nestlings exposed to ALAN having lower haptoglobin and higher nitric oxide levels after the immune challenge compared with dark-night nestlings. Unexpectedly, nitric oxide levels were overall lower after the immune challenge than before. These effects were probably mediated by melatonin, as ALAN-treated birds had on average 49% lower melatonin levels than the dark-night birds. ALAN exposure did not have any clear effects on nestling growth. This study provides a potential physiological mechanism underlying the documented differences in immune function between urban and rural birds observed in other studies. Moreover, it gives evidence that ALAN exposure affects nestling physiology, potentially causing long-term effects on physiology and behaviour, which ultimately can affect their fitness.

Epigenetics and Early Life Stress: Experimental Brood Size Affects DNA Methylation in Great Tits (Parus major)

Early developmental conditions are known to have life-long effects on an individual’s behavior, physiology and fitness. In altricial birds, a majority of these conditions, such as the number of siblings and the amount of food provisioned, are controlled by the parents. This opens up the potential for parents to adjust the behavior and physiology of their offspring according to local post-natal circumstances. However, the mechanisms underlying such intergenerational regulation remain largely unknown. A mechanism often proposed to possibly explain how parental effects mediate consistent phenotypic change is DNA methylation. To investigate whether early life effects on offspring phenotypes are mediated by DNA methylation, we cross-fostered great tit (Parus major) nestlings and manipulated their brood size in a natural study population. We assessed genome-wide DNA methylation levels of CpG sites in erythrocyte DNA, using Reduced Representation Bisulfite Sequencing (RRBS). By comparing DNA methylation levels between biological siblings raised in enlarged and reduced broods and between biological siblings of control broods, we assessed which CpG sites were differentially methylated due to brood size. We found 32 differentially methylated sites (DMS) between siblings from enlarged and reduced broods, a larger number than in the comparison between siblings from control broods. A considerable number of these DMS were located in or near genes involved in development, growth, metabolism, behavior and cognition. Since the biological functions of these genes line up with previously found effects of brood size and food availability, it is likely that the nestlings in the enlarged broods suffered from nutritional stress. We therefore conclude that early life stress might directly affect epigenetic regulation of genes related to early life conditions. Future studies should link such experimentally induced DNA methylation changes to expression of phenotypic traits and assess whether these effects affect parental fitness to determine if such changes are also adaptive.

Exploring the faecal microbiome of the Eurasian nuthatch (Sitta europaea).

Gastrointestinal microbiota fulfill pivotal roles in providing a host with nutrition and protection from pathogenic microorganisms. Up to date, most microbiota research has focused on humans and other mammals, whereas birds and especially wild birds lag behind. Within the field of the avian gut microbiome, research is heavily biased towards poultry. In this study, we analyzed the gut microbiome of the Eurasian nuthatch (Sitta europaea), using faecal samples of eight nestlings originating from three nuthatch nests in the south of Ghent (Belgium), using Illumina sequencing of the 16S rRNA gene. Relative frequency analysis showed a dominance of Firmicutes and Actinobacteria and to a lesser extent Proteobacteria. Bacteroidetes and other phyla were relatively rare. At higher taxonomic levels, a high degree of inter-individual variation in terms of overall microbiota community structure as well as dominance of certain bacteria was observed, but with a higher similarity for the nestlings sharing the same nest. When comparing the nuthatch faecal microbiome to that of great tit nestlings that were sampled during the same breeding season and in the same forest fragment, differences in the microbial community structure were observed, revealing distinct dissimilarities in the relative abundancy of taxa between the two bird species. This study is the first report on the nuthatch microbiome and serves as a reference study for nuthatch bacterial diversity and can be used for targeted screening of the composition and general functions of the avian gut microbiome.

Sex-specific effects of experimental ectoparasite infestation on telomere length in great tit nestlings.

Telomere length is a biomarker of biological ageing and lifespan in various vertebrate taxa. Evidence is accumulating that telomeres shorten more rapidly when an individual is exposed to environmental stressors. Parasites are potent selective agents that can cause physiological stress directly or indirectly through the activation of the host's immune system. Yet to date, empirical evidence for a role of parasites in telomere dynamics in natural populations is limited. Here, we show experimentally that exposure to ectoparasitic hen fleas (Ceratophyllus gallinae) during growth results in shorter telomeres in female, but not male, great tit (Parus major) nestlings. Females had longer telomeres than males when growing up in experimentally deparasitized nests but, likely because of the sex-specific effects of ectoparasitism on telomere length, this sexual dimorphism was absent in birds growing up in experimentally infested nests. Our results provide the first experimental evidence for a role of ectoparasitism in telomere dynamics in a natural vertebrate population, and suggest that the costs of infection manifest in sex-specific ways.

Food availability limits avian reproduction in the city: An experimental study on great tits Parus major.

The altered ecological and environmental conditions in towns and cities strongly affect demographic traits of urban animal populations, for example avian reproductive success is often reduced. Previous work suggests that this is partly driven by low insect availability during the breeding season, but robust experimental evidence that supports this food limitation hypothesis is not yet available. We tested core predictions of the food limitation hypothesis using a controlled experiment that provided supplementary insect food (nutritionally enhanced mealworms supplied daily to meet 40%-50% of each supplemented brood's food requirements) to great tit nestlings in urban and forest habitats. We measured parental provisioning rates and estimated the amount of supplementary food consumed by control and experimental nestlings, and assessed their body size and survival rates. Provisioning rates were similar across habitats and control and supplemented broods, but supplemented (and not control) broods consumed large quantities of supplementary food. As predicted by the food limitation hypothesis we found that nestlings in (a) urban control broods had smaller body size and nestling survival rates than those in forest control broods; (b) forest supplemented and control broods had similar body size and survival rates; (c) urban supplemented nestlings had larger body size and survival rates than those in urban control broods; and crucially (d) urban supplemented broods had similar body size and survival rates to nestlings in forest control broods. Our results provide rare experimental support for the strong negative effects of food limitation during the nestling rearing period on urban birds' breeding success. Furthermore, the fact that supplementary food almost completely eliminated habitat differences in survival rate and nestling body size suggest that urban stressors other than food shortage contributed relatively little to the reduced avian breeding success. Finally, given the impacts of the amount of supplementary food that we provided and taking clutch size differences into account, our results suggest that urban insect populations in our study system would need to be increased by a factor of at least 2.5 for urban and forest great tits to have similar reproductive success.

Human recreation decreases antibody titre in bird nestlings: an overlooked transgenerational effect of disturbance.

Outdoor recreational activities are booming and most animals perceive humans as predators, which triggers behavioural and/or physiological reactions [e.g. heart rate increase, activation of the hypothalamic-pituitary-adrenal (HPA) axis]. Physiological stress reactions have been shown to affect the immune system of an animal and therefore may also affect the amount of maternal antibodies a female transmits to her offspring. A few studies have revealed that the presence of predators affects the amount of maternal antibodies deposited into eggs of birds. In this study, using Eurasian blue and great tit offspring (Cyanistes caeruleus and Parus major) as model species, we experimentally tested whether human recreation induces changes in the amount of circulating antibodies in young nestlings and whether this effect is modulated by habitat and competition. Moreover, we investigated whether these variations in antibody titre in turn have an impact on hatching success and offspring growth. Nestlings of great tit females that had been disturbed by experimental human recreation during egg laying had lower antibody titres compared with control nestlings. Antibody titre of nestling blue tits showed a negative correlation with the presence of great tits, rather than with human disturbance. The hatching success was positively correlated with the average amount of antibodies in great tit nestlings, independent of the treatment. Antibody titre in the first days of life in both species was positively correlated with body mass, but this relationship disappeared at fledging and was independent of treatment. We suggest that human recreation may have caused a stress-driven activation of the HPA axis in breeding females, chronically increasing their circulating corticosterone, which is known to have an immunosuppressive function. Either, lower amounts of antibodies are transmitted to nestlings or impaired transfer mechanisms lead to lower amounts of immunoglobulins in the eggs. Human disturbance could, therefore, have negative effects on nestling survival at early life-stages, when nestlings are heavily reliant on maternal antibodies, and in turn lead to lower breeding success and parental fitness. This is a so far overlooked effect of disturbance on early life in birds.

Increased glucocorticoid concentrations in early life cause mitochondrial inefficiency and short telomeres.

Telomeres are DNA structures that protect chromosome ends. However, telomeres shorten during cell replication and at critically low lengths can reduce cell replicative potential, induce cell senescence and decrease fitness. Stress exposure, which elevates glucocorticoid hormone concentrations, can exacerbate telomere attrition. This phenomenon has been attributed to increased oxidative stress generated by glucocorticoids ('oxidative stress hypothesis'). We recently suggested that glucocorticoids could increase telomere attrition during stressful periods by reducing the resources available for telomere maintenance through changes in the metabolic machinery ('metabolic telomere attrition hypothesis'). Here, we tested whether experimental increases in glucocorticoid levels affected telomere length and mitochondrial function in wild great tit (Parus major) nestlings during the energy-demanding early growth period. We monitored resulting corticosterone (Cort) concentrations in plasma and red blood cells, telomere lengths and mitochondrial metabolism (metabolic rate, proton leak, oxidative phosphorylation, maximal mitochondrial capacity and mitochondrial inefficiency). We assessed oxidative damage caused by reactive oxygen species (ROS) metabolites as well as the total non-enzymatic antioxidant protection in plasma. Compared with control nestlings, Cort-nestlings had higher baseline corticosterone, shorter telomeres and higher mitochondrial metabolic rate. Importantly, Cort-nestlings showed increased mitochondrial proton leak, leading to a decreased ATP production efficiency. Treatment groups did not differ in oxidative damage or antioxidants. Hence, glucocorticoid-induced telomere attrition is associated with changes in mitochondrial metabolism, but not with ROS production. These findings support the hypothesis that shortening of telomere length during stressful periods is mediated by glucocorticoids through metabolic rearrangements.

Arsenic-related oxidative stress in experimentally-dosed wild great tit nestlings

Arsenic (As) is broadly distributed due to natural and anthropogenic sources, and it may cause adverse effects in birds. However, research on other elements (Pb, Hg and Cd) has been prioritized, resulting in scarce data on As exposure and related effects in wild birds. One of the mechanisms responsible for As toxicity is oxidative stress. Therefore, the aim of this study was to investigate if environmentally relevant As levels affected oxidative stress biomarkers in great tits (Parus major). This is the first field experiment studying the effects of As on oxidative stress in wild passerines. Wild great tit nestlings were orally dosed with sodium arsenite (Control: water, Low dose: 0.2 μg g−1 d−1 and High dose: 1 μg g−1 d−1; from day 3 to day 13 post-hatching). We intended to reach As concentrations similar to those at which passerines are exposed to at actual polluted areas. We compared the responses to the experimental manipulations (High, Low and Control groups) with those in an As/metal-exposed population breeding close to a Cu–Ni smelter in Finland (Smelter group). A set of antioxidants (tGSH, GSH:GSSG ratio, CAT, SOD, GST and GPx), and oxidative damage biomarkers (lipid peroxidation, protein carbonylation, 8-hydroxy-2′-deoxyguanosine formation in DNA, and telomere length) were explored in blood. Arsenic administration had no significant effect on most of the biomarkers measured: only the CAT activity was lower in the High As group and the GPx activity was enhanced in the Smelter group compared to the Control. Our results suggest that the dose and duration of the As exposure was not enough to induce oxidative damage in red cells of great tit nestlings. In spite of this, nestlings dosed with 1 μg g−1 d−1 of sodium arsenite showed non-significantly higher oxidative stress biomarkers than controls, suggesting that we were close to an effect level for the redox-defense system. Oxidative effects at equivalent As levels combined with other stressors cannot be dismissed.