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Migratory birds may either upregulate their immune system during migration as they might encounter novel pathogens or downregulate their immune system as a consequence of trade‐offs with the resource costs of migration. Support for the latter comes not least from a study that reports a positive correlation in autumn migrating birds between fuel stores and parameters of innate and acquired immune function, that is, energy‐exhausted migrants appear to have lowered immune function. However, to our knowledge, no study has tested whether this pattern exists in spring migrating birds, which may face other trade‐offs than autumn migrants. Here, we investigate if in spring there is a relationship between fuel stores and microbial‐killing ability, a measure of innate immune function, and total immunoglobulin (IgY), a measure of acquired immune function, in four migrating bird species: chaffinches (Fringilla coelebs), dunnocks (Prunella modularis), song thrushes (Turdus philomelos) and northern wheatears (Oenanthe oenanthe). Our findings indicate no significant correlation between fuel stores and either microbial killing ability or IgY levels when considering all species collectively. When analysing species separately, we found a significant negative correlation between fuel stores and microbial‐killing ability in chaffinches and a positive correlation between fuel stores and IgY levels in wheatears. In song thrushes, but not in any of the other species, there was a significant negative correlation between relative arrival date and microbial‐killing ability and between arrival date and IgY levels. Sex did not affect immune function in any of the species. Our study suggests that the relationship between immune function and fuel stores may be different during spring migration compared to autumn migration. Differences in the speed of migration or pathogen pressure may result in different outcomes of the resource trade‐off between investment in immune function and migration among the seasons. [ABSTRACT FROM AUTHOR]

Migratory birds may either upregulate their immune system during migration as they might encounter novel pathogens or downregulate their immune system as a consequence of trade‐offs with the resource costs of migration. Support for the latter comes not least from a study that reports a positive correlation in autumn migrating birds between fuel stores and parameters of innate and acquired immune function, that is, energy‐exhausted migrants appear to have lowered immune function. However, to our knowledge, no study has tested whether this pattern exists in spring migrating birds, which may face other trade‐offs than autumn migrants. Here, we investigate if in spring there is a relationship between fuel stores and microbial‐killing ability, a measure of innate immune function, and total immunoglobulin (IgY), a measure of acquired immune function, in four migrating bird species: chaffinches (Fringilla coelebs), dunnocks (Prunella modularis), song thrushes (Turdus philomelos) and northern wheatears (Oenanthe oenanthe). Our findings indicate no significant correlation between fuel stores and either microbial killing ability or IgY levels when considering all species collectively. When analysing species separately, we found a significant negative correlation between fuel stores and microbial‐killing ability in chaffinches and a positive correlation between fuel stores and IgY levels in wheatears. In song thrushes, but not in any of the other species, there was a significant negative correlation between relative arrival date and microbial‐killing ability and between arrival date and IgY levels. Sex did not affect immune function in any of the species. Our study suggests that the relationship between immune function and fuel stores may be different during spring migration compared to autumn migration. Differences in the speed of migration or pathogen pressure may result in different outcomes of the resource trade‐off between investment in immune function and migration among the seasons. [ABSTRACT FROM AUTHOR]

In many animals, catabolic and anabolic periods are temporally separated. Migratory birds alternate energy expenditure during flight with energy accumulation during stopover. The size of the energy stores at stopover affects the decision to resume migration and thus the temporal organization of migration. We now provide data suggesting that it is not only the size of the energy stores per se that may influence migration scheduling, but also the physiological consequences of flying. In two subspecies of the northern wheatear Oenanthe oenanthe , a long-distance migrant, estimated energy stores at a stopover during autumn migration were positively related with both constitutive innate and acquired immune function, and negatively related with oxidative damage to lipids. In other words, migrants' physiological condition was associated with their energetic condition. Although time spent at stopover before sampling may have contributed to this relationship, our results suggest that migrants have to trade-off the depletion of energy stores during flight with incurring physiological costs. This will affect migrants' decisions when to start and when to terminate a migratory flight. The physiological costs associated with the depletion of energy stores may also help explaining why migrants often arrive at and depart from stopover sites with larger energy stores than expected. We propose that studies on the role of energy stores as drivers of the temporal organization of (avian) migration need to consider physiological condition, such as immunological and oxidative states. [ABSTRACT FROM AUTHOR]

Numerous species adopt inducible defence strategies; that is, they have phenotypically plastic traits that decrease the risk of capture and consumption by potential predators. The benefits of expressing alternative phenotypes in high‐ vs. low‐risk environments are well documented. However, inducible anti‐predator traits are also expected to incur costs, as they are not expressed when predators are absent, yet empirical evidence of such costs remains scarce.Virtually, all animals in nature are simultaneously under strong selection to evade both capture by predators and infection by parasites or pathogens and, hence, display a diverse arsenal of defences to combat these threats, raising the possibility of trade‐offs between defences. A classic example of a predator‐induced morphological defence is the deep‐bodied shape of crucian carp that reduces risk of predation from gape‐size‐limited predators. The goal of this study was to examine whether predator exposure affects also immune function in crucian carp, and whether the degree of expressed morphological defence is traded off against immune function in individuals.Following exposure to manipulations of perceived risk (predator presence/absence) in a long‐term experiment (8 months), key aspects of innate immune function and individual differences in the expression of inducible morphological defence were quantified.Predator‐exposed individuals showed lower haptoglobin levels and complement activity, but higher natural antibody titres than fish from predator‐free conditions. When experimentally challenged with a mimicked bacterial infection (LPS injection), fish reared in the presence of a natural predator showed a weaker immune response. Moreover, among predator‐exposed individuals, the magnitude of morphological defence expression correlated with both baseline immune function and the ability to mount an immune response. However, these relationships were not consistently supportive of a general trade‐off among defences.Our results suggest that fish exposed to predators on average reduce investment in immune function, and, further, the observed relationships among defences in predator‐exposed individuals can best be explained from individual fitness and pace‐of‐life perspectives. [ABSTRACT FROM AUTHOR]

Billions of animals migrate between breeding and non-breeding areas worldwide. Partial migration, where both migrants and residents coexist within a population, occurs in most animal taxa, including fish, insects, birds and mammals. Partial migration has been hypothesised to be the most common form of migration and to be an evolutionary precursor to full migration. Despite extensive theoretical models about partial migration and its potential to provide insight into the ecology and evolution of migration, the physiological mechanisms that shape partial migration remain poorly understood. Here, we review current knowledge on how physiological processes mediate the causes and consequences of avian partial migration, and how they may help us understand why some individuals migrate and others remain resident. When information from birds is missing, we highlight examples from other taxa. In particular, we focus on temperature regulation, metabolic rate, immune function, oxidative stress, telomeres, and neuroendocrine and endocrine systems. We argue that these traits provide physiological pathways that regulate the ecological and behavioural causes and/or consequences of partial migration, and may provide insight into the mechanistic basis of wintering decisions. They may, thus, also help us to explain why individuals switch strategies among winters. We also highlight current gaps in our knowledge and suggest promising future research opportunities. A deeper understanding of the physiological mechanisms mediating the causes and consequences of partial migration will not only provide novel insights into the ecology and evolution of migration in general, but will also be vital to precisely modelling population trends and predicting range shifts under global change. [ABSTRACT FROM AUTHOR]

Stopovers play a crucial role for the success of migrating animals and are key to optimal migration theory. Variation in refuelling rates, stopover duration and departure decisions among individuals has been related to several external factors. The physiological mechanisms shaping stopover ecology are, however, less well understood. Here, we explore how immune function and blood parasite infections relate to several aspects of stopover behaviour in autumn migrating short- and long-distance migrating songbirds. We blood sampled individuals of six species and used an automated radio-telemetry system in the stopover area to subsequently quantify stopover duration, ‘bush-level’ activity patterns (~ 0.1-30 m), landscape movements (~ 30-6000 m), departure direction and departure time. We show that complement activity, the acute phase protein haptoglobin and blood parasite infections were related to prolonged stopover duration. Complement activity (i.e., lysis) and total immunoglobulins were negatively correlated with bush-level activity patterns. The differences partly depended on whether birds were long-distance or short-distance migrants. Birds infected with avian malaria-like parasites showed longer landscape movements during the stopover than uninfected individuals, and birds with double blood parasite infections departed more than 2.5 h later after sunset/sunrise suggesting shorter flight bouts. We conclude that variation in baseline immune function and blood parasite infection status affects stopover ecology and helps explain individual variation in stopover behaviour. These differences affect overall migration speed, and thus can have significant impact on migration success and induce carry-over effects on other annual-cycle stages. Immune function and blood parasites should, therefore, be considered as important factors when applying optimal migration theory. [ABSTRACT FROM AUTHOR]

Migration usually consists of intermittent travel and stopovers, the latter being crucially important for individuals to recover and refuel to successfully complete migration. Quantifying how sickness behaviours influence stopovers is crucial for our understanding of migration ecology and how diseases spread. However, little is known about infections in songbirds, which constitute the majority of avian migrants.We experimentally immune‐challenged autumn migrating passerines (both short‐ and long‐distance migrating species) with a simulated bacterial infection. Using an automated radiotelemetry system in the stopover area, we subsequently quantified stopover duration, "bush‐level" activity patterns (0.1–30 m) and landscape movements (30–6,000 m).We show that compared to controls, immune‐challenged birds prolonged their stopover duration by on average 1.2 days in long‐distance and 2.9 days in short‐distance migrants, respectively (100%–126% longer than controls, respectively). During the prolonged stopover, the immune‐challenged birds kept a high "bush‐level" activity (which was unexpected) but reduced their local movements, independent of migration strategy. Baseline immune function, but not blood parasite infections prior to the immune challenge, had a prolonging effect on stopover duration, particularly in long‐distance migrants.We conclude that a mimicked bacterial infection does not cause lethargy, per se, but restricts landscape movements and prolongs stopover duration, and that this behavioural response also depends on the status of baseline immune function and migration strategy. This adds a new level to the understanding of how acute inflammation affect migration behaviour and hence the ecology and evolution of migration. Accounting for these effects of bacterial infections will also enable us to fine‐tune and apply optimal migration theory. Finally, it will help us predicting how migrating animals may respond to increased pathogen pressure caused by global change. The authors show that migratory birds prolong their stopovers when challenged with a mimicked bacterial infection. The effect was smaller in birds that fly long distances (to winter in Africa) compared to birds that migrate shorter distances (to winter in Europe). Their baseline immune function further influenced the increase in stopover duration. Understanding how infections impact migrating song birds, which constitute the majority of avian migrants, is crucial for understanding disease ecology and the ecology and evolution of migration. [ABSTRACT FROM AUTHOR]

Background: Over the past couple of decades, measuring immunological parameters has become widespread in studies of ecology and evolution. A combination of different immunological indices is useful for quantifying different parts of the immune system and comprehensively assessing immune function. Running multiple immune assays usually requires samples to be repeatedly thawed and re-frozen. There is some evidence that repeated freezing and thawing can affect assay results, but this has never been comprehensively studied in some common ecological immunology assays. We tested the effect of multiple (1, 2, 3, 4, 5, 10) freeze-thaw cycles on the results of four commonly used immunological assays: haemolysis-haemagglutination titres, haptoglobin concentration, bacterial killing capacity and total immunoglobulins (IgY). We tested five different bird species from four different bird orders (Passeriformes, Columbiformes, Charadriiformes and Galliformes), and we included both captive and free-living individuals. In addition, we tested for haptoglobin concentrations and the haemolysis-haemagglutination assay if re-analysing samples 1 year apart led to different results. For the haemolysis-haemagglutination assay we also tested two different sources of rabbit blood, and we compared untreated microtitre plates with plates that were "blocked" to prevent nonspecific interactions between the plate and assay reagents. Results: Repeated freezing and thawing of plasma had no effect on lysis titres, haptoglobin concentrations, bacterial killing capacity, or total immunoglobulin levels. Agglutination titres were unaffected by up to five cycles but were lower after ten freeze-thaw cycles. For the haemolysis-haemagglutination assay and haptoglobin concentrations, re-analysing samples 1 year apart yielded highly correlated data. For the haemolysis-haemagglutination assay, the source of rabbit blood did not influence the results, and the untreated vs. blocked plates differed slightly overall, but at the individual level assay results were highly correlated. Using different rabbit blood sources or different types of microtitre plates yielded highly correlated data. Conclusions: Our data suggest that repeated freeze-thaw cycles do not impair assay results to the point of influencing ecological or evolutionary conclusions. Plasma samples can be safely stored in one tube and thawed repeatedly for different assays. Nevertheless, we recommend consistent treatment of samples in terms of freeze-thaw cycles or other laboratory treatments to minimize the potential for introducing a systematic bias. [ABSTRACT FROM AUTHOR]

Many animal species have populations in which some individuals migrate and others remain on the breeding grounds. This phenomenon is called partial migration. Despite substantial theoretical work, empirical data on causes and consequences of partial migration remain scarce, mainly because of difficulties associated with tracking individuals over large spatial scales. We used stable hydrogen isotopes in claw material to determine whether skylarks Alauda arvensis from a single breeding population in the Netherlands had migrated or remained resident in the previous winter and investigated whether there were causes or consequences of either strategy. Age and sex had no influence on the propensity to migrate, but larger individuals were more likely to be residents. The wintering strategy was not fixed within individuals. Up to 45% of individuals measured in multiple years switched strategies. Reproductive parameters were not related to the wintering strategy, but individuals that wintered locally experi-enced lower future return rates, and this was directly correlated with two independent measures of immune function. Our results suggest that partial migration in skylarks is based neither on genetic dimorphism nor on an age-and sex-dependent condition. Instead, the wintering strategy is related to structural size and immune function. These new insights on causes and consequences of partial migration advance our understanding of the ecology, evolution, and coexistence of different life-history strategies. [ABSTRACT FROM AUTHOR]

Investment in immune defences is predicted to covary with a variety of ecologically and evolutionarily relevant axes, with pace of life and environmental antigen exposure being two examples. These axes may themselves covary directly or inversely, and such relationships can lead to conflicting predictions regarding immune investment. If pace of life shapes immune investment then, following life history theory, slow-living, arid zone and tropical species should invest more in immunity than fast-living temperate species. Alternatively, if antigen exposure drives immune investment, then species in antigen-rich tropical and temperate environments are predicted to exhibit higher immune indices than species from antigen-poor arid locations. To test these contrasting predictions we investigated how variation in pace of life and antigen exposure influence immune investment in related lark species (Alaudidae) with differing life histories and predicted risks of exposure to environmental microbes and parasites. We used clutch size and total number of eggs laid per year as indicators of pace of life, and aridity, and the climatic variables that influence aridity, as correlates of antigen abundance. We quantified immune investment by measuring four indices of innate immunity. Pace of life explained little of the variation in immune investment, and only one immune measure correlated significantly with pace of life, but not in the predicted direction. Conversely, aridity, our proxy for environmental antigen exposure, was predictive of immune investment, and larks in more mesic environments had higher immune indices than those living in arid, low-risk locations. Our study suggests that abiotic environmental variables with strong ties to environmental antigen exposure can be important correlates of immunological variation. [ABSTRACT FROM AUTHOR]

Non-lethal infections are common in free-living animals and the associated sickness behaviours can impact crucial life-history trade-offs. However, little is known about the duration and extent of such sickness behaviours in free-living animals, and consequently how they affect life-history decisions. Here, free-living Eurasian blackbirds, Turdus merula, were immune-challenged with lipopolysaccharide (LPS) to mimic a bacterial infection and their behaviour was monitored for up to 48 days using accelerometers. As expected, immune-challenged birds were less active than controls within the first 24 h. Unexpectedly, this reduced activity remained detectable for 20 days, before both groups returned to similar activity levels. Furthermore, activity was positively correlated with a pre-experimental index of complement activity, but only in immune-challenged birds, suggesting that sickness behaviours are modulated by constitutive immune function. Differences in daily activity levels stemmed from immune-challenged birds resting earlier at dusk than control birds, while activity levels between groups were similar during core daytime hours. Overall, activity was reduced by 19% in immune-challenged birds and they were on average almost 1 h less active per day for 20 days. This unexpected longevity in sickness behaviour may have severe implications during energy-intense annual-cycle stages (e.g. breeding, migration, winter). Thus, our data help to understand the consequences of non-lethal infections on free-living animals. [ABSTRACT FROM AUTHOR]

In recent decades, Skylark ( Alauda arvensis) populations in Europe have declined sharply due to agricultural intensification. Insufficient reproduction rates are one reason. Increased winter mortality may also be important, but studies outside the breeding season are scarce and mostly limited to the UK. We studied habitat selection of wintering Skylarks in an agricultural area in the Netherlands. We monitored Skylarks between November 2008 and March 2009 on 10 survey plots including 77 different arable fields and permanent grasslands and covering in total 480 ha. We simultaneously measured food availability, vegetation structure and field boundary characteristics. We also analysed 158 faecal pellets collected on potato and cereal stubble fields to relate Skylark diet to seasonal changes in food availability and foraging habitat. We show that cereal stubble fields larger than 4.3 ha, surrounded by no or low boundary vegetation and a density of dietary seeds of more than 860 seeds m, were most suitable for wintering Skylarks. Skylark group densities were low on permanent grasslands and on maize stubble fields. Densities of dietary seeds were highest in soils of potato stubble fields followed by cereal stubble fields, grasslands and maize stubble fields. Skylarks showed a strong preference for cereal grains, but their proportion in the diet fell sharply at the end of November, indicating that cereal grains were depleted and birds had to switch to less profitable food sources, such as weed seeds and leaves. We conclude that Skylarks wintering in agricultural landscapes possibly suffer from a lack of energy-rich food sources and only a few fields provide sufficient food. Conservation measures should strive to improve the wintering situation by creating food-rich habitats such as over-winter stubble with a rich layer of weeds on large fields and localised in open areas. [ABSTRACT FROM AUTHOR]

Trade-offs between immune function and other physiological and behavioural processes are central in ecoimmunology, but one important problem is how to distinguish a reallocation of resources away from the immune system from a reallocation or redistribution within the immune system. While variation in baseline values of individual immune parameters is well established, studies in wild animals on multiple parameters during an immune response are lacking. It also remains to be tested whether and how immune responses correlate with baseline values that vary, for example, over the course of an annual cycle. We studied immunological responses to an endotoxin challenge in skylarks (Alauda arvensis), a partial migrant bird breeding in temperate zones. We compared birds injected with the endotoxin LPS with un-injected controls, characterizing immunological responses with leukocyte profiles, titres of lytic enzymes and natural antibodies, and concentrations of haptoglobin and heat shock proteins. We did this in five annual-cycle stages to test whether the response varied throughout the year. The endotoxin challenge affected six of 10 measured parameters. Lysis titres and proportions of heterophils increased; haptoglobin concentrations and proportions of lymphocytes, basophils and eosinophils decreased. The variable effects on different immune components demonstrate the complexity of an Immune response. We found no evidence that the response differed between annual-cycle stages. The response was independent of baseline measures taken directly upon capture in the field, indicating that birds were facing no immunological ceiling when mounting an immune response. Values of five parameters collected under field conditions were significantly related to values taken under standardized laboratory conditions. We conclude that multiple parts of the immune system are modulated during an immunological response and that responses are not re-organized throughout the annual cycle. [ABSTRACT FROM AUTHOR]

Introduction Life-history theory predicts that organisms trade off survival against reproduction. However, the time scales on which various consequences become evident and the physiology mediating the cost of reproduction remain poorly understood. Yet, explaining not only which mechanisms mediate this trade-off, but also how fast or slow the mechanisms act, is crucial for an improved understanding of life-history evolution. We investigated three time scales on which an experimental increase in body mass could affect this trade-off: within broods, within season and between years. We handicapped adult skylarks (Alauda arvensis) by attaching extra weight during first broods to both adults of a pair. We measured body mass, immune function and return rates in these birds. We also measured nest success, feeding rates, diet composition, nestling size, nestling immune function and recruitment rates. Results When nestlings of first broods fledged, parent body condition had not changed, but experimental birds experienced higher nest failure. Depending on the year, immune parameters of nestlings from experimental parents were either higher or lower than of control nestlings. Later, when parents were feeding their second brood, the balance between selfmaintenance and nest success had shifted. Control and experimental adults differed in immune function, while mass and immune function of their nestlings did not differ. Although weights were removed after breeding, immune measurements during the second brood had the capacity to predict return rates to the next breeding season. Among birds that returned the next year, body condition and reproductive performance a year after the experiment did not differ between treatment groups. Conclusions We conclude that the balance between current reproduction and survival shifts from affecting nestlings to affecting parents as the reproductive season progresses Furthermore, immune function is apparently one physiological mechanism involved in this trade-off. By unravelling a physiological mechanism underlying the trade-offs between current and future reproduction and by demonstrating the different time scales on which it acts, our study represents an important step in understanding a central theory of life-history evolution. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A central hypothesis of eco-immunology proposes trade-offs between immune defences and competing physiological and behavioural processes, leading to immunological variation within and among annual-cycle stages, as has been revealed for some species. However, few studies have simultaneously investigated patterns of multiple immune indices over the entire annual cycle in free-living birds, and none has investigated the consistency of seasonal patterns across multiple years. We quantified lysis, agglutination, haptoglobin, leukocyte profiles, and body mass in free-living skylarks ( Alauda arvensis) through two complete annual cycles and within and between four breeding seasons. The skylarks' annual cycle is characterised by annually repeated changes in energy and time budgets, social structure and diet. If trade-offs relating to these cyclic changes shape evolution, predictable intra-annual immune patterns may result. Alternatively, intra-annual immune patterns may vary among years if fluctuating environmental changes affect the cost-benefit balances of immune function. We found significant variation in immune indices and body mass across the annual cycle, and these patterns differed between years. Immune parameters differed between four breeding seasons, and in all years, lysis and agglutination increased as the season progressed independent of average levels. Population-level patterns (intra-annual, inter-annual, within breeding season) were consistent with within-individual patterns based on repeated measurements. We found little evidence for sex differences, and only haptoglobin was correlated (negatively) with body mass. We conclude that immune modulation is not simply a pre-programmed phenomenon that reflects predictable ecological changes. Instead, fluctuating environmental conditions that vary among years likely contribute to the immunological variation that we observed. [ABSTRACT FROM AUTHOR]

Immune defense may vary as a result of trade-offs with other life-history traits or in parallel with variation in antigen levels in the environment. We studied lark species (Alaudidae) in the Arabian Desert and temperate Netherlands to test opposing predictions from these two hypotheses. Based on their slower pace of life, the trade-off hypothesis predicts relatively stronger immune defenses in desert larks compared with temperate larks. However, as predicted by the antigen exposure hypothesis, reduced microbial abundances in deserts should result in desert-living larks having relatively weaker immune defenses. We quantified host-independent and host-dependent microbial abundances of culturable microbes in ambient air and from the surfaces of birds. We measured components of immunity by quantifying concentrations of the acute-phase protein haptoglobin, natural antibody-mediated agglutination titers, complement- mediated lysis titers, and the microbicidal ability of whole blood. Desert-living larks were exposed to significantly lower concentrations of airborne microbes than temperate larks, and densities of some bird-associated microbes were also lower in desert species. Haptoglobin concentrations and lysis titers were also significantly lower in desert-living larks, but other immune indexes did not differ. Thus, contrary to the trade-off hypothesis, we found little evidence that a slow pace of life predicted increased immunological investment. In contrast, and in support of the antigen exposure hypothesis, associations between microbial exposure and some immune indexes were apparent. Measures of antigen exposure, including assessment of host-independent and host-dependent microbial assemblages, can provide novel insights into the mechanisms underlying immunological variation. [ABSTRACT FROM AUTHOR]

The Skylark Alauda arvensis is a European passerine species lacking plumage dimorphism and having a complete post-nuptial moult in adults, and a complete post-juvenile moult in young birds. Tongue spots were thought to allow age discrimination after moult. Based on data from a study population in the Netherlands, where the age of many birds was known, we show that tongue spots are not useful for ageing Skylarks. Some nestlings lack tongue spots and half of all birds had tongue spots when three years or older. Regarding sex discrimination, males and females can usually be distinguished from a bimodal wing length distribution, but the threshold and amount of overlap in wing length between sexes vary greatly between studies. Here, we show that 98% of all individuals from our Dutch breeding population could be sexed correctly by wing length. However, during migration and winter, when populations from other regions co-occur in the same area, molecular analyses revealed that the overlap in wing length between sexes increased. Therefore, ringers need to be cautious when assigning sex, and small males especially are often misclassified. For example, based on criteria used in France, 14% of Dutch male Skylarks, which are partial migrants, would be misclassified as females. [ABSTRACT FROM AUTHOR]

A central hypothesis of ecological immunology is that immune defences are traded off against competing physiological and behavioural processes. During energetically demanding periods, birds are predicted to switch from expensive inflammatory responses to less costly immune responses. Acute phase responses (APRs) are a particularly costly form of immune defence, and, hence, seasonal modulations in APRs are expected. Yet, hypotheses about APR modulation remain untested in freeliving organisms throughout a complete annual cycle. We studied seasonal modulations in the APRs and in the energy budgets of skylarks Alauda arvensis, a partial migrant bird from temperate zones that experiences substantial ecological changes during its annual cycle. We characterized throughout the annual cycle changes in their energy budgets by measuring basal metabolic rate (BMR) and body mass. We quantified APRs by measuring the effects of a lipopolysaccharide injection on metabolic rate, body mass, body temperature, and concentrations of glucose and ketone. Body mass and BMR were lowest during breeding, highest during winter and intermediate during spring migration, moult and autumn migration. Despite this variation in energy budgets, the magnitude of the APR, as measured by all variables, was similar in all annual cycle stages. Thus, while we find evidence that some annual cycle stages are relatively more energetically constrained, we find no support for the hypothesis that during these annual cycle stages birds compromise an immune defence that is itself energetically costly. We suggest that the ability to mount an APR may be so essential to survival in every annual cycle stage that skylarks do not trade off this costly form of defence with other annual cycle demands. [ABSTRACT FROM AUTHOR]

The article presents a study on the food intake in a Montagu's Harrier with the use of two methods of pellet analysis. The study was conducted by a group of researchers using an eight-month-old captive male Montagu's Harrier which was kept on a diet of dead day-old preys. The excess food and pellets were collected and the ingested prey mass was calculated. An estimate of food intake was also taken from the number of individuals found in pellets. Based from the result of the calculation, the researchers concluded that the food intake on the previous day is best represented by pellets.

Research on telomeres in the fields of ecology and evolution has been rapidly expanding over the last two decades. This has resulted in the formulation of a multitude of, often name‐given, hypotheses related to the associations between telomeres and life‐history traits or fitness‐facilitating processes (and the mechanisms underlying them). However, the differences (or similarities) between the various hypotheses, which can originate from different research fields, are often not obvious. Our aim here is therefore to give an overview of the hypotheses that are of interest in ecology and evolution and to provide two frameworks that help discriminate among them. We group the hypotheses (i) based on their association with different research questions, and (ii) using a hierarchical approach that builds on the assumptions they make, such as about causality of telomere length/shortening and/or the proposed functional consequences of telomere shortening on organism performance. Both our frameworks show that there exist parallel lines of thoughts in different research fields. Moreover, they also clearly illustrate that there are in many cases competing hypotheses within clusters, and that some of these even have contradictory assumptions and/or predictions. We also touch upon two topics in telomere research that would benefit from further conceptualization. This review should help researchers, both those familiar with and those new to the subject, to identify future avenues of research. [ABSTRACT FROM AUTHOR]

Recent tracking technology has shown that some birds migrate at extreme altitudes, potentially to avoid predators. Predation risk during migration is a significant factor contributing to high mortality rates among birds. A study tracking Grey Plovers revealed that one bird was killed by a Peregrine Falcon at 3000m above ground, challenging the idea that flying at extreme altitudes can always protect against predators. The research was funded by the Dutch Research Council and conducted by a team of scientists. [Extracted from the article]

Although animals are born with a protective immune system, even the innate immune system is under development from birth to adulthood and this development may be affected by sex and growth. However, most knowledge comes from captive animals or long-lived slow growing species. Moreover, little is known about how innate immune function, the important first line of defence, develops during early life in fast-growing animals such as free-living passerines. We studied development of innate baseline immune function in nestlings of free-living jackdaws Corvus monedula. We measured four immune parameters (hemolysis, hemagglutination, bacterial-killing capacity, haptoglobin concentration) and structural body size (body mass, wing length, tarsus length) at day 12 and day 29 post-hatching. We found that three out of four immune parameters (hemolysis, hemagglutination, bacterial-killing capacity) substantially increased with nestling age and had roughly reached adult levels shortly prior to fledging. We found little differences in immune development between males and females despite them differing in structural development. We also found no evidence that the nestlings traded off immune development with growth. That nestlings rapidly increase innate baseline immune function during early life and similarly in males and females indicates the importance of a well-functioning immune system already during the nestling phase. • Innate immune function substantially increases with nestling age. • Immune parameters are roughly at adult levels shortly prior to fledging. • Little difference in immune development between sexes despite different growth rates. • No trade-off between immune development and growth. [ABSTRACT FROM AUTHOR]

Conditions experienced during early life can have long-term individual consequences by influencing dispersal, survival, recruitment and productivity. Resource allocation during development can have strong carry-over effects onto these key parameters and is directly determined by the quality of parental care. In the black sparrowhawk (Accipiter melanoleucus), a colour-polymorphic raptor, parental morphs influence nestling somatic growth and survival, with pairs consisting of different colour morphs ('mixed-morph pairs') producing offspring with lower body mass indices, but higher local apparent survival rates. Resource allocation theory could explain this relationship, with nestlings of mixed-morph pairs trading off a more effective innate immune system against somatic growth. We quantified several innate immune parameters of nestlings (hemagglutination, hemolysis, bacteria-killing capacity and haptoglobin concentration) and triggered an immune response by injecting lipopolysaccharides. Although we found that nestlings with lower body mass index had higher local survival rates, we found no support for the proposed hypothesis: neither baseline immune function nor the induced immune response of nestlings was associated with parental morph combination. Our results suggest that these immune parameters are unlikely to be involved in providing a selective advantage for the different colour morphs' offspring, and thus innate immunity does not appear to be traded off against a greater allocation of resources to somatic growth. Alternative hypotheses explaining the mechanism of a low nestling body mass index leading to subsequent higher local survival could be related to the post-fledgling dependency period or differences in dispersal patterns for the offspring from different morph combinations. [ABSTRACT FROM AUTHOR]

A correction to the article "Environmental proxies of antigen exposure explain variation in immune investment better than indices of pace of life" published in a previous issue is presented.

Abstract: Migration is energetically demanding and physiologically challenging. Migrating birds, for example, need to boost their antioxidant defenses to defeat the pro‐oxidants produced during high energetic activity. The enhanced antioxidant defense possibly withdraws limited resources (e.g., energy or micronutrients) from other physiological functions, such as immune defense. Such a trade‐off might not occur outside the migration seasons or in resident individuals. Here, we investigate whether there is a negative relationship between innate immune function and antioxidant defense by sampling both migrating and resident blackbirds (Turdus merula) at the same location during the same period of the annual cycle. We show that in migrating blackbirds microbial killing capacity (BKA), an integrative measure of baseline innate immune function was negatively correlated with total nonenzymatic antioxidant capacity. In contrast, in resident conspecifics, sampled at the same time and location, these two physiological measures were not correlated. This suggests that migrating birds trade off innate immune function and antioxidant defense. Furthermore, and likely a consequence of this trade‐off, in migrant blackbirds BKA was positively correlated with oxidative damage to lipids. In resident blackbirds BKA and degree of lipid oxidation were uncorrelated. The mechanism and currencies of the supposed trade‐off are currently unknown, but energetic investments or micronutrients are likely candidates. Future experimental studies could provide more conclusive evidence for this trade‐off; yet, our results open up a new level of thinking about the physiological costs of migration. [ABSTRACT FROM AUTHOR]

Prey choice by predators may be based on the potential prey's condition, for example resulting in substandard individuals running a higher risk of being predated. Over 5 years, we studied young jackdaws, Corvus monedula , to determine whether size and innate baseline immune function may predict predation risk by goshawks, Accipiter gentilis , during the nestling and early fledging phases. We measured body mass, wing length, tarsus length and four innate immune indices two to four times when nestlings were 12–29 days old. To determine which individuals had been predated during the nestling phase and shortly after fledging, we searched for metal rings of the jackdaws in the only goshawk territory close to the jackdaw colony. Nestling mortality before 12 days of age was entirely due to starvation, whereas between 12 days of age and fledging, mortality was mainly due to predation. Nestlings with smaller size (mass, wing, tarsus) and low lysis titre and haptoglobin concentrations were at a higher risk of being predated before fledging. Directly after fledging, individuals with short wings were preferentially predated, with no effects of body mass, tarsus length or any of the four immune indices measured at day 29 (i.e. shortly before fledging). That lower immune function and smaller size predict predation risk in nestlings may reflect that these individuals are of poor quality and/or lag behind in development. We hypothesize that hunger makes these nestlings sit closest to the entrance hole and hence become the first to be predated. For fledglings, our results suggest that poor flight ability makes individuals with short wings the easiest targets for avian predators. • Nestling mortality before day 12 was entirely due to starvation. • Mortality after day 12 was mainly due to predation. • Both size and immune function influenced predation risk in nestlings. • Postfledging, individuals with short wings had the highest risk of predation. [ABSTRACT FROM AUTHOR]

Background: In contrast with macroorganisms, that show well-documented biogeographical patterns in distribution associated with local adaptation of physiology, behavior and life history, strong biogeographical patterns have not been found for microorganisms, raising questions about what determines their biogeography. Thus far, large-scale biogeographical studies have focused on free-living microbes, paying little attention to host-associated microbes, which play essential roles in physiology, behavior and life history of their hosts. Investigating cloacal gut microbiota of closely-related, ecologically similar free-living songbird species (Alaudidae, larks) inhabiting desert, temperate and tropical regions, we explored influences of geographical location and host species on α-diversity, co-occurrence of amplicon sequence variants (ASVs) and genera, differentially abundant and dominant bacterial taxa, and community composition. We also investigated how geographical distance explained differences in gut microbial community composition among larks. Results: Geographic location did not explain variation in richness and Shannon diversity of cloacal microbiota in larks. Out of 3798 ASVs and 799 bacterial genera identified, 17 ASVs (< 0.5%) and 43 genera (5%) were shared by larks from all locations. Desert larks held fewer unique ASVs (25%) than temperate zone (31%) and tropical larks (34%). Five out of 33 detected bacterial phyla dominated lark cloacal gut microbiomes. In tropical larks three bacterial classes were overrepresented. Highlighting the distinctiveness of desert lark microbiota, the relative abundances of 52 ASVs differed among locations, which classified within three dominant and 11 low-abundance phyla. Clear and significant phylogenetic clustering in cloacal microbiota community composition (unweighted UniFrac) showed segregation with geography and host species, where microbiota of desert larks were distinct from those of tropical and temperate regions. Geographic distance was nonlinearly associated with pairwise unweighted UniFrac distances. Conclusions: We conclude that host-associated microbiota are geographically structured in a group of widespread but closely-related bird species, following large-scale macro-ecological patterns and contrasting with previous findings for free-living microbes. Future work should further explore if and to what extent geographic variation in host-associated microbiota can be explained as result of co-evolution between gut microbes and host adaptive traits, and if and how acquisition from the environmental pool of bacteria contributes to explaining host-associated communities. [ABSTRACT FROM AUTHOR]

Urban areas provide breeding habitats for many species. However, animals raised in urban environments face challenges such as altered food availability and quality, pollution and pathogen assemblages. These challenges can affect physiological processes such as immune function and antioxidant defences which are important for fitness.Here, we explore how levels of urbanisation influence innate immune function, immune response to a mimicked bacterial infection and antioxidant capacity of nestling Black Sparrowhawks Accipiter melanoleucus in South Africa. We also explore the effect of timing of breeding and rainfall on physiology since both can influence the environmental condition under which nestlings are raised. Finally, because urbanisation can influence immune function indirectly, we use path analyses to explore direct and indirect associations between urbanisation, immune function and oxidative stress.We obtained measures of innate immunity (haptoglobin, lysis, agglutination, bactericidal capacity), indices of antioxidant capacity (total non‐enzymatic antioxidant capacity (tAOX) and total glutathione from nestlings from 2015 to 2019. In addition, in 2018 and 2019, we mimicked a bacterial infection by injecting nestlings with lipopolysaccharide and quantified their immune response.Increased urban cover was associated with an increase in lysis and a decrease in tAOX, but not with any of the other physiological parameters. Furthermore, except for agglutination, no physiological parameters were associated with the timing of breeding. Lysis and bactericidal capacity, however, varied consistently with the annual rainfall pattern. Immune response to a mimicked a bacterial infection decreased with urban cover but not with the timing of breeding nor rainfall. Our path analyses suggested indirect associations between urban cover and some immune indices via tAOX but not via the timing of breeding.Our results show that early‐life development in an urban environment is associated with variation in immune and antioxidant functions. The direct association between urbanisation and antioxidant capacity and their impact on immune function is likely an important factor mediating the impact of urbanisation on urban‐dwelling animals. Future studies should explore how these results are linked to fitness and whether the responses are adaptive for urban‐dwelling species. [ABSTRACT FROM AUTHOR]

Introduction All bird eggs are exposed to microbes in the environment, which if transmitted to the developing embryo, could cause hatching failure. However, the risk of trans-shell infection varies with environmental conditions and is higher for eggs laid in wetter environments. This might relate to generally higher microbial abundances and diversity in more humid environments, including on the surface of eggshells, as well as the need for moisture to facilitate microbial penetration of the eggshell. To protect against microbial infection, the albumen of avian eggs contains antimicrobial proteins, including lysozyme and ovotransferrin. We tested whether lysozyme and ovotransferrin activities varied in eggs of larks (Alaudidae) living along an arid-mesic gradient of environmental aridity, which we used as a proxy for risk of trans-shell infection. Results Contrary to expectations, lysozyme activity was highest in eggs from hotter, more arid locations, where we predicted the risk of trans-shell infection would be lower. Ovotransferrin concentrations did not vary with climatic factors. Temperature was a much better predictor of antimicrobial protein activity than precipitation, a result inconsistent with studies stressing the importance of moisture for trans-shell infection. Conclusions Our study raises interesting questions about the links between temperature and lysozyme activity in eggs, but we find no support for the hypothesis that antimicrobial protein deposition is higher in eggs laid in wetter environments. [ABSTRACT FROM AUTHOR]

Many hypotheses attempt to explain parasite–host associations, but rarely are they examined together in a single community. For hosts, key traits are the proportion of infected individuals (prevalence) and the diversity of parasites infecting them. A key parasite trait is host specificity, ranging from specialists infecting one or a few closely related species to generalists infecting many species. We tested 10 hypotheses to explain host‐parasite associations; five 'host‐centric' (e.g. prevalence is related to host abundance) and five 'parasite‐centric' (e.g. parasite abundance is related to host specificity). We analyzed a community of 67 locally transmitted avian haemosporidian parasite lineages (genera Plasmodium, Haemoproteus or Leucocytozoon), sampled from 2726 birds (64 species) in southern Sweden. Among host‐centric hypotheses, Haemoproteus and Leucocytozoon prevalence and Haemoproteus diversity were related to host habitat preferences, whereas there were no relationships with host abundance or body mass. Haemoproteus and Leucocytozoon prevalences were more similar among closely related than among distantly related host species. Haemoproteus prevalence and diversity were lower in host species with few close relatives ('evolutionarily distinct' hosts). Among parasite‐centric hypotheses, most lineages, even relative generalists, infected closely related host species more often than expected by chance. However, the host species of Haemoproteus and Leucocytozoon lineages overlapped less among lineages than expected by chance. Specialists did not reach higher prevalences than generalists on single host species. However, the abundance of Haemoproteus lineages was related to host specificity with generalists more common than specialists; this was driven by three closely related generalists. Host specificity of parasites was unrelated to the abundance or evolutionarily distinctiveness of their hosts. Parasite communities are likely structured by many factors and cannot be explained by hypotheses focusing solely on hosts or parasites. However, we found consistent effects of host phylogenetic relationships, plausibly a result of evolutionarily conserved host immune systems limiting parasite distributions. [ABSTRACT FROM AUTHOR]

Stopover periods in between migratory flights determine the speed of migration and as such may affect birds' fitness. The decision to leave a stopover site is affected by, for example, energy stores and weather conditions. We investigated whether immunological state influences autumn stopover departure decisions in Northern Wheatear (Oenanthe oenanthe), a nocturnal long-distance migrant. We measured three parameters of constitutive immune function: microbial killing capacity, haptoglobin, and total immunoglobulins. None of them explained variation in departure probability, minimum stopover duration, or departure time within the night. Thus, we did not find an effect of the birds' immunological state on stopover departure decisions. [ABSTRACT FROM AUTHOR]