Nest-site selection is an important determinant of avian reproductive success, mainly through its effect on predation risk. However, how environmental and social factors affect nest-site selection and predation risk remains less well understood. Optimal nest positioning may depend on the balance of many distinct factors such as nest predation, food availability, extra-pair mating opportunities, and interactions with neighbouring conspecifics. We investigated how these factors may affect nest-site selection and nest survival in the Seychelles warbler (Acrocephalus sechellensis), a facultatively cooperative-breeding passerine that defends stable territories year-round. We found that nest survival was higher when nests were built in higher vegetation layers and that nests were more likely to be located in food-rich parts of the territory, especially in territories with lower food availability. Further, we found that nests in territories with helpers were built in areas with higher food availability, whereas nests in territories without helpers were built in areas with fewer nest-predators. Finally, we found that females paired with younger males tended to build their nest closer to the territory border. Our results suggest that nest-site selection in the Seychelles warbler is affected by nest-predation, food availability and, possibly, the female’s opportunity for extra-pair copulations. Clarifying the relative importance of these different selective factors is key to the understanding of optimal nest-site selection to maximize reproductive success.

In cooperatively breeding species, sexually mature individuals may delay natal dispersal and become subordinates, helping a dominant pair raise offspring. To understand how cooperative breeding evolved, it is important to determine the mechanisms leading to delayed dispersal. Adult sex ratio (ASR) variation may affect dispersal by limiting breeding vacancies available to the more abundant sex, and cooperative breeders often have a more biased ASR than noncooperative breeders. However, no studies of cooperative breeders have related ASR at both the local and population level with dispersal. Using the long-term Seychelles warbler, Acrocephalus sechellensis, data set, we tested the influence of population-wide and local ASR, and density, on natal dispersal of yearlings. Our ASR - density hypothesis predicts that the probability of natal dispersal is lower when the ASR is biased towards the sex of the focal individual, but only when the population density is high. Dispersal was associated with population density and population-wide ASR in males, but not in females; males were more likely to delay dispersal when ASR was male biased and density was high. Our findings illustrate a complex association between demographic factors and cooperative breeding and suggest that individuals alter their dispersal behaviour in response to the demographic composition of the population.

Abstract Environmental conditions experienced during early life may have long‐lasting effects on later‐life phenotypes and fitness. Individuals experiencing poor early‐life conditions may suffer subsequent fitness constraints. Alternatively, individuals may use a strategic “Predictive Adaptive Response” (PAR), whereby they respond—in terms of physiology or life‐history strategy—to the conditions experienced in early life to maximize later‐life fitness. Particularly, the Future Lifespan Expectation (FLE) PAR hypothesis predicts that when poor early‐life conditions negatively impact an individual's physiological state, it will accelerate its reproductive schedule to maximize fitness during its shorter predicted life span. We aimed to measure the impact of early‐life conditions and resulting fitness across individual lifetimes to test predictions of the FLE hypothesis in a wild, long‐lived model species. Using a long‐term individual‐based dataset, we investigated how early‐life conditions are linked with subsequent fitness in an isolated population of the Seychelles warbler Acrocephalus sechellensis. How individuals experience early‐life environmental conditions may vary greatly, so we also tested whether telomere length—shorter telomers are a biomarker of an individual's exposure to stress—can provide an effective measure of the individual‐specific impact of early‐life conditions. Specifically, under the FLE hypothesis, we would expect shorter telomeres to be associated with accelerated reproduction. Contrary to expectations, shorter juvenile telomere length was not associated with poor early‐life conditions, but instead with better conditions, probably as a result of faster juvenile growth. Furthermore, neither juvenile telomere length, nor other measures of early‐life conditions, were associated with age of first reproduction or the number of offspring produced during early life in either sex. We found no support for the FLE hypothesis. However, for males, poor early‐life body condition was associated with lower first‐year survival and reduced longevity, indicating that poor early‐life conditions pose subsequent fitness constraints. Our results also showed that using juvenile telomere length as a measure of early‐life conditions requires caution, as it is likely to not only reflect environmental stress but also other processes such as growth.

Even in well-studied organisms, it is often challenging to uncover the social and environmental determinants of fitness. Typically, fitness is determined by a variety of factors that act in concert, thus forming complex networks of causal relationships. Moreover, even strong correlations between social and environmental conditions and fitness components may not be indicative of direct causal links, as the measured variables may be driven by unmeasured (or unmeasurable) causal factors. Standard statistical approaches, like multiple regression analyses, are not suited for disentangling such complex causal relationships. Here, we apply structural equation modeling (SEM), a technique that is specifically designed to reveal causal relationships between variables, and which also allows to include hypothetical causal factors. Therefore, SEM seems ideally suited for comparing alternative hypotheses on how fitness differences arise from differences in social and environmental factors. We apply SEM to a rich data set collected in a long-term study on the Seychelles warbler (Acrocephalus sechellensis), a bird species with facultatively cooperative breeding and a high rate of extra-group paternity. Our analysis reveals that the presence of helpers has a positive effect on the reproductive output of both female and male breeders. In contrast, per capita food availability does not affect reproductive output. Our analysis does not confirm earlier suggestions on other species that the presence of helpers has a negative effect on the reproductive output of male breeders. As such, both female and male breeders should tolerate helpers in their territories, irrespective of food availability.

Communal breeding, wherein multiple conspecifics live and reproduce together, may generate short-term benefits in terms of defence and reproduction. However, its carry-over effects remain unclear. We experimentally tested the effects of communal breeding on parental care and reproduction in burying beetles (Nicrophorus vespilloides), which use carcasses as breeding resources and provide parental care to offspring. We subjected individuals to communal or non-communal breeding (i.e. pair breeding) during their first breeding event and to non-communal breeding during their second breeding event. We measured the parental care of individuals and of groups and the reproductive success of groups during both breeding events. In communal groups, large individuals became dominant and largely monopolized the carcass, whereas small individuals (i.e. subordinates) had restricted access to the carcass. At the first breeding event, large males in communal groups spent more time providing care than large males in non-communal groups, whereas such an effect was not observed for large females and small individuals. Reproductive successes were similar in communal and non-communal groups, indicating no short-term benefits of communal breeding in terms of reproduction. Compared with males from non-communal groups, males originating from communal groups produced a larger size of brood during their second breeding event, whereas such an effect was not observed for females. Our results demonstrate the sex-specific effects of communal breeding experience on parenting performance and fitness.

In socially monogamous species, extra-pair paternity (EPP) is predicted to increase variance in male reproductive success (RS) beyond that resulting from genetic monogamy, thus, increasing the "opportunity for selection" (maximum strength of selection that can act on traits).This prediction is challenging to investigate in wild populations because lifetime reproduction data are often incomplete.Moreover, age-specific variances in reproduction have been rarely quantified.We analyzed 21 years of near-complete social and genetic reproduction data from an insular population of Seychelles warblers (Acrocephalus sechellensis).We quantified EPP's contribution to lifetime and age-specific opportunities for selection in males.We compared the variance in male genetic RS vs social ("apparent") RS (RS ap ) to assess if EPP increased the opportunity for selection over that resulting from genetic monogamy. Despite not causinga statistically significant excess (19%) of the former over the latter, EPP contributed substantially (27%) to the variance in lifetime RS, similarly to within-pair paternity (WPP, 39%) and to the positive WPP-EPP covariance (34%).Partitioning the opportunity for selection into age-specific (co)variance components, showed that EPP also provided a substantial contribution at most ages, varying with age.Therefore, despite possibly not playing the main role in shaping sexual selection in Seychelles warblers, EPP provided a substantial contribution to the lifetime and age-specific opportunity for selection, which can influence evolutionary processes in age-structured populations.

Parental age can have considerable effects on offspring phenotypes and health. However, intergenerational effects may also have longer term effects on offspring fitness. Few studies have investigated parental age effects on offspring fitness in natural populations while also testing for sex- and environment-specific effects. Further, longitudinal parental age effects may be masked by population-level processes such as the selective disappearance of poor-quality individuals. Here, we used multigenerational data collected on individually marked Seychelles warblers (Acrocephalus sechellensis) to investigate the impact of maternal and paternal age on offspring life span and lifetime reproductive success. We found negative effects of maternal age on female offspring life span and lifetime reproductive success, which were driven by within-mother effects. There was no difference in annual reproductive output of females born to older versus younger mothers, suggesting that the differences in offspring lifetime reproductive success were driven by effects on offspring life span. In contrast, there was no association between paternal age and female offspring life span or either maternal or paternal age and male offspring life span. Lifetime reproductive success, but not annual reproductive success, of male offspring increased with maternal age, but this was driven by between-mother effects. No paternal age effects were found on female offspring lifetime reproductive success but there was a positive between-father effect on male offspring lifetime reproductive success. We did not find strong evidence for environment-dependent parental age effects. Our study provides evidence for parental age effects on the lifetime fitness of offspring and shows that such effects can be sex dependent. These results add to the growing literature indicating the importance of intergenerational effects on long-term offspring performance and highlight that these effects can be an important driver of variation in longevity and fitness in the wild.

Behavioural patterns often differ consistently across individuals and are linked to fitness. In species with biparental care, the defence behaviour of both parents can affect reproductive success through offspring survival. In addition to the intensity of defence behaviour by both pair members, the similarity in this behaviour among parents may affect offspring survival. However, few studies have investigated the relative impact of both the intensity and similarity of defence behaviour. Here, we examined nest defence behaviour of males and females during the incubation stage in an Arctic population of barnacle geese Branta leucopsis . We calculated the repeatability of defence behaviour to test whether this behaviour is consistent within individuals and investigated how it is associated with age. In addition, we investigated how daily survival rate (DSR) of the nests until hatching is associated with nest defence behaviour and age of the parents, as well as the effect of parent similarity in nest defence behaviour as an emergent trait of the pair bond. Both male and female defence behaviour were highly repeatable. The ages of both partners within breeding pairs were positively related, but age was only significantly associated with defence behaviour in females. Further, we found high similarity in defence behaviour within breeding pairs, but the similarity and intensity of defence behaviour within breeding pairs did not predict DSR. Finally, male defence behaviour positively predicted DSR, but female defence behaviour and male and female age did not. Our results suggest that nest protection is adaptive in males but behavioural similarity of pair members does not enhance nest survival, indicating behavioural similarity itself is not adaptive but rather a by‐product of different effects.

The environment experienced during development, and its impact on intrinsic condition, can have lasting outcomes for individual phenotypes and could contribute to variation in adult senescence trajectories. However, the nature of this relationship in wild populations remains uncertain, owing to the difficulties in summarizing natal conditions and in long-term monitoring of individuals from free-roaming long-lived species. Utilizing a closely monitored, closed population of Seychelles warblers (Acrocephalus sechellensis), we determine whether juvenile body mass is associated with natal socioenvironmental factors, specific genetic traits linked to fitness in this system, survival to adulthood, and senescence-related traits. Juveniles born in seasons with higher food availability and into smaller natal groups (i.e., fewer competitors) were heavier. In contrast, there were no associations between juvenile body mass and genetic traits. Furthermore, size-corrected mass-but not separate measures of natal food availability, group size, or genetic traits-was positively associated with survival to adulthood, suggesting juvenile body mass is indicative of natal condition. Heavier juveniles had greater body mass and had higher rates of annual survival as adults, independent of age. In contrast, there was no association between juvenile mass and adult telomere length attrition (a measure of somatic stress) nor annual reproduction. These results indicate that juvenile body mass, while not associated with senescence trajectories, can influence the likelihood of surviving to old age, potentially due to silver-spoon effects. This study shows that measures of intrinsic condition in juveniles can provide important insights into the long-term fitness of individuals in wild populations.

Understanding where and how genetic variation is maintained within populations is important from an evolutionary and conservation perspective. Signatures of past selection suggest that pathogen-mediated balancing selection is a key driver of immunogenetic variation, but studies tracking contemporary evolution are needed to help resolve the evolutionary forces and mechanism at play. Previous work in a bottlenecked population of Seychelles warblers (Acrocephalus sechellensis) show that functional variation has been maintained at the viral-sensing Toll-like receptor 3 (TLR3) gene. Here, we characterise evolution at this TLR3 locus over a 25-year period within the original remnant population of the Seychelles warbler, and in four other derived, contained populations. Results show a significant and consistent temporal decline in the frequency of the TLR3C allele in the original population, and that similar declines in the TLR3C allele frequency occurred in all the derived populations. Individuals (of both sexes) with the TLR3CC genotype had lower survival, and males - but not females - that carry the TLR3C allele had significantly lower lifetime reproductive success than those with only the TLR3A allele. These results indicate that positive selection, caused by an as yet unknown agent, is driving TLR3 evolution in the Seychelles warblers. No evidence of heterozygote advantage was detected. However, whether the positive selection observed is part of a longer-term pattern of balancing selection (through fluctuating selection or rare-allele advantage) cannot be resolved without tracking the TLR3C allele in the populations over an extended period of time.

Understanding where genetic variation exists, and how it influences fitness within populations is important from an evolutionary and conservation perspective. Signatures of past selection suggest that pathogen-mediated balancing selection is a key driver of immunogenetic variation, but studies tracking contemporary evolution are needed to help resolve the evolutionary forces and mechanism at play. Previous work in a bottlenecked population of Seychelles warblers (Acrocephalus sechellensis) show that functional variation has been maintained at the viral-sensing Toll-like receptor 3 (TLR3) gene, including one nonsynonymous SNP, resulting in two alleles. Here, we characterise evolution at this TLR3 locus over a 25-year period within the original remnant population of the Seychelles warbler, and in four other derived, populations. Results show a significant and consistent temporal decline in the frequency of the TLR3C allele in the original population, and that similar declines in the TLR3C allele frequency occurred in all the derived populations. Individuals (of both sexes) with the TLR3CC genotype had lower survival, and males - but not females - that carry the TLR3C allele had significantly lower lifetime reproductive success than those with only the TLR3A allele. These results indicate that positive selection on the TLR3A allele, caused by an as yet unknown agent, is driving TLR3 evolution in the Seychelles warbler. No evidence of heterozygote advantage was detected. However, whether the positive selection observed is part of a longer-term pattern of balancing selection (through fluctuating selection or rare-allele advantage) cannot be resolved without tracking the TLR3C allele over an extended time period.

Offspring from elderly parents often have lower survival due to parental senescence. In cooperatively breeding species, where offspring care is shared between breeders and helpers, the alloparental care provided by helpers is predicted to mitigate the impact of parental senescence on offspring provisioning and, subsequently, offspring survival. We test this prediction using data from a long-term study on cooperatively breeding Seychelles warblers (Acrocephalus sechellensis). We find that the nestling provisioning rate of female breeders declines with their age. Further, the total brood provisioning rate and the first-year survival probability of offspring decline progressively with age of the female breeder, but these declines are mitigated when helpers are present. This effect does not arise because individual helpers provide more care in response to the lower provisioning of older dominant females, but because older female breeders have recruited more helpers, thereby receiving more overall care for their brood. We do not find such effects for male breeders. These results indicate that alloparental care can alleviate the fitness costs of senescence for breeders, which suggests an interplay between age and cooperative breeding.

Abstract Understanding trade‐offs in wild populations is difficult, but important if we are to understand the evolution of life histories and the impact of ecological variables upon them. Markers that reflect physiological state and predict future survival would be of considerable benefit to unraveling such trade‐offs and could provide insight into individual variation in senescence. However, currently used markers often yield inconsistent results. One underutilized measure is hematocrit, the proportion of blood comprising erythrocytes, which relates to the blood's oxygen‐carrying capacity and viscosity, and to individual endurance. Hematocrit has been shown to decline with age in cross‐sectional studies (which may be confounded by selective appearance/disappearance). However, few studies have tested whether hematocrit declines within individuals or whether low hematocrit impacts survival in wild taxa. Using longitudinal data from the Seychelles warbler (Acrocephalus sechellensis), we demonstrated that hematocrit increases with age in young individuals (<1.5 years) but decreases with age in older individuals (1.5–13 years). In breeders, hematocrit was higher in males than females and varied relative to breeding stage. High hematocrit was associated with lower survival in young individuals, but not older individuals. Thus, while we did not find support for hematocrit as a marker of senescence, high hematocrit is indicative of poor condition in younger individuals. Possible explanations are that these individuals were experiencing dehydration and/or high endurance demands prior to capture, which warrants further investigation. Our study demonstrates that hematocrit can be an informative metric for life‐history studies investigating trade‐offs between survival, longevity, and reproduction.

Extra-pair paternity (EPP) is often linked to male age in socially monogamous vertebrates; that is, older males are more likely to gain EPP and less likely to be cuckolded. However, whether this occurs because males improve at gaining paternity as they grow older, or because "higher quality" males that live longer are preferred by females, has rarely been tested, despite being central to our understanding of the evolutionary drivers of female infidelity. Moreover, how extra-pair reproduction changes with age within females has received even less attention. Using 18 years of longitudinal data from an individually marked population of Seychelles warblers (Acrocephalus sechellensis), we found considerable within-individual changes in extra-pair reproduction in both sexes: an early-life increase and a late-life decline. Furthermore, males were cuckolded less as they aged. Our results indicate that in this species age-related patterns of extra-pair reproduction are determined by within-individual changes with age, rather than differences among individuals in longevity. These results challenge the hypothesis-based on longevity reflecting intrinsic quality-that the association between male age and EPP is due to females seeking high-quality paternal genes for offspring. Importantly, EPP accounted for up to half of male reproductive success, emphasizing the male fitness benefits of this reproductive strategy. Finally, the occurrence of post-peak declines in extra-pair reproduction provides explicit evidence of senescence in infidelity in both males and females.

It remains poorly understood how effects of anthropogenic activity, such as large-scale habitat fragmentation, impact sociality in animals. In cooperatively breeding species, groups are mostly formed through delayed offspring dispersal, and habitat fragmentation can affect this process in two opposite directions. Increased habitat isolation may increase dispersal costs, promoting delayed dispersal. Alternatively, reduced patch size and quality may decrease benefits of philopatry, promoting dispersal. Here, we test both predictions in a cooperatively breeding bird (placid greenbul, Phyllastrephus placidus) from an Afrotropical cloud forest archipelago. Males born in fragmented forest dispersed about 1 year earlier than those born in continuous forest. Contrary to females, males also started to reproduce earlier and mostly settled within their natal patch. Females only rarely delayed their dispersal for more than 1 year, both in fragmented and continuous forests. Our results suggest that early male dispersal and reproduction is jointly driven by a decrease in the value of the natal territory and an increase in local breeding opportunities in fragmented forest. While plasticity in dispersal strategies of cooperative breeders in response to anthropogenic change is believed to optimize reproduction-survival trade-offs, to what extent it shapes the ability of species to respond to rapid environmental change remains to be studied.

Life-history theory predicts a trade-off between current and future reproduction to maximize lifetime fitness. In cooperatively breeding species, where offspring care is shared between breeders and helpers, helper presence may influence the female breeders' egg investment, and consequently, survival and future reproductive success. For example, female breeders may reduce egg investment in response to helper presence if this reduction is compensated by helpers during provisioning. Alternatively, female breeders may increase egg investment in response to helper presence if helpers allow the breeders to raise more or higher quality offspring successfully. In the facultatively cooperative-breeding Tibetan ground tit Pseudopodoces humilis, previous studies found that helpers improve total nestling provisioning rates and fledgling recruitment, but have no apparent effects on the number and body mass of fledglings produced, while breeders with helpers show reduced provisioning rates and higher survival. Here, we investigated whether some of these effects may be explained by female breeders reducing their investment in eggs in response to helper presence. In addition, we investigated whether egg investment is associated with the female breeder's future fitness. Our results showed that helper presence had no effect on the female breeders' egg investment, and that egg investment was not associated with breeder survival and reproductive success. Our findings suggest that the responses of breeders to helping should be investigated throughout the breeding cycle, because the conclusions regarding the breeders' adjustment of reproductive investment in response to being helped may depend on which stage of the breeding cycle is considered.

Helping by group members is predicted to lead to delayed senescence by affecting the trade-off between current reproduction and future survival for dominant breeders. Here we investigate this prediction in the Seychelles warbler, Acrocephalus sechellensis , in which mainly female subordinate helpers (both co-breeders and non-breeding helpers) often help dominants raise offspring. We find that the late-life decline in survival usually observed in this species is greatly reduced in female dominants when a helper is present. Female dominants with a female helper show reduced telomere attrition, a measure that reflects biological ageing in this and other species. Finally, the probability of having female, but not male, helpers increases with dominant female age. Our results suggest that delayed senescence is a key benefit of cooperative breeding for elderly dominants and support the idea that sociality and delayed senescence are positively self-reinforcing. Such an effect may help explain why social species often have longer lifespans.

In cooperatively breeding species, care provided by helpers may affect the dominant breeders' investment trade-offs between current and future reproduction. By negatively compensating for such additional care, breeders can reduce costs of reproduction and improve their own chances of survival. Alternatively, helper care can be additive to that of dominants, increasing the fledging fitness of the current brood. However, the influence helpers have on brood care may be affected by group size and territory quality. Therefore, the impact of helping needs to be disentangled from other factors determining offspring investment before conclusive inferences about the effect of help on additive and compensatory care can be made. We used 20 years of provisioning data to investigate the effect of helping on provisioning rates in the facultative cooperatively breeding Seychelles warbler Acrocephalus sechellensis. Our extensive dataset allowed us to statistically disentangle the effects of helper presence, living in larger groups and different food availability. We show compensatory and additive care (i.e., partial compensation) in response to helper provisioning. Helpers lightened the provisioning load of the dominant male and female and increased total provisioning to nestlings. This was irrespective of group size or territory quality (food availability). Moreover, our results illustrate sex-specific variation in parental care over the course of the breeding event. We discriminate between temporal variation, group size, and territory quality processes affecting cooperative care and as such, gain further insight into the importance of these factors to the evolutionary maintenance of helping behavior.

Haemoparasitic infections in wild birds have been widely studied in order to understand parasite–host relation- ships, but our understanding of their impact on the host's fitness is still limited. In this study, we quantified the associations between microfilarial infections and body mass and between microfilarial infections and haemato- crit values in Village Weavers Ploceus cucullatus. We screened blood samples (thin smear and buffy coat) for the presence of haemoparasitic infections and measured haematocrit values. Fifty-seven percent of 91 individuals screened carried a microfilarial infection but, contrary to expectations, no other haemoparasitic infections were detected in the blood samples. We found a negative relationship between microfilarial infection status and body mass, but no relationship between infection status and haematocrit values. Our results suggest that microfilarial infections may be associated with body mass loss in wild birds.

Dear editor, Mimetic protection is most effective when mimics are relatively rare (Pfennig et al., 2001). In polymorphic damselfly species, male-colored female morphs may avoid costly male mating attempts because they are not immediately recognized as a suitable mating partner (van Gossum et al., 2008). We investigated morphological resemblance of male-colored females to males across six populations of the polymorphic blue-tailed damselfly Ischnura elegans (Vander Linden). We found that male-colored females resembled males more closely with an increasing ratio of male-colored females to other female morphs. Our results suggest that the degree of mimetic fidelity is frequencydependent. Genetically determined color polymorphisms have evolved throughout the tree of life and studying them helps in understanding the selective forces that affect the maintenance of genetic diversity (Gray & McKinnon, 2007). In some species, color polymorphism is limited to one sex. Such sex-limited color polymorphisms has probably evolved in response to sex-specific predation, sexual competition, or sexual conflict (Stamps & Gon III, 1983; Gross, 1996; Svensson et al., 2009). In I. elegans, mature males occur in one color (blue) and mature females occur in three genetically controlled color morphs (Sánchez-Guillén et al., 2005): infuscans (olive green), rufescens-obsoleta (brown-red) and androchrome (“male-colored”: blue or green-blue). In this species, females suffer from excessive male mating harassment, which is costly in terms of female fitness (Gosden & Svensson, 2009). Female morph frequencies differ greatly between populations (Cordero-Rivera & Sánchez-Guillén, 2007) and frequency-dependent male