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Converging lines of inquiry from across the social and biological sciences target the adult sex ratio (ASR; the proportion of males in the adult population) as a fundamental population-level determinant of behavior. The ASR, which indicates the relative number of potential mates to competitors in a population, frames the selective arena for competition, mate choice, and social interactions. Here we review a growing literature, focusing on methodological developments that sharpen knowledge of the demographic variables underlying ASR variation, experiments that enhance understanding of the consequences of ASR imbalance across societies, and phylogenetic analyses that provide novel insights into social evolution. We additionally highlight areas where research advances are expected to make accelerating contributions across the social sciences, evolutionary biology, and biodiversity conservation.

In Supplementary Table 1 of this Article, 23 samples (B10K-DU-029-32, B10K-DU-029-33, B10K-DU-029-36 to B10K-DU-029-44, B10K-DU- 029-46, B10K-DU-029-47, B10K-DU-029-49 to B10K-DU-029-53, B10K-DU- 029-75 to B10K-DU-029-77, B10K-DU-029-80, and B10K-DU-030-03; styled in boldface in the revised table) were assigned to the incorrect institution. Supplementary Table 1 has been amended to reflect the correct source institution for these samples, and associated data (tissue, museum ID/source specimen ID, site, state/province, latitude, longitude, date collected and sex) have been updated accordingly. The original table is provided as Supplementary Information to this Amendment, and the original Article has been corrected online.

Whole-genome sequencing projects are increasingly populating the tree of life and characterizing biodiversity1-4. Sparse taxon sampling has previously been proposed to confound phylogenetic inference5, and captures only a fraction of the genomic diversity. Here we report a substantial step towards the dense representation of avian phylogenetic and molecular diversity, by analysing 363 genomes from 92.4% of bird families-including 267 newly sequenced genomes produced for phase II of the Bird 10,000 Genomes (B10K) Project. We use this comparative genome dataset in combination with a pipeline that leverages a reference-free whole-genome alignment to identify orthologous regions in greater numbers than has previously been possible and to recognize genomic novelties in particular bird lineages. The densely sampled alignment provides a single-base-pair map of selection, has more than doubled the fraction of bases that are confidently predicted to be under conservation and reveals extensive patterns of weak selection in predominantly non-coding DNA. Our results demonstrate that increasing the diversity of genomes used in comparative studies can reveal more shared and lineage-specific variation, and improve the investigation of genomic characteristics. We anticipate that this genomic resource will offer new perspectives on evolutionary processes in cross-species comparative analyses and assist in efforts to conserve species.

Social monogamy, typically characterized by the formation of a pair bond, increased territorial defense, and often biparental care, has independently evolved multiple times in animals. Despite the independent evolutionary origins of monogamous mating systems, several homologous brain regions and neuropeptides and their receptors have been shown to play a conserved role in regulating social affiliation and parental care, but little is known about the neuromolecular mechanisms underlying monogamy on a genomic scale. Here, we compare neural transcriptomes of reproductive males in monogamous and nonmonogamous species pairs of Peromyscus mice, Microtus voles, parid songbirds, dendrobatid frogs, and Xenotilapia species of cichlid fishes. We find that, while evolutionary divergence time between species or clades did not explain gene expression similarity, characteristics of the mating system correlated with neural gene expression patterns, and neural gene expression varied concordantly across vertebrates when species transition to monogamy. Our study provides evidence of a universal transcriptomic mechanism underlying the evolution of monogamy in vertebrates.

Migrating animals are thought to be important spillover sources for novel pathogens. Haemosporidians (malaria‐related parasites) are one such group of pathogens that commonly spillover into novel host communities if competent vectors are present. In birds, shorebirds (sandpipers, plovers and allies) perform some of the longest avian migrations, yet they are traditionally perceived as relatively free from haemosporidians. Although low prevalence fits several theories, such as effective immune responses or low exposure to vectors, few studies have been carried out in freshwater inland sites, where the vectors of haemosporidians (e.g. mosquitoes) are abundant, with a mixture of actively migrating (staging) and breeding hosts. Here we report the prevalence of three haemosporidian parasites, Haemoproteus, Leucocytozoon and Plasmodium, screened in 214 shorebirds from 15 species sampled in a freshwater marshland, southern Belarus. Contrary to most previous studies, we found that haemosporidians were frequent, with an overall prevalence in the community of 16.36%, including the locally breeding shorebirds (23.13%, 134 individuals of 10 species). However, actively migrating shorebirds had much lower prevalence (0.05%, 55 individuals of five species). We suggest that blood parasite infections are more common in shorebirds than currently acknowledged. Yet, actively migrating species may be free from haemosporidians or carry suppressed infections, leading to lower prevalence or even apparent absence in some species. Taken together, we theorize that a combination of sampling biases has driven our understanding of haemosporidian prevalence in shorebirds and future studies should take the migratory status of individuals into account when reporting prevalence. Furthermore, we argue that birds undergoing active migration may be less likely sources of spillover events than previously assumed. [ABSTRACT FROM AUTHOR]

Size differences between males and females are common across the tree of life (termed sexual size dimorphism; SSD), and have fundamental implications for ecology, life history and behaviour of both sexes. Conventionally, SSD is thought to evolve in response to sex‐specific sexual selection but more recent work suggests that ecological processes can also promote sex‐differences in size. Here, we provide a global test for the role of sexual selection in the evolution of sexual size dimorphism using data from 77 comparative studies spanning the major classes of the animal kingdom. We show that intense sexual selection typically correlates with male‐biased SSD across species. Importantly, pre‐copulatory but not post‐copulatory sexual selection predicts SSD, suggesting a pervasive role of premating male–male competition and female choice to drive sex differences in body size. Collectively, our findings suggest that pre‐copulatory sexual selection plays a major role in the evolution of male‐biased SSD. [ABSTRACT FROM AUTHOR]

A combination of life history traits and environmental conditions has been highlighted as the main drivers of avian breeding success. While drivers of breeding success are well known in some species, especially birds in northern, temperate regions; species in other parts of the world have received relatively little attention. In this study, we used a long‐term dataset on breeding success of tropical plovers from south‐west Madagascar to investigate whether nest survival changed over time and whether the drivers of nest survival were similar for multiple species breeding in the same arid habitat. In the 12‐year period of 2009–2020, we monitored 2077 nests for three species with different breeding strategies: 1185 nests of Kittlitz's plovers (Anarhynchus pecuarius) with a flexible breeding strategy and uniparental care; and 565 nests of white‐fronted plovers (A. marginatus) and 327 nests of Madagascar plovers (A. thoracicus) which both have biparental care. We found that nest survival was associated with a combination of clutch‐size, age of nest and year among the three plover species. In addition, annual variation in climatic conditions associated with El Niño/La Niña events were included in the most supported survival models for Kittlitz's and white‐fronted plovers, but the effects were not significant. Overall estimates of daily nest survival were similar for all three species: Kittlitz's plover: 0.950 ± 0.002 SE, Madagascar plover: 0.919 ± 0.007 SE, and white‐fronted plover: 0.862 ± 0.047 SE. Estimates of nest success for the breeding season, based on increases in daily nest survival with the clutch age during the incubation periods (26 days for Kittlitz's plovers and 29 days for Madagascar and white‐fronted plovers), were relatively low: Kittlitz's plover: 0.161 ± 0.056 SE, Madagascar plover: 0.287 ± 0.022 SE, and white‐fronted plover: 0.228 ± 0.019 SE. All three species had a combination of factors affecting nest survival, both environmental and life history traits. [ABSTRACT FROM AUTHOR]

Males and females often differ in ecology, behaviour and lifestyle, and these differences are expected to lead to sex differences in parasite susceptibility. However, neither the sex differences in parasite prevalence, nor their ecological and evolutionary drivers have been investigated across a broad range of taxa using phylogenetically corrected analyses. Using the most extensive dataset yet that includes 755 prevalence estimates from 151 wild bird species in a meta-analytic framework, here we compare sex differences in blood and gastrointestinal parasites. We show that despite sex differences in parasite infection being frequently reported in the literature, only Haemoproteus infections were more prevalent in females than in males. Notably, only seasonality was strongly associated with the sex-specific parasite prevalence of both Leucocytozoon and Haemoproteus, where birds showed greater female bias in prevalence during breeding periods compared to the non-breeding period. No other ecological or sexual selection variables were associated with sex-specific prevalence of parasite prevalence. We suggest that much of the variation in sex-biased prevalence could be idiosyncratic, and driven by local ecology and behavioural differences of the parasite and the host. Therefore, breeding ecology and sexual selection may only have a modest influence on sex-different parasite prevalence across wild birds. [ABSTRACT FROM AUTHOR]

Adult sex ratio (ASR, the proportion of males in the adult population) is a central concept in population and evolutionary biology, and is also emerging as a major factor influencing mate choice, pair bonding and parental cooperation in both human and non-human societies. However, estimating ASR is fraught with difficulties stemming from the effects of spatial and temporal variation in the numbers of males and females, and detection/capture probabilities that differ between the sexes. Here, we critically evaluate methods for estimating ASR in wild animal populations, reviewing how recent statistical advances can be applied to handle some of these challenges. We review methods that directly account for detection differences between the sexes using counts of unmarked individuals (observed, trapped or killed) and counts of marked individuals using mark-recapture models. We review a third class of methods that do not directly sample the number of males and females, but instead estimate the sex ratio indirectly using relationships that emerge from demographic measures, such as survival, age structure, reproduction and assumed dynamics. We recommend that detection-based methods be used for estimating ASR in most situations, and point out that studies are needed that compare different ASR estimation methods and control for sex differences in dispersal.This article is part of the themed issue 'Adult sex ratios and reproductive decisions: a critical re-examination of sex differences in human and animal societies'.

In birds, males are homogametic and carry two copies of the Z chromosome ('ZZ'), while females are heterogametic and exhibit a 'ZW' genotype. The Z chromosome evolves at a faster rate than similarly sized autosomes, a phenomenon termed 'fast-Z evolution'. This is thought to be caused by two independent processes—greater Z chromosome genetic drift owing to a reduced effective population size, and stronger Z chromosome positive selection owing to the exposure of partially recessive alleles to selection. Here, we investigate the relative contributions of these processes by considering the effect of role-reversed polyandry on fast-Z in shorebirds, a paraphyletic group of wading birds that exhibit unusually diverse mating systems. We find stronger fast-Z effects under role-reversed polyandry, which is consistent with particularly strong selection on polyandrous females driving the fixation of recessive beneficial alleles. This result contrasts with previous research in birds, which has tended to implicate a primary role of genetic drift in driving fast-Z variation. We suggest that this discrepancy can be interpreted in two ways—stronger sexual selection acting on polyandrous females overwhelms an otherwise central role of genetic drift, and/or sexual antagonism is also contributing significantly to fast-Z and is exacerbated in sexually dimorphic species. [ABSTRACT FROM AUTHOR]

Global biodiversity loss is a major environmental concern. The wildlife on islands are particularly vulnerable to threats posed by alien predators, habitat loss and overexploitation. Effective conservation management of vulnerable species requires reliable information on vital population rates for all life stages and an understanding of key environmental drivers. However, demographic data are often not available for island populations before they decline or are extirpated. Here, we use Cormack–Jolly–Seber (CJS) models and 15 years of data for 1370 juveniles and 687 adults to estimate apparent survival for a genetically distinct resident population of Kentish plovers Charadrius alexandrinus on the island of Maio, Cabo Verde. We report two main findings. First, environmental conditions have a large effect on demographic performance since chicks that hatch during dry years experience a tenfold reduction in first‐year survival compared to chicks that hatch during wet years. Second, female and male plovers in Maio are expected to live for 7.41 ± 0.69 (mean ± SE) years and 6.75 ± 0.64 years, respectively, due to relatively high annual survival among comparable‐sized shorebirds. High adult survival thus could buffer the population against low reproductive success that this population has experienced over the last decade. Cabo Verde is typical of tropical islands with increased development that can impact native breeders and/or will accelerate habitat loss. Thus, more frequent droughts associated with climate change may exacerbate the prospects of native wildlife on many islands. [ABSTRACT FROM AUTHOR]

Preen oil, the secretion from the uropygial gland of birds, may have a specific function in incubation. Consistent with this, during incubation, the chemical composition of preen oil is more likely to differ between sexes in species where only one sex incubates than in species where both sexes incubate. In this study, we tested the generality of this apparent difference, by investigating sex differences in the preen oil composition of a shorebird species, the Kentish plover (Anarhynchus, formerly Charadrius, alexandrinus). As both sexes incubate in this species, we predicted the absence of sex differences in preen oil composition during incubation. In the field, we sampled preen oil from nine females and 11 males during incubation, which we analysed with gas chromatography–mass spectrometry (GC–MS). Consistent with predictions, we found no sex difference in preen oil composition, neither in beta diversity (Bray-Curtis dissimilarities) nor in alpha diversity (Shannon index and number of substances). Based on these results, we cannot conclude whether preen oil has a function during incubation in Kentish plovers. Still, we discuss hypothetical roles, such as olfactory crypsis, protection against ectoparasites or olfactory intraspecific communication, which remain to be tested. [ABSTRACT FROM AUTHOR]

Males and females often exhibit different behaviours during mate acquisition, pair‐bonding and parenting, and a convenient label to characterize these behaviours is sex role. The diverse roles that male and female shorebirds (plovers, sandpipers and allies) exhibit in mating and parenting have played a key role in advancing mainstream theories in avian ecology and behavioural biology including sexual selection, sexual conflict and parental cooperation. Recent advances in shorebird research have also highlighted the significance of the social environment in driving sex role behaviours by linking the adult sex ratio with breeding behaviour and population demography. Here we review the key advances in sex role research using shorebirds as an ecological model system. We identify knowledge gaps and argue that shorebirds have untapped potential to accelerate diverse research fields including evolutionary genomics, movement ecology, social networks and environmental changes. Future studies of sex roles will benefit from individual‐based monitoring using advanced tracking technologies, and from multi‐team collaborations that are facilitated by standardized data collection methodologies across different species in the field. These advances will not only contribute to our understanding of reproductive strategies, but they will also have knock‐on effects on predicting population resilience to environmental changes and on prioritizing species for conservation. [ABSTRACT FROM AUTHOR]

Agricultural intensification has affected wildlife across Europe, triggering steep declines and regional extinctions of farmland birds. Effective conservation activities are essential for the preservation of biodiversity in an agricultural landscape, but current efforts have not succeeded in halting these declines. Here we investigate a ground-nesting shorebird, the collared pratincole Glareola pratincola , which has shifted its habitat use in Central Europe over the last 20 years from alkaline grasslands to intensively managed agricultural fields. We show that nesting success was different between three agricultural habitat types, with the highest nesting success in fallow lands and the lowest in row crops. Nesting success was also associated with the timing of breeding and breeding density, as nests produced early in the breeding season and those in high-breeding-density areas hatched more successfully than those produced later in the season and at low density. We implemented direct conservation measures including marking nests and negotiating with farmers to avoid cultivating the field between nest markers, controlling nest predators and, most recently, creating suitable nesting sites and foraging areas for pratincoles. As a result of these conservation actions, nest survival increased from 11.2% to 83.5% and the size of the breeding population increased from 13 to 56 pairs during 2012–2021. Thus, we show that agricultural landscapes can continue to provide suitable habitats, and targeted conservation actions have the potential to reverse the declines of farmland species. [ABSTRACT FROM AUTHOR]