Journal of Animal Ecology

AimElucidating the mechanisms shaping parasite diversity patterns is critical because parasites encompass about 40% of known species, and are crucial for ecosystem structure and function. In free-living species, diversity patterns in the Anthropocene are shaped by niche-breadth because specialists (narrow niche-breadth taxa) are more sensitive to environmental disturbance compared to generalists (broad niche-breadth taxa). Like free-living species, parasites too can be categorized as specialists or generalists according to their niche-breadth (i.e., diversity of hosts they can infect). However, unlike free-living species, the effects of niche-breadth on parasite diversity patterns remain unclear. Here, we used haemosporidian parasites as a model system to identify factors affecting parasite diversity patterns, and test if these patterns differ between specialist (Haemoproteus) and generalist (Plasmodium) parasites. LocationSouthern India TaxonHaemoproteus spp. and Plasmodium spp. (Haemosporida) MethodsBlood samples from wild birds were screened using molecular tools to identify haemosporidian parasite lineages. Statistical analyses, including random forest models and generalized dissimilarity models, were utilized to evaluate how environmental and host factors drive spatial patterns of parasite and {beta} diversity. ResultsOur results reveal that phylogenetic diversity is primarily shaped by host-related variables in the specialist parasites, but by numerous host- and environment-related factors in the generalists. In keeping with ecological theory, the specialist parasites showed higher diversity and lower evenness compared to the generalists. Additionally, while {beta} diversity of the specialist parasites was primarily driven by spatial differences in richness (e.g., taxon nestedness) rather than replacement (e.g., taxon turnover), the opposite pattern was found in the generalist. Main conclusionThe differential patterns and drivers of diversity in specialist vs. generalist parasites demonstrates why specialists parasites are good indicators of ecosystem health and elucidates the mechanism by which anthropogenic disturbance increases the risk of emerging infectious diseases which are primarily caused by generalist parasites.

Age-dependent survival is central to understanding population dynamics and life-history evolution. We analysed carcass weight and age-at-harvest data from 6022 red foxes (Vulpes vulpes) collected across Sweden between 1967 and 1971 to evaluate latitudinal effects on body mass and age-dependent survival. Carcass weights decreased from south to north in both adults and sub-adults, contrary to Bergmann's rule, with southern foxes weighing approximately 1.27 times more than northern foxes. The latitudinal weight gradient exceeded the sex difference in both age classes, and no sex x region interaction was detected. The decrease in weight with latitude is consistent with reduced prey availability and harsher winter conditions in the north, which limit growth and body size during development. Using a Bayesian age-at-harvest model with region-specific population growth rates (lambda), we estimated age-dependent survival probabilities for four latitudinal regions and both sexes. Despite the strong latitudinal gradient in weight, survival did not show a corresponding pattern - regional differences were uncertain, with all credible intervals spanning zero. Regional population growth rates were consistent with slight decline in the north and near-stability in the south-central region, which suggests that body condition and population dynamics are coupled at the regional scale despite no survival gradient. The decoupling of body condition and survival across regions suggests that mortality patterns are similar across the latitudinal gradient. We discuss these patterns in terms of latitudinal productivity gradients, prey availability, and life-history trade-offs in a widely distributed carnivore.

Social relationships are consequential and affect survival, longevity and reproductive success in many species. Prior studies have successfully identified the fitness consequences of variation in sociality relative to other individuals. However, we have yet to quantify the fitness consequences of social variation within an individual over time. By using a within-individual centering approach, we can identify the consequences of deviations from an individuals average sociality. This approach has not been applied to the study of animal sociality. We used 20 years of data from a population of yellow-bellied marmots (Marmota flaviventer) to understand the fitness consequences of an individuals average affiliative social network position over their lifetime, and of individuals annual deviations from their average social position. We found that an individuals deviation from its average sociality was very strongly associated with both winter and summer survival. Deviations from some average measures of sociality were also associated with the likelihood of reproducing, but not in a consistent direction. Lifetime average social network positions had limited associations with survival and reproduction. Interestingly, litter size results varied between males to females; neither lifetime averages of social network traits nor deviations from these averages were associated with litter size in females, but deviations in sociality were associated with larger numbers of weaned offspring in males. Future studies of the consequences of social relationships may benefit from the use of an individual-centering approach, and further work is required to understand the environmental and social drivers of this intra-individual adaptive plasticity. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=184 SRC="FIGDIR/small/728517v1_ufig1.gif" ALT="Figure 1"> View larger version (44K): org.highwire.dtl.DTLVardef@380d04org.highwire.dtl.DTLVardef@1deba24org.highwire.dtl.DTLVardef@18ea2dborg.highwire.dtl.DTLVardef@165eb20_HPS_FORMAT_FIGEXP M_FIG C_FIG HighlightsO_LILifetime averages of social network traits are not associated with fitness C_LIO_LIAnnual deviations from lifetime average sociality improves survival and reproduction C_LIO_LIIntra-individual behavioral plasticity may be an understudied fitness driver C_LIO_LIWithin-individual centering is a useful approach for the study of animal behavior C_LI

Identifying how and why species vary in their ability to adjust to rapidly changing climates is a key challenge in ecology. While phenological shifts are well documented for birds and often studied in the context of tracking resource availability, less is known about the extent to which adjustments in phenology allow populations to track a consistent thermal niche. In particular, there has been little examination of how the extent of phenological thermal niche tracking compares over time versus space; a comparison that has the potential to inform on the underlying mechanisms. Here, we use data on breeding phenology derived from BTO Nest Record Scheme data, to examine the extent to which 13 passerine bird species track a consistent incubation thermal niche across years (both interannually and a year gradient) and along latitudinal and elevational gradients, and whether migrant and resident species differ in their tracking ability. Overall, we found support across species for partial tracking, with all species showing trends consistent with partial tracking across one or more axis, though for one species we could not reject the null hypothesis of no tracking. When we looked at average trends across species, we found significant tracking across interannual variation, latitude, and elevation, but not across a year trend. However, we found no evidence that tracking differs between residents and migrants, and for only a few species did we found evidence that species incubation thermal niche impacts on fitness. Taken together, our findings highlight the extent to which shifts in phenology can allow birds to track a thermal niche in a changing climate. The timing of a thermal niche provides a useful and widely-applicable yardstick to examine how changes in climate will impact on the abiotic conditions that populations experience.

Reproduction in species with parental care involves sustaining a brood of offspring through an energetically demanding period, when shifts in resource availability, weather, predation risk, and parental condition can strongly alter offspring survival. The most extreme outcome is complete brood failure (death of all offspring), which is relatively frequent in many bird species and may occur when conditions cross a viability threshold. Although complete brood failure is important for shaping fitness variation and population dynamics, we have limited understanding of how intra- and interspecific density dependence governs these events, or of how factors such as habitat quality and disease burden contribute to them, because deriving this requires fine-scale, individual-level data collected across generations for multiple overlapping species. Using a dataset totalling 38,509 nesting attempts from great tits (Parus major) and blue tits (Cyanistes caeruleus) in Wytham Woods, Oxford, UK, we examined how brood failure is shaped by local conspecific and heterospecific density, habitat structure, and avian malaria infection for a subset. Complete brood failure was frequent (14.75%), mostly involving chick mortality in the nest consistent with starvation, rather than brood removal by predators. Relationships between density and brood failure were strong but species-specific. Specifically, great tit failure risk was higher in neighbourhoods that remained densely populated across years, whereas blue tit failure risk was lower where annual great tit or combined density was high, but not where annual blue tit density itself was high. This suggests that local overall density reflects continuing constraint for great tits, while local annual density may partly track favourable within-year conditions and settlement patterns for blue tits. In great tits, failure was also more common where oak density was low and farther from the closest river (Thames), while habitat associations were weak in blue tits. Malaria infection was spatially heterogeneous and covaried with density and habitat, but infection status did not significantly explain complete brood failure. Together, these results show that complete brood failure is shaped by spatially structured local ecological context, and how density dependence in these events can differ in direction and timescale between sympatric species.

Mass mortality events (MMEs) create sudden pulses of carrion that can alter how vertebrate scavengers use carcass resources, including the frequency, duration, and timing of species-carcass contacts. These changes could have implications for pathogen transmission at the scavenger-carcass interface. We aimed to develop and apply a reproducible analytical framework for using camera-trap data to quantify transmission-relevant vertebrate activity at carcass sites under differing carrion biomass scenarios. We studied experimental carcass plots (single carcass ~43 kg; 'mass mortality' plots [10 carcasses, >350 kg total]; 6 of each) in Australia's alpine ecosystem. The framework integrated descriptive summaries (bipartite network analysis, Kaplan-Meier curves) and marked temporal point-process models to characterise structural and temporal dimensions of species-carcass activity. Mass mortality plots had greater overall visitation duration, occurring as sustained activity (50% of visitation event volume by day 17), compared with intense then rapidly declining activity at single carcasses (50% by day 8). Mass mortality plots also had higher predicted daily arrival probability and contact hours across most species, indicating an extended window for pathogen transmission. This framework provides empirically derived contact parameters for MME-related disease spread models using camera-trap data to identify potential transmission pathways at the scavenger-carcass interface.

O_LILocal environmental conditions during the breeding season can limit bird populations. Identifying which variables, when, and how they affect key biological traits such as body condition is crucial for understanding long-term population trends under ongoing climate change. C_LIO_LIWe analysed the relationships between environmental variables and body condition during the breeding season in European robins (Erithacus rubecula), aiming to uncover links between short-term environmental influences and long-term trends in body condition in the context of local climate change. C_LIO_LIUsing data from a robin population monitored between 2007 and 2021, we applied weather sliding-window analyses to identify periods when temperature, soil moisture, and vegetation productivity best predicted individual body condition. For each variable, we identified critical time windows (CTWs) influencing (1) body condition across the season and (2) individual changes within two weeks. Juveniles and adults were analysed separately, with adult males and females distinguished during pre- and post-fledging periods. We also assessed long-term trends in environmental variables and body condition, and examined how body condition was correlated with apparent survival. C_LIO_LIBody condition variation across the season was explained by different environmental variables depending on age, sex, and period. Body condition declined with increasing minimum temperatures in adult males and juveniles, and with low soil moisture in adults of both sexes. We did not identify reliable CTWs explaining short-term within-individual changes in body condition. Across 2007-2021, body condition in adult males during the post-fledging period declined with rising minimum temperatures, while fledging dates advanced. Apparent survival was positively associated with body condition only in juvenile robins. C_LIO_LIOur results reveal multiple seasonal environmental influences that may contribute to short- and long-term declines in body condition in European robins, with effects particularly strong (or most detectable) in adult males. Reduced body condition may have demographic consequences by lowering juvenile survival, although shifts in breeding phenology could mitigate this impact. Overall, these findings highlight how environmental effects on body condition can shape long-term population trends and species vulnerability to climate change. C_LI

Theory predicts that virulence evolves as a consequence of selection to optimize transmission, generating a trade off in which increased exploitation enhances transmission but shortens the infectious period. Despite its central role in evolutionary epidemiology, empirical support for this virulence-transmission trade off remains limited, has focused largely on microparasites, and often overlooks variation in host immunity which can fundamentally alter links between virulence and transmission. Here, we provide a rare empirical test of virulence-transmission dynamics in a macroparasite with a complex life cycle. Using a field survey of the helminth parasite, Schistocephalus solidus, and its second intermediate host, threespine stickleback (Gasterosteus aculeatus), we quantify how host immune variation shapes relationships among parasite burden (a proxy for virulence) and transmission potential to definitive hosts, piscivorous birds. Importantly, host populations in the focal lakes span a gradient of evolved immune strategies, from low to high fibrosis, a strong anti-growth resistance mechanism. We find that variation in immune timing across host populations constrains the window in which parasites can reach transmissible stages. Subsequent changes in parasite burden scale up to alter transmission potential and reveal a nonlinear relationship consistent with a virulence-transmission trade off. Transmission potential is highest at intermediate parasite burdens, which also corresponds to intermediate immune responses. Together, this work links within-host processes to population-level epidemiological outcomes and demonstrates how host immune variation can shape virulence-transmission relationships. Incorporating immune heterogeneity may therefore help reconcile the mixed empirical support for trade off theory.

Escape behaviour directly influences survival, yet individuals often vary substantially in escape performance. Laboratory studies have documented trade-offs between anti-predator responses and life-history traits, but it remains unclear whether such trade-offs occur under natural predation risk. We studied a natural population of the field cricket Gryllus campestris. Mortality risk and behavioural performance are known to change with age in this species. We aimed to determine whether individuals expressing a higher escape response pay a cost in terms of a faster increase in mortality risk with age or a shorter lifespan. We quantified escape speed in response to a vibrational predation cue. We found no clear evidence for a trade-off between escape performance and lifespan or age-specific mortality risk. The relationship between escape speed and the among-individual effect of age differed between sexes: older males showed faster escape speeds compared with younger males, whereas younger females were faster than older females. This pattern is consistent with sex-specific selective disappearance. Individual baseline mortality risk varied with sex and escape speed, but age-dependent mortality did not. It suggests that such trade-offs in the wild may be context- or condition-dependent rather than reflecting a universal life-history trade-off.

Demographic models are crucial for uncovering the mechanisms underlying changing populations trajectories and structure and to identify the drivers of such changes. Common pochard populations of North east/North west Europe have experienced a sharp decline accompanied by an increase in male proportions among adults since the mid-1990. We used a two-sex, two-stage deterministic matrix population model, to perform a prospective perturbation analysis and to explore, through simulations, some plausible causes underlying the observed decline and change in sex structure. We show that Common pochard populations are more sensitive to changing survival than to changes in productivitys components (clutch size, nest survival...). However, due to an environmental variance much higher than that of survival, components of productivity, especially nest survival, would be the main drivers of Common pochard populations growth rate, a finding supported by empirical data. More importantly, we show that although sex-specific changes in survival at any stage of the life cycle are potent drivers of both population growth rate and changing sex ratio, there is no need to resort to them for explaining the increasing proportions of males such as observed in Common pochard. Because adult males display higher survival than adult females (0.74 against 0.64 on average), any factor affecting recruitment (nest or first year survival) increases the weight of adults into the populations and hence the proportion of males. Thus, in species displaying sex-biased mortality, such as many ducks, decreasing recruitment can underly declining population size and changes in sex structure at the same time, emphasising the importance of accounting for males in monitoring schemes and demographic models.

Arctic and Boreal raptor communities will continue to be affected by borealization and other climate change related processes, providing a challenge for ecologists predicting future sates. However, by using community assembly theory and species traits, future communities may be predictable. In this study, we analyzed variation in reproduction traits as a consequence of diet specialization for 29 raptors, 2 skuas and 3 corvids. We assessed and implemented foraging traits for specialists and generalists into predator-prey models from which successful invasion conditions were derived. Specialist raptors produced larger clutch sizes, had a higher proportion of fledged per clutch and also expressed more variation compared to generalist raptors. These results suggest a relationship between diet specialization and reproductive traits which was also observed within phylogenetic orders. Specialist owls (Strigiformes) produced higher clutch sizes with a larger clutch range compared to generalist owls. The same pattern was observed for falcons (Falconiformes). No clear difference in reproduction was observed for specialist and generalist hawks, kites and eagles (Accipitriformes). Corvids expressed clutch sizes similar to that of specialist raptors while having the lowest proportion of fledged per clutch. Differences in foraging traits between specialists and generalists could be distinguished using functional response curves. A predator-prey model parameterized with foraging trait data showed that a generalist can coexist with a resident specialist if it has access to prey unavailable to the resident specialist. Otherwise, the native specialist outcompetes the invading generalist due to foraging efficiency. The combined empirical and theoretical findings in this study show how diet specialization affects both reproduction and the potential invasion success of raptors.

Migrating bats alternate between stopover periods and directed flights. When departing from a stopover site, bats select the night, the specific time within the night, and the flight direction to resume migration. Despite their ecological importance, the factors shaping these stopover departure decisions remain poorly understood. To identify the intrinsic and environmental factors driving departure decisions and movement patterns, we tagged Nathusius pipistrelles Pipistrellus nathusii at three coastal locations in the Netherlands and tracked 178 individuals during autumn migration, using the MOTUS Wildlife Tracking System. We examined movement patterns and analysed departure probability in relation to a set of individual and environmental covariates in a Bayesian capture-recapture model in state-space formulation. Additionally, we modelled within-night variation in departure timing. Seasonal patterns were strongly influenced by reproductive behaviour, with decreased migration probability during the mating period. Regardless of their seasonal timing, bats departed under moderate tailwinds and dry conditions, optimizing energy efficiency, while avoiding crosswinds and cloud cover, enhancing navigational safety. Most individuals departed shortly after sunset, whereas headwinds delayed nocturnal departure. Movement patterns were diverse, including migration towards lower latitudes, coastal barrier movements, and long-distance roundtrips, suggesting the use of multiple movement strategies. Our study demonstrates that migration patterns in bats emerge from the interaction between intrinsic factors and external conditions, and highlights the importance of both energy efficiency and safety in shaping stopover departure decisions. The presence of multiple movement strategies complicates predictions of spatiotemporal occurrence, emphasising the need to account for behavioural variability in conservation planning, for example in the context of wind energy developments.

Frugivorous insect larvae are dependent on fruiting plants for development, leading to complex host-parasite interactions that may be influenced by many factors at various scales. We compared the relative effects of factors at the individual, neighborhood, and landscape scales in forest patches. Our results suggest that in areas like upstate New York, where agricultural land uses are dominant, individual scale factors are the most influential. Specifically, parasitism increased with host fruit crop size, but was not associated with host species richness or proximity to forest edge. Notably, the most parasitized hosts were non-native species, including Frangula alnus Mill. (Glossy Buckthorn), indicating a potential role of invasive species to shape host-parasite interactions in our system. Our results underscore the importance of host-specific traits in structuring parasitism patterns and suggest management could consider both the ecological context of host traits and the influence of invasive species at multiple scales.

Understanding how symbiosis generates biodiversity requires comprehensive insight into the underlying ecology to reveal broad evolutionary patterns, and the clownfish-anemone symbiosis serves as an excellent model. Recently, a new paradigm revealed sea anemone host associations of adult clownfish drove convergent evolution of host specialists and generalists. An alternative study downplayed host specificity in explaining clownfish evolution due partly to an incongruence between the behavior, biology, and host specificity of a single species--Amphiprion polymnus. Previously classified as a host specialist, we revisit the host specificity and ecology of A. polymnus by studying its host associations, behavioral, and dietary ecology compared to a co-occurring host generalist and specialist. By incorporating n = 358 observations, we find A. polymnus associates with five sea anemone species, including a newly documented host, and four at the adult life stage. We show that A. polymnus swim consistently far from their hosts and have broad dietary niches, an ecology that aligns with generalists. Cumulatively, our data indicate A. polymnus is a host generalist, and not a specialist as previously classified. Our findings serve to reconcile previously conflicting studies about A. polymnus, and reinforce the hypothesis that host is the key ecological variable for understanding clownfish evolution. Suitable subject areasbehaviour, ecology, evolution

Phenological shifts under climate change often arise through phenotypic plasticity and, where this is insufficient to track shifts in optimum timing, genetic adaptation may also play a role. Understanding the contributions of these two processes is critical for predicting species persistence in a changing climate. While many species show phenological plasticity, we know surprisingly little about the contributions that genetic adaptation of the plasticity reaction norm elevation (timing in the mean environment) and slope (shift in timing as a response to temperature) make to phenological shifts. With the aim of disentangling plasticity from adaptation in temperature-phenology reaction norms, we applied a statistical approach to long-term first egg-laying data from 44 Finnish bird species represented by 69 populations spanning six decades. Applying phylogenetic meta-analysis to parameter estimates obtained from the individual time series, we estimated average plasticity and adaptation effect sizes and tested whether migratory strategy, generation length, and mean laying-date explained among-species variation. Egg-laying phenology was strongly plastic, advancing by 2.5 days {degrees}C{square}{superscript 1}. We found no evidence for a steeper reaction norm between 5-year periods versus within them, consistent with no adaptation of the reaction norm elevation. However, we detected a significant steepening of slopes over time (-0.04 days {degrees}C{square}{superscript 1} year{square}{superscript 1}), consistent with plasticity across the whole study area increasing from -2.5 to -5.1 days {degrees}C{square}{superscript 1} and in the northernmost area (-0.07 days {degrees}C{square}{superscript 1} year{square}{superscript 1}) from -2 to -6.5 days {degrees}C{square}{superscript 1} over the 64-year study period. Trait analyses revealed no significant effect of migratory strategy, generation length, nor mean phenology on adaptation. We show that plasticity enables substantial short-term tracking of warming accompanied by noteworthy evidence consistent with widespread evolution of. Our approach demonstrates how observational data can help reveal evolutionary signals, offering a tool for improved understanding of the processes that underpin phenological responses.

Despite the ecological importance of ungulate migrations, we lack a complete understanding of why some ungulates migrate and others do not. Though progress has been made towards understanding differences across species and between populations, migratory behavior varies even within populations: in many populations, some individuals remain behind as residents (partial migration). Theoretical population-level work has suggested that these different migratory tactics can coexist, but such approaches stop short of providing insights into how individuals make the decision to stay or go each year. Using long-term data from three ungulate populations, we find that individuals probabilities of migrating are highly variable across years, which points to a non-trivial context-dependent decision-making process, whose underlying mechanisms must be probed via individual-level modeling. Drawing on existing knowledge, we propose a decision-making model of ungulate migration onset wherein individuals probabilistically decide to start migrating based on the local intensity of environmental and/or social cues. Residents arise as a robust collective organization phenomenon in our model. At sufficiently large population sizes, the number of residents is invariant with total population size, consistent with empirical patterns. Instead, resident numbers are influenced by the severity of the bad season, by relevant character differences among individuals, and by how individuals contribute and respond to environmental and/or social cues; for instance, when social cues contribute to decision-making in addition to environmental ones, fewer residents result, and migration is more likely to be complete. Overall, our model provides a potential mechanistic explanation for how residents might emerge within migratory ungulate populations.

Helminths are widespread parasites that can modulate host immunity, potentially increasing susceptibility to viral infections. However, evidence for these effects varies across systems and environments, and links between laboratory and wild populations remain unclear. We developed a tractable system using wood mice, Heligmosomoides spp. nematodes, and wood mouse herpes virus (WMHV) to bridge this gap. Combining laboratory and field experiments with population modelling, we examined how helminth infection, anthelmintic treatment and diet affect viral dynamics. Across lab and field data, helminth infection consistently increased WMHV risk, with stronger effects at higher worm burdens. Field results showed that anthelmintic treatment reduced viral infection, and laboratory experiments showed that improved nutrition mitigates helminth-induced increases in viral susceptibility. Our population-level modelling suggested that helminth burden-dependent facilitation can generate nonlinear effects on viral spread, dependent on helminth virulence. Our findings highlight the potential importance of helminths as facilitators of viral infections, and suggest that anthelmintic treatment may provide indirect benefits for viral control. We also show the value of integrating lab and field approaches on the same (or closely related) species, in particular the potential offered by the wood mouse - Heligmosomoides - WMHV system, to understand the drivers and consequences of host-helminth-viral interactions.

Winter can affect animal population dynamics by limiting resource availability and increasing energetic costs of movement caused by deep snow. Given the rapid alteration of snowpack properties due to climate change, quantifying how snow characteristics influence reproduction and physical condition is critical. We evaluated how snow cover duration, depth, and density affect spring body mass, reproduction probability, and subsequent autumn body mass of bighorn sheep (Ovis canadensis) using 45 years of individual-based data at Ram Mountain, Alberta, Canada, along with historical snow records reconstructed via the SNOWPACK model. Using Bayesian structural equation modeling, we quantified the direct and indirect effects of snow across different sex and age classes. Long and deep snow covers reduced spring body mass across all demographic groups, with yearlings, especially males, losing up to 0.12 kg per additional cm of snow depth. Harsh snow conditions reduced the probability of reproduction for adult females and generated a compensatory indirect effect on mass by avoiding the energetic costs of reproduction. In contrast, yearlings showed no compensatory responses and entered the following autumn in poor condition (up to 14% lighter for males and 8% for females following the deepest snow years). The impact of snow density on autumn mass of adult males was density-dependent, shifting from beneficial at low density (+0.09 kg per kg/m3) to detrimental at high density (-0.04 kg per kg/m3). The effects of snow conditions generate persistent, context-dependent carry-over effects across seasons. Our study suggests that distinct demographic groups rely on different mechanisms to cope with environmental constraints, highlighting complex, time-lagged consequences of changing winter climate on alpine herbivore populations.

Carotenoid- and melanin-based plumage coloration traits are key signals in avian communication and sexual selection as they are often thought to provide "honest" information about individual condition and fitness. These traits arise through distinct but interconnected physiological and genetic pathways. Recent work suggests that there may be a link between host-associated gut microbiota and the functional pathways leading to pigment-based plumage coloration, but this remains largely unexplored in wild populations. To address this gap, we tested whether variation in plumage coloration, as well as breeding condition, is associated with gut microbiome variation in wild populations of male Common Yellowthroats (Parulidae: Geothlypis trichas). We quantified multiple plumage coloration traits and characterized gut microbiome bacterial diversity using 16S rRNA metabarcoding. Through a comprehensive modeling framework, we found that individuals with brighter, more orange-tinted breast feathers and smaller cloacal protuberances (a proxy for breeding condition) exhibited higher gut microbiome diversity. At the taxonomic level, Methylobacterium-Methylorubrum, a carotenoid-producing bacteria, showed strong associations with multiple plumage traits, including mask area, breast feather hue, and saturation. Our results demonstrate that gut microbiome diversity is associated with variation in carotenoid-based coloration traits and breeding condition in Common Yellowthroats. More broadly, these results highlight the potential for host-microbiome interactions to shape phenotypic variation through physiological pathways in wild animal populations.

O_LIThe frequency and intensity of heat events is increasing across marine and terrestrial ecosystems. Within the same ecological community, the relative exposure and sensitivity to heat stress may vary considerably among interacting species, like predators and prey. This can be especially true for species that interact at the aquatic-terrestrial interface, as well as for interactions between primarily nocturnal and diurnal species, making it difficult to predict how such communities will respond to habitat warming. C_LIO_LIThermal limit metrics such as CTmax are often assumed to equate with ecological death because such temperatures impair behavioral activity and/or physiological functioning. Prey that are diurnally active can be more frequently exposed to temperatures that approach CTmax compared to their nocturnal predators, which may use thermal refuges during the day. Yet the impacts of daytime heat exposure on nighttime predation risk remain unknown. C_LIO_LIHere, we compared the thermal environment, performance, and heat tolerance between the predatory blue crab, Callinectus sapidus and one of its prey species, the mangrove periwinkle Littoraria anguilifera in a tropical mangrove ecosystem. We examined how exposing prey to heat stress at and below their CTmax affected their capacity to avoid predation in the field at night when predation risk is highest. C_LIO_LIWe found that acute exposure to temperatures near CTmax during the day increased the prey species susceptibility to predation during recovery at night. Although both interacting predator and prey have high thermal tolerance, prey are exposed to conditions that already reach CTmax, suggesting that current extremes in temperatures may already be influencing vulnerability to predation in this ecosystem. C_LIO_LIOur results suggest that differential exposure to sublethal heat stress in diurnal prey relative to their predator, along with the subsequent impact of these exposures on predation risk, will play a role in shaping these interacting as climate warms. C_LI