Journal of Animal Ecology

O_LISeasonal forcing is a dominant factor shaping host-pathogen interactions and disease dynamics across many wildlife systems, including species impacted by environmentally transmitted parasites. How seasonality in parasite dynamics translates to the host when the infection and disease impacts operate at different timescales, however, remains poorly understood. C_LIO_LIWe investigate how seasonality shapes sarcoptic mange in bare-nosed wombats, Vombatus ursinus, a disease caused by the environmentally transmitted parasitic mite Sarcoptes scabiei, causing a protracted clinical time-course in the host. Using an empirically informed state-based deterministic model we explore how wombat population trajectories are influenced by (i) seasonal constraints to mite survival and (ii) in context of host-pathogen encounter rates, as measured by the ratio of burrows to wombats. C_LIO_LIWe demonstrate three long-term outcomes of wombat-mange: host and parasite extinction, endemic disease, and disease-free. We find seasonal environments narrow the range of host-pathogen encounter rates that support S. scabiei persistence relative to stable environments, and prevalence and population sizes vary more in seasonal compared to stable environments except under moderate host-pathogen encounter rates when seasonal effects are less apparent. We also find that a protracted infectious period is essential for host-parasite coexistence in the wombat-mange system. C_LIO_LIOur seasonal model results are consistent with field observations, such that mange prevalence in natural populations increases during seasons of longer off-host mite survival. Application of these findings suggest management efforts could reduce host population impacts through disease management in seasons with longer off-host parasite survival or reduce the environmental reservoir through disease management in seasons with shorter off-host survival. C_LIO_LIWe provide novel, mechanistic explanations for distinctive population trajectories that arise from a seasonally forced wildlife disease, including climate factors that operate independently on parasites, host demography, and disparate timescales over which seasonality affects parasites and hosts. Broadly, linking seasonality to long-term population dynamics can improve the predictability and management of wildlife diseases, but requires an understanding of how local intrinsic factors interact with seasonal pressures over time. C_LI

O_LISeasonal migration in birds involves a substantial spatial redistribution of avian biodiversity each year and drives seasonal changes in community composition. Migrants experience different combinations of species interactions over space and time, generating regular disassembly and reassembly of bird communities throughout their annual cycles. However, the effects of seasonal migration on phylogenetic community structure remain poorly understood. C_LIO_LIWe assess spatiotemporal variation in phylogenetic community structure of North American passerines to test how seasonal migration restructures the evolutionary relatedness and dominant assembly mechanisms in bird communities throughout the annual cycle. Using distributional projections, we calculated metrics describing the phylogenetic dispersion of passerine communities each week of the year. We then tested the relationship between seasonal turnover in community phylogenetic dispersion and seasonal variation in species richness and proportion of migratory species. C_LIO_LISeasonal migration, by changing spatial patterns of avian diversity, simultaneously drives a complex continental redistribution of phylogenetic community structure. We find evidence of taxonomic scale dependency to our results, wherein throughout North America, the seasonal influx of migrant passerines yields communities that are overall more phylogenetically clustered, yet also exhibit greater phylogenetic overdispersion at smaller taxonomic scales. C_LIO_LISeasonal shifts in phylogenetic dispersion, though complex, track changes in diversity, manifesting as fluctuations in phylogenetic dispersion between northern and southern regions as seasonal migrants move between these regions. Our findings reveal a dynamic continental landscape of phylogenetic community structure directed by the movements of seasonal migrants. C_LI

Space-for-time Cormack-Jolly-Seber (CJS) models have been developed to estimate survival of migrating animals that are imperfectly detected at spatially discrete sampling stations. However, space-for-time CJS models typically ignore survival over time and age-specific probability of movement, missing the diversity of life history strategies. To address these limitations, we extended space-for-time Cormack-Jolly-Seber models to explicitly incorporate time, age, and individual-level covariates to account for diverse life history strategies. We incorporate a sub-model that includes uncertainty in individuals age using a finite-mixture model. The detection and transition probabilities were parameterized using generalized linear models, facilitating flexible model specification and inclusion of spatiotemporal and individual covariates. We apply the model to detections of juvenile steelhead (Oncorhynchus mykiss) from two populations in the Snake River Basin in Idaho, USA, to estimate trends in survival with respect to population, age, time, and the length of individuals. Additionally, we generalized the model so that it can be applied to other systems and populations with different life history strategies and monitoring infrastructure.

Immune defenses may vary across environments due to differences in life history or abiotic and biotic characteristics, but few studies have compared the strength of immune responses across broad geographical gradients. Here we investigated how bacteria killing ability (BKA), a measure of constitutive innate immune function, varies among populations of tree swallows (Tachycineta bicolor) inhabiting different environments. We hypothesized that populations living in unpredictable environments, which also have shorter breeding seasons and thus fewer opportunities to renest in the event of reproductive failure, would invest in reproduction at the expense of immunity. To test this hypothesis, we measured BKA and monitored reproductive activity in four populations located across most of the breeding range of tree swallows, in Tennessee, New York, Wyoming and Alaska. Consistent with our predictions, BKA was lowest in Alaska and Wyoming and highest in Tennessee. In most populations BKA was also lower during incubation than during nestling provisioning, and lower in males than in females. These results suggest that the high reproductive investment seen in populations experiencing more unpredictable breeding conditions may come at a cost to immune function. While reduced immune investment could increase susceptibility to parasites, this potential cost is likely offset by lower parasite pressure in these environments, limiting the need for maintaining strong constitutive innate defenses like BKA.

Understanding the processes governing community assembly remains a central challenge in ecology. Although phylogenetic niche conservatism predicts that closely related species should occupy similar ecological niches, contemporary environmental conditions may weaken this relationship by promoting ecological convergence among distantly related species. Whether such decoupling occurs in vector mosquito communities remains largely unknown despite its importance for predicting disease transmission under global environmental change. Here, we integrated species distribution models, niche comparison tests, and phylogenetic generalized linear mixed models as a unified analytical framework to investigate the relative contributions of environmental filtering, species-specific environmental responses, and phylogenetic relatedness to the assembly of Japanese vector mosquito communities. Species distributions were primarily determined by climatic and land-cover variables. Although several mosquito species exhibited highly similar ecological niches, phylogenetic relatedness explained only a small proportion of variation in species occurrence. Instead, environmental filtering together with species-specific environmental responses consistently dominated community assembly across spatial scales. Our findings demonstrate that ecological similarity can become decoupled from evolutionary relatedness under contemporary environmental conditions. This study provides a community-level framework that links species distributions, niche relationships, and evolutionary history to improve vector surveillance, anticipate community reorganization under environmental change, and support evidence-based strategies for preventing mosquito-borne disease.

Wildlife diseases cause well-documented and often dramatic reductions in host survival. However, the impact of infectious diseases on host reproduction remains understudied, especially with respect to effects of prior and/or current pathogen exposure on reproductive development. Here we experimentally tested how prior and/or current infection with a common bacterial pathogen, Mycoplasma gallisepticum ( MG), alters reproductive development for female versus male house finches (Haemorhous mexicanus). Finches were inoculated with either MG or sterile media while in wintering condition and subsequently received one of these treatments while in breeding condition. In females, MG exposure had both immediate and carry-over effects on reproduction: controls had higher odds of laying eggs compared to females inoculated with MG in spring only, higher odds than females inoculated in both winter and spring, and higher odds than females given MG in the winter only. Conversely, breeding-condition males inoculated with MG in spring had higher testosterone levels than males receiving only control inoculations, and there were no carryover effects of winter MG inoculation or inoculations during both seasons on testosterone. Sex bias in the reproductive impacts of infectious diseases may have important knock-on effects on the epidemiology and population-regulating effects of pathogens, thereby warranting further study.

Climate change is thought to have altered the timing of avian migration, with species-specific responses varying across spatial and temporal scales. This is notable as ecological mismatches may occur when birds arrive or depart out of sync with the peak flowering period of their associated food plant resources. This study compares first arrival and last departure dates of Archilochus colubris (Ruby-throated Hummingbird) using citizen science data from the North American Bird Phenology Program for historical records (1880-1969) and recent records from eBird (2006-2024) and Journey North (2001-2024). Results show significantly earlier spring arrivals (14.2-27.9 days) and later fall departures (17.3-33.7 days) in the recent period compared to historical periods, while migration pace remained relatively consistent across datasets. Although temperatures are increasing in both the nonbreeding and breeding ranges of A. colubris, winter and fall temperatures were not strong independent predictors of migration timing. This study is the first to document shifts in fall departure dates for the species and highlights the need for continued research to uncover the mechanisms driving these phenological changes and assess their broader ecological implications.

In many ecosystems, anthropogenic warming is reshaping thermal regimes, leading to resource quality declines and imposing a dual constraint for ectotherms: elevated metabolic demand coupled with reduced assimilable energy. We tested whether plasticity in gut morphology and gut microbiome can buffer amphibian larvae against these concurrent stressors. Common frog (Rana temporaria) tadpoles were reared at two temperatures (18 vs. 24.5{degrees}C) crossed with three food-quality treatments (low, medium, high). We quantified growth and developmental rates, critical thermal limits (CTmax, CTmin), gut morphology (mass, relative length), and gut bacterial diversity and composition, together with predicted functional pathways. Warming accelerated growth and development and increased CTmax. Food quality increased growth and development, with temperature-dependent effects on developmental rate and CTmax. Gut mass declined at higher temperature and low-quality diets, but relative gut length showed only modest diet effects and no temperature dependence. Bacterial community composition and structure shifted with temperature and food quality. Predicted pathways suggest functional reconfiguration under warming and low food quality, consistent with sustaining energy acquisition and mitigating metabolic and oxidative stress. Together, these results implicate microbiome plasticity, rather than gut morphological plasticity, as a candidate mechanism supporting larval performance and heat-tolerance acclimation under warming and low food quality.

Population dynamics and their component vital rates may be driven by weather, climate teleconnections between sea and air (e.g. ENSO), or biotic interactions. These drivers operate directly or indirectly and on different temporal scales. We used a Bayesian structural equation model to characterize effects among weather, climate, and incidental intraguild predation (IGP) on the butterfly Euphydryas gillettii's vital rate of pre-diapause survival, using an 18 year dataset. IGP was a major determinant of pre-diapause survival, along with direct and indirect effects of weather and spring climate teleconnections. The direction of climate effects was reversed when mediated through IGP. Our analysis illustrates the need for sequential hypotheses to capture the cascading effects of abiotic factors via biotic interactions. Using sequential hypotheses addresses the debate on weather -- climate teleconnection roles by disentangling their contributions from one another. Finally, vital rates must be decomposed to component rates in order to detect their drivers.

Migration can increase population connectivity and gene flow but may also facilitate pathogen transmission among hosts and habitats. The way life-history strategies shape viral diversity in closely related freshwater fishes remains poorly understood. Here, we used metatranscriptomics to compare the viromes of two fish in the family Galaxiidae with contrasting life-history strategies: amphidromous [i]nanga (Galaxias maculatus) and river-resident Gollum galaxias (Galaxias gollumoides). We analysed 54 pooled libraries derived from 416 individuals collected across 27 sites spanning multiple catchments and fish-passage barriers in southern New Zealand. A total of 13 novel fish-associated viruses were identified from five viral families (Astroviridae, Circoviridae, Coronaviridae, Hepeviridae, and Picornaviridae). Virome composition differed markedly between species, with only a single virus-positive pool detected among [i]nanga, whereas Gollum galaxias harboured 11 novel viruses, with circoviruses dominating the virome. No fish-associated viruses were detected in post-larval juveniles of either species, and alpha diversity did not differ significantly among catchments, sites or positions relative to fish-passage barriers. However, beta diversity analyses revealed significant spatial structuring of viral communities among sites and catchments, indicating localised variation in virome composition. Contrary to our predictions, the migratory species exhibited substantially lower viral diversity than the river-resident species. The absence of detectable viruses in juveniles, combined with the greater viral richness of the longer-lived Gollum galaxias, suggests that host longevity and cumulative environmental exposure may be important drivers of virome assembly. These findings demonstrate that host life-history strategy influences virome composition and reveal unexpectedly low viral diversity in a globally widespread migratory galaxiid, providing new insights into virus-host ecology and connectivity in freshwater fishes.

1. Organisms rely on environmental cues, such as moonlight illumination and weather, to modulate nocturnal behaviors that reduce predation risk and conserve energy during demanding periods. Urbanization can alter the reliability of cues, potentially impairing organisms' ability to respond appropriately with consequences in increasingly urbanized landscapes. 2. We leveraged the Motus Wildlife Tracking System to identify roost initiation, roost departure and nocturnal restlessness of dark-eyed juncos (Junco hyemalis) overwintering along an urban to rural gradient to determine if activity patterns are affected by weather, moonlight illumination, and urbanization. 3. Nocturnal behavior of overwintering juncos varied with environmental factors, including moonlight illumination and temperature. However, the strength of these responses differed among sites, suggesting that natural cues of moonlight illumination were masked in urban areas, where individuals were more responsive to temperature. 4. Our results suggest that urbanization weakens birds' responsiveness to natural light cues, likely because artificial lighting reduces the visibility of moonlight; representing a key mechanism by which human-altered landscapes can disrupt nocturnal ecology in birds.

Individual variation in singing behavior is influenced by cultural evolution, environmental constraints, and population dynamics. In closed-ended songbird species, aging may be associated to temporal song variations in individual birds due to vocal apparatus deterioration. However, song variations may also arise as a result of extrinsic factors that induce behavioral changes. In this study, we aimed to examine song-type consistency over 13 years in four populations of the White-eared Ground-sparrow (Melozone leucotis, Aves: Passerellidae) along an urban-rural gradient. We also compared within and between-individual variation to determine potential population-level responses. We used long-term data from singing males to quantify five fine structural characteristics (minimum frequency and maximum frequencies, bandwidth, peak frequency, and song duration) for two most common song types from each population. Song consistency differed between fine-structure characteristics, song types, and populations. Contrary to our expectations, we found that the Highly urbanized population had greater between-individual variance, associated with anthropogenic disturbances such as habitat structure and noise pollution, suggesting non-uniform individual responses. The Medium, Low urbanized, and Rural populations, on the other hand, had higher levels of within-individual variance, associated to individuals' age. Our study brings insight into how both intrinsic and extrinsic factors can influence song consistency and highlights how cultural evolution within a species can diverge across populations experiencing distinct environmental contexts.

Social relationships can buffer individuals against environmental challenges, reducing the physiological costs of disturbance. Assessing how social and environmental conditions jointly shape physiological stress is critical for predicting individual responses to disturbance, particularly in managed populations. We evaluated whether social and environmental stressors act additively or whether social context modifies physiological responses to environmental challenges. We combined behavioural data and faecal glucocorticoid metabolite samples from a small, intensively monitored population of African elephants (Loxodonta africana) in a fenced reserve. Using a Bayesian framework, we tested how social factors interacted with variation in the biophysical environment, including natural environmental and anthropogenic disturbances, to predict glucocorticoid concentrations. Natural environmental variables, including weather, water availability, and season, did not predict glucocorticoid concentrations. In contrast, helicopter use, a common management intervention, was associated with higher glucocorticoid concentrations. Social effects were context dependent. Following disturbance, low-ranking individuals showed higher glucocorticoid concentrations than high-ranking individuals, whereas this pattern reversed under low-disturbance conditions. Individuals with more social connections had lower glucocorticoid concentrations, but this effect was evident only under non-disturbance conditions. Measures of betweenness centrality did not predict glucocorticoid levels. These results demonstrate that social factors mediate physiological responses to anthropogenic disturbance and that management actions can impose uneven physiological costs across individuals. Quantifying interactions between social factors and the biophysical environment may improve conservation and management strategies for social species. Integrating physiological responses to interactions between social factors and the biophysical environment can guide conservation strategies and improve management interventions in social species.

Agricultural intensification is a major cause of population declines in farmland birds and insects worldwide. While these declines are expected to disrupt trophic interactions through density-dependent cascading effects, examples outside of crop pest control or involving more than two trophic levels remain poorly documented. Here we quantify such cascading effects across three trophic levels involving a declining aerial insectivore, the Tree Swallow (Tachycineta bicolor), which breeding success decreases with agricultural intensity, Protocalliphora bird blowflies (Diptera: Calliphoridae) that parasitize its nestlings, and Nasonia parasitoid jewel wasps (Hymenoptera: Pteromalidae) that feed upon blowfly pupae within swallow nests. Agriculture intensity may affect each of these actors either directly via, for instance, toxicological effects from pesticide use, or indirectly through density-dependent trophic interactions, which can exacerbate or mitigate one another. Using Bayesian models within a causal inference framework, we quantified how nestling host density, meteorological conditions, and agricultural landscape context affect Nasonia parasitoid infestation of Protocalliphora pupae, and estimated Protocalliphora emergence success. Analyses were based on 3,340 Protocalliphora puparia collected from 248 Tree Swallow nests over a 16-year period (2004-2019) across a 10,200-km2 agricultural intensity gradient in Southern Quebec, Canada. Model predictions indicated that Protocalliphora emergence probability increased steeply with nestling host availability (i.e., nestling-days). Whereas Nasonia parasitoid infestation occurrence at the nest level increased with Protocalliphora host availability, the proportion of infested Protocalliphora pupae within nest decreased and led to a dilution effect at the individual level. Although Protocalliphora abundance within nests decreased with agricultural intensity, their emergence probability peaked in agrointensive landscapes. This increase was associated with reduced Nasonia infestation rates, suggesting that agricultural intensity indirectly mitigated parasitoid pressure on Protocalliphora host populations. Our results highlight how agricultural intensification can shape multitrophic interactions and life-history traits (emergence success and infestation rates) by simultaneously reducing host availability and weakening top-down control by parasitoids.

Forecasting infectious disease dynamics is important for ecological management and emerging disease preparedness, yet many systems lack the long-term datasets required to develop reliable predictions. Spatial replication may provide an alternative by allowing information to be shared among populations, but the extent to which this improves forecasting remains unclear. Here, I used epidemic and temperature data from 20 replicated semi-natural Daphnia-parasite pond populations monitored across four seasons (80 epidemics) to test whether information from multiple populations improves forecasts and whether environmental similarity identifies informative populations. I forecasted disease prevalence, infected host density, and healthy host density using a benchmark model, autoregressive integrated moving average (ARIMA), and time-series regression models. Models were trained using focal-population data alone, mean dynamics across other populations, or temperature-weighted mean dynamics from environmentally similar populations. Epidemics showed strong seasonal structure, with prevalence and infected host density generally peaking during the warmest period of the season. However, no forecasting approach consistently outperformed others across all variables. Temperature improved forecasts of prevalence and infected host density but not healthy host density. Multi-population training improved forecasts in some cases, particularly for ARIMA models, whereas temperature-weighted averaging provided no additional benefit over simple averaging. Regression models consistently produced the most accurate forecasts of healthy host density. These results show that borrowing information among populations can improve forecasts under specific conditions but is not universally beneficial, highlighting the importance of matching forecasting approaches and data sources to the ecological processes being predicted.

Disentangling causation from correlation is the foundation of the scientific method. Yet, growing evidence suggests that much observational ecology research has not correctly made this distinction due to inappropriate statistical modelling and unappreciated time-delays. Here, we apply time-lagged causal inference techniques to assess the drivers of seabird declines, using multi-decadal North Atlantic seabird data across the behaviour, mass, survival, reproduction and population size of two species ecology (Atlantic puffin, Fratercula arctica, and black-legged kittiwake, Rissa tridactyla). We demonstrate that both climate and anthropogenic activity can suppress breeding success and cause population declines. Moreover, population size is specifically impacted by delayed recruitment effects where both species decline after a lag corresponding to their estimated age of first reproduction. These North Atlantic seabirds are therefore at risk from future environmental and anthropogenic changes, as time-delays may result in populations already on an extinction trajectory prior to changes being detectable in their abundance.

Vaccination of free-roaming dogs (Canis lupus familiaris) and cats (Felis catus) remains a major public health challenge. Rapid urbanization forces these species into complex contact zones, where structural failure of pulsed vaccination under high demographic turnover undermines standard One Health interventions. In such cases species-specific intervention cycles are needed to reduce zoonotic disease risk. We integrated field-data within a simulated vaccination campaign to determine how species-specific turnover rates drive the erosion of herd immunity at an Amazonian urban sentinel site (university campus). We monitored free-roaming populations (72 dogs, 75 cats) using a non-invasive photographic mark-resight protocol from 2023 to 2025. We modelled time-to-disappearance using Cox Proportional Hazards and simulated the trajectory of effective vaccination coverage against a 40% herd immunity threshold, distinguishing between loss of vaccinated individuals and recruitment of susceptible individuals. The campus functioned as a high-turnover system, with 72% of dogs and 48% of cats classified as transients. Species significantly predicted persistence (Hazard Ratio = 0.56; 95% CI: 0.33-0.94; p=0.029), with cats exhibiting double the median residency of dogs (432 vs. 193 days). Consequently, the species experienced divergent epidemiological failure modes. For dogs, simulated vaccination coverage collapsed below a 40% herd immunity threshold in 160 days, driven by rapid immunity attrition (the loss of vaccinated individuals). Although cats persisted longer, their effective coverage was eroded by immunity dilution due to recruitment of naive juveniles, creating a 33% gap between cohort survival and population-level immunity by day 365. Annual vaccination campaigns are likely insufficient in this high-turnover urban dog population. Effective One Health zoonotic control strategies must transition from static abundance-based targets to dynamic, species-specific and turnover-adjusted intervention schedules.

Quantifying how individuals adjust the seasonal timings of life-history events (i.e. their phenology) in response to seasonal environmental conditions is fundamental to predicting impacts of changing seasonality. However, the current standard approach of inferring phenological responses by directly analysing observed event dates relies on foundational assumptions that could be invalidated by changing seasonality, and excludes individuals that do not express focal events, potentially generating misleading inferences. To overcome these limitations, we devised a Phenological Threshold Model (PTM) that conceptualizes phenological events as switches between discrete life-history states and primarily estimates variation in underlying latent dynamic responses to seasonal progression, from which event dates secondarily arise. We demonstrate that the PTM can accurately retrieve simulated parameter values, including given incomplete seasonal field observations. Application to multi-year data on annual breeding in black-legged kittiwakes (Rissa tridactyla) demonstrated substantial among-individual variation in latent responses to seasonally varying environmental drivers, and associated daily probabilities that individuals will initiate breeding, revealing how annual distributions of breeding dates and proportions of non-breeders jointly arise. We thereby formalise a new conceptual and statistical framework for quantifying the potential for, and constraints on, rapid plastic and micro-evolutionary changes in phenology in changing seasonal climatic and environmental regimes.

Morphological variation in insects can indicate responses to habitat change and land use intensification, with potential consequences for trophic interactions. In insect host-parasitoid systems, host body is often a key determinant of offspring development and emergence success, as well as parasitoid host-selection behavior. Yet, most studies in agricultural landscapes have focused on size variation in crop-associated insects (i.e.: pests, pollinators and enemies), leaving those relying on non-crop resources largely unexplored. Here, we investigated how agroenvironmental contexts and trophic resource availability influence puparial size in Protocalliphora sialia (Diptera: Calliphoridae), an obligate hematophagous ectoparasite of Tree Swallow (Tachycineta bicolor) nestlings. We further tested whether puparial size affects emergence probability and risk of parasitoidism by Nasonia spp. (Hymenoptera: Pteromalidae) jewel wasps. Using 1,513 P. sialia puparia from 88 nests within a nest-box monitoring system spanning a 10,200-km^2 east-west intensification gradient in southern Quebec, Canada, we evaluated how agrointensive contexts, avian host availability, and ectoparasite densities influence puparial size, parasitoidism risk, and emergence success. The agricultural context appears to shape puparial size through survival biases and/or altered growth rates. In agroextensive contexts (i.e.: pastures, forages with high tree cover), where Protocalliphora are more numerous, P. sialia puparia were smaller and exhibited greater size variation. Conversely, in agrointensive environments (maize and soybeans monocultures with higher doses of pesticides), where Protocalliphora are fewer, small individuals were underrepresented, suggesting a selective mortality in P. sialia population. However, puparial size explained less than 10% of the variation in emergence or parasitism, likely reflecting consistently high parasitic pressure from Nasonia wasps, which infested 50-90% of puparia, depending on site and year. Our findings highlight the importance of selective mortality in structuring trait distributions and modulating multi-trophic interactions in human-modified landscapes.

Schooling is a widespread behaviour among pelagic fish that reduces locomotion costs of migration and predation risk. It is characterised by coordinated movements, speed and orientation of group members. Although schooling can vary both between populations and among individuals of the same population, these levels of variation are seldom considered together. Here, we investigated whether schooling tendencies in three-spined sticklebacks (Gasterosteus aculeatus) vary between populations and among individuals within-populations depending on their phenotype. We first compared the schooling behaviour of anadromous (migrants) and recently landlocked (residents) populations and tested whether individual sociability explained within-population variation in schooling. We then studied whether schooling behaviour is modulated when migrants and residents are in mixed groups. Migrant groups displayed better schooling performance than residents with higher alignment. Within migrant groups, less social individuals displayed more leadership, were better aligned, swam closer to others and fatigued less than more social fish. These patterns were absent in residents. When the two populations were mixed, overall schooling performance declined, particularly in migrants, with schools exhibiting reduced compaction and alignment, resembling resident groups. Thus, individuals did not modulate their schooling behaviour to the group social composition and phenotype-dependent schooling patterns were disrupted in migrants when mixed. These findings suggest that migration-related behaviours can disappear quickly post-isolation. They also highlight the importance of individual sociability in self-organisation and shaping collective behaviour, especially in populations that retain migratory behaviour. Considering behavioural variation across multiple hierarchical levels is thus key to understanding complex social systems.