Interlinked population dynamic and evolutionary responses to spatial and seasonal environmental variation, stemming from interactions and feedbacks among phenotypic variation, genetic variation, selection and demography, could generate complex eco-evolutionary dynamics that span temporal and spatial scales. Partially migratory metapopulations (PMMPs), featuring sequential seasonal sympatry and allopatry of different sets of resident and seasonally migrant individuals, have clear potential for such eco-evolutionary outcomes. This is because ongoing evolution of reversible seasonal migration affects spatio-seasonal population dynamics and densities, which could in turn shape forms and magnitudes of selection on migration, causing feedbacks on evolution. However, key environmental and genetic conditions that maintain migratory polymorphisms, and resulting eco-evolutionary dynamics of PMMPs given stochastic environmental variation and strong spatially restricted seasonal perturbations, have not been characterized. We built a general individual-based model that tracks eco-evolutionary dynamics in PMMPs inhabiting spatially structured and seasonally varying landscapes, with seasonal migration formulated as a quantitative genetic threshold trait. Simulations showed that such genetic architectures and landscape structures, which are common in nature, readily produce stable partially migratory systems given diverse regimes of environmental variation. Partial migration is maintained whenever sites differ in non-breeding season suitability, defined as variation in density-dependence, causing 'ideal free' non-breeding distributions where residents and migrants occur with frequencies generating similar survival probabilities. Further, bet-hedging can cause stable partial migration without any fixed differences in non-breeding season density-dependence among sites and even without density-dependence at all, given sufficiently large stochastic environmental fluctuations among sites and years. Importantly, major local non-breeding season mortality events, as could result from extreme climatic events, generate eco-evolutionary dynamics that ripple out to affect breeding and non-breeding season space use of subpopulations throughout the PMMP, on both short and longer timeframes. These effects result from spatially divergent selection on the occurrence and destination of migration. Our model thus shows how changing partial seasonal migration acts as a key mediator of eco-evolutionary dynamics in (meta)populations occupying spatially and seasonally varying environments. It thereby initiates new steps towards predicting responses of natural partially migratory populations to ongoing changes in spatio-seasonal patterns of environmental variation.

Diel rhythms synchronized to Earth's photic cycle are near-ubiquitous among animals living in regions with distinct day-night cues. Where such cues are reduced or absent, however, activity patterns may weaken, reorganize or become decoupled from the light-dark cycle, which may allow for more flexible behavioural expression. Using a dataset of >900 free-ranging black-legged kittiwakes from colonies spanning a broad latitudinal gradient (40-81°N), we show considerable population-level variation in diel activity patterns, from clear 24 h rhythms with distinct peaks at dawn and dusk at lower latitudes, to variable and non-diel rhythms with flattened activity patterns across the 24 h day at high latitudes. These patterns were consistent with the wide spectrum of light-dark cycles experienced across their breeding range, supported by our finding that activity was strongly predicted by sun elevation, suggesting alignment with the light-dark cycle. Together, our findings reveal substantial within-species variation in rhythmicity across latitudes and demonstrate that changes in the photic environment can reshape the temporal organization of activity even within a single, widely distributed species.

Phenotypic responses to climate affect individual fitness, but the extent to which this translates into effects on population dynamics remains poorly understood. We assemble 213 time series on phenotypes and population sizes of wild vertebrates globally and match them with local climate data. Our meta-analysis shows that morphological traits are mostly climate insensitive. However, phenology is earlier in warmer-than-average years, which contributes positively to population growth in most species. At lower latitudes, temperature has weaker effects on phenology but stronger direct negative effects on population growth, likely because these populations are less capable of tracking climate via plasticity. Variation in the phenology-mediated effect of temperature on population growth cannot be explained by latitude, generation time, migratory mode, or diet. This suggests that simple relationships between species characteristics and population responses to warming may not occur in nature. Instead, we may need to embrace ecological complexity by considering local-scale predictors that capture intra-specific variation.

Many seabird populations along the UK’s East Coast have declined over the past three decades. A key concern is reduced availability of their primary breeding season prey, sandeel (Ammodytes marinus). While some seabirds have partially switched to sprat (Sprattus sprattus), this is not universal. Studying these predator–prey dynamics requires fine-scale estimates of prey distributions that are both spatially and temporally resolved—yet such data are limited, especially for sandeel due to sampling challenges. We evaluate the utility of large-scale ICES bottom trawl surveys to model sandeel distribution, combining them with winter dredge surveys designed specifically for this species. For sprat, we combine bottom trawl data with the Herring Acoustic Survey. Using hierarchical Bayesian spatio-temporal models (R-INLA), we estimate the biomass of small and medium sandeel (<= 16 cm) and sprat across the entire North Sea from 2000 to 2020, accounting for spatial and temporal dependencies. We validate our sandeel models by comparison with other modelling approaches and ICES stock assessments. Results reveal marked regional variation: sandeel increased in Area 3 (Norwegian EEZ), declined in Areas 1 and 2, and fluctuated in Area 4. Sprat biomass increased over time, with a modest northward expansion. This study provides the first North Sea-wide, fine-scale, multi-decadal distribution models for these two key seabird prey species and identifies important regional drivers of their populations.

Estuaries are particularly at risk from persistent organic pollutants (POPs), since they form the boundary between freshwater and marine environments. We investigated spatial and seasonal trends of 7 organochlorine pesticides (OCs), 35 polychlorinated biphenyls (PCBs), and 25 polybrominated diphenyl ethers (PBDEs) in sediments (n = 155) from a range of sites in an urbanised and industrialised estuary (the Forth, south-east Scotland). PCBs (∑PCB mean concentration 3.3 ng g-1 ww, detection frequency 86%) and OCs (∑OC mean 2.5 ng g-1 ww; DF 95%) were the most abundant and frequently detected POPs, with PBDEs less widespread and at lower concentrations (∑PBDE mean 0.17 ng g-1 ww, DF 59%). POP profiles reflected historic patterns of production and use, and, for PCBs, congener-specific degradation profiles. PCB congeners 28, 118, 149, 101, 153, and 138, and the pesticides DDE (a breakdown product of DDT) and hexachlorobenzene were amongst the most frequently detected and abundant compounds. We found no evidence for seasonal variation in ∑OC and ∑PBDE concentrations, although ∑PCB and some individual compounds, e.g. dieldrin, exhibited seasonal differences. Key drivers of POP fate were sediment organic matter and distance from source areas. The weight of evidence suggested that the majority of POPs pose a relatively low risk in the estuary. However some compounds, e.g. Lindane exceeded the Probable Effect Level in 8% of samples. This work demonstrates the spatial variation in presence and potential risk of POPs in the UK environment and the need to continue to monitor the levels of these contaminants.

ABSTRACT Parasites can impose substantial fitness costs on hosts, yet infection intensity varies markedly among individuals and across life stages. Age‐related variation in parasite burden may reflect shifts in behaviour or physiology that alter exposure and susceptibility, or arise from selective disappearance of heavily infected individuals. Distinguishing between these processes is essential for understanding the ecological and evolutionary consequences of parasitism in wild populations. Age at first reproduction (AFR) is a fundamental life‐history trait that shapes lifelong infection dynamics through trade‐offs between reproductive investment, somatic maintenance, immune defence and survival. Because reproductive investment and timing often differ between sexes, age at first reproduction may generate sex‐specific infection trajectories across adulthood, yet whether early‐life reproductive strategies are linked to adult parasite dynamics in a sex‐specific manner remains largely unexplored. Using 10 years of individual‐based monitoring in European shags (Gulosus aristotelis), we tested whether age at first reproduction predicts within‐individual changes in gastrointestinal nematode burden across adulthood and whether these trajectories differ between sexes. Parasite burdens were quantified by repeated endoscopy, and a within‐subject centring approach was used to separate within‐ from between‐individual age effects and explicitly account for selective disappearance. Age at first reproduction predicted sex‐specific infection trajectories in male but not female shags. Early‐ and modal‐recruiting males showed within‐individual declines in parasite burden with age, whereas late‐recruiting males exhibited progressive increases. A marginal trend in early‐recruiting males suggests possible selective disappearance in this group. Females showed consistent within‐individual declines regardless of age at first reproduction, with no evidence of selective disappearance. These findings provide the first evidence that age at first reproduction is associated with sex‐specific parasite infection trajectories in a long‐lived bird, revealing how early‐life reproductive strategies are linked to adult host–parasite dynamics. Integrating life‐history variation with longitudinal, within‐individual approaches exposes mechanisms that cross‐sectional analyses cannot detect and highlights the population‐level consequences of sex‐specific infection trajectories.

Abstract Seabird populations are exposed to a wide range of threats from anthropogenic activity, and many species are rapidly declining. For decision‐makers and practitioners to give advice on the impacts of human activities and design of mitigation or compensation strategies, understanding how impacts to key demographic rates influence population dynamics and trajectories across different species is essential. Importantly, practitioners must do so in the context of the resources and information that are available to them. In this study, we build a quantitative resource using population viability analysis (PVA) that allows comparative assessment of resultant population outcomes following a wide range of impacts to breeding success and survival. We project population responses at different sizes and under varying density‐dependent regulation scenarios using a commonly utilised impact assessment tool. We examine positive and negative impact scenarios, with a view to quantifying the effectiveness of theoretical management measures. We use five UK seabird species representing a range of life‐history strategies. By running over 100,000 population models testing the responses of five seabird species to a variety of impact scenarios, we present a wide range of population projections in response to impacted seabird demographic rates. Broadly, we find that species differ as expected in sensitivity to changes in breeding success and survival according to their life‐history strategies. We demonstrate the importance, within and between species, of including density‐dependent regulation in population dynamics, even within a simple comparative resource such as this. Finally, we show population responses to interactions between degree and duration of impact to demographic rates, although we found little interspecific variation here. Practical implications. We provide a comprehensive and valuable resource that could help inform decisions around development planning and consenting. This resource allows practitioners and users to readily consider quantitative scales of impact for various ‘realistic’ scenarios when making preliminary assessments of potential impacts of new developments on seabirds, without necessarily performing detailed quantitative population modelling, while incorporating critical processes such as density dependence. It could also inform the design and evaluation of potential interventions as part of strategic compensation and wider conservation measures to sustain declining populations.

The abundances of zooplankton species supporting Northeast Atlantic food webs have declined over the past 60 years, and their future is uncertain given continuing climate change. Here, we develop a new high-resolution machine-learning model of key taxa, driven by past ocean conditions and trained on Continuous Plankton Recorder observations. We apply it to an ensemble of regional climate projections for the Northwest European Shelf under the high emissions scenario RCP8.5 to enhance the detectability of a clear signal of climate-driven change in zooplankton abundance. The results project large declines in abundance: ensemble mean of 58%-72% by 2050 and 84%-93% by 2100 for small copepods and large Calanus. The projections are not geographically uniform, with evidence of regional refugia. The future declines are primarily driven by increasing temperature and decreasing salinity and nutrient concentrations, which shift conditions away from the species’ environmental niches and water mass associations. Impacts on food webs would be severe unless alternative zooplankton assemblages emerged to support existing planktivorous fish and their predators. Large declines in abundance of key zooplankton species on Northwest European Shelf can be driven by rising temperatures, falling nutrient concentrations, and a slowdown in the Atlantic Meridional Overturning Circulation, according to machine learning projections of a high emissions scenario.

The form, magnitude and temporal dynamics of selection on phenotypic plasticity will fundamentally shape eco-evolutionary responses to environmental variation, but such attributes have not been fully conceptualized or quantified in nature. We provide a general framework that conceptualizes the dynamics of selection on phenotypic plasticity in labile dichotomous traits, which commonly shape behaviour and life history. Specifically, we highlight distinctions between selection on expressed plasticity and selection on resulting phenotypes, effects of phenotypic switches in opposite directions, and the full selection dynamics emerging across temporal sequences of environmental conditions. To enact this framework, we quantified selection on early-life plasticity in the ecologically critical trait of seasonal migration versus residence, by fitting a novel multi-state model to spatio-seasonal resighting data from 13 newly fledged cohorts of partially migratory European shags (Gulosus aristotelis). We demonstrate strong and consistent directional selection against early-life plasticity, manifested as substantially lower juvenile survival after phenotypic switches from resident to migrant, but not after reverse switches from migrant to resident. Yet, evident short-term costs translated into weaker and fluctuating selection on plasticity given sequences of phenotypes expressed throughout initial months. We thereby reveal how complex forms of selection against early-life plasticity can arise yet be rapidly attenuated in nature.

Declining body sizes are prevalent in marine fish and have been suggested to be a response to increasing temperatures. However, the evidence is mixed, and the underlying causes are often unknown. Here, we explore drivers of spatio-temporal patterns in size in juvenile lesser sandeel Ammodytes marinus, focusing on ongoing size declines in parts of the North Sea. We combine experimental and field data with theory to develop a biologically realistic dynamic energy budget model that explicitly models feeding, metabolism and energy allocation to produce daily predictions of sandeel length during the growth season from 1979 to 2016 in 4 North Sea sub-populations. When forced with daily temperature estimates and zooplankton data from the Continuous Plankton Recorder, model predictions largely match observed spatio-temporal patterns. Our results suggest that the most plausible driver of observed size declines in the western North Sea is declining prey densities. In contrast, the direct effect of temperature on sandeel size is small but interacts with local prey availability so that increasing temperatures may boost growth rates in areas with high food availability but reduce growth rates in areas with low food availability. Our results thus suggest that to understand the effects of climate change on fish size, we need to account for both direct physiological effects and changes in resource availability. Finally, we show that early-life phenology and turbidity (via its impact on intake rates in the visually foraging sandeel) may also impact sandeel size, highlighting the importance of broadening our view of potential drivers of size declines.

Seabirds experience variable extrinsic and intrinsic pressures throughout the annual cycle that affect their ability to forage. Consequently, their foraging strategies may vary between breeding and non-breeding seasons due to the constraints of central-place foraging during the former. Here, we studied a population of a generalist seabird, the Great Black-backed Gull Larus marinus, breeding at a colony on the Isle of May, Scotland. We quantified seasonal variation in sexual segregation, trophic position, trophic niche width, and resource use by examining stable isotopes in feathers collected from adults. We found no sexual segregation, but we detected population- and individual-level shifts in trophic position, trophic niche width, and resource use throughout the annual cycle, providing novel information about the ecology of the Great Black-backed Gull. The population was most specialised during the late non-breeding period, when marine resources made up over 95% of the population's diet. During breeding, terrestrial resources made up 20% of the population's diet, and a much greater percentage for some individuals. We highlight the importance of undertaking trophic studies beyond the breeding period to advance collective knowledge of species' ecology and to improve assessments of the potential impacts of environmental change and other anthropogenic threats during the non-breeding season, which is critical for seabird survival.

Life-history traits expressed in early life can exhibit considerable among-cohort variation, which could substantially affect population age-structure and dynamics if initial variation persists into later life-stages. Yet, initial among-cohort variation could be reinforced, rapidly dissipated, or else completely reshaped by dynamic combinations of age-specific phenotypic plasticity and selective disappearance acting within and among cohorts. However, such effects have not been comprehensively quantified for any trait, precluding full prediction of the form and implications of phenotypic dynamics, and emerging age-specific life-history variation, in varying environments. We provide a framework for conceptualising phenotypic change resulting from joint and interacting effects of cohort-specific and age-specific plasticity and selective disappearance. We implement this framework by quantifying overall early-life age-specific phenotypic change (or stasis), and dissecting underlying dynamics of plasticity and selection, for the ecologically critical life-history trait of seasonal migration versus residence. We achieve this by fitting multi-state models to extensive multi-year ring-resighting data from 9358 colour-ringed European shags (Gulosus aristotelis) from 11 cohorts in a partially migratory population. The overall cross-cohort mean proportion of migrants versus residents remained approximately constant across the four winters following fledging, implying no overall change in the degree of seasonal migration with age. This stasis was underlain by consistently high cross-year individual phenotypic repeatability, and by average plasticity towards residence that was counter-acted by average selective disappearance of sub-adult residents. However, these cross-cohort means obscured substantial among-cohort variation in the initial degree of partial migration, and in subsequent joint effects of plasticity and selective disappearance. Here, plasticity and selection were not systematically associated within or across cohorts or ages, but rather reinforced versus counter-acted each other at different times, thereby reshaping the pattern of among-cohort variation in partial migration across ages. These results demonstrate that an absence of overall age-specific change in a key life-history trait, seasonal migration versus residence, obscures substantial underlying variation in both early-life plasticity and selective disappearance, generating complex phenotypic dynamics within individual cohorts. Standard cross-cohort analyses may therefore inadequately predict future spatio-seasonal dynamics, since novel age-specific life-histories could readily emerge given changing environmental drivers of plasticity and selection.

In iteroparous, socially monogamous species, individuals vary in the extent of mate fidelity across breeding attempts, often with important fitness consequences. Numerous studies have demonstrated intrinsic drivers of mate fidelity, notably previous breeding success and parental age. Environmental conditions may also influence mate fidelity, and the habitat-mediated hypothesis predicts that fidelity will be lower when environmental conditions are poor. However, limited testing of this hypothesis has been undertaken in longitudinal studies of single populations. Furthermore, studies have mainly focused on environmental conditions during the breeding season, yet conditions prior to breeding may be important for mate fidelity because this is a critical period for pair bond formation. We investigated the effects of prebreeding environmental conditions (onshore wind component and sea surface temperature) on mate fidelity over a 20-year period in the socially monogamous, iteroparous, long-lived marine bird, the European shag, Gulosus aristotelis. Average fidelity rate varied three-to four-fold between years. Mate fidelity was affected by prebreeding environmental conditions, being lower when onshore winds were more prevalent and sea surface temperature was higher. However, mate fidelity was more strongly affected by intrinsic factors, with higher rates when breeding success in the previous attempt and population density were higher, and among older females and middle-aged males. We found that mate fidelity affected timing of breeding, with faithful pairs laying earlier, and early laying pairs bred more successfully, but there was no independent effect of mate fidelity on breeding success. Our results support the habitat-mediated hypothesis whereby prebreeding environmental conditions affect individual pairing decisions. Given environmental conditions are predicted to change globally, further investigation of their impact on aspects of social behaviour in a range of species is warranted. Crown Copyright (c) 2025 Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour. This is an open access article under the CC BY license (http://creativecommons.org/licenses/ by/4.0/).

An understanding of the relationships between plastics and plasticizers is vital in order to assess their environmental risk. We investigated spatial trends and relationships between microplastics and plasticizers in sediments of an urbanized estuary subject to contemporary and historic sources of contamination (Forth estuary, Scotland, UK). As such, this study represents one of the first to investigate the co-occurrence of emerging plasticizers, phthalates, and microplastics in an estuary system. We determined the concentration of 7 legacy (phthalate) and 3 emerging (adipate, terephthalate, trimellitate) plasticizers and 21 microplastic polymer types. The most abundant microplastics were polyethylene (PE), polypropylene (PP), polyurethane (PU), and poly(vinyl chloride) (PVC). Plasticizers were dominated by diethylhexyl phthalate (DEHP), although emerging plasticizers (e.g., diethylhexyl terephthalate, DEHTP) were frequently detected at low concentrations (mean 7.3 ng g-1 ww). There was strong evidence that concentrations of microplastics and plasticizers were significantly lower in the outer estuary. However, we found no evidence for a spatial relationship between the concentration of microplastics and plasticizers at individual site level. Our results indicate that microplastics in the size range analyzed (∼25-1000 μm) may not be a good predictor of the spatial distribution of plasticizers in estuaries. This could result from release of plasticizers prior to plastic fragmentation and deposition and differences in transport and fate.

Climate-driven ocean warming is profoundly reshaping marine ecosystems, with cascading effects on biodiversity and trophic interactions. For migratory marine predators such as seabirds, demographic responses to warming depend on when and where populations are exposed across the annual cycle. Therefore, integrating demographic monitoring and tracking data, across broad geographic and temporal scales, is essential, given the spatial and seasonal variability in ocean warming. Here, we integrated long-term demographic data, seasonal distributions, and sea surface temperatures (SSTs) for 26 populations of five seabird species breeding in the North-East Atlantic to assess the effects of SSTs on reproduction, survival, and population growth trajectories. Demographic responses varied widely among populations and seasons, but negative effects were most consistently associated with warming during the autumn period postbreeding, particularly in the Barents and East Greenland Seas. Winter warming also corresponded to reduced survival, while breeding-season SSTs showed fewer significant effects on reproductive rates. Populations with dual responses to warming in both the breeding and nonbreeding seasons had the lowest projected population growth rates under future SSTs given a high emissions scenario. These results demonstrate that population vulnerability reflects the interaction between seabirds' year-round distributions and regional ocean warming. This underlines the need to integrate year-round tracking and long-term monitoring to inform conservation strategies and marine spatial planning to ensure climate-resilient marine ecosystems.

CapsuleAnalyses of ring recovery data can relatively rapidly detect excessive mortality in wild bird populations, including that arising from outbreaks of high pathogenicity avian influenza (HPAI).AimsWild bird populations in the United Kingdom and Republic of Ireland were significantly impacted by HPAI between 2021 and 2023, causing mass mortality particularly among seabirds and waterbirds. We assessed whether reporting of dead ringed birds ('recoveries') provided a rapid way of assessing population impacts of HPAI across multiple species.MethodsWe quantified ring recoveries from 58 bird species found dead between October 2021 and March 2023, coinciding with an HPAI outbreak in the United Kingdom and Republic of Ireland. To detect and quantify unusual mortalities, we compared these recoveries to baseline recovery trends from 2010/11 to 2020/21, preceding the HPAI outbreak, calculated using generalized linear models and 95% prediction intervals to project the expected upper and lower bounds of recovery numbers into 2021/22 and 2022/23. If observed recoveries exceeded prediction intervals, a species was identified as experiencing excess mortality. Population proportions of impacted species likely to have died during 2021/22 and 2022/23 were estimated from adult survival estimates and ring recovery analysis.ResultsOf 53 species investigated, excessive ring recoveries were detected in 14 species: one raptor, eight seabirds and five waterbirds. Striking increases in ring recoveries were displayed by seabirds, aligning with HPAI monitoring via other methods. Mortality estimates varied between species, potentially dependent on biases in ringing effort or detectability.ConclusionRing recovery data can rapidly indicate excessive mortality occurring within a population and rapidly estimate annual mortality rates to aid, in this case, quantification of the impact of HPAI on bird populations. Meaningful estimates of excess mortality are most likely where the distribution of ringed birds and search effort are evenly spread across populations.

Anthropogenic habitat change is having a detrimental impact on biodiversity worldwide, altering the foraging behaviour and population dynamics of many species. Generalist species often adapt by broadening their resource use and/or exploiting human-modified environments. However, habitat changes that reduce the availability of good quality resources can lead to increased interspecific competition among sympatric species and increased conflict with human activities. We investigated the breeding season foraging ecology of three sympatric gull species, Lesser Black-backed (Larus fuscus), Herring (Larus argentatus) and Great Black-backed Gulls (Larus marinus), from the same colony in Scotland. Using GPS tracking data, we analysed foraging ranges, spatial distributions and habitat preferences to determine the extent of the gulls' niche partitioning and use of human-modified landscapes. Our findings revealed considerable overlap in resource use between species. However, species-level differences in spatial distributions and habitat selection demonstrated partial niche partitioning. Lesser Black-backed Gulls had significantly larger foraging ranges than Herring and Great Black-backed Gulls, indicating spatial segregation. Herring and Great Black-backed Gulls strongly selected for landfill and coastal habitats. Lesser Black-backed Gulls also selected for these habitats but primarily used agricultural areas. Individual-level analysis revealed that most species-level selection for urban, landfill and harbour habitats was driven by a subset of individuals. The observed limited niche partitioning indicates that further habitat loss or degradation could negatively impact all three gull species unless the extent of niche partitioning changes. Given that most habitats used were linked to human activities, further anthropogenic change may displace gulls from preferred foraging areas, increasing competition for limited resources and exacerbating conflicts with human activities in alternative habitats. By simultaneously tracking sympatric species, we can better understand how shifts in resource availability may impact interspecific competition and interactions with human activities to help inform management actions and mitigate conflict with humans, particularly around licensed control.

Light-immersion data from geolocators can be used to estimate behavioural budgets and energetics in seabirds throughout the annual cycle. However, all methods used to categorise time spent in behaviours rely on assumptions that are difficult to validate. Additional data, such as pressure and temperature data from time-depth recorders (TDRs), can help to refine these assumptions. We explore the utility of previous methods to derive behavioural budgets from light-immersion data using a dataset from Common Guillemots Uria aalge, where individuals were equipped with both a TDR and a solar Global Location Sensor (GLS), also known as a ‘geolocator’. We compared behavioural allocations from previous methods to those derived when also using TDR data. Previous methods used light-immersion data to distinguish between time foraging, active, and resting on the water, but the addition of TDR data revealed that these activities resulted in similar light and immersion levels. It was also more difficult to differentiate between rest and flight using light-immersion data alone. However, by using insights gained from combined light-immersion and TDR data, we developed an improved method to assign behaviours using light-immersion data alone, and provide an adjusted equation to use these data to calculate energetics in Guillemots. We recommend using our approach when processing light-immersion data; however, if detailed activity budgets (particularly foraging information) are required, we recommend using higher resolution loggers, e.g. integrated light-immersion-temperature-pressure devices. Our findings are likely to be relevant for studies of other seabird species (particularly other auks) that dive and spend most of their time at sea during winter.