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Migration is a common trait among many animals allowing the exploitation of spatiotemporally variable resources. It often implies high energetic costs to cover large distances, for example between breeding and wintering grounds. For flying or swimming animals, the adequate use of winds and currents can help reduce the associated energetic costs. Migratory seabirds are good models because they dwell in habitats characterized by strong winds while undertaking very long migrations. We tested the hypothesis that seabirds migrate through areas with favourable winds. To that end, we used the SEATRACK dataset, a multi-colony geolocator tracking dataset, for two North Atlantic seabirds with contrasting flight capabilities, the black-legged kittiwake Rissa tridactyla and the Atlantic puffin Fratercula arctica, and wind data from the ERA5 climate reanalysis model. Both species had on average positive wind support during migration. Their main migratory routes were similar and followed seasonally prevailing winds. The general migratory movement had a loop-shape at the scale of the North Atlantic, with an autumn route (southward) along the east coast of Greenland, and a spring route (northward) closer to the British Isles. While migrating, both species had higher wind support in spring than in autumn. Kittiwakes migrated farther and benefited from higher wind support than puffins on average. The variation in wind conditions encountered while migrating was linked to the geographical location of the colonies. Generally, northernmost colonies had a better wind support in autumn while the southernmost colonies had a better wind support in spring, with some exceptions. Our study helps understanding how the physical environment shapes animal migration, which is crucial to further predict how migrants will be impacted by ongoing environmental changes.

Understanding the responses of seabirds to climate-induced variations in phenology and abundance of their prey is key to developing ecosystem-based fisheries management measures that benefit higher trophic levels. The match/mismatch hypothesis (MMH) emphasizes the need to consider synchrony in the seasonal cycles of predators and prey, while the match/mismatch/abundance hypothesis (MMAH) proposes that prey abundance may reinforce/compensate mismatch effects. This study considers the effects of both variations in seasonal availability and abundance of lesser sandeel Ammodytes marinus on hatching, fledging and breeding success of 5 seabird species: blacklegged kittiwake Rissa tridactyla, Atlantic puffin Fratercula arctica, razorbill Alca torda, common guillemot Uria aalge and European shag Gulosus aristotelis. Consistent with MMH, temporal asynchrony between sandeel availability and seabird breeding schedules affected productivity in 4 species. The effects of trophic asynchrony were either reinforced or compensated by sandeel abundance for some species, supporting MMAH. Breeding success in the late-breeding kittiwake was high when conditions favoured both high sandeel abundance and temporal synchrony while the cost of asynchrony could be compensated by high sandeel abundance in the earlier-breeding puffin. Differential effects of sandeel abundance and trophic synchrony at different stages of the seabird breeding season suggest that distinct mechanisms are in volved. The effects were most evident in the most sandeel-reliant seabirds. As further disruption of sandeel phenology and abundance is anticipated under the current climate crisis, the present study is an important step towards understanding bottom-up effects of environmental change on higher trophic levels.

Knowledge of seabird distributions plays a key role in seabird conservation and sustainable marine management, underpinning efforts to designate protected areas or assess the impact of human developments. Technological advances in animal tracking devices increasingly allow researchers to acquire information on the movement of birds from specific colonies. Nevertheless, most seabird colonies have not been subject to such tracking and another means must be found to assess their likely foraging distribution. Consequently, foraging range data collated and summarized across other tracking studies has often been used to estimate species-level foraging distances for use within applied settings. However, generic species-specific foraging ranges must be used with caution due to the amount of variation in seabird foraging behaviour at both the individual and colony level. Specifically, while current reviews of seabird foraging ranges provide summary estimates of maximum foraging range, they typically do not assess the extent of among-colony or among-individual variation around such estimates. To address this, we conducted a variance component analysis of the maximum distance reached from the breeding colony per foraging trip (foraging range) using multi-colony tracking datasets to estimate the degree of between-individual, between-year, and between-colony variation in foraging range in four UK breeding seabirds (Black-legged Kittiwake Rissa tridactyla, Common Guillemot Uria aalge, Razorbill Alca torda & European Shag Gulosus aristotelis). We also provide updated estimates of typical foraging ranges for each species and quantified the influence of breeding stage and colony size. Overall, between-colony variation was typically the largest variance component, explaining 20% - 30% of the observed variation in foraging range across the four species. Individual-level variation was also relatively large among Shags. In Kittiwake, Guillemot, and Shag, but not Razorbill, average forag

Dissecting joint micro-evolutionary and plastic responses to environmental perturbations requires quantifying interacting components of genetic and environmental variation underlying expression of key traits. This ambition is particularly challenging for phenotypically discrete traits where multiscale decompositions are required to reveal nonlinear transformations of underlying genetic and environmental variation into phenotypic variation, and when effects must be estimated from incomplete field observations. We devised a joint multistate capture–recapture and quantitative genetic animal model, and fitted this model to full-annual-cycle resighting data from partially-migratory European shags (⁠Gulosus aristotelis⁠) to estimate key components of genetic, environmental and phenotypic variance in the ecologically critical discrete trait of seasonal migration versus residence. We demonstrate non-negligible additive genetic variance in latent liability for migration, resulting in detectable micro-evolutionary responses following two episodes of strong survival selection. Further, liability-scale additive genetic effects interacted with substantial permanent individual and temporary environmental effects to generate complex nonadditive effects on expressed phenotypes, causing substantial intrinsic gene-by-environment interaction variance on the phenotypic scale. Our analyses therefore reveal how temporal dynamics of partial seasonal migration arise from combinations of instantaneous micro-evolution and within-individual phenotypic consistency, and highlight how intrinsic phenotypic plasticity could expose genetic variation underlying discrete traits to complex forms of selection.

1. Climate warming can reduce food resources for animal populations. In species exhibiting parental care, parental effort is a ‘barometer’ of changes in environmental conditions. A key issue is the extent to which variation in parental effort can buffer demographic rates against environmental change. 2. Seabirds breed in large, dense colonies and globally are major predators of small fish that are often sensitive to ocean warming. We explored the causes and consequences of annual variation in parental effort as indicated by standardised checks of the proportions of chicks attended by both, one or neither parent, in a population of common guillemots Uria aalge over four decades during which there was marked variation in marine climate and chick diet. We predicted that, for parental effort to be an effective buffer, there would be a link between environmental conditions and parental effort, but not between parental effort and demographic rates. 3. Environmental conditions influenced multiple aspects of the prey delivered by parents to their chicks with prey species, length and energy density all influenced by spring sea surface temperature (sSST) in the current and/or previous year. Overall, the mean annual daily energy intake of chicks declined significantly when sSST in the current year was higher. 4. In accordance with our first prediction, we found that parental effort increased with sSST in the current and previous year. However, the increase was insufficient to maintain chick daily energy intake. In contrast to our second prediction, we found that increased parental effort had major demographic consequences such that growth rate and fledging success of chicks, and body mass and overwinter survival of breeding adults all decreased significantly. 5. Common guillemot parents were unable to compensate effectively for temperature-mediated variation in feeding conditions through behavioural flexibility, resulting in immediate consequences for breeding population size

The non-breeding season presents significant energetic challenges to birds that breed in temperate or polar regions, with clear implications for population dynamics. In seabirds, the environmental conditions at non-breeding sites drive food availability and the energetic cost of regulatory processes, resulting in variation in diet, behaviour and energetics; however, very few studies have attempted to understand if and how these aspects vary between populations. We investigated whether non-breeding location influenced diet, behaviour and energetics in the common guillemot Uria aalge. We studied guillemots from four UK breeding colonies, two located on the west coast of Scotland and two on the east. We quantified non-breeding distribution, foraging behaviour and activity budgets of 39 individuals from July to March, using geolocation–immersion loggers and time-depth recorders, and used feather stable isotope signatures to infer diet during the post-breeding moult. We calculated energy expenditure and investigated whether the peak (an indicator of the potential vulnerability to marine threats) varied between colonies. Individuals were spatially segregated according to the coastline they breed on, with west coast guillemots distributed off the west coast of the UK and east coast guillemots distributed off the east coast. Diet and behaviour were more similar in guillemots that shared a breeding coastline than those that did not, as west coast guillemots foraged at a lower trophic level, spent less time diving and engaged in more pelagic foraging than east coast guillemots. However, energy expenditure was remarkably similar between colonies, peaking during late February/early March, indicating that, during our study period, there was high synchrony between colonies in the timing of potential vulnerability to marine threats. Therefore, any anthropogenic changes that result in decreased food availability or increased energy expenditure during late winter may have greater im

Population responses to environmental variation ultimately depend on within-individual and among-individual variation in labile phenotypic traits that affect fitness and resulting episodes of selection. Yet complex patterns of individual phenotypic variation arising within and between time periods, as well as associated variation in selection, have not been fully conceptualized or quantified. We highlight how structured patterns of phenotypic variation in dichotomous threshold traits can theoretically arise and experience varying forms of selection, shaping overall phenotypic dynamics. We then fit novel multistate models to 10 years of band-resighting data from European shags to quantify phenotypic variation and selection in a key threshold trait underlying spatioseasonal population dynamics: seasonal migration versus residence. First, we demonstrate substantial among-individual variation alongside substantial between-year individual repeatability in within-year phenotypic variation (“flexibility”), with weak sexual dimorphism. Second, we demonstrate that between-year individual variation in within-year phenotypes (“supraflexibility”) is structured and directional, consistent with the threshold trait model. Third, we demonstrate strong survival selection on within-year phenotypes—and hence on flexibility—that varies across years and sexes, including episodes of disruptive selection representing costs of flexibility. By quantitatively combining these results, we show how supraflexibility and survival selection on migratory flexibility jointly shape population-wide phenotypic dynamics of seasonal movement.

1. Timing of breeding, an important driver of fitness in many populations, is widely studied in the context of global change, yet despite considerable efforts to identify environmental drivers of seabird nesting phenology, for most populations we lack evidence of strong drivers. Here we adopt an alternative approach, examining the degree to which different populations positively covary in their annual phenology to infer whether phenological responses to environmental drivers are likely to be (i) shared across species at a range of spatial scales, (ii) shared across populations of a species, or (iii) idiosyncratic to populations. 2. We combined 51 long-term datasets on breeding phenology spanning 50 years from nine seabird species across 29 North Atlantic sites and examined the extent to which different populations share early versus late breeding seasons depending on a hierarchy of spatial scales comprising breeding site, small-scale region, large-scale region and the whole North Atlantic. 3. In about a third of cases we found laying dates of populations of different species sharing the same breeding site or small-scale breeding region were positively correlated, which is consistent with the hypothesis that they share phenological responses to the same environmental conditions. In comparison we found no evidence for positive phenological covariation among populations across species aggregated at larger spatial scales. 4. In general we found little evidence for positive phenological covariation between populations of a single species, and in many instances the inter-year variation specific to a population was substantial, consistent with each population responding idiosyncratically to local environmental conditions. Black-legged kittiwake (Rissa tridactyla) was the exception, with populations exhibiting positive covariation in laying dates that decayed with the distance between breeding sites, suggesting that populations may be responding to a similar driver. 5. Our