C Lambert

J Fort

No evidence that seasonal changes in large-scale environmental conditions drive migration in seabirds

Keywords: Biogeography, Ecological niche, Marine Ecoregions of the World, Niche dislocation, Niche dynamics, Niche overlap, Niche stability, Seasonality

No evidence that seasonal changes in large-scale environmental conditions drive migration in seabirds

Charlotte Lambert, Jérôme

Introduction

Environmental conditions faced by living organisms are highly variable in space and time, from hours to decades. Seasonal variability is one of the most predictable dimensions of temporal variability, and is considered one of the main drivers of seasonal movements such as migration [START_REF] Alerstam | Long-distance migration: evolution and determinants[END_REF][START_REF] Winger | A long winter for the Red Queen: rethinking the evolution of seasonal migration[END_REF][START_REF] Dufour | Reconstructing the geographic and climatic origins of long-distance bird migrations[END_REF]. Migration is an adaptation strategy ensuring persistence in face of climate or resources fluctuations through optimisation of fitness. Its evolution might be driven by fidelity to reproductive and wintering sites, increasing both reproductive success (breeding sites) and survival (non-breeding sites; [START_REF] Shaw | Migration or Residency? The Evolution of Movement Behavior and Information Usage in Seasonal Environments[END_REF][START_REF] Winger | A long winter for the Red Queen: rethinking the evolution of seasonal migration[END_REF]. Although today we have a good understanding of many aspects of migration, our general knowledge of how migratory strategy affects the realized ecological niche of species [START_REF] Hutchinson | Concluding remarks[END_REF], and in particular the temporal dynamics of species occupancy within their niche, is still limited.

Many birds are thought to engage in seasonal migration from high latitude breeding grounds to avoid resource shortage or bad weather during the unfavourable season, optimising survival by tracking the spatial displacement of optimal energetic conditions (the Green Wave Surfing; [START_REF] Somveille | Where the wild birds go: explaining the differences in migratory destinations across terrestrial bird species[END_REF][START_REF] Winger | A long winter for the Red Queen: rethinking the evolution of seasonal migration[END_REF]. For example, common cuckoo (Cuculus canorus) tracked the peak in vegetation greenness and adjusted the timing and direction of migration to fit seasonal changes in resources across its range [START_REF] Thorup | Resource tracking within and across continents in long-distance bird migrants[END_REF][START_REF] Williams | A temporally explicit species distribution model for a long distance avian migrant, the common cuckoo[END_REF]. Such patterns have been supported by recent efforts suggesting that resident species display more pronounced fluctuations in their ecological niche across seasons [START_REF] Gómez | Niche-tracking migrants and niche-switching residents: evolution of climatic niches in new world warblers (parulidae)[END_REF], while migrants preferentially track the same niche [START_REF] Thorup | Resource tracking within and across continents in long-distance bird migrants[END_REF][START_REF] Williams | A temporally explicit species distribution model for a long distance avian migrant, the common cuckoo[END_REF][START_REF] Zúñiga | Migration confers winter survival benefits in a partially migratory songbird[END_REF][START_REF] Zurell | Do long-distance migratory birds track their niche through seasons?[END_REF][START_REF] Somveille | Where the wild birds go: explaining the differences in migratory destinations across terrestrial bird species[END_REF].

Yet, most of the research on the relationship between niche characteristics and migration has focused on terrestrial systems (e.g. [START_REF] Gómez | Niche-tracking migrants and niche-switching residents: evolution of climatic niches in new world warblers (parulidae)[END_REF][START_REF] Thorup | Resource tracking within and across continents in long-distance bird migrants[END_REF][START_REF] Zurell | Do long-distance migratory birds track their niche through seasons?[END_REF], and whether niche-tracking tends to characterise migrant species in marine systems is still unknown. Marine pelagic systems are characterised by a strong spatio-temporal structuring [START_REF] Hunt | Scale-dependent processes in the physical and biological environment of marine birds[END_REF] with their seasonal fluctuations peaking at temperate latitudes and in large upwellings, but being lower at equatorial latitudes (Figure 1). Seabirds are among the best known groups of mobile marine species at the global scale, with diversified ecological strategies and a wide array of seasonal movements (Box 1). Some species are resident year-round, some disperse widely during the non-breeding period, others engage in basin-scale or pole to pole migrations [START_REF] Hunt | Scale-dependent processes in the physical and biological environment of marine birds[END_REF][START_REF] Gaston | Seabirds: a natural history[END_REF], supposedly to track favourable environmental conditions [START_REF] Shaffer | Spatiotemporal habitat use by breeding sooty shearwaters Puffinus griseus[END_REF][START_REF] Block | Tracking apex marine predator movements in a dynamic ocean[END_REF][START_REF] Péron | Tracking through Life Stages: Adult, Immature and Juvenile Autumn Migration in a Long-Lived Seabird[END_REF]. Seabirds perform some of the longest known migrations, as the 70,900 km of arctic terns (Sterna paradisea; [START_REF] Egevang | Tracking of Arctic terns Sterna paradisaea reveals longest animal migration[END_REF] or the 64,000 km of sooty shearwaters (Ardenna grisea; [START_REF] Shaffer | Migratory shearwaters integrate oceanic resources across the Pacific Ocean in an endless summer[END_REF].

One of the main characteristics of seabirds that may impact the relationship between niche dynamics and migratory strategy is their particular relationship to land [START_REF] Gaston | Seabirds: a natural history[END_REF]. While feeding within marine waters, seabirds have to go back to land to breed (Schreiber & Burger, 2002). Yet, sites with the physical and environmental conditions requested to ensure successful breeding (e.g. nest substrate, wind, rain, predation, disturbance, food availability) are relatively rare for many species [START_REF] Coulson | Biology of Marine Birds chapter Colonial breeding in seabirds[END_REF]. This low availability of breeding sites is a major constraint, and probably one of the factors favouring colonial breeding for most of seabird species [START_REF] Siegel-Causey | The evolution of coloniality[END_REF]. This enhances competition for resources and increases constraints on habitats where sufficient resources should be available to sustain the very high energetic demands of densely aggregated breeding individuals [START_REF] Hamer | Biology of Marine Birds chapter Breeding biology, life histories, and life history-environment interactions in seabirds[END_REF][START_REF] Gaston | Seabirds: a natural history[END_REF]. When released from reproductive duties, seabirds are also released from the necessity of remaining close to shore (central-place foraging; [START_REF] Gaston | Seabirds: a natural history[END_REF] and are free to move to virtually any favourable habitat (free-ranging foraging), thanks to their exceptional flight capabilities. Furthermore, seabirds have a broad thermal tolerance range (Schreiber & Burger, 2002), permitting them to live in polar areas and to cross the equatorial regions without harm. Several polar species are also able to cope with the polar night and adverse weather of the highest latitudes (some Alcid species for example; [START_REF] Fort | Energetic consequences of contrasting winter migratory strategies in a sympatric Arctic seabird duet[END_REF], as well as extreme winds associated with storm and cyclones [START_REF] Weimerskirch | Cyclone avoidance behaviour by foraging seabirds[END_REF][START_REF] Clairbaux | North Atlantic winter cyclones starve seabirds[END_REF]. This work had two objectives which we fulfilled at global scale using the monthly distribution of 62 seabird species across marine ecoregions. First, we quantified the niche occupancy during the breeding and non-breeding periods for all species, and compared the niche dynamics between groups of species with different seasonal movement patterns to test the hypothesis that migrants track their ecological niche throughout the year, while residents switch their niche seasonally.

Second, we tested the hypothesis that the niche used by migrants during one season becomes unavailable during the other, 1 as would be if niche availability triggered niche occupancy dynamics through avoidance of adverse conditions. Here, the ecological niche was characterised as the realized part of the fundamental niche resulting from the restrictions imposed on the species distribution in geographical space by the interaction between abiotic, biotic, dispersal and historical factors [START_REF] Peterson | Ecological niches and geographic distributions number 49[END_REF]. Although knowledge regarding seabird distribution and migration has accumulated at the species level, there has been, to our knowledge, no attempt to concurrently describe the global-scale ecological niches of a wide array of seabird species, ranging from polar to tropical waters. As such, this study is the first integrated analysis drawing perspectives on the link between the seasonal dynamics of the ecological niche of seabirds and migratory status.

Materials and methods

Species seasonal distributions and species phenology

For this study, we focused on the species for which extensive knowledge exists on seasonal movements during both the breeding and non-breeding periods over a set of colonies representative of the entire distribution range. This resulted in the inclusion of 62 species of flying seabirds from seven families distributed worldwide (Supporting Information S1): 10 Alcidae species, 10 Diomedeidae, four Fregatidae, two Laridae, three Phaethontidae, 23 Procellaridae and 10 species of Sulidae. This set of species represents 21% of the total number of seabird species, including 40% of Alcidae species, 45% of Diomedeidae, 80% of Fregatidae, 100% of Phaethontidae and Sulidae, 25% of Procellaridae, and 2% of Laridae. For this latter family, the two included species represented 50% of the species with an exclusively pelagic marine distribution year-round. No Stercoraridae species were included, due to the relatively poor knowledge on the global scale distribution ranges and its dynamics across season and populations. Similarly, we were not able to incorporate any Oceanitidae, Hydrobatidae or Pelecanoididae as they are small and elusive species, for which knowledge of their at-sea distributions and seasonal movements remains very scarce.

We compiled validated information from the literature and from the BirdLife database (see Supporting Information S1 for detailed references; Supporting Information S2 for the maps) to build the breeding and non-breeding distributions of seabird species at the scale the Marine Ecoregions of the World. This publicly available dataset (Figure 2 and Supporting Information S2; [START_REF] Spalding | Marine ecoregions of the world: a bioregionalization of coastal and shelf areas[END_REF][START_REF] Spalding | Pelagic provinces of the world: a biogeographic classification of the world's surface pelagic waters[END_REF] classifies the coastal, shelf and surface pelagic waters into biogeographic ecoregions. Their scale matched the one of seabird distribution: all studied species had ranges wider than the ecoregions and always encompassed more than one ecoregion, without species ranging at a smaller scale. As such, this approach did not blur the seasonal variations in distribution through an over-representation of unused areas.

We also assessed the species migratory strategies from the literature and the BirdLife database (see Supporting Information S1). Four seasonal movement patterns were considered: migrant, partial migrant, dispersive, resident (Box 1). 44% of species were migratory (27 species), 26% dispersive (16 species), 16% resident (10 species) and 16% partially migratory (10 species), with species ranging in tropical, temperate and polar areas in each category. This work did not require ethical approval.

Environmental space

The environmental space is the representation of the environmental conditions (climate, resource) available to the species when removing all geographical and temporal attributes, so that contiguous points in that space are not necessarily contiguous in the geographical space, nor in time [START_REF] Peterson | Ecological niches and geographic distributions number 49[END_REF]. The niche of species is then described as the volume used by the species within this environmental space, which corresponds to the so-called "multi-dimensional hypervolume" defined by [START_REF] Hutchinson | Concluding remarks[END_REF]. Here, we built the environmental space by summarizing monthly climatologies (i.e., longterm averages) of 29 variables characterizing the pelagic ecosystem at the global scale by means of a Principal Component Analysis (PCA; Husson et al., 2016, Supporting Information S3).

Dynamic environmental variables were derived from GLObal ReanalYSis (GLORYS) of environmental conditions, using the "GLORYS Phys" dataset for sea surface temperature (SST), sea surface salinity (SSS), sea surface current (SSC), sea surface height (SSH), sea ice fraction (SIF), mixed layer depth (MLD); and the "GLORYS Bio" dataset for chlorophyll a surface concentration (CHL) and plankton concentration in carbon (PKN). For each month, the mean, minimum and maximum values of SST, SSS, SSC, SSH, SIF, MLD, CHL and PKN were summarised over the complete available period (2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017) to compute monthly climatologies. Minimum and maximum were considered in addition to the mean to take into account the temporal variability of environmental conditions within each month. The mean solar radiance (UVB) was used as a proxy for day length. UVB was extracted from the "Global UV-B radiation dataset for macro-ecological studies", using the monthly climatologies constructed from daily measurements summarised into monthly means over the period of 2004-2013. As UVB is null over polar areas during winter, we set all pixels with no values to zero, to avoid removing these areas from the analysis. Similarly, SIF values were set to zero in all pixels without sea ice. Two static variables, bathymetry and roughness, were extracted from the BioORACLE database. Monthly maps and detailed sources for each variables can be found in the Supporting Information S3.

All variables were resampled at a 1°resolution, centred and scaled before being included in the PCA. CHL and PKN were logged. Each 1°cell was considered 12 times in the analysis (one per month). The PCA was computed in R version 3.5.1 with the FactoMineR package [START_REF] Husson | FactoMineR: Multivariate Exploratory Data Analysis and Data Mining r package version 1[END_REF][START_REF] Core | R: A Language and Environment for Statistical Computing R Foundation for Statistical Computing Vienna[END_REF]. A single PCA was performed to ensure comparison of niche dynamics across species.

One inherent risk in characterizing the realised niche is the use of uninformative or irrelevant environmental gradients, i.e. with very small effects on species fitness, thereby impairing relevant assessment of niche dynamics. To reduce this risk, we considered only environmental variables with recognized importance to seabird ecology. They can have direct effects on bird physiology (temperature, salinity, solar radiance) or diet (plankton concentration), or be proxies of either prey availability (MLD, chlorophyll and plankton concentration) or oceanographic processes favouring prey aggregation (upwellings, eddies, river plumes, tidal fronts, described by roughness, SST, SSS, SSC and SSH; [START_REF] Weimerskirch | Are seabirds foraging for unpredictable resources?[END_REF][START_REF] Wakefield | Quantifying habitat use and preferences of pelagic seabirds using individual movement data: a review[END_REF]. We incorporated minimum and maximum monthly values to include a full description of temporal variability and potential effects of extremes in the analysis: the monthly mean of one environmental parameter might not constrain the presence of a species at a given time, but its maximum value might impair species survival through threshold effects (e.g. Šímová et al., 2011). Although we took great care in using only relevant ecological parameters, we were not able to include wind patterns in our analysis, despite its importance to seabird migration flyways [START_REF] Felicísimo | Ocean surface winds drive dynamics of transoceanic aerial movements[END_REF] due to large gaps in high latitude data.

Niche dynamics under true distribution

The ecological niche of seabirds during the breeding period is composed of two subniches: the at-sea and the nesting subniches. The at-sea subniche is characterised by the set of environmental conditions occurring at sea that conditions the survival of immatures and adults, and indirectly that of chicks (through food supply by adults), so the at-sea subniche have a large effect on the overall species growth rate. The nesting subniche is characterised by the set of environmental conditions occurring at the reproductive sites on land (wind, rain, snow, predatory pressure), and impacts the species growth rate by directly conditioning the survival of the brood (but not that of adults). This terrestrial subniche being by definition deserted during the non-breeding period, the breeding and non-breeding niches are always partially different.

The overall reduced availability of this nesting subniche in the geographical space combined with central-place foraging constraints makes large portions of the ocean inaccessible that would otherwise be included in the fundamental or potential niche. Here, we focused on the at-sea subniche resulting from these accessibility limitations, and did not consider the nesting subniche (which would be quantified at a finer scale than the one considered here).

We analysed the seasonal niche dynamics of seabirds following four steps and using the R package ecospat [START_REF] Broennimann | Measuring ecological niche overlap from occurrence and spatial environmental data[END_REF][START_REF] Core | R: A Language and Environment for Statistical Computing R Foundation for Statistical Computing Vienna[END_REF]. ( 1) Species presences in ecoregions during the breeding vs non-breeding months were projected over the environmental space depicted by the first two PCA dimensions (see Supporting Information S1 for species-specific periods) for the respective periods. [START_REF] Petitpierre | Climatic niche shifts are rare among terrestrial plant invaders[END_REF] For each period, occurrences on the environmental space were smoothed using a kernel function, then the space occupied by the species in the environmental space (its niche) was described by dividing the species' density of occurrence by the density of occurrence of the entire environment. (3) Niche dynamics were quantified with four metrics (Figure 3a): (A) the niche overlap between seasons was estimated using the Schoener's D, as

1 -1 /2( (|z b -z nb |))
, where z b is the breeding niche and z nb the non-breeding niche (see [START_REF] Broennimann | Measuring ecological niche overlap from occurrence and spatial environmental data[END_REF]; (B) the breeding specificity was quantified as the proportion of occurrences in the breeding niche located in environmental conditions not occupied during the non-breeding season [START_REF] Petitpierre | Climatic niche shifts are rare among terrestrial plant invaders[END_REF]; (C) the non-breeding specificity was the proportion of occurrences in the non-breeding distribution located in environmental conditions not occupied during the breeding season; (D) the stability of the non-breeding niche was the proportion of occurrences in the non-breeding distribution located in environmental conditions also occupied during the breeding season. The four metrics ranged from 0 to 1.

We formulated five theoretical patterns that could characterise seasonal niche dynamics (Figure 3), from near-complete overlap (i.e., niche tracking) to complete dislocation (i.e., niche switching). Extreme niche tracking behaviour would result in a strong overlap between seasonal niches, low breeding and non-breeding specificity but high stability (Figure 3B).

Extreme niche switching behaviour would result in low to no overlap, with high breeding and non-breeding specificity but low stability (Figure 3B). We tested for the equality of medians across seasonal movement types using a Kruskal-Wallis test, as variances were non-normal and non-homogeneous. The comparison was done for each metric, comparing all types together and then each pair separately if the first test was significant.

Finally, we computed the difference between the average position of species occurrences across PCA dimensions during the breeding and non-breeding periods to investigate any changes in niche positions and the orientation of such changes.

We also computed the difference in niche breadth between periods (niche breadth was the difference between the minimum and maximum positions of species occurrences on the PCA dimensions).

Niche dynamics under simulated residency

To test if migration was triggered by the avoidance of adverse conditions during the non-breeding period, we simulated ( 1) species residency at the breeding grounds by considering species to range only in the breeding grounds during the nonbreeding period, and ( 2) residency in the non-breeding grounds by considering species to range only in the non-breeding grounds during the breeding period. If the assumption that migration is driven by avoidance of adverse conditions was true, there should be strong seasonal differences due to changing environmental conditions between the two periods.

Overlap, breeding specificity, non-breeding specificity and stability were computed for the two simulated residencies.

We then compared the metrics obtained with the true distribution and the two simulated residencies separately for each movement type using a paired Wilcoxon test, and visually inspected the deviation of observed niches from simulated residencies for each species.

Results

Environmental space

The first two PCA dimensions were retained to define the environmental space, explaining 56.3% of the total variance of the dataset (Supporting Information S4). The first dimension, explaining 34.8% of the variance, represented a gradient from areas of high temperature, high plankton concentration and high sea surface height to polar areas with an important sea ice fraction. The second dimension was a productivity gradient (21.5% of the variance).

Niche dynamics under true distribution

All theoretical patterns of seasonal niche dynamics except for complete dislocation (Figure 3B) were identified in at least one of the 62 studied species. The overlap between seasonal niches of all species was high, with an average of 0.68 ± 0.21 (mean ± standard deviation; range: 0.24-1.00). Overall, breeding specificity and non-breeding specificity were low, averaging 0.05 ± 0.08 (from 0.00 to 0.37) and 0.06 ± 0.11 (from 0.00 to 0.59), respectively. Niche stability was strong with an average of 0.94 ± 0.11 (from 0.41 to 1.00).

Kruskal-Wallis tests confirmed that niche dynamics strongly depended on the migratory status (Figure 4 and Supporting Information S4), as at least one of the statuses had a median differing from the others for each of the four metrics (overlap: p < 0.01; breeding specificity : p = 0.04, stability and non-breeding specificity: p < 0.01). Pairwise tests showed that migrant species had a significantly lower overlap between niches than resident, partial migrant and dispersive species (p < 0.01, p = 0.03, p < 0.01, respectively; Figure 4A). Migrants also had higher breeding specificity than dispersive species (p < 0.01; Figure 4B). Migrant and partial migrants exhibited lower stability and higher non-breeding specificity than resident and dispersive species (Figures 4C,D): p < 0.01 for migrant-dispersive and migrant-resident comparisons, p < 0.01 for partial migrant-dispersive and p < 0.01 for partial migrant-resident.

The difference in niche positioning and breadth between the two seasons also varied according to migratory status (Figure 5A and Supporting Information S4). Dispersive species did not shift their niches between seasons (null differences for the two dimensions, Figure 5A) but exhibited wider niches on the second dimension during the breeding period (Figure 5B). Resident species experienced, on average, warmer and saltier (dimension 1) but also more productive waters (dimension 2) during the breeding period compared to the non-breeding one (positive differences for both dimensions; Figure 5A).

They displayed no difference in niche breadth between breeding and non-breeding niches on the second dimension, but wider breeding niches on the first one (Figure 5B). Migrant and partial migrant species experienced more productive waters during the breeding period (dimension 2, Figure 5A). They shifted positioning between season for the first dimension as well, but in both directions with species experiencing warmer and saltier waters during the breeding season or the opposite.

Both groups exhibited non-breeding niche wider than the breeding one (Figure 5B), on both dimensions for migrants but mostly on the first dimension for partial migrants.

Niche dynamics under simulated residency

Niche dynamics were similar when residency was simulated in the breeding and non-breeding ranges for resident and dispersive species (non-significant differences; Figure 4 and Supporting Information S4). Partial migrants exhibited similar niche overlap and breeding specificity if remaining resident in the breeding or non-breeding areas (non-significant differences), but exhibited smaller ranges of niche stability and non-breeding specificity under simulated residency than for the true distribution (p = 0.03 and 0.02 for stability and non-breeding specificity under the two simulations, respectively; Figure 4). Simulated seasonal niches for migrants resident in either the breeding or non-breeding ranges were more similar across seasons than the observed seasonal niches of migrants (Figure 4). For migrants, the overlap increased to values similar to that of the other groups for both simulated residencies (p < 0.01 for residency in breeding areas and in non-breeding areas), while breeding specificity was strongly reduced compared to the true distribution (difference only significant for residency in non-breeding area, p = 0.02). Stability and non-breeding specificity remained the same in the case of residency in the breeding range (p > 0.05), but stability was larger and non-breeding specificity smaller in the case of residency in the non-breeding range (p < 0.01 in both cases).

Visual inspection of the relationships between metrics obtained from the true distribution and simulated residency (Figure 6) confirmed that migrants deviated more from equivalence (1:1 line) than others species (except for two partial migrant species, the yelkouan (Puffinus yelkouan) and Balearic (P. mauretanicus) shearwaters). The overlap between true breeding and non-breeding niches was lower for migrants than when residency was simulated in the breeding and nonbreeding ranges (the largest deviations were observed for the Antarctic (Pachyptila desolata) and slender-billed (P. belcheri) prions; Figure 6A). Yet, some species (including dispersive and partial migrants) exhibited the opposite pattern, with a true overlap larger than under simulated residency, such as the yelkouan shearwater, the Chatham albatross (Thalassarche eremita), the black-footed albatross (Phoebastria nigripes) and the Barolo shearwater (Puffinus baroli). The majority of species had breeding specificity similar to the one obtained from simulated residency in the breeding and non-breeding ranges (Figure 6B), but a set of species exhibited strong deviations from simulated residency: breeding niches were either more different (higher breeding specificity; in particular the sooty and short-tailed (Ardenna tenuirostris) shearwaters) or more similar (lower breeding specificity; yelkouan and Balearic shearwaters) than non-breeding niches. There was less deviation from what would occur under simulated residency for stability and specificity of non-breeding niches (Figures 6C,D), but niche stability was still lower and specificity higher for migrants (i.e., non-breeding niches differed more from breeding niches than under simulated residency). A set of three species stood out, exhibiting much lower stability and much higher expansion than under simulated residency (higher dissimilarity in seasonal niches): the Antarctic and slender-billed prions (migrants) and the Balearic shearwater (partial migrant).

Discussion

This study represents the first investigation of the seasonal dynamics of seabird ecological niches at the global, ecoregion and inter-specific scales, by analysing the up-to-date knowledge about their spatial distribution. The comparison of the niche dynamics among the four existing seasonal movement patterns highlighted that, contrary to our predictions, migratory species tracked less their ecological niche across seasons than resident and dispersive ones. Achieved with a macro-ecological perspective, both spatially (global) and temporally (climatological), it hence provides new insights as to what processes could drive seabird migratory strategy at the species scale. Yet, such a macro-ecological scale limits interpretations at individual or population levels (e.g. drivers of specific trajectories or of distribution at specific sites) as it does not investigate conditions tracked by individuals, but rather the conditions experienced by the species as a whole.

Therefore, the present study opens new perspectives and calls for further down-scaling investigations, exploring the niche dynamics at the population and individual levels as to further disentangle the drivers of migration and understand their consequences on the temporal variations of the ecological niche. Thanks to the rapid increase in individual tracking for more and more seabirds species and in various environments, we are confident that such approaches should become possible in a near future.

Seasonal dynamics of ecological niches

Our quantification of the ecological niche dynamics of 62 seabird species demonstrated that they all tracked their ecological niche between seasons to some extent, with strong niche overlap (68% on average) between the breeding and non-breeding periods. Thus, there was a high stability of environmental conditions between the two niches: on average, 94% of the non-breeding niche was included in the breeding niche and only 5% of conditions encountered in the breeding niche were not encountered in the non-breeding niche (breeding specificity).

Seasonal niche dynamics nonetheless varied between species with different migratory status. Resident and dispersive species had an average overlap >80% between their seasonal niches, with almost complete stability of environmental conditions between the breeding and non-breeding niches. Resident and dispersive species were distributed mainly at low latitudes (boobies, tropicbirds and frigatebirds) in the least seasonal environments, where encountered environmental conditions barely change across the year, explaining this strong stability. However, some resident and dispersive species exhibiting niche tracking were also distributed in highly seasonal subpolar to polar waters. This was the case for the black guillemot (Cepphus grylle), the marbled murrelet (Brachyramphus marmoratus), the light-mantled albatross (Phoebetria palpebrata), the sooty albatross (Phoebetria fusca), the Indian yellow-nosed albatross (Thalassarche carteri), the northern fulmar (Fulmarus glacialis), the Barolo shearwater and the blue petrel (Halobaena caerulea). These species exhibited similar breeding and non-breeding niche contours, but their highest densities were located in different parts of their niches due to environmental seasonality. We can thus propose that these species have developed strategies other than migration to cope with unfavourable environmental conditions [START_REF] Shaffer | Functional significance of sexual dimorphism in wandering albatrosses, Diomedea exulans[END_REF][START_REF] Fort | Energetic consequences of contrasting winter migratory strategies in a sympatric Arctic seabird duet[END_REF][START_REF] Winger | A long winter for the Red Queen: rethinking the evolution of seasonal migration[END_REF].

Partial migrant species exhibited an intermediate pattern between resident/dispersive and migrant species, as found for the thermal niche by [START_REF] Dufour | Reconstructing the geographic and climatic origins of long-distance bird migrations[END_REF]. The seasonal niche overlap of partial migrants was lower than in resident and dispersive species but larger than in migrants. Stability and non-breeding niche specificity of partial migrants were similar to that of migrants, but their breeding specificity was similar to resident and dispersive species. This pattern indicates that the non-breeding niche incorporates and expands the breeding one, as confirmed by the difference in niche breadth between seasons. As a result, partial migrants do not switch, but rather expand, their ecological niche during the non-breeding period (third theoretical niche dynamics in Figure 3B). The intermediate pattern observed in partial migrants might originate in the migratory portions of the population exhibiting niche switching, while the resident portions track the same niche year-round, although testing this hypothesis would require examining the niche dynamics of the migrant and resident portions of the population separately.

Contrary to studies showing niche-tracking migrants [START_REF] Gómez | Niche-tracking migrants and niche-switching residents: evolution of climatic niches in new world warblers (parulidae)[END_REF][START_REF] Zurell | Do long-distance migratory birds track their niche through seasons?[END_REF][START_REF] Somveille | Where the wild birds go: explaining the differences in migratory destinations across terrestrial bird species[END_REF], the migratory species included in our study displayed more divergent breeding and non-breeding niches than other species [START_REF] Dufour | Reconstructing the geographic and climatic origins of long-distance bird migrations[END_REF]. They exhibited the lowest overlap coupled with the lowest stability of the environmental conditions between seasonal niches, and the largest breeding niche specificity. Migrants also exhibited the largest differences in niche breadth between seasons. These species engaged in short to long-range migrations, including basin-scale, trans-equatorial migrations. It has previously been suggested that long-range migration in seabirds might be linked to niche tracking, allowing individuals to follow an endless summer, such as in sooty shearwaters in the Pacific Ocean [START_REF] Shaffer | Migratory shearwaters integrate oceanic resources across the Pacific Ocean in an endless summer[END_REF] or in Scopoli shearwaters (Calonectris diomedea) from the Mediterranean Sea to the equatorial Atlantic [START_REF] Péron | Tracking through Life Stages: Adult, Immature and Juvenile Autumn Migration in a Long-Lived Seabird[END_REF]. This result cautions against considering niche tracking a golden rule for marine migrants, given the stronger level of niche divergence observed in migrant species when considering a large set of environmental conditions, rather than productivity only, as has previously been done. Some trans-equatorial migrants did exhibit similar (but not identical) niches between the breeding and non-breeding seasons, despite performing long-range, basin-scale migrations: Chatham albatross, Buller's albatross (Thalassarche bulleri), Cook's petrel (Pterodroma cooki), Deserta's petrel (Pterodroma deserta). Yet, others species performing similar basin-scale migrations exhibited some of the most divergent seasonal niches among studied species: sooty shearwater, great shearwater (Ardenna gravis, unlike in [START_REF] Shaffer | Migratory shearwaters integrate oceanic resources across the Pacific Ocean in an endless summer[END_REF], and short-tailed shearwater.

Environmental drivers of seabird migration

Migrant species exhibiting some variation in their niche occupancy across seasons points towards a more complex pattern than migrant tracking favourable conditions. However, the question remains open as to whether migration is triggered by avoidance of unfavourable conditions, which could optimise the individual survival during the non-breeding period [START_REF] Alerstam | Long-distance migration: evolution and determinants[END_REF][START_REF] Shaw | Migration or Residency? The Evolution of Movement Behavior and Information Usage in Seasonal Environments[END_REF][START_REF] Somveille | Where the wild birds go: explaining the differences in migratory destinations across terrestrial bird species[END_REF][START_REF] Winger | A long winter for the Red Queen: rethinking the evolution of seasonal migration[END_REF]. Here, we tested this hypothesis by simulating the residency of seabirds in their breeding and non-breeding grounds. If migration was triggered by favourable conditions becoming unavailable, simulated residency should show important variations in niche occupancy. This was, however, found for two partial migrant species only, the yelkouan and the Balearic shearwaters. Both species breed in the Mediterranean Sea from October to May/July [START_REF] Guilford | Geolocators reveal migration and pre-breeding behaviour of the critically endangered Balearic Shearwater Puffinus mauretanicus[END_REF][START_REF] Raine | Migration strategies of the Yelkouan Shearwater Puffinus yelkouan[END_REF]. Environmental conditions encountered during this higher productivity period changed drastically during the non-breeding period (Supporting Information S4), and both species cope with this change by performing short and medium scale migrations to track their niche, to the eastern Mediterranean, Black Sea (yelkouan shearwater; [START_REF] Raine | Migration strategies of the Yelkouan Shearwater Puffinus yelkouan[END_REF] and the Bay of Biscay (Balearic shearwater [START_REF] Guilford | Geolocators reveal migration and pre-breeding behaviour of the critically endangered Balearic Shearwater Puffinus mauretanicus[END_REF]. The environmental envelop ensuring individual survival became too scarce to ensure the maintenance of the complete population within the breeding range, eventually driving the migration of a large portion of the populations.

Conversely, the stand-out result arising from this work is that, for all other species and whatever their migratory strategies, environmental conditions used during the breeding and non-breeding periods remained available during the other period. Migrants exhibited stronger seasonal variations in niche occupancy when migrating than if they stayed in either the breeding or non-breeding grounds. Thus, our results suggest that the unavailability of favourable conditions is not a primary driver of migration, contrary to what was hypothesized. This striking result is opposite to what was found in terrestrial birds [START_REF] Gómez | Niche-tracking migrants and niche-switching residents: evolution of climatic niches in new world warblers (parulidae)[END_REF][START_REF] Thorup | Resource tracking within and across continents in long-distance bird migrants[END_REF][START_REF] Zurell | Do long-distance migratory birds track their niche through seasons?[END_REF] and opens the question as to why seabirds engage in migration if favourable conditions remain available from one season to the other? For a migratory strategy to be viable, it must come with a fitness benefit, either through enhanced survival or through an improved reproductive success [START_REF] Zúñiga | Migration confers winter survival benefits in a partially migratory songbird[END_REF]. As mentioned above, one of the main characteristics of seabirds is their reliance on land for reproduction. This characteristic entails strong constraints on species distribution as it induces central-place foraging behaviour, and requires breeding sites suitable enough to support reproduction in densely aggregated colonies and ensure the survival of both adults and their chicks.

During the breeding season, the most energy demanding period in seabirds, aggregations of individuals around colonies induce the strongest level of competition faced by seabirds during their phenological cycle [START_REF] Gaston | Seabirds: a natural history[END_REF]. However, once the breeding duties are completed, seabird energy requirements drop and the central-place foraging constraint disappears.

This results in a dilution of intra-specific competition during the non-breeding period [START_REF] Pettex | Spatial segregation between immatures and adults in a pelagic seabird suggests age-related competition[END_REF], but also in the possibility for individuals to move to more favourable areas otherwise inaccessible during the breeding period.

We can then reasonably suggest that the larger shift observed in the seasonal niches of migrating seabirds reflects the fact that large-scale seasonal movements of seabirds are not driven by niche availability alone but rather by a combination of biological (reproductive duty) and environmental (breeding site availability and fidelity) constraints imposed during the breeding period [START_REF] Gaston | Seabirds: a natural history[END_REF][START_REF] Shaw | Migration or Residency? The Evolution of Movement Behavior and Information Usage in Seasonal Environments[END_REF][START_REF] Winger | A long winter for the Red Queen: rethinking the evolution of seasonal migration[END_REF]. This combination eventually leads species to switch their niche between seasons, where individuals move between areas offering an environment that optimises their reproductive success (as exemplified by the more productive systems used in average by migrants during the breeding period) and areas that are optimal for the survival during the free-ranging period.

However, while all species have to cope with strong levels of competition during the breeding period, they do not all migrate or disperse. Migration is only one of the many strategies available to animals to cope with environmental seasonal fluctuations [START_REF] Winger | A long winter for the Red Queen: rethinking the evolution of seasonal migration[END_REF], and resident species must chose alternative strategies to cope with the unfavourable period (low temperature, polar night or extreme winds). Several seabirds have been shown to sustain their needs in seasonal environments by adjusting other parameters than spatial distribution, such as their energy budget, their foraging effort, or their diet [START_REF] Fort | Energetic consequences of contrasting winter migratory strategies in a sympatric Arctic seabird duet[END_REF]. Those alternative strategies are less common than migration in seabirds, as in birds in general (see Figure (a) in Box 1), because birds are characterised by exceptional flight capabilities [START_REF] Gaston | Seabirds: a natural history[END_REF][START_REF] Winger | A long winter for the Red Queen: rethinking the evolution of seasonal migration[END_REF]). Yet, alternative strategies are necessary to ensure species maintenance in seasonally fluctuating environments in the absence of migration. As a result, a needful extension to the present work is to better describe the strategies used by resident species to face fluctuating conditions (e.g. activity budgets, diets). Thanks to the increasing availability of tracking dataset and biological information for a large amount of species and populations, it would become possible to complement this work in that direction in a near future, to more precisely quantify seasonal niches of seabirds at a finer scale, and ultimately to better understand what triggers seabird migration.

Finally, other factors not considered in the present study might also influence seabird seasonal movements. In particular, we did not control for phylogenetic relationships among species, nor for past evolutionary constraints on bird spatial distributions: some species might engage in migration through inheritance processes although current environmental conditions do not favour such a strategy [START_REF] Pulido | The genetics and evolution of avian migration[END_REF]. Migratory strategies might also be influenced by morphological characteristics (the bird capacity to carry out long-range migration; [START_REF] Dias | Do seabirds differ from other migrants in their travel arrangements? On route strategies of Cory's shearwater during its trans-equatorial journey[END_REF], foraging tactics, and life history parameters (e.g. maturity, life expectancy; [START_REF] Péron | Tracking through Life Stages: Adult, Immature and Juvenile Autumn Migration in a Long-Lived Seabird[END_REF]. These factors might interact with the effect of the environmental availability to condition the extent of niche tracking exhibited by seabird species. The present work thus opens several research avenues to further investigate underlying drivers of seabird migratory movements and to better understand the evolution of migration.

To conclude, we, for the first time, described the global seasonal dynamics of a wide array of seabird species, ranging from polar to tropical waters. We demonstrated that all seabirds are niche trackers, yet contrasting dynamics occurred depending on seasonal movement types. Resident and dispersive seabirds displayed high levels of niche tracking throughout the year, regardless of the environmental seasonality, while migrants exhibited more divergent seasonal niches. In most cases, breeding and non-breeding niches of migrants were still available during the other period, suggesting that migration is not primarily driven by the availability of the favourable niche. This result contrasts with previous results obtained on terrestrial systems, and might originate from the particular biological constraints experienced by seabirds. Specifically, seabird migration might in part be driven by the scarcity of breeding sites, which may force individuals to move between areas where the environment allows for optimization of reproductive success and non-breeding grounds that are optimal for survival during the free-ranging period.

Box 1. The four types of seasonal movement patterns in seabirds: Four movement types were considered in this work:

Residency: the population remains sedentary within a similar area along the year; e.g. Cape gannet Morus capensis [START_REF] Sabarros | Fine-scale recognition and use of mesoscale fronts by foraging Cape gannets in the Benguela upwelling region[END_REF].

Dispersal: the population disperses using non-directed movements during the non-breeding period over a large range often including the breeding range, without a clearly defined non-breeding "residency area"; e.g.

white-tailed tropicbird (Phaethon lepturus; [START_REF] Mejías | Distribution and at-sea behavior of Bermudan White-tailed Tropicbirds (Phaethon lepturus catesbyi) during the non-breeding season[END_REF].

Partial migration: one fraction of the population is sedentary, another migrant; e.g. Balearic and yelkouan shearwaters (Puffinus mauretanicus and yelkouan), wandering albatross (Diomedea exulans; [START_REF] Raine | Migration strategies of the Yelkouan Shearwater Puffinus yelkouan[END_REF][START_REF] Guilford | Geolocators reveal migration and pre-breeding behaviour of the critically endangered Balearic Shearwater Puffinus mauretanicus[END_REF][START_REF] Weimerskirch | Extreme variation in migration strategies between and within wandering albatross populations during their sabbatical year and their fitness consequences[END_REF].

Migration: populations completely desert their breeding grounds during the non-breeding period with directed movements and precise schedules.

Direct migration: individuals perform direct trips from breeding to non-breeding grounds, optionally with some stop-over sites; e.g. sooty shearwater (Ardenna grisea; [START_REF] Shaffer | Migratory shearwaters integrate oceanic resources across the Pacific Ocean in an endless summer[END_REF].

Leap-frog migration: individuals from northerly colonies migrate further south than those from southerly colonies; e.g. Bulwer's petrel (Bulweria bulwerii; [START_REF] Ramos | Leapfrog migration and habitat preferences of a small oceanic seabird, Bulwer's petrel (Bulweria bulwerii)[END_REF].

Chain migration: populations from all colonies migrate the same distance, resulting in a sequential displacement where a colony breeding ground becomes the non-breeding ground for another colony during the non-breeding period; e.g. northern gannet (Morus bassanus; [START_REF] Fort | Meta-population evidence of oriented chain migration in northern gannets (Morus bassanus)[END_REF].

A majority of the studied species were migrant or dispersive (70%), a minority resident (16%;

Figure S2- 1 ,FiguresFigure 1 .

 11 Figures

Figure 2 .

 2 Figure 2. Number of species present in each Marine Ecoregion of the World[START_REF] Spalding | Marine ecoregions of the world: a bioregionalization of coastal and shelf areas[END_REF][START_REF] Spalding | Pelagic provinces of the world: a biogeographic classification of the world's surface pelagic waters[END_REF] during (a) their breeding and (b) non-breeding periods. Note that breeding and non-breeding periods do not necessarily overlap among species present in similar ecoregions, meaning they are not necessarily present at the same time.

Figure 3 .

 3 Figure 3. (a) -Seasonal niches projected on environmental spaces and the four metrics derived from their comparisons: niche overlap, breeding specificity (portion of the niche occupied only during the breeding season), niche stability (portion of the niche occupied in both seasons) and non-breeding specificity (portion of the niche occupied only during the non-breeding season); (b) -Five different possible niche dynamics along the continuum from niche tracking to niche switching, and their resulting comparison metrics (overlap, breeding specificity, stability and non-breeding specificity), with examples from observed seabird seasonal niches. One species is shown as an example for each category (Chatham albatross Thalassarche eremita, blue petrel Halobaena caerulea, Desertas petrel Pterodroma deserta and Antarctic prion Pachyptila desolata). The breeding (left panel) and non-breeding (centre panel) niches are shown displaying the densities of presence inside the niche as shaded grey. The overlap between the two niches is shown in the right panel, with breeding niche in dark grey, non-breeding niche in light grey and overlap in mid-tone grey. The solid contour lines depict 100% of the available environment, the dashed contour lines 50% of the most common background environment. There was no seabird species exhibiting complete dislocation of seasonal niches.

Figure 4 .

 4 Figure 4. (a) -Species niche overlap, (b) -breeding niche specificity, (c) -stability of niches and (d) -non-breeding niche specificity for each type of seasonal movement pattern for the true distribution (observed niche, in black), simulated residency in breeding area (orange) and simulated residency in non-breeding area (blue). The highlighted pairs of seasonal movement patterns are those exhibiting significantly different medians (for true distributions only) as determined by Kruskal-Wallis tests, with the corresponding significance levels below.

Figure 5 .

 5 Figure 5. (a) -Differences in averaged position of species occurrences on PCA dimensions 1 and 2 between the breeding and non-breeding periods. Positive differences correspond to warmer, saltier and shallower waters (dimension 1) or more productive waters (dimension 2) during the breeding period compared to the non-breeding one. (b) -Differences in niche breadth between the breeding and non-breeding periods. Positive differences indicate a wider breeding niche compared to the non-breeding one.

Figure 6 .

 6 Figure 6. Relationship between observed niche (true distribution) and simulated niche (left: simulated residency in breeding area; centre: simulated residency in non-breeding area). (a) -Overlap of seasonal niches: species occurring above the line display stronger overlap in the observed distribution than under simulated residency (right panel). (b) -Breeding niche specificity: species occurring above the line have conditions encountered during the breeding period that differ more from conditions encountered during the non-breeding period than under simulated residency (right panel). (c) -Stability of seasonal niches: species occurring below the line have conditions encountered during the non-breeding period that differ more from conditions encountered during the breeding period than under simulated residency (right panel). (d) -Non-breeding niche specificity: species occurring above the line have conditions encountered during the non-breeding period that differ more from conditions encountered during the breeding period than under simulated residency (right panel). Some species displaying contrasting behaviours are specified: 1 -Yelkouan shearwater (Puffinus yelkouan); 2 -Chatham albatross (Thalassarche eremita); 3 -Black-footed albatross (Phoebastria nigripes); 4 -Barolo shearwater (Puffinus baroli); 5 -Antarctic prion (Pachyptila desolata); 6 -Slender-billed prion (Pachyptila belcheri); 7 -Sooty shearwater (Ardenna grisea); 8 -Short-tailed shearwater (Ardenna tenuirostris); 9 -Black-legged kittiwake (Rissa tridactyla); 10 -Christmas frigatebird (Fregata andrewsi); 11 -Balearic shearwater (Puffinus mauretanicus).

Acknowledgements

We warmly thank Auriane Virgili for constructive discussion about the topic and suggestions that improved the work. We also are deeply grateful to Drs Melissa and Andrea Grunst for proof-reading of the English as well as to Prof. Allen Hurlbert and five anonymous referees for constructive comments on previous versions of this manuscript.

Conflict of interest

We have no conflict of interest to declare.

Author contributions

Charlotte Lambert conceived the ideas and designed the methodology; Charlotte Lambert collected and analysed the data; Charlotte Lambert and Jérôme Fort wrote the manuscript. Both authors contributed critically to the drafts and gave final approval for publication.

Data Accessibility Statement

Codes available from the Zenodo repository https://doi.org/10.5281/zenodo.6617889 [START_REF] Lambert | No evidence that seasonal changes in large-scale environmental conditions drive migration in seabirds[END_REF].