Delord Particle

Karine Suffix

Cherel Particle

Yves Suffix

Christophe Suffix

Olivier Suffix

Weimerskirch Particle

Henri Suffix

Karine Delord
email: karine.delord@cebc.cnrs.fr

Yves Cherel

Christophe Barbraud

Olivier Chastel

Henri Weimerskirch

High variability in migration and wintering strategies of brown skuas (Catharacta antarctica lonnbergi) in the Indian Ocean

Keywords: separated by '-') Activity -Feather -Geolocator -Intraspecific strategies -Phenology -Stable isotopes Activity, Feather, Geolocator, Intraspecific 41 strategies, Phenology, Stable isotopes

Movements of brown skuas (Catharacta antarctica lonnbergi) originating from two populations in the southern Indian Ocean were studied during the nonbreeding period using geolocation. A total of 33 individuals were equipped resulting in 34 annual tracks recovered from 50 deployments. Brown skuas varied extensively in their post-breeding movements, from true long range migrations to reach distant wintering zones, to short movements in the vicinity of breeding grounds. Overall, brown skuas migrated northward to overwinter in different areas in the southern hemisphere; individuals remained in the Indian Ocean, except two that overwintered in the Benguela Current (Atlantic Ocean). Wintering grounds were generally situated in productive dynamic upwelling waters or frontal systems. Brown skuas avoided the less productive area of the South Subtropical Gyre in the Central Indian Ocean. Individuals clearly differed in migratory strategies, targeting areas in a continuum from the sub-Antarctic to the tropics. Interindividual differences were not sex-dependent. The migration dates varied between sexes with females leaving the breeding sites earlier and returning later compared to males. The duration of migration depended on wintering area and sex. Males had shorter migrations than females, regardless of the wintering area. Isotopic signatures clearly indicated that birds moulted in the wintering area and during migration. The low δ 15 N values of feathers that grew in mixed subtropical-sub-Antarctic waters suggest that skuas fed on low trophic level prey in these areas. The origin and consequences of such strong interindividual variation in migratory strategies requires further investigation.

Introduction 43

Migration is commonly considered to be a response to 44 seasonal variation in the environment and particularly to 45 prey availability (Jessopp et al. 2013). Seabirds are central 46 place foragers during breeding but can range over much 47 larger areas in search of food outside the breeding period.

48 Migration often necessitates large-scale movements 49 beyond the breeding home range and is an essential com-50 ponent of the life-history of flying seabirds (Shaffer et al. 51 2006;Egevang et al. 2010;[START_REF] Weimerskirch | Lifetime foraging patterns of the wandering 982 albatross: life on the move![END_REF].

52 Migration strategies of seabirds can differ markedly, 53 exhibiting intraspecific variations linked to population, sex 54 or age (Guilford et al. 2009;Dias et al. 2011;Fijn et al. 55 2013). Individuals from the same breeding colony could 56 migrate using different routes, thereby reaching distant and 57 distinct wintering grounds [START_REF] Yamamoto | 996 At-Sea Distribution and Behavior of Streaked Shearwaters 997 (Calonectris Leucomelas) During the Nonbreeding Period[END_REF]58 Weimerskirch et al. 2015a, b). Timing for the outbound or 59 the inbound movements could also differ between indi-60 viduals and the sexes, [START_REF] Yamamoto | 996 At-Sea Distribution and Behavior of Streaked Shearwaters 997 (Calonectris Leucomelas) During the Nonbreeding Period[END_REF]Hedd et al. 61 2014;[START_REF] Thiebot | Gentlemen first? 955 'Broken stick' modelling reveals sex-related homing decision 956 date in migrating seabirds[END_REF]. Hence, describing and under-62 standing the individual differences in migration strategy 63 that may impact winter survival of individuals is a key 64 issue in population dynamics of long-lived species [START_REF] Barbraud | Climate and density shape population dynamics of a marine top predator[END_REF]Frederiksen et al. 2008;[START_REF] Reiertsen | Prey density in non-breeding areas affects adult survival of 938 black-legged kittiwakes Rissa tridactyla[END_REF].

U N C O R R E C T E D P R O O F
The identification of migration strategies is furthermore crucial to understand how individuals are differently affected by climate change [START_REF] Weimerskirch | Changes 978 in wind pattern alter albatross distribution and life-history traits[END_REF], pollution, particularly oiling (Montevecchi et al. 2012a, b), interaction with fisheries including the associated incidental bycatch [START_REF] Phillips | Summer distribution and migration of nonbreeding albatrosses: individual consistencies and and implications for conservation[END_REF][START_REF] Watkins | Interactions between 975 seabirds and deep-water hake trawl gear: an assessment of 976 impacts in South African waters[END_REF][START_REF] Maree | Significant reductions in mortality of threatened seabirds in a South African trawl fishery[END_REF]) and exposure to contaminants [START_REF] Ble ´vin | Wide range of mercury contamination in chicks of Southern Ocean seabirds[END_REF];L e a te ta l .2013; Carravieri et al. 2014a). Individual responses to environmental variability or to anthropogenic factors can have strong implications in terms of survival and population dynamics (Harris et al. 2013;Goutte et al. 2014). Individuals, from the same or different populations, might share wintering areas and experience similar threats (Gonzalez-Solis et al. 2007;Frederiksen et al. 2012;[START_REF] Tranquilla | Individual Winter Movement 963 Strategies in Two Species of Murre (Uria spp.) in the Northwest 964 Atlantic[END_REF], or individuals might have different strategies, resulting in differences in threat exposure [START_REF] Rayner | Contemporary and historical separation of transequatorial migration between genetically distinct seabird populations[END_REF]Hedd et al. 2012;[START_REF] Carey | Trans-equatorial migration of Short-tailed Shearwaters revealed by geolocators[END_REF].

Large skuas (Stercorariidae, Catharacta sp.) are colonial seabirds that breed mainly in the Southern Ocean and in Antarctica (except one Northern Atlantic species). A high degree of interspecific variation exists in migration strategies in skuas, even within species and populations [START_REF] Phillips | Movements, winter distribution and activity patterns of Falkland and brown skuas: insights from loggers and isotopes[END_REF]Kopp et al. 2011;Weimerskirch et al. 2015a, b;[START_REF] Krietsch | There were notable differences in migratory routes 533 and wintering areas between individuals as found in other 534[END_REF]. However, few studies on migration movements of skuas have been conducted and intraspecific variations in migration patterns and in wintering areas remain poorly understood. For species that employ specialized predatory strategies (such as skuas), understanding how individuals can adjust their foraging behaviour to changing conditions to fulfil their life-history needs is a cornerstone of their ecology. The reason for such high variation is unknown, but it should clearly have different fitness consequences for individuals (because the different areas might differ in their productivity and in the experienced threats).

The aim of this study was to contribute such information for two brown skua populations (Catharacta antarctica lonnbergi, Mathews, 1912) breeding in the sub-Antarctic French Southern Territories. The brown skua is a longlived seabird that mainly breeds on sub-Antarctic islands and the Antarctic Peninsula. The diet of brown skuas is relatively well documented during the breeding season, with birds feeding on local abundant resources; they often specialize as predators or scavengers of petrels and penguins (Furness 1987; Olsen and Larsson1997). However, the diet and foraging ecology of brown skuas remain poorly known outside the breeding season (Furness 1987). Food availability usually varies throughout the year and is most likely a driver of migration and wintering strategies (Lack 1968;[START_REF] Alerstam | Long-distance migration: evolution and determinants[END_REF]. The timing of the migration is expected to be synchronized with the 118 phenology of the prey species, especially with migratory 119 species, to avoid a mistiming in breeding cycles (Visser 120 and Both 2005). The nonbreeding distribution of brown 121 skuas is only known for South Georgia [START_REF] Phillips | Movements, winter distribution and activity patterns of Falkland and brown skuas: insights from loggers and isotopes[END_REF]122 Carneiro et al. 2016) and King George Island (Krietsch 123 et al. 2017) 144 concomitant with a change in the isotopic signature 145 between the two periods, similar to recent results for the 146 south polar skua (Weimerskirch et al. 2015a, b). 166 Processing of the GLS daylight data was carried out as 167 described previously (Pe ´ron et al. 2010). Migration timing darkness each day (consecutive light and dark period) was assessed directly from the logger daylight data. Loggers also recorded sea surface temperature (SST) when the logger was immersed for more than 10 min. The average SST recorded monthly was used as an indicator of the water masses where the birds foraged during the nonbreeding period.

We then fitted generalized linear models (GLMs) to investigate the effects of sex, wintering location (three different water masses sensu Longhurst (2007): subtropical, tropical or sub-Antarctic) and their interaction on migration schedules (date of departure, date of arrival and duration) using a Poisson distribution. We selected only one track for the bird tracked over two consecutive years to prevent pseudo-replication. General linear mixed-effects models (GLMMs), fitted in the lme4 package (Zuur et al. 2010), were used to model activity patterns (daily proportion of time spent on the water during daytime or during darkness). Sex, wintering location, month (independent variables) and two by two interactions were included as fixed effects. Individual identity was fitted as a random effect to account for pseudo-replication, as the same individual could contribute several values. Model assumptions were visually validated based on normalized residuals, following the protocol presented by Zuur et al. ( 2010). Unsexed individuals were excluded from the dataset. Due to sample size, year could not be used as a fixed factor in the models. The effect of breeding colony could not be included due to small sample size for the Crozet population. Models were ranked using the Akaike Information Criterion (AIC) and Akaike weights [START_REF] Burnham | Model selection and multimodel inference: a practical information-theoretic approach[END_REF]). The model with the lowest AIC was considered the best model. When DAIC between models was \2, the models were considered equally good descriptors of the data and we chose the most parsimonious model-that with the lowest number of parameters. Spatial and statistical analyses were performed using R 2. 15.3 (R Development Core Team 2013).

Following (Jaeger et al. 2009), carbon and nitrogen stable isotopes ratios (d 13 C and d 15 N, respectively) were measured on four different fully grown body feathers from the lower back. Feathers were collected upon recapture of each individual bird, thus corresponding to the previous moulting period at sea recorded by the GLS. In seabirds, including skuas, feather isotope values represent the foraging habitat (d 13 C) and diet/trophic position (d 15 N) during the nonbreeding period because adult birds replace their plumage at that time (Higgins and Davies 1996;Cherel et al. 2008b; but see Gran ˜a Grilli and Cherel 2017). To verify that skuas moulted body feathers outside the breeding period, ten large chicks from Kerguelen Islands were feather sampled as control birds reflecting the skua diet during the summer breeding period. At that time

U N C O R R E C T E D P R O O F
brown skuas from Mayes Island prey upon seabirds on land, mainly blue petrels [START_REF] Mougeot | Predation on burrowing petrels by the brown shua (Catharacta skua lo ¨nnbergi)a t Mayes Island, Kerguelen[END_REF], a small Procellariiforme that forages at sea in southern Antarctic waters (Cherel et al. 2014). A single chick body feather was used for isotopic analyses, because chick feathers grow almost synchronously and thus present low inter-feather d 13 C and d 15 N variations (Carravieri et al. 2014b). Feather preparation and isotopic analyses were detailed by (Jaeger et al. 2009). In brief, feathers were cleaned using a 2:1 chloroform: methanol solution and then oven dried for 48 h at 50 °C. Each whole body feather was homogenized by cutting it with scissors into small fragments and a subsample of *0.3 mg was packed into tin containers for stable isotope analysis. The relative abundances of carbon and nitrogen isotopes were determined with a continuous flow mass spectrometer (Thermo Scientific Delta V Advantage) coupled to an elemental analyzer (Thermo Scientific Flash EA 1112) 1). In the former analysis, isotopic values of the four feathers per 301 bird were assigned to the main wintering zone identified by 302 the GLS data of the corresponding individual (tropical, 303 subtropical and sub-Antarctic). In the latter analysis, each 304 single body feather was tentatively assigned to a moulting 305 zone (Weimerskirch et al. 2015a, b). We first looked at the 306 GLS tracks of birds that wintered in only one marine area 307 to assign the corresponding feather isotope values to that 308 area. We then carefully examined the feather isotope val-309 ues of skuas that spent the nonbreeding period in more than 310 one area to correctly assign their isotopic values. We were 311 unable to reliably assign isotopic values to a wintering area 312 for a few feathers using this 2-step protocol (see 'Un-313 known' in Table 1).

314

Results

315

A total of 34 successful GLS deployments (Crozet, n = 5; 316 Kerguelen, n = 29; 1 individual from Crozet was tracked 317 over two successive years and returned to the same area-318 results not presented) were made on 33 individuals. The GLS 319 recovery rates varied from 40% at Crozet to 73% at Ker-320 guelen. The loggers provided simultaneous records of loca-321 tion and immersion data during 34 annual tracks, which were 322 mainly located within the Indian Ocean (Fig. 1). Body 323 feathers were collected from 19 adult brown skuas (9 females 324 and 10 males) from Kerguelen Islands only. Individuals consistently started their post-breeding 367 movements to reach their wintering areas in late February-368 early March, with a mean departure date around the 1st 369 March (range 28th January-8th April; Table 2). Similarly, 370 skuas consistently left their wintering area for their pre-371 breeding migration in late August with a mean return date 372 around 21st August (range 30th July-24th September). The migration dates (outbound or inbound movements) varied between sexes (Table 2; Fig. 3). Females tended to leave earlier and to return later at the breeding sites compared to males. Similarly, the duration of migration substantially differed between the wintering areas and sexes (Table 3; Fig. 3). Contrary to the inbound migration, where brown skuas left their wintering grounds at the same time independently of their wintering areas (21st August), the average outbound migration dates occurred later for birds overwintering in subtropical waters (2nd March vs. 27th February) and migration lasted longer (20 ± 16 vs. 8 ± 13 days; Tables 2,3) compared to those in southern- 2,3). Males 389 tended to migrate (outward and inward migration) during 390 shorter periods than females did (Table 2) whatever the 391 wintering area. 392 Activity (percentage of time spent on water daily) was 393 measured during the whole inter-breeding period, including 394 outward and inward migrations and the wintering zone. 395 Brown skuas spent a very high proportion of time sitting on 396 the water during the inter-breeding period (80-81%; 397 Online Resource Fig S3). The activity pattern (during 398 daytime and during night-time) varied with month and 399 wintering area (Table 4). Individuals tended to spend a 400 greater proportion of time on the water at the beginning of 401 the wintering period than at the end of the period; Online 402 Resource Fig S3 1). However, both intra-and inter-individual isotopic variations within the same group (sub-Antarctic, subtropical and tropical) were high with large d 13 C overlaps among groups (Fig. 4). This indicates that most individual birds did not moult all their body feathers in the wintering zone. Rather, individuals moulted over different water masses during the whole inter-nesting period, i.e. during migratory movements and before migration.

Combining GLS data and isotopic values at the feather level depicted a more informative pattern (Fig. 4). Feather d 13 C and d 15 N values from different moulting zones were significantly different (Kruskal-Wallis: H = 55.0 and 64.3 for d 13 C and d 15 N, respectively, both p \ 0.0001). Post hoc pairwise Conover-Inman tests documented three notable features (Table 1): (i) feather d 13 C values overall increased with decreasing latitudes, from the lower values of chicks to the higher values of adults that moulted in tropical waters (Madagascar and Java); (ii) feather isotopic values were identical for birds that moulted in western (Madagascar) and eastern (Java) tropical waters of the Indian Ocean; (iii) Feathers of birds that foraged in latitudes between *35 and 50°S (subtropical and sub-Antarctic) presented remarkably low d 15 N values (8.5%) that differed from all the other groups (11.1-14.1%). Long-distance northward migration of brown skuas was 458 long-suspected based on coastal and at-sea resightings and 459 observations (Furness 1987). Brown skuas had long been 460 erroneously reported as migrants in the northern hemi-461 sphere until it was clearly established that these reports 462 were misidentifications of south polar skuas (Devillers 463 1977;Furness 1987). Both brown and south polar skuas are 464 regularly mentioned as visitors in the Northern Indian 465 Ocean during the southern monsoon (Sri Lanka's coastal 466 waters; [START_REF] Van Den Berg | Barau's Petrel Pterodroma baraui, 968 Jouanin's Petrel Bulweria fallax and other seabirds in the 969 northern indian ocean in June-July 1984 and 1985[END_REF]De Silva 2011). 467 Unbanded and formerly misidentified Catharacta birds 468 observed at sea, in large numbers off India or Sri Lanka, as well as museum specimens from the same region, supported the belief that they regularly migrate northward from sub-Antarctic and Antarctic breeding sites. Recent tracking studies suggest that they must be predominantly south polar skuas originating from Antarctic breeding colonies located in the southern Atlantic Ocean sector (Weimerskirch et al. 2015a, b). Our study indicates that some of these birds observed in tropical waters are brown skuas. Timing and migratory patterns of brown skuas appeared similar between the two populations from Kerguelen and Crozet Islands, which concurs with typical return dates of untagged birds at the breeding sites [START_REF] Paulian | Pinnipe `des, ce ´tace ´s, oiseaux des iles Kerguelen et Amsterdam[END_REF]Barre ´1976;authors' unpubl. data). Adult brown skuas from both populations principally migrated over the Indian Ocean Basin to overwinter, except for a few individuals that wintered in the very productive Benguela Current System in the eastern Atlantic Ocean. All individuals remained within the southern hemisphere, thus contrasting with the trans-equatorial migrations performed by many individuals of the closely related South Polar skua (Kopp et al. 2011;Weimerskirch et al. 2015a, b). Previous studies on brown skuas from the Southern Atlantic Ocean showed a consistent northward migration to winter within the Argentine Basin, mainly in sub-Antarctic waters [START_REF] Phillips | Movements, winter distribution and activity patterns of Falkland and brown skuas: insights from loggers and isotopes[END_REF][START_REF] Carneiro | Consistency in migration strategies and habitat preferences of brown skuas over two winters, a decade apart[END_REF][START_REF] Krietsch | There were notable differences in migratory routes 533 and wintering areas between individuals as found in other 534[END_REF]. Considering recent results [START_REF] Phillips | Movements, winter distribution and activity patterns of Falkland and brown skuas: insights from loggers and isotopes[END_REF][START_REF] Carneiro | Consistency in migration strategies and habitat preferences of brown skuas over two winters, a decade apart[END_REF][START_REF] Krietsch | There were notable differences in migratory routes 533 and wintering areas between individuals as found in other 534[END_REF]) and those from this study on the nonbreeding movements of individuals from 4 distant colonies, it appears that brown skuas might winter in a belt around Antarctica, as suggested by Furness (1987). During this period, wintering strategies differ among populations, although variation exists within populations. Single individuals use only a small proportion of the entire nonbreeding range, a pattern that might partly result from competition. Our data show that there was high individual variability in wintering areas among populations, and differences in migratory strategy between sexes. The population-level migratory strategy of brown skuas can be described as longitudinal or dispersive movements versus long-distance and anticlockwise loop movements (Online Resource Fig S1). Brown skuas targeted neritic or oceanic waters corresponding to three biomes during the nonbreeding period, namely sub-Antarctic, subtropical and tropical waters. Brown skuas avoided the South Subtropical Gyre in the Central Indian Ocean, which corresponds to less productive areas. The wintering grounds correspond to productive dynamic upwelling ecosystems [Benguela Current System or equatorial Eastern Indian Ocean [START_REF] Chen | Intraseasonal variability of upwelling in the equatorial Eastern Indian Ocean[END_REF]] or frontal systems [salinity front southwest of Sumatra (Hunt and Schneider 1987)] and to oligotrophic tropical waters. Some of these zones are important areas for several taxa, specifically seabirds (Le [START_REF] Corre | Individual wintering strategies and patterns of 525 winter site fidelity are highly variables among seabird 526 species[END_REF]Delord et al. 2013[START_REF] Weimerskirch | Lifetime foraging patterns of the wandering 982 albatross: life on the move![END_REF]. The nonbreeding distribution of Values are mean ± SD of all body feathers synthesized within the same habitat (see Table 1) skuas from South Georgia [START_REF] Phillips | Movements, winter distribution and activity patterns of Falkland and brown skuas: insights from loggers and isotopes[END_REF][START_REF] Phillips | Stable isotopes reveal individual variation in migration strategies and habitat preferences in a suite of seabirds during the nonbreeding period[END_REF] and South Polar skuas when they forage at similar latitudes in the southern Indian Ocean (Weimerskirch et al. 2015a, b). This suggests that skuas fed on low trophic level prey in the area, but this remains to be confirmed. A comparison of the skua d 15 N values with those of other sub-Antarctic and subtropical organisms suggests that the unknown prey was not marine mammals, seabirds, cephalopods or fish, but instead macrozooplankton, probably crustaceans (Cherel et al. 2008a(Cherel et al. , 2010;;[START_REF] Stowasser | Food web dynamics in the Scotia Sea in summer: a 953 stable isotope study[END_REF]; this hypothesis needs to be thoroughly investigated. Migratory strategy and foraging during the nonbreeding period between members of breeding pairs is poorly documented in seabirds [START_REF] Anderson | Diet, individual specialisation and breeding of brown skuas (Catharacta antarctica lonnbergi): an investigation using stable isotopes[END_REF][START_REF] Mueller | Seabirds mated for life migrate separately to the same places: behavioural coordination or shared proximate causes?[END_REF][START_REF] Thiebot | Mates but not sexes differ in 959 migratory niche in a monogamous penguin species[END_REF]. Our results showed that throughout their inter-breeding period, members of the same pairs were spatially segregated (GLS data) and exhibited different foraging habitats and diets (stable isotope values). To the best of our knowledge, the only previous studies on the foraging strategy of pairs of brown skuas focused on the breeding period [START_REF] Anderson | Diet, individual specialisation and breeding of brown skuas (Catharacta antarctica lonnbergi): an investigation using stable isotopes[END_REF][START_REF] Carneiro | Foraging behaviour and habitat use by brown skuas Stercorarius lonnbergi breeding at South Georgia[END_REF]). The results also indicated no sex-related effects on foraging time, habitat use or diet during the nonbreeding period.

Catharacta skuas have diverse feeding techniques and feed on a wide variety of prey (Le Morvan et al. 1967;Barre ´1976;Hemmings 1984;[START_REF] Peter | Ecological and morphological investigations on south polar Skuas (Catharacta maccormicki) and brown Skuas (C. skua lonnbergi) on Fildes peninsula, King George Island, South Shetland Islands[END_REF][START_REF] Mougeot | Predation on burrowing petrels by the brown shua (Catharacta skua lo ¨nnbergi)a t Mayes Island, Kerguelen[END_REF]. During the breeding season, brown skuas breeding at sub-Antarctic islands show some degree of specialization related to local availability of resources. On Mayes Island at Kerguelen, brown skuas are highly dependent on burrowing petrels and feed mainly on two species, the blue petrel (Halobaena caerulea) and the thinbilled prion (Pachyptila belcheri). At Possession Island, birds rely on rookeries of penguins (Barre ´1976;[START_REF] Stahl | La se ´gre ´gation alimentaire chez le Skua 946 subantarctique Stercorarius skua lo ¨nnbergi dans l'archipel 947[END_REF]. Migratory patterns appeared to be partly synchronized with the phenology of the skuas' prey (see Online Resource Table S6). The timing of migration of brown skuas coincided with the presence on land and the breeding cycle of their two main prey species. Brown skuas and their prey species arrive and stay at colonies until fledging. A complete shift in foraging ecology and diet between the breeding and nonbreeding period is suspected for skuas, from predation of seabirds on land during the breeding period to exclusively marine during the interbreeding season. This specialization during the breeding season appears not to be maintained outside this period as evidenced by activity pattern and stable isotopic data. There is strong evidence that at least during moulting adult skuas do not rely on small petrels for feeding (Carravieri et al. 2014a). At South Georgia, results suggest a mixed diet of zooplankton and low trophic-level prey over the wintering grounds [START_REF] Phillips | Movements, winter distribution and activity patterns of Falkland and brown skuas: insights from loggers and isotopes[END_REF]). Compared to south

  studied on two sub-Antarctic islands in 149 the southern Indian Ocean: Possession Island (46°25 0 S, 150 51°45 0 E) in the Crozet archipelago and Mayes Island 151 (49°28 0 S, 69°57 0 E) in the Kerguelen archipelago. Tracking 152 data were used to characterise the at-sea distribution of 153 adults during their breeding and nonbreeding periods. 154 Global location sensing (GLS) loggers (British Antarctic 155 Survey, Cambridge) were deployed on adults during the 156 breeding season (2008-2009 and 2011-2012 at Kerguelen; 157 n = 40 and 2010-2011 at Crozet; n = 10). GLS loggers 158 (Mk5, Mk15) weighed 3.6 and 2.5 g, respectively, i.e. 159 &0.14% of body mass and well below the recommended 160 threshold of 3% of body mass (Phillips et al. 2003). These 161 were mounted on plastic or metal leg bands. Adults were 162 captured on the nest during the breeding season (brooding 163 or chick rearing stage). Sex of the birds, except those from 164 Crozet, was determined using molecular methods (Blan-165 chard et al. 2007)( n = 29; 13 males and 16 females).

  ± 0.01 Note that birds may moult in several different zones. Within wintering or moulting zones, values sharing the same superscript letters are not significantly different at the 0.05 level. Values are mean ± SD U N C O R R E C T E D P R O O F Movements and activity Brown skuas showed extensive variation in their postbreeding movements, from true long range migrations to distant wintering zones, to short movements near breeding grounds (Fig. 1). Wintering zones of brown skuas from Kerguelen and Crozet Islands (which are located *1400 km apart) overlapped considerably (Fig. 1). All individuals overwintered in the southern hemisphere, remaining in the Indian Ocean (10°E-150°E), except two individuals that overwintered in the Benguela Current (Atlantic Ocean). At the end of the breeding season adult skuas left the breeding grounds typically to move northwards (Online Resource Fig S1). Skuas undertook their migration to wintering areas over three different water masses (sensu Longhurst (2007). Each bird wintered within a single area: (i) in the tropics, in waters off Java, the Mascareignes Region and the Seychelles Archipelago (SST in July: 25.8 ± 2.2 °C), (ii) in the subtropics, in waters off Australia or in the Benguela Current System (north of the northern subtropical front, 15.4 ± 2.2 °C), or (iii) in sub-Antarctic waters of the southern Indian Ocean (between the northern subtropical front and the polar front 10.3 ± 3.0 °C) (Figs. 1, 2). The population-level migratory strategy can be described as being one of longitudinal or dispersive migrations (individuals wintering in the sub-Antarctic area; n = 12) or long-distance and anticlockwise loop migrations (individuals wintering in subtropical or tropical areas; n = 17). Males tended to migrate more 353 towards tropical and subtropical waters (67%) than females 354 did (47%; Fig. 3) but differences were not significant (Chi 355 square test, all p [ 0.614). While birds that wintered in 356 subtropical waters, including coastal waters, left the colony 357 and headed directly to their wintering zones, the birds 358 which targeted tropical areas first moved eastward to sub-359 Antarctic staging zones until April-May before reaching 360 their wintering zones (Online Resource Fig S1). In contrast,

Fig. 1

 1 Fig. 1 Kernel densities or utilization distributions (25%) of individual adult brown skuas (Catharacta antarctica lonnbergi) during the inter-breeding period. Individual brown skuas from Crozet (n = 5,

Fig. 3

 3 Fig. 3 Maximum distances from the breeding colony reached by individual brown skuas (Catharacta antarctica lonnbergi) from Crozet and Kerguelen during the inter-breeding period versus their departure (upper panel) and arrival (lower panel) dates from and to the breeding sites, respectively. Symbols indicate sex of individuals (dots females, stars males, inverted triangle unknown). Colours

a

  Departure date from the breeding site b Arrival date at the wintering area c Departure date from the wintering area d Arrival date at the breeding site . The return migration tended to last longer for 386 birds coming from tropical and subtropical areas 387 (20.3 ± 7.3 days) compared to birds coming from sub-388 Antarctic areas (10.5 ± 10.1 days; Tables

439

  DAIC) indicates the AIC difference between the selected model and the null model a Departure date from the breeding site b Arrival date at the breeding site Table 4 Selected generalized linear mixed models of the variation of the activity patterns of brown skua (Catharacta antarctica lonnbergi) Area, random = * 1|ID a * Month:Area, random = * 1|ID a DAIC = 345.3 DAIC = 211 Delta AIC (DAIC) indicates the AIC difference between the selected model and the null model a Including individual as random effect Females and males from the same pairs (n = 4) that 440 were studied over the same inter-breeding period had dif-441 ferent feather isotopic values (Mann-Whitney U tests, data 442 not detailed). One pair (blue) differed in their feather d 13 C 443 values (but not d 15 N values), two pairs (green and red) 444 differed in their d 15 N values (but not d 13 C values) and one 445 pair (grey) differed in both their d 13 C and d 15 N values 446 (Online Resource Fig S2). 447 Discussion 448 Using geolocation, we showed that brown skuas from two 449 populations in the southern Indian Ocean tracked in this 450 study migrated northward to overwinter in different areas 451 in the southern hemisphere, remaining in the Indian Ocean 452 (10°E-150°E), except two individuals that overwintered in 453 the Benguela Current (Atlantic Ocean). Inter-individual 454 variability in migratory strategies was clear, with individ-455 uals targeting areas in a continuum from the sub-Antarctic 456 to the tropics. 457

Fig. 4

 4 Fig. 4 Body feather d 15 N values versus d 13 C values of adult (squares) and chick (circles) brown skuas (Catharacta antarctica lonnbergi) from Kerguelen Islands according to their moulting zones.Values are mean ± SD of all body feathers synthesized within the same habitat (see Table1)

  . To date, the wintering ranges for Southern

	Indian Ocean populations have been inferred primarily	124
	from at-sea sightings or recoveries of ringed birds. This	125
	information suggests that during the nonbreeding period	126
	some individuals stay close to their breeding range, while	127
	others are migratory or dispersive (Eklund 1961; Olsen and	128
	Larsson 1997). Some individuals winter around Australia,	129
	with two ring recoveries of Kerguelen birds there (Higgins	130
	and Davies 1996).	131
	The present study focuses on the intraspecific compar-	132
	ison of migration strategies, wintering ranges, activity	133
	patterns and diet of two populations of brown skuas in the	134
	Southern Indian Ocean by combining tracking (geoloca-	135
	tors) and stable isotope analyses. Using biotelemetry data,	136
	we investigated whether (1) individuals vary in their	137
	migratory behaviour, (2) sexes differ in their migration	138
	timing and wintering areas, and (3) individuals modify	139
	their activity patterns between the breeding, migration and	140
	nonbreeding period. We hypothesized that a shift might be	141
	accompanied by modification of the activity pattern (i.e.	142
	time spent on the water recorded by the geolocators),	143

  . Results are presented in the usual d notation relative to Vienna PeeDee Belemnite and atmospheric N 2 for d 13 C and d 15 N, respectively. Replicate measurements of internal laboratory standards (acetanilide) indicated measurement errors \0.15% for both d 13 C and d 15 N values. For statistical analyses, feather d 13 C and d 15 N values were either grouped at the individual level (wintering zones) or at the feather level (moulting zone) (Table

Table 1

 1 Isotopic niches of brown skuas (Catharacta antarctica lonnbergi) according to individual wintering zones and to feather moulting zones (see text)

	Wintering areas and habitats	Individuals	Body feathers	Feather d 13 C	Feather d 15 N	C:N mass
		(n)( n)( %)( %)	ratios
	Wintering zones
	Tropical	7	28	-17.4 ± 1.5 a	11.2 ± 2.2 a	3.14 ± 0.02
	Subtropical	5	19 (1 outlier)	-18.3 ± 1.2 b	12.5 ± 2.2 b	3.15 ± 0.02
	Sub-Antarctic	7	27 (1 outlier)	-18.3 ± 0.6 b	8.5 ± 0.6 c	3.14 ± 0.01
	Chicks (Kerguelen)	10	10	-21.5 ± 0.3 c	11.1 ± 0.2 a,b	3.17 ± 0.01
	Moulting zones
	Madagascar	2	7	-15.7 ± 0.1 a	13.4 ± 1.0 a	3.14 ± 0.02
	Java	2	3	-15.8 ± 0.1 a	13.0 ± 0.2 a,d,f	3.13 ± 0.01
	West Australia	2	5	-17.7 ± 0.7 b	11.7 ± 0.9 b,e,f	3.14 ± 0.01
	Southern Australia	4	9	-18.0 ± 1.2 b	14.1 ± 1.2 a	3.16 ± 0.01
	Subtropical and sub-Antarctic	13	37	-18.2 ± 0.6 b	8.5 ± 0.6 c	3.14 ± 0.02
	Sub-Antarctic	6	7	-19.9 ± 0.9 c	12.3 ± 1.6 b,d	3.15 ± 0.02
	Unknown	3	8	-	-	-
	Chicks (Kerguelen)	10	10	-21.5 ± 0.3 c	11.1 ± 0.2 e	3.17

Table 2

 2 Inter-breeding behaviour of brown skuas (Catharacta antarctica lonnbergi) from Crozet and Kerguelen islands

	Wintering area Sex n	Outward migration	Inward migration		Wintering
				Departure date a Arrival date b Duration (days) Departure date c Arrival date d Duration (days) Duration
								(days)
	Subtropical	M	3	27 Feb ± 3	2 Apr ± 10 34 ± 13	19 Aug ± 13	9 Sep ± 11 21 ± 7	140 ± 3
		F	5	19 Feb ± 16	14 Mar ± 32 24 ± 19	27 Aug ± 10	19 Sep ± 18 23 ± 9	166 ± 38
	Tropical	M	5	5 Mar ± 8	15 Mar ± 12 11 ± 11	17 Aug ± 8	3 Sep ± 81 7 ± 5	155 ± 10
		F	2	4 Mar ± 11	31 Mar ± 42 8 ± 15	11 Aug ± 15	7 Sep ± 82 7 ± 6	133 ± 18
	Sub-Antarctic M	4	25 Feb ± 3	27 Feb ± 23 ± 2	24 Aug ± 16	31 Aug ± 96 ± 9	179 ± 18
		F	6	26 Feb ± 10	10 Mar ± 18 13 ± 18	22 Aug ± 19	4 Sep ± 13 13 ± 10	165 ± 24
	Total	All 29 29 Feb ± 14	15 Mar ± 20 15 ± 16	21 Aug ± 13	6 Sep ± 14 16 ± 10	159 ± 24

  ). Feather isotopic values of adult brown skuas from Ker-405 guelen Islands ranged widely, from -21.3 to -15.5% (a 406 5.7% difference), and from 7.1 to 16.1% (9.0%) for d 13 C 407 and d 15 N values, respectively. Only one adult body feather 408 had isotopic values similar to the chick values, thus veri-409 fying that adult feathers did not grow during the breeding 410 period. d 13 C and d 15 N values of feathers from individuals that wintered in different zones were overall significantly different (ANOVA: F (3,80) = 34.8 and 24.0 for d 13 C and d 15 N, respectively, both p \ 0.0001) (Table

	403 Stable isotopes
	404

Table 3

 3 Selected generalized linear models used to test for the effects of sex and wintering area on variation of the migration patterns of brown skuas (Catharacta antarctica lonnbergi) from Crozet and Kerguelen islands

		Independent variables	Outward migration		Inward migration
			Departure date a	Duration (days)	Arrival date b	Duration (days)
	All birds	Sex, area	*Sex	*Sex ? Area ? Sex. Area	*Sex	*Sex ? Area ? Sex.Area
			DAIC = 50

Author Proof

Author Proof

628 polar skuas, brown skuas spent a significantly greater 629 proportion of their time in flight during darkness, indicative 630 of greater search effort in similar wintering grounds 631 (Weimerskirch et al. 2015a, b). Body size might be a factor 632 driving part of this difference in behaviour, as suggested by 633 comparing brown skuas and Falkland skuas (Phillips et al. 634 2007). Brown skuas are larger than south polar skuas 635 (Furness 1987), suggesting differences in manoeuvrability 636 which might allow different feeding behaviours. To our 637 knowledge there is no study on the foraging habits of 638 brown skuas during migration and winter. Skuas are well-639 documented opportunistic kleptoparasites at some times 640 during their annual cycle (Furness 1987). Observations of 641 skuas attacking flocks of seabirds (terns, gulls or shear-642 waters) and robbing them food (Furness 1987) confirm this 643 seasonal shift in feeding strategy. Brown skuas have been 644 observed attacking seabirds such as white-chinned petrels 645 (Procellaria aequinoctialis) or sooty albatrosses (Phoebe-646 tria fusca); KD's pers. obs.) at sea.

647

The existence of such a variety of migration strategies 648 within brown skua populations is puzzling. Evaluating the 649 demographic correlates as well as the repeatability in 650 individual migratory routes and wintering locations will be 651 valuable to understand this diversity and its fitness conse-652 quences. It would be crucial to track the ontogeny of 653 migration strategies, to evaluate heritability and conse-654 quences for survival. Identification of carry-over effects in 655 seabird behaviour during the wintering period and identi-656 fication of their impact on population dynamics (e.g. the 657 effect of winter conditions faced by individual on its sub-658 sequent reproduction), should be investigated (Harris et al. 659 2013). Skuas and other seabird species are good indicators 660 of environmental contaminants (Corsolini et al. 2011;661 Carravieri et al. 2014a;Goutte et al. 2014). Such individual 662 variability in migratory strategies should also have conse-663 quences in terms of susceptibility to environmental stres-664 sors such as contaminants and climate change.

665 Acknowledgments The authors thank many field workers, namely K 666 Coustaut, N El Ksabi, O Gore, T Gouello, T Lacombe, M Loubon, M 667 Passerault, R Perdriat, A Pe ´rot, B Planade, A Prudor, F The ´ron. We 668 also thank S Ruault for molecular sexing, M Brault-Favrou for 669 preparing feather samples and G Guillou for running stable isotope 670 analysis. We are grateful to H Demarcq and P Lehodey for valuable 671 insights into oceanographic characteristics of Indian Ocean. The 672 authors would also like to thank three anonymous referees and D 673 Piepenburg for helpful reviews and constructive suggestions to 674 improve the manuscript. We are grateful to R Reisinger for kindly 675 revising the English of this manuscript. The Ethics Committee of 676 IPEV and Comite ´Environnement Polaire approved the field proce- 677 dures. Author Proof et al. 2016) were mentioned in the manuscript; however, these were not included in the reference list. As a rule, all mentioned references should be present in the reference list. Please provide the reference details to be inserted in the reference list.

AQ2 Reference (Lascelles 2014) was provided in the reference list; however, this was not mentioned or cited in the manuscript. As a rule, if a citation is present in the text, then it should be present in the list. Please provide the location of where to insert the reference citation in the main body text.