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To mediate competition, similar sympatric species are assumed to use different resources, or the same but geographically separated resources. The two giant petrels (Macronectes spp.) are intriguing in that they are morphologically similar seabirds with overlapping diets and distributions. To better understand the mechanisms allowing their coexistence, we investigated intra- and interspecific niche segregation at Marion Island (Southern Indian Ocean), one of the few localities where they breed in sympatry. We used GPS tracks from 94 individuals and remote-sensed environmental data to quantify habitat use, combined with blood carbon and nitrogen stable isotope ratios from 90 individuals to characterize their foraging habitat and trophic ecology. Females of both species made distant at-sea foraging trips and fed at a similar trophic level. However, they used distinct pelagic habitats. By contrast, males of both species mainly foraged on or near land, resulting in significant sexual segregation, but high interspecific habitat and diet overlap. However, some males showed flexible behavioural strategies, also making distant, pelagic foraging trips. Using contemporaneous tracking, environmental and stable isotope data we provide a clear example of how sympatric sibling species can be segregated along different foraging behaviour dimensions.

Supplementary Tables S1, S2 and Supplementary Figures S1-S3.

Additional file 1: Supplementary Material 1. Approximate dates of different breeding stages throughout the year for wandering, grey-headed and sooty albatrosses, northern and southern giant petrels and gentoo, macaroni and eastern rockhopper penguins breeding at Marion Island, sub-Antarctic. Yellow dots represent approximate times when GPS data loggers were deployed on birds and blood plasma was collected for stable isotope analysis. Supplementary Material 2. Species-specific normalizing equations calculated from plasma samples (N) with both delipidated and raw plasma for wandering, grey-headed and sooty albatrosses and gentoo and macaroni penguins breeding at Marion Island, sub-Antarctic. Supplementary Material 3. Corresponding plasma a) δ13C (‰) and b) δ15N (‰) values given as mean ± SD (range) for different water zones and fronts (AZ: Antarctic zone, PF: polar front, PFZ: polar frontal zone, SAF: sub-Antarctic front, SAZ: sub-Antarctic zone, STF: sub-tropical front and STZ: sub-tropical zone) estimated from isoscapes interpolated from mean foraging locations and plasma isotopic values of wandering, grey-headed and sooty albatrosses, northern and southern giant petrels breeding at Marion Island.

Blue Petrels (Halobaena caerulea) are known to breed at seven locations in the Southern Ocean. Population estimates have been made recently for the two major breeding sites, but accurate estimates are lacking for the remaining locations. We used a systematic survey technique to estimate the size of the population breeding at Marion Island (290 km2), the larger of the two Prince Edward Islands. A combination of colony area and density estimates suggested there were 214 700 Blue Petrel burrows on Marion Island in 2012. Burrow occupancy rates at the mid-incubation stage averaged 82% (range 36–98%), suggesting a total breeding population of 145 000 pairs (95% confidence interval 110 000–180 000). There appeared to be some range expansion since the population was mapped in the mid-1980s. Predation of chicks and eggs by introduced house mice (Mus musculus) could be affecting the recovery of Blue Petrels since feral cats (Felis catus) were eradicated in 1991. Based on our count from Marion Island alone, the Prince Edward Islands support the third largest population of Blue Petrels globally, after Diego Ramirez Islands and the Kerguelen Islands.

International audience; Sex-specific feeding segregation related to sexual bill dimorphism has been described in several oystercatcher species, including the African black oystercatcher. For the latter, studies concerned only a small number of breeding pairs and were done prior the invasion of the South African rocky shores by the Mediterranean mussel, which is believed to have benefited oystercatchers by increasing overall biomass. Here, we investigated geographic variability in the segregation of diet, biometrics and body condition between sexes in the African species, in relation to changes in foraging habitats along the South African coastline, using stable isotope analyses. Males and females and their potential prey (mussels, limpets, polychaetes and ascidians) were sampled on the southern African west, south-west and south-east coasts for stable isotope analyses and biometrics and body conditions of birds were measured. Bill dimorphism occurred throughout the study area and south-west males had lower body conditions than other males and females in general. Sexes displayed little differences in their δ13C ratios and in the relative consumption of the different prey throughout the study area, except on the south-east coast where males were slightly depleted in 13C relative to females and the most abundant prey elsewhere (the Mediterranean mussel) is rare. Females were slightly but significantly enriched in 15N by 0.3‰ compared to their breeding partners and this did not link clearly to differences in diet.We argue that the combined effect of biogeographic variations in rocky shores diversity and biomass, heterogeneous invasion by the Mediterranean mussel on the South African coastline and bill dimorphism may have altered the sex-specific feeding behaviour of oystercatchers differently between coastal regions and possibly had an additional cost for male oystercatchers faced with lower prey biomass and diversity on the south-west coast.

International audience; The diet of seabirds is usually studied by the identification of prey items recovered from their stomachs. This method is however limited to recently ingested prey and to non-digestible hard parts, precluding the determination of marine resources consumed by birds during long foraging trips. Thus, alternative indirect approaches are necessary to assess the potential importance of digested prey from long-term foraging activity. In this study, we present three complementary techniques to determine the prey of breeding short-tailed shearwaters (Puffinus tenuirostris) when they feed for themselves during long foraging trips: (1) conventional food analysis, (2) stable isotope analysis of carbon and nitrogen signatures (δ13C and δ15N) of plasma, and (3) lipid analysis of stomach oil and the use of fatty acids and fatty alcohols as trophic markers (stomach oil is of dietary origin). Dietary analysis showed that fish dominated by mass over crustaceans (82 and 18%, respectively). Two euphausiids Euphausia vallentini (a sub-Antarctic species) and Nyctiphanes australis (a Tasmanian species), and fish postlarvae represented more than 94% of the total number of food items, with myctophid fish of larger size dominating by mass. Plasma isotopic signature of birds suggested that shearwaters foraged mainly in Antarctic waters (δ13C = -23.8‰), and fed at a trophic level close to that of a myctophid-eater, the king penguin (δ15Nshort-tailed shearwater = 8.7‰, δ15Nking penguin = 9.8‰). Comparisons between fatty -acid and -alcohol patterns of stomach oil wax esters with those of potential prey also suggested a food based on myctophids (Electrona antarctica, Krefftichthys anderssoni and Gymnoscopelus braueri). To conclude, both lipid and stable isotope methods emphasized the importance of myctophids in the nutrition of short-tailed shearwaters during the chick-rearing period when adult birds feed for themselves. This study illustrates the interest of using both direct and indirect methods to determine trophic relationships between marine organisms.