Background Precise and accurate retrospective geolocation of marine predators via their tissues’ isotopic composition relies on quality reference maps of relevant isotopic gradients (“isoscapes”). Additionally, a good working knowledge of any discrimination factors that may offset a marine predator’s isotopic composition from baseline isotopic values, as well as tissue specific retention rates, are imperative. We provide a critical assessment of inter-specific differences among marine predator-level isoscapes within the Indian Sector of the Southern Ocean. Methods We combined fine-scale GPS tracking data and concurrent blood plasma δ13C and δ15N values of eight seabird species (three albatross, two giant petrel and three penguin species) breeding at Marion Island to produce species- and guild-specific isoscapes. Results Overall, our study revealed latitudinal spatial gradients in both δ13C and δ15N for far-ranging seabirds (albatrosses and giant petrels) as well as inshore-offshore gradients for near-ranging seabirds (penguins). However, at the species level, latitudinal spatial gradients were not reflected in the δ13C and δ15N isoscapes of two and three, respectively, of the five far-ranging species studied. It is therefore important when possible to estimate and apply species-specific isoscapes or have a good understanding of any factors and pathways affecting marine predators’ isotopic composition when estimating the foraging distribution of marine predators via their tissues’ stable isotope compositions. Conclusions Using a multi-species approach, we provide evidence of large and regional scale systematic spatial variability of δ13C and δ15N at the base of the marine food web that propagates through trophic levels and is reflected in the isotopic composition of top predators’ tissues.

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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.

We report the breeding success of four species of burrow-nesting petrels at sub-Antarctic Marion Island where house mice Mus musculus are the sole introduced mammal. Feral cats Felis catus were present on Marion for four decades from 1949, killing millions of seabirds and greatly reducing petrel populations. Cats were eradicated by 1991, but petrel populations have shown only marginal recoveries. We hypothesize that mice are suppressing their recovery through depredation of petrel eggs and chicks. Breeding success for winter breeders (grey petrels Procellaria cinerea (34±21%) and great-winged petrels Pterodroma macroptera (52±7%)) were lower than for summer breeders (blue petrels Halobaena caerulea (61±6%) and white-chinned petrels Procellaria aequinoctialis (59±6%)) and among winter breeders most chick fatalities were of small chicks up to 14 days old. We assessed the extent of mouse predation by monitoring the inside of 55 burrow chambers with video surveillance cameras (4024 film days from 2012–16) and recorded fatal attacks on grey (3/18 nests filmed, 17%) and great-winged petrel chicks (1/19, 5%). Our results show that burrow-nesting petrels are at risk from mouse predation, providing further motivation for the eradication of mice from Marion Island.