downwelling (,148uE–162uE, Figure 6). While the sooty shear- waters did not utilise the region around 165uE, this is a region of upwelling and was utilised by the Tasmanian short-tailed shearwaters. The lack of use of this particular area by the sooty shearwaters might therefore reflect differences in preferred (learned) foraging areas of the birds. Tracking studies on other species have shown foraging distributions in similar areas to the shearwaters. Southern elephant seals Mirounga leonina from Macquarie Island, like the sooty shearwaters, showed a bimodal south-east/south-west foraging pattern (Figure 1b in [95]), although these seals foraged farther south than did the sooty shearwaters. Grey-headed albatrosses Thalassarche chrysostoma from Macquarie Island foraged just west of the sooty shearwaters’ south-western foraging zone, around 180uE– 190uE (Figure 5 in [96]), as did grey-headed and black-browed T. melanophrys albatrosses from Campbell Island [97]. Light-mantled sooty albatrosses Phoebetria palpebrata, also from Macquarie Island, foraged just south of the south-western zone (approximately 130– 140uE, 64uS) [98]. King penguins Aptenodytes patagonicus from Macquarie Island foraged east and south of the island [99], including the Polar Front zone at ,165uE where the Tasmanian short-tailed shearwaters foraged. Stomach samples from these penguins consisted of fish (primarily the myctophids K. anderssoni and E. carlsbergi) and squid (Moroteuthis ingens and Mastigoteuthis sp.). M. ingens is a predator of myctophids, particularly K. anderssoni [100]. The dives made by these penguins in the Polar Front zone were similar in pattern to those of the sooty shearwaters reported here, occurring from sunrise until mid-morning, and again from mid- afternoon until sunset, with little dive activity around midday (B. Wienecke, unpublished). The dive depths were also similar: shallowest at sunrise and sunset, increasing in depth closer to mid- day; consistent with the diurnal vertical migration of myctophids and their predators. The penguins’ deepest dives (up to 170m) were well beyond the maximum diving depths of shearwaters, suggesting that such prey would be accessible to the shearwaters for a much shorter period of each day. However, they can cover a much greater distance over this shorter time period. Upper-level trophic predators, including seabirds, are commonly used as indicators for ecosystem monitoring [e.g. 101,102,103] and fish stock assessment [e.g. 104]. Clearly, their value in this role is determined by the extent to which variations in indices derived from these predators reflect changes occurring at lower trophic levels of the ecosystem [e.g. 105]. The use of sooty and short-tailed shearwaters as indicators of Southern Ocean ecosystems is compromised to some extent by their annual migrations and corresponding exposure to factors external to the Southern Ocean [e.g. 106,107]. However, our results suggest that during the breeding season, these birds are consistent in their foraging habitats. The foraging overlap between the two species, and between populations, and the similarities in their foraging areas to those of other marine predators, suggests that these birds might be useful as indicators of References 1. Shaffer SA, Tremblay Y, Weimerskirch H, Scott D, Thompson DR, et al. (2006) Migratory shearwaters integrate oceanic resources across the Pacific Ocean in an endless summer. Proceedings of the National Academy of Sciences 103: 12799–12802. 2. So¨hle IS, Robertson CJR, Nicholls DG, Mouritsen H, Frost B, et al. (2007) Satellite tracking of sooty shearwaters (Puffinus griseus) during their pre-laying ‘‘exodus’’ and incubation. Notornis 54: 180–188. 3. Serventy DL, Serventy V, Warham J (1971) The Handbook of Australian Sea- Birds. Sydney: Reed. 254 p. 4. Moller H, Lyver POB, Bragg C, Newman J, Clucas R, et al. (2009) Guidelines for cross-cultural participatory action research partnerships: a case study of a customary seabird harvest in New Zealand. New Zealand Journal of Zoology 36: 211–241. Shearwater Foraging aspects of the Southern Ocean ecosystem, particularly of the Polar Front zone. Combining indices from multiple colonies and/or different species might allow variations in breeding success due to localised factors (e.g. density dependence, or predation from introduced mammals) to be disentangled from those due to changes in the ecosystem of the Polar Front zone. Combining indicator information from populations at multiple sites, and from multiple species, can assist in reducing single-site effects, and potentially make indicators based on seabird breeding performance more robust [108]. An index of the abundance of Australian krill (Nyctiphanes australis) based on a combination of indices of the growth of short- tailed shearwater chicks in Tasmania and mortality occurrences in Japan has previously been proposed [109]. Summary and conclusions Electronic tagging technologies can provide detailed insights into the foraging and migratory behaviours of wide-ranging marine predators. In this study we combined tracking data from two shearwater species that originate from colonies separated by thousands of kilometres in New Zealand and Australia, along with ship-based and remote-sensed information to evaluate and describe the predators’ foraging behaviour within the wider context of their marine ecosystem. The birds displayed foraging site fidelities at individual and population levels, with overlap in foraging habitat between species and populations. Wind cost modelling yielded new information about the putative mechanisms that influence foraging destination. The distributions of foraging effort at broad spatial scales were consistent with patterns in oceanic winds. Prey availability modulated these patterns, and influenced foraging effort at smaller spatial scales. The spatial distribution of shearwater foraging effort in the Polar Front zone was matched by patterns in large-scale upwelling, primary production, and abundances of copepods and myctophid fish. These results offer great promise for modelling and projecting possible changes in foraging behaviour and distribution related to environmental factors. Acknowledgments We thank D. Pinaud, C. Bragg, R. Mules, B. Newton, S. McKecknie, and J. Adams for assistance in the field with sooty shearwater deployments, R. Williams for providing the trawl data, B. Wienecke the king penguin foraging data, and V. Wadley for support and coordination. H. Moller and J. Adams provided valuable comments on an earlier version of this manuscript. Author Contributions Conceived and designed the experiments: BR SAS EJW DPC MH. Analyzed the data: BR SAS SS MP AS. Wrote the paper: BR SAS SS EJW DPC GH MP. Conducted and supported fieldwork: SAS EJW DPC LE MH GH PMS DS DRT CV HW. 5. Skira IJ, Wapstra JE, Towney GN, Naarding JA (1985) Conservation of the short-tailed shearwater Puffinus tenuirostris in Tasmania, Australia. Biological Conservation 37: 225–236. 6. McKechnie S (2006) Biopedturbation by an island ecosystem engineer: burrowing volumes and litter deposition by sooty shearwaters (Puffinus griseus). New Zealand Journal of Zoology 33: 259–265. 7. Croll DA, Maron JL, Estes JA, Danner EM, Byrd GV (2005) Introduced predators transform subarctic islands from grassland to tundra. Science 307: 1959–1961. 8. Fukami T, Wardle DA, Bellingham PJ, Mulder CPH, Towns DR, et al. (2006) Above- and below-ground impacts of introduced predators in seabird-dominated island ecosystems. Ecology Letters 9: 1299– 1307. PLoS ONE | www.plosone.org 12 June 2010 | Volume 5 | Issue 6 | e10960