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Aim Species that breed sympatrically often occupy different foraging niches to mitigate competition for prey. When resource availability declines at the end of the breeding season, some animals migrate to regions with more favourable environmental conditions. When these life-history traits combine, foraging habitat preferences may continue to influence migration patterns and habitat utilization. The Bering Sea is home to the red-legged kittiwake ( RLKI), Rissa brevirostris, which is endemic, and the black-legged kittiwake ( BLKI), Rissa tridactyla, which has a circumpolar breeding distribution. Since the 1970s, numbers of RLKIs at the largest colony have declined and then recovered, whilst the BLKI population has remained stable. Knowledge of the migration ecology of kittiwakes is key to understanding differences in population trajectories, and predicting possible future responses of these species to climate change. Location Pribilof Islands, Bering Sea, subarctic North Pacific. Methods Using geolocation loggers, we tracked adult RLKIs and BLKIs during their non-breeding migrations. We used iterative methods to assess suitable sample sizes for determining space use. Kittiwakes are surface foragers; therefore we used wet-dry data to distinguish active foraging behaviour and to test the species' responses to environmental conditions. Stable isotope ratios of feathers grown during the non-breeding period were used to assess dietary niche. Results RLKIs remained largely in the Bering Sea, where they experienced colder conditions and shorter days; individual birds used multiple habitats, including the continental shelves, the sea-ice edge and pelagic waters. In contrast, BLKIs migrated to the subarctic North Pacific, where they dispersed laterally across the basin; the majority of birds travelled to the western subarctic. RLKIs spent less time actively foraging than BLKIs, and consumed higher trophic-level prey. Main conclusions The disparate wintering ranges and foraging behaviour of BLKIs and RLKIs suggest distinct environmental factors drive variation in overwinter survival. A strong association with sea ice, and specialization both in diet and foraging behaviour, may make RLKIs particularly vulnerable to climatic change. [ABSTRACT FROM AUTHOR]

Aim Pelagic seabirds exploit large areas of ocean when acting as central-place foragers during the breeding season, and ranges are even more extensive outside the breeding period. Spatial niche partitioning is known to occur among species that breed sympatrically, but is less apparent during the non-breeding period when there is increased potential for overlap among closely related species from neighbouring island groups. This applies to several species of prion, Pachyptila spp., in the Southern Ocean; although extremely abundant, their at-sea distribution was virtually unknown because they are difficult to distinguish while at sea. To understand spatial niche partitioning at large scales, we investigated the year-round distribution of thin-billed prions ( Pachyptila belcheri) from the Falkland Islands (Islas Malvinas) and Antarctic prions ( Pachyptila desolata) from South Georgia. Location South Atlantic Ocean. Methods Recently, geolocation devices have become small enough to be deployed on small seabirds. During 2009-10, we tracked 20 thin-billed prions and 9 Antarctic prions with miniaturized geolocators. We applied ecological niche models to compare environmental conditions in the habitat utilized year-round. Results We show that two prion species from the south-west Atlantic Ocean have divergent patterns of migration, and that this has resulted in nearly complete spatial segregation (0-5% overlap by month in the 95% kernel density polygons). Nineteen of 20 thin-billed prions migrated to an area > 3000 km east of their breeding site, whereas all Antarctic prions migrated a much shorter distance, and to the north-west. The non-breeding distribution of thin-billed prions included the waters around South Georgia, but only when the Antarctic prions were absent. The models highlighted large differences in the realized niche between the two species, and between the habitat characteristics of breeding and non-breeding areas of thin-billed and Antarctic prions. Main conclusions Our results are consistent with the prediction that spatial niche partitioning occurs at large scales, allowing the co-existence of related species. The methods applied here will enable predictive maps of the distributions of other prion populations to be created, once data become available from other breeding sites in the Southern Ocean. [ABSTRACT FROM AUTHOR]