Trans-equatorial migration of Short-tailed Shearwaters revealed by geolocators

Until recent decades, details of the migratory movements of seabirds remained largely unknown owing to the difficulties in following individuals at sea. Subsequent advances in biologging technology have greatly increased our knowledge of seabird migration and distribution, particularly of highly pelagic species. Short-tailed Shearwaters (Ardenna tenuirostris) (similar to 500 g) have been studied extensively during their breeding season but our understanding of their movements outside this period remains poor. Here, we present the first tracks of the trans-equatorial migration of Short-tailed Shearwaters from a colony on Great Dog Island, Tasmania, Australia. Data were obtained from global location sensors (GLS loggers or geolocators), which enable the estimation of location twice per day based on ambient light levels. After breeding, tracked Shearwaters flew south of the Antarctic Polar Front to a previously unknown stopover site, where they remained for several weeks, before travelling rapidly northward through the western Pacific Ocean to coastal waters off Japan. Short-tailed Shearwaters spent the bulk of the northern hemisphere summer, either in this region or further north in the Bering Sea, before returning south through the central Pacific to their breeding sites. Our results, for the first time, describe in detail the complete migration of this long-lived seabird, reveal individual variation in timing and distribution, and describe the environmental characteristics of their key non-breeding habitats.