| Summary: | Acknowledgements: We would like to thank James Fox at Migrate Technology for advice and help with device management, Danilo and Guillermo Araya for crucial on-island support, the owner and crew of Tio Lalo for inter-island transport and Iva Vasquez and Valentina Colodro of Oikonos plus Jose Luis Cabello and María José Vilches of Island Conservation for logistical support. We thank Associate Editor Vitor Paiva and two anonymous reviewers who provided useful comments on an earlier version of the manuscript, John Horsfall for facilitating COVID-19-impeded transit through continental Chile, and Samantha Andrzejaczek for providing guidance on the preparation of one of the figures. <jats:title>Abstract</jats:title><jats:p>Gadfly petrels are among the widest-ranging birds and inhabit oceanic regions beyond the legislative protection of national jurisdictions (the High Seas). Detailed information on breeding phenology, at-sea distributions, and habitat requirements is crucial for understanding threats and designing conservation measures for this highly threatened group. We tracked 10 Stejneger’s petrels <jats:italic>Pterodroma longirostris</jats:italic>, endemic to Isla Alejandro Selkirk, Juan Fernández Islands in the southeast Pacific Ocean, with geolocator-immersion loggers over two years to examine year-round movements, phenology, habitat use, and activity patterns. Birds conducted round-trip trans-equatorial migrations of 54,725 km to the northwest Pacific Ocean between Hawaii and Japan. Across the boreal summer, birds followed the <jats:italic>c.</jats:italic> 1000 km northward movement of the North Pacific Transition Zone Chlorophyll Front, before their return migration which took a long detour south toward New Zealand before heading east at 40–50°S, presumably benefitting from Antarctic circumpolar winds. To our knowledge, a comparable triangular migration is unique among seabirds. During the pre-laying exodus, birds traveled southwest to the Sub-Antarctic Front, and ...
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