How a winner can also be a loser ? At-sea vulnerability of a critically endangered endemic seabird to climate change Fernandez, R. /Guilhaumon, François Saunier, M. Pinet, P. Humeau, L. Le Corre, M. Jaeger, A. climate change global location sensing migratory species species distribution modelling tracking vulnerability Aim: The last decades have been marked by a global decline of many migratory species, and predictions are even more alarming when climate change is considered. We investigated the migration patterns and marine habitat selection processes of a critically endangered endemic seabird of the tropical western Indian Ocean, and projected future habitat suitability under climate-change scenarios. Location: Indian Ocean. Taxon: Mascarene petrel (Pseudobulweria aterrima). Methods: Non-breeding distribution of the Mascarene petrel was estimated using Global Location Sensors (GLS) and key habitat selection predictors were identified using Species Distribution Models (SDM). The best-performing models were averaged to build an Ensemble model, which was projected under three Shared Socioeconomic Pathways (SSP) to assess future changes in non-breeding habitat suitability. Results: The Mascarene petrel exhibits a remarkably wide distribution throughout the tropical Indian Ocean during its non-breeding period. We identified seven main core areas which highlights high inter-individual variability. The species selected warm (similar to 28 degrees C) and deep water (-4000 m and -2000 m) with low gradient of bathymetry. Our predictive models suggest that the suitable marine habitat of this species will increase in surface area by 7%-9% as a consequence of climate change. Main Conclusions: Our projections indicate that the Mascarene petrel may experience stable or slightly improved climatic suitability in its non-breeding marine habitat, suggesting a potential 'climate change winner' signal at sea. However, as many migratory species, it relies on multiple habitats and accumulates threat exposure throughout its entire life cycle. The species therefore remains highly vulnerable due to strong terrestrial pressures and its restricted endemic range during breeding, supporting its characterization as a 'global change loser'. More broadly, our results highlight the importance of assessing threats across the entire annual cycle, particularly for migratory and wide-ranging species, to avoid misestimating overall vulnerability. 2026 text https://www.documentation.ird.fr/hor/fdi:010097079 fdi:010097079 Fernandez R., Guilhaumon François, Saunier M., Pinet P., Humeau L., Le Corre M., Jaeger A.. How a winner can also be a loser ? At-sea vulnerability of a critically endangered endemic seabird to climate change. 2026, 53 (4), e70234 [11 p.] EN