Understanding the migratory patterns of declining species is essential to guide targeted conservation efforts. We studied the migratory dynamics of the critically endangered Balearic shearwater (Puffinus mauretanicus) using geolocator data spanning 88 annual cycles from 53 individuals tracked between 2017 and 2022. Breeding birds were tagged at colonies across their main breeding islands in the Balearic archipelago (Ibiza, Mallorca and Menorca). Post-breeding movements revealed distinct migratory routes: all individuals from Ibiza and Mallorca (n = 78) migrated to the Atlantic, whereas 80 \% of those from Menorca (n = 10) remained within the Mediterranean. Among Atlantic migrants, 54 \% established core non-breeding areas in the Bay of Biscay, 41 \% in the Western Iberia, and 5 \% in the Gulf of Cadiz. Of the 27 individuals tracked across multiple years, 89 \% consistently returned to the same non-breeding area, demonstrating strong site fidelity. Duration of stay in these regions ranged from one to seven months, with earlier arrivals typically remaining longer. Migration timing was influenced by breeding success: failed breeders departed approximately one month earlier than successful ones. These results underscore the importance of long-term monitoring programmes in capturing individual-level migratory patterns. By revealing consistent use of specific non-breeding areas, such programmes help identify sites that are repeatedly important across both space and time. Enabled by biologging technology, these insights are invaluable for the conservation of declining species. Site-based conservation actions focused on key Atlantic areas could deliver tangible benefits during the non-breeding season.