Reassessment of post-breeding migratory passage of the Critically Endangered Balearic Shearwater at the Strait of Gibraltar

The Balearic Shearwater ( Puffinus mauretanicus ), Europe’s most threatened seabird, is classified as Critically Endangered. Previous migration counts at the Strait of Gibraltar suggested substantially higher population estimates than breeding colony surveys, but no updated assessment has been conducted for more than 15 years. We reassessed post-breeding migratory passage through the Strait using the same census framework and analytical approach applied during 2008–2010. Daily standardised counts were conducted between 15 May and 15 July in 2024 and 2025 from Isla de Tarifa (Spain). Passage rates were compared with those from 2008 to 2010 using negative binomial generalised linear mixed models, while migration-based population estimates were obtained using generalised additive models to reconstruct passage during unsurveyed hours within the daily census window. Mean net westward passage was approximately 15 birds per hour higher in 2024–2025 than in 2008–2010 (95% CI: 0.09–28.11), corresponding to a nearly 40% higher mean migratory flux. Migration-based estimates reached approximately 39,500 individuals in 2024 and 28,000 in 2025, indicating substantial interannual variability. Overall mean passage during 2024–2025 was approximately 32% higher than the previous estimate. These findings contrast with demographic declines inferred from breeding colonies and at-sea surveys, suggesting that migration counts at geographic bottlenecks provide complementary information on population status. Standardised long-term monitoring at the Strait of Gibraltar offers a cost-effective tool for tracking population change and improving conservation assessments of this threatened seabird.