Severe Population Decline of Great Skua Stercorarius skua Following a HPAI Outbreak Reflected in Migration Counts

The H5N1 avian influenza (HPAI) outbreak that began in 2021 has led to widespread mortality in birds and mammals globally, significantly impacting seabird populations. This study examines the changes in the number of migratory great skuas ( Stercorarius skua ), a species particularly vulnerable due to its kleptoparasitic behavior and reliance on communal baths. Using a 59‐year migration time series from Cape Estaca de Bares (Galicia, NW Spain), we first characterized long‐term migration phenology. To evaluate the potential impact of HPAI, we then focused on the most standardized 20‐year period (2004–2023; 35,828 observations). We first built a climate‐based model using pre‐HPAI data (2004–2020) to quantify the influence of weather conditions on the number of migrating great skuas. This model was subsequently used to generate predictions for the post‐HPAI period (2021–2023) under observed weather conditions, allowing us to assess whether recent declines exceeded expectations based on weather variability alone. Migration counts revealed substantial interannual variation, with the sharpest decline in 2022 and 2023, coinciding with the HPAI outbreak. Importantly, observed migration numbers during the post‐HPAI period were consistently lower than predicted based on weather conditions alone, indicating that weather cannot explain the recent decline. These results might suggest that the reduction in the number of migrating great skuas reflects the impact of an additional large‐scale process consistent with the HPAI outbreak, mirroring similar trends observed in breeding colonies. Our findings highlight the critical role of long‐term monitoring of migration studies in understanding the broader ecological implications of HPAI. Further investigations are necessary to determine how predation pressures by great skua and the virus's effects are influencing seabird species that it kleptoparasites.