Spatial and temporal variation in mortality from avian influenza in Greenland Barnacle Geese Branta leucopsis in their wintering grounds

Capsule: Avian influenza caused the loss of more than 20% of the Greenland Barnacle Goose population, but this impact varied between wintering areas and over time. Aims: The primary objective was to investigate the spatial and temporal mortality patterns due to avian influenza (H5N1) in wintering Greenland Barnacle Geese. Methods: We analysed a comprehensive dataset of marked individuals spanning six years, with observations from a network of observers across their wintering range. The study specifically compared the mortality rates of Greenland Barnacle Geese during the H5N1 outbreak years (2021/2022 and 2022/2023) with the three previous winters (2018/2019–2020/2021). Results: The study found significant spatial and temporal variation in mortality resulting from avian influenza outbreaks within the Greenland Barnacle Goose population on their wintering grounds in Scotland and Ireland. Some sites (Islay, Tiree and Sligo) experienced 30–56% reductions in survival rates, while others (Uist and Mayo) showed little or no impact. The timing of the main outbreaks also differed between sites. Excess deaths (in comparison with the previous baseline), estimated using mark-resighting data, indicated that mortality was considerably higher than suggested from direct field counts, reaching at least 20% of the global population in the peak outbreak winter (2022/2023). Conclusion: The results highlight the importance of spatial and temporal dynamics in avian influenza impacts. Disease dynamics should be integrated into population management models and used for setting appropriate thresholds for minimum population levels, to ensure resilience to disease outbreaks and long-term viability.