Highly pathogenic avian influenza (HPAI) has caused unprecedented mortality in wild birds and led to the culling of millions of poultry worldwide. Since 2021, this ongoing global outbreak has severely affected seabird populations, yet key aspects of how the virus spreads among wild birds remain poorly understood.

BioSS researcher David Ewing collaborated with Dr Sandra Bouwhuis at The Institute of Avian Research to model data from a long-term monitoring programme of the common terns colony that breeds at Banter See in Germany. This colony suffered major outbreaks of HPAI in 2022 and 2023. The team developed advanced Bayesian modelling techniques that use daily carcass count data to estimate how efficiently the virus was transmitted and how deadly it was to infected birds.

The study, published in the Journal fo Animal Ecology, found that each infected tern infected, on average, 3-4 others (R₀ between 3 and 4). The estimated probability of death following infection decreased from approximately 26% in 2022 to 14% in 2023, potentially suggesting either increased immunity within the population or a shift in the circulating virus strain. Results also indicated that direct bird-to-bird transmission was the main route of infection, with little evidence of environmental spread—possibly due to effective carcass removal during the outbreaks.

These findings highlight that managing breeding density by facilitating “social distancing”, for example by increasing the availability of suitable nesting habitat, could help to reduce transmission risk in colonial seabirds. Such insights are vital for designing evidence-based conservation and disease management strategies in the face of future HPAI outbreaks.

This work was conducted in collaboration with The Institute for Bird Research and was funded by the Scottish Government Rural and Environmental Science and Analytical Services (RESAS) and the Scottish Centre of Expertise on Animal Disease Outbreaks (EPIC).

The full publication can be accessed here:

Ewing, D. A., & Bouwhuis, S. (2025). Estimating epidemiological parameters of highly pathogenic avian influenza in common terns using exact Bayesian inference. Journal of Animal Ecology, 00, 1–13. https://doi.org/10.1111/1365-2656.70145

The cumulative number of transponder-marked common terns registered daily at the Banter See colony (solid lines) is shown alongside the cumulative number of transponder-marked birds found dead in the colony and its surrounding area (dashed lines) over the duration of the HPAI outbreaks in 2022 (left) and 2023 (right).