Avian influenza mainly affects the oldest Sandwich terns: the loss of experienced birds may hinder population recovery
The outbreak of avian influenza (H5N1) in 2022, which claimed the lives of over 20,500 Sandwich terns (more than 17 per cent of the European population), mainly affected the oldest birds. This is according to a study in Nature Communications, led by INBO. The study was based on 28 years of ringing data from over 50,000 birds.
Whilst mortality among young breeding birds ranged between 5 and 10 per cent, more than 40 per cent of the oldest birds died. This may have implications for the population’s recovery: older birds have the most breeding experience and often make the greatest contribution to reproductive success. The massive loss of precisely these experienced birds could undermine the species’ resilience and ability to recover.
The results show that avian influenza is not just about how many birds die, but also about which birds are lost. It is therefore essential to continue monitoring populations over the long term, also after an outbreak, paying particular attention to age structure, survival and breeding success.
Wouter Courtens
Read more: Courtens, W., Ballmann, M., Fijn, R. et al. Avian influenza amplified age-related mortality in a long-lived seabird. Nat Commun 17, 8653 (2026). https://doi.org/10.1038/s41467-026-76726-7