Avian influenza amplified age-related mortality in a long-lived seabird
- Wouter Courtens
- Mónika Ballmann
- Ruben Fijn
- Morten Frederiksen
- Susanne Kühn
- Luc Lens
- Mardik Leopold
- Sander Lilipaly
- Florian Packmor
- Eric W. M. Stienen
- Peter van Horssen
- Ulrich Knief
2026-08-19
In recent years, highly pathogenic avian influenza (HPAI) A (H5N1) clade 2.3.4.4b viruses have caused mass mortality events in seabirds worldwide, raising concern for long-lived species with low reproductive rates. Using 28 years of ringing data together with individual-level data from the 2022 mass mortality event in northwestern European Sandwich terns ( Thalasseus sandvicensis ), we show that mortality was biased towards older individuals, while no sex bias was observed. HPAI-related mortality increased from approximately 5–10% in young breeders to more than 40% in the oldest individuals. This age-biased loss likely removed the most experienced individuals from the population. Our findings highlight a previously underappreciated mechanism through which HPAI outbreaks may impair the resilience of long-lived avian populations.