Bird flu hit Europe's oldest Sandwich Terns hardest
Mortality during the 2022 H5N1 outbreak rose from about 5-10% among young breeders to more than 40% in the oldest birds.
Europe's oldest Sandwich Terns were far more likely to die during the catastrophic 2022 outbreak of highly pathogenic avian influenza than younger breeding birds, a new study has found.
Researchers combined 28 years of ringing records with individual data from the H5N1 mass mortality event that swept through Sandwich Tern colonies in north-western Europe. Estimated outbreak mortality rose from approximately 5-10% among young breeders to more than 40% in the oldest age groups.
The result shows that disease did not remove adults at random. Instead, it disproportionately affected birds likely to possess the greatest breeding and foraging experience, potentially weakening the ability of colonies to recover.
The analysis drew on more than 93,000 records involving 50,646 Sandwich Terns ringed during their first year of life in the Netherlands and Belgium. During the 2022 outbreak, 832 ringed birds of known age from the two study regions were recovered dead across Europe.
Predicted mortality among four-year-old birds ranged from approximately 5% to 11%, depending on the region and model used. For 28-year-old birds it rose to between 26% and almost 50%. Each additional year of age increased the estimated odds of outbreak mortality by about 8-9%.
There was no equivalent difference between the sexes. Of 243 dead adults that could be sexed, 123 were female and 120 male.
Our findings highlight a previously underappreciated mechanism through which HPAI outbreaks may impair the resilience of long-lived avian populations.
The researchers examined whether older birds might simply have arrived at breeding colonies earlier and therefore experienced greater exposure. Although first-time breeders arrived later, every age group included in the analysis was present well before the outbreak began, making arrival timing an unlikely explanation.
Older birds may have differed in their exposure within densely packed colonies or been more susceptible to infection because of age-related physiological changes. However, the study found no firm evidence identifying the mechanism and says further work is needed.
Sandwich Terns are long-lived seabirds with low annual reproductive output. Experienced adults can contribute disproportionately to population growth, so an outbreak concentrated among the oldest birds may have consequences extending well beyond the immediate death toll.
More than 20,500 adult Sandwich Terns, over 17% of the north-western European population, were reported dead during the 2022 outbreak. The study area covered 12 colonies - 11 in the Netherlands and one in Belgium - at the centre of the mortality event.
Recruitment of first-time breeders, surviving non-breeders and immigrants may initially replace some lost adults. The authors warn, however, that repeated outbreaks could exhaust this reserve and that removing experienced birds may reduce future breeding performance.
The study, Avian influenza amplified age-related mortality in a long-lived seabird, is published in Nature Communications.
25 Aug 2026
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