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H5N1 Outbreak Killed at Least 8,137 Black-headed Gulls and 614 Common Terns in Germany

Juvenile black-headed gull swimming on calm water
Research · Disease, parasites & microbiology

A reconstruction of Germany’s 2023 H5N1 outbreak documented high mortality in both black-headed gulls (Chroicocephalus ridibundus) and common terns (Sterna hirundo). H5N1 is a subtype of influenza A virus, and the euBB genotype involved here belonged to the highly pathogenic H5N1 clade 2.3.4.4b. At least 8,137 adult black-headed gulls and 614 adult common terns were reported dead. Compared with the breeding-population figures available to the researchers, these corresponded to minimum estimates of about 3.0% and 3.5%, but the reference census was already several years old and not every dead bird would have been found, so the true mortality fraction was probably higher.

The study, published online on 8 June 2026, combined virus genomes, ringing records, outbreak timing and breeding-population data. The euBB genotype had emerged in France in spring 2022 through reassortment between a gull-associated low-pathogenicity influenza virus and highly pathogenic H5N1. Reassortment occurs when influenza viruses exchange genome segments, creating a new combination of viral genes.

Several independent introductions reached German colonies

Phylogeographic analyses — comparisons of how virus genomes are related across places and dates — indicated that euBB reached German breeding colonies through several separate incursions rather than one single spread event. When these genetic relationships were combined with ringing records, the pattern was most consistent with black-headed gulls returning from wintering areas in the Netherlands, Belgium and France. The timing then supported subsequent spillover into common terns, which typically return to the shared breeding colonies several weeks later.

National surveillance also recorded a sharp rise in H5-positive samples during the outbreak year. Among sampled black-headed gulls, the proportion positive for HPAIV H5 rose from 21% in 2022 to 53% in 2023. Among common terns it rose from 25% to 36%. Most samples came from dead or diseased birds, so these proportions describe the surveillance sample rather than infection prevalence in the populations.

Mortality included adults and chicks

In addition to the adult carcasses, 29,812 black-headed gull chicks and 1,166 common tern chicks were reported dead. The authors emphasise that not every bird that died would have been found and reported, making the documented mortality a minimum estimate.

Breeding-pair counts were also lower in 2024 than in 2022. In the study’s main comparison, black-headed gull pairs declined by 15.9%, from 66,139 to 55,622, while common tern pairs declined by 5.8%, from 6,972 to 6,569. When data from eastern Mecklenburg-Western Pomerania were included, the declines increased to 24.0% and 10.2%, respectively. The authors also discuss redistribution among colonies as a factor that can influence regional breeding-pair changes.

The genotype later faded from surveillance

By 2024, H5 was detected in only 1.1% of sampled black-headed gulls, while all six common terns tested that year were negative. The authors discuss increasing immunity as one possible contributor to the apparent fade-out of euBB. In adult common terns, antibodies against influenza A rose to 40% in 2024 and H5-specific antibodies also became more common. Antibodies indicate previous exposure and an immune response, not an active infection at the time of sampling, so the serology supports survival after infection and possible population immunity rather than showing that 40% of the birds were currently infected.

The study shows how a highly pathogenic avian influenza genotype can be introduced repeatedly by migratory birds and then affect another colonial species. The authors conclude that HPAI should be recognised as an additional pressure on seabird populations alongside the other threats they already face.

Sources and image credit

  1. Knief, U., Ahrens, A. K., Allendorf, V., Behringer, C. J., Bertram, J., Bouwhuis, S., Fiedler, W., Globig, A., Günther, A., Herrmann, C., Knauf, S., Marchowski, D., Piro, S., Pohlmann, A., Rollins, R. E., Staubach, C., & Harder, T. (2026). Outbreak dynamics of high pathogenicity avian influenza virus H5N1, clade 2.3.4.4b euBB, in black-headed gulls and common terns in Germany in 2023. Avian Pathology, 55(4), 487–507. https://doi.org/10.1080/03079457.2026.2659834Primary peer-reviewed study, published online 8 June 2026. It analysed H5N1 euBB spread, virus-related mortality and population-level effects in black-headed gulls and common terns in Germany, including likely incursions from wintering areas in the Netherlands, Belgium and France.

Image: Donald Hobern / Wikimedia Commons — juvenile black-headed gull photographed at Utterslev Mose, Denmark, 11 July 2015; representative species image, not a bird from the German H5N1 study · CC BY 2.0 · Source

About this content: This story was produced with AI assistance within an editorial workflow developed by Wildlife Vagabond. Editorial responsibility remains with Wildlife Vagabond.How AI is used

Disease, parasites & microbiology

SPECIES IN THIS STORY

Species in this story

Black-headed Gull Chroicocephalus ridibundus Explore species Common Tern Sterna hirundo Explore species

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