First reports of dead sandwich terns in colonies in France caused by the high pathogenic avian influenza (HPAI) virus, or bird flu, were released at the end of May. Within a few days, the virus swept through the breeding colonies, killing hundreds and up to thousands of adult birds. Within the Wadden Sea, the first cases were reported from the Netherlands shortly after. While definite numbers are hard to get by in this highly mobile species, the breeding colonies of sandwich terns in the Dutch part of the Wadden Sea have been largely deserted. On Texel alone over 3,500 sandwich terns have been found dead in June (nearly 40% of the birds breeding on the island), with more animals expected to have died at sea or in areas outside the main breeding sites. From colonies in Lower Saxony, losses of at least 20% among adult birds have been reported. The north-eastern part of the Wadden Sea seems to be less affected.
In the last Trend Report on Breeding Birds (Koffijberg et al., 2020), the numbers of sandwich terns in the Wadden Sea were estimated to be stable at around 15,000 individuals. However, the species is confined to only a few specific breeding sites on the Wadden Sea islands, most of which have been affected by the outbreak. As the colonies in the Wadden Sea represent a large part of the Northwest European breeding population, the deaths of such an extensive number of adult birds can have large-scale consequences for the development of the sandwich tern population in Northwest-Europe. As a result of the high adult mortality, fewer chicks are raised to fledging, meaning that this year’s reproductive success will also be dampened.
Scientists assume that the dense colonies in which sandwich terns breed have facilitated the spread of the virus. Other colony-breeding birds in the Wadden Sea (common terns, black-headed gulls), in and around the North Sea (common terns, black-headed gulls, gannets, guillemots, great skuas) and in the Baltic Sea (cormorants), have also been affected. Another common characteristic of the species most affected this summer is that they are mainly fish-eating species. If and how this relates to the spread of the disease is currently unknown.
The scale of the recent outbreak is unprecedented, and it is unclear which management measures can help curbing the spread of the virus. A trilateral meeting with scientists and managers is planned for autumn to discuss the effectiveness of different management strategies to be prepared for potential future outbreaks.
In consultation with the members of the trilateral Expert Groups on birds, group members from the National Park Lower Saxony and the Common Wadden Sea Secretariat developed a first outline for a bird flu workshop. In this framework, scientists, site managers, and policy makers working on sandwich terns in the Wadden Sea or adjacent colonies, as well as experts on the bird flu virus and its transmission in colony breeding birds are invited to join a first online meeting on 18-19 October 2022 (noon to noon). More information & registration.
If you find any sick or dead birds, please do not touch them but inform one of the respective agencies listed in a previous article in relation to bird flu on our website.