Jeroen Nienhuis, Roy Slaterus & Frank Majoor (2025) High summer mortality and low breeding success due to highly pathogenic avian influenza among Black-headed Gulls Chroicocephalus ridibundus wintering in Groningen in 2023/24. LIMOSA 98 (1): 37-41.

Black-headed gulls are social birds that live in groups all year round. As a result, infectious diseases can spread quickly. In the winter of 2022/23, a variant of highly pathogenic avian influenza (HPAI) emerged to which Black-headed Gulls were highly susceptible. From late 2022, HPAI rapidly spread and was eventually observed throughout the species’ European range. At the wintering sites, Black-headed Gulls from much of the breeding range congregate, making it possible to get an impression of average mortality and reproduction with a local study in the Dutch city of Groningen. In Groningen, Black-headed Gulls have been colour-ringed annually since 1996, gradually building up a wintering population of individually recognisable birds. Frequent searches provide information of which individuals reside in the city. Of birds seen at least three times in one winter, it was calculated how many were seen again in the subsequent winter. Breeding success was determined by counts of the proportion of juvenile birds. There are differences between neighbourhoods in the proportion of young and frequency of visits. Therefore a yearly city wide weighted average was calculated for frequently visited neighbourhoods. The average summer survival til 2022/23 was 78.8%. After the HPAI outbreak, this was significantly lower at 64.7%. Most deaths occurred after leaving Groningen for the Northeast-European breeding grounds. After the HPAI outbreak breeding success was lowest in a series of eight years. This was not significant due to a large variation in breeding success in the preceeding years.

[pdf only for members] [dutch summary]