Beached bird surveys in The Netherlands : summer 2024 & winter 2024/25

This is the annual report on beached bird survey (BBS) combining results obtained in The Netherlands during summer 2024 (May-Oct) and the winter 2024/25. The long-term study showed a steady, long-term decline in oil pollution rates among guillemots, accelerating since 2005. All recent seasons have shown exceptionally low oil rates, but two pollution incidents occurred in the winter of 2024/25. An oil rate of 5.8% (n = 86) was found among guillemots, the highest value since 2017/18. The multi-year average reached a level of 2.0 ± 2.3%, which still meets the OSPAR targets for 2030. In addition to a localised oiling incident, the discharge of polyisobutylene into the Southern Bight in December 2024 caused significant mortality among seabirds, many of which stranded alive (i.e. outside de beached bird survey protocols). The cause of this incident was initially attributed in the media to paraffin contamination, despite the chemical's sticky nature being recognized. Avian influenza again claimed many victims, including many Barnacle Geese Branta leucopsis and Herring Gulls Larus argentatus, as well as Northern Fulmars Fulmarus glacialis.