The Belgian part of the North Sea (BNS) is an important biotope for seabirds. Due to the large supply of food, it serves as a foraging area. In addition, the BNS also functions as a wintering and migration area. For example, during the migration period, significant numbers (more than 1% of the global biogeographical population) of the lesser black-backed gull (Larus fuscus), little gull (Hydrocoloeus minutus), grote stern (Thalasseus sandvicensis) and common tern (Sterna hirundo) are regularly counted (Stienen et al. 2007).

In order to protect bird species, various European agreements have been made. For example, conservation measures must be taken in accordance with the Ramsar Convention if at least 1% of the global population of a certain bird species occurs in a site. In addition, the European Birds Directive contains a list of rare or threatened bird species. In order to protect these birds, each Member State must designate Special Protection Areas (SPA-V or Birds Directive areas) that form part of the Natura 2000 network.

Despite the efforts and international agreements on the conservation of bird species, the revision of the initial assessment for Belgian marine waters shows that the situation of seabirds is deteriorating (MSFD 2018). Long-term monitoring of the number of breeding pairs in the 3 OSPAR regions (Arctic waters, North Sea and Celtic Sea) shows that the number of breeding seabirds has fallen sharply. In the BNS, 25% of the species are currently below the reference level. Possible reasons for this decline are habitat destruction, deterioration in habitat quality and predation (Stienen et al. 2017).

Research on seabirds is carried out in various ways. Bird counts are carried out through a combination of observations and calculations. On the other hand, migration patterns of birds are recorded via colour rings and GPS tracking systems (UvA-BiTS). A transmitter on a bird's back measures the bird's position and height at regular intervals. This is done within the framework of the LifeWatch project for the European herring gull (Larus argentatus), the lesser black-backed gull and the Western marsh harrier (Circus aeruginosus). These data are available on the LifeWatch data explorer. Within LifeWatch, the behaviour of birds is also observed using video images from Birdcams (live video).

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