Population trends of marine versus terrestrial bird species on Skúgvoy, FaroeIslands: 1961-2023

Capsule: Long-term breeding bird censuses on the Faroese Island of Skúgvoy showed differingtrends of declining seabird populations and more stable terrestrial birds.Aim: To examine the population trends from 1961 to 2023 using periodic breeding bird censuseson Skúgvoy, in the Faroe Islands, encompassing most terrestrial and marine species.Methods: Skúgvoy is the only Faroese Island with periodic bird censuses, conducted since 1961 atapproximately 20-year intervals, with a new census in 2023. Inland birds were counted by mappingbreeding territories, with colonial seabirds counted individually on cliff ledges from the sea surface.These historical censuses provided a long-term quantitative assessment of species’ populationdynamics and exploration of potential causes of observed declines, especially among seabirds.Results: Seabird populations have not only declined drastically since the previous comprehensivecensus in 2001 but have also experienced acceleration in the scale of decline. In contrast, terrestrialspecies have exhibited only minor fluctuations.Conclusion: The differing trends between seabirds and inland birds suggest that the declines arelikely driven by regional-scale processes, such as reduced food availability for pelagic seabirds,rather than local-scale processes operating within the island system.