Population changes in the declining Turnstone (Arenaria interpres) and other waders in the northern Baltic Sea based on past and current breeding observations

Many waders are declining based on winter surveys, but assumed trends need to be validated against changes in breeding numbers. Comparing breeding population changes between threatened and non-threatened closely related species allows assessment of the species- specificity of threats. To identify biogeographical and biological correlates of population trends in declining Ruddy Turnstones (Arenaria interpres) and four other waders in the Archipelago Sea, SW Finland, we re-censused 209 islets in 2015-2018 hosting breeding Turnstones in the late 1980s-early 1990s. We used island-specific geographical variables and pair numbers of five gull and three tern species as predictors in species-specific logistic regression models of breeding probability. The number of islands with Turnstones (29) decreased by 86.1%. The proportion of islands with breeding Ringed Plovers (Charadrius hiaticula) significantly increased, while there were no trends for Redshanks (Tringa tetanus), Oystercatchers (Haematopus ostralegus) orCommon Sandpipers (Actitis hypoleucos). However, the proportion of islands with more than one breeding pair of Oystercatchers increased. Ruddy Turnstone breeding occurrence was positively associated with Arctic (Sterna paradisaea) and Caspian Tern (Hydroprogne caspia) breeding numbers, island exposure and area, and negatively associated with forest area and Common Tern (S. hirundo) breeding numbers. Probable reasons for the decline are the presence of American mink (Neovison vison), a significantly declining proportion of islands with breeding Arctic Terns and natural vegetation succession. Our study highlights that population trends varygreatlybetween waders, butconfirms the population decline of the Turnstone, now increasingly confined to exposed thinly scattered islands. Maintaining viable populations of small larids is crucial in improving its conservation status.