Wintering Waterbirds Along Sandy Littorals of the Northern Adriatic Sea (Italy): Long-Term Changes in Abundance, Diversity, and Functional Composition

Abstract

Sandy littorals are highly dynamic coastal ecosystems but remain poorly studied as wintering habitats for waterbirds in Mediterranean regions. We analysed mid-winter International Waterbird Census data collected along the 100 km sandy littoral of the province of Venice, northern Adriatic Sea (Italy), between 1994 and 2022. The dataset included 28 winter censuses, 70 species, and 150,083 individuals. The assemblage was rich but highly uneven: the Yellow-legged Gull Larus michahellis, Dunlin Calidris alpina, Black-headed Gull Chroicocephalus ridibundus, and Great Cormorant Phalacrocorax carbo accounted for most individuals. Total abundance was stable, despite strong interannual variability, whereas species richness increased significantly. Eco-functional guilds showed contrasting trends: benthic feeders and diving piscivores increased moderately, omnivores declined moderately, whereas the trends of the remaining four guilds were classified as uncertain. The Community Temperature Index did not show a significant directional trend, either for the whole assemblage or after excluding dominant species, suggesting that the observed changes cannot be interpreted as progressive thermophilisation. Overall, the Venetian sandy littoral supports a diverse and functionally heterogeneous wintering bird assemblage, whose long-term changes reflect shifts in community structure and the changing use of open-coast, nearshore, and artificial roosting habitats. The results provide a long-term evidence base for identifying conservation-relevant shoreline sectors, protecting feeding and high-tide roosting habitats, regulating disturbance and beach-management practices, and maintaining continued IWC monitoring of open-coast waterbird assemblages.