Rapid interannual reorganisation of Whiskered Tern Chlidonias hybrida breeding colonies within an urban wetland network

Colonial waterbirds breeding in unstable wetland habitats are expected to show flexible colony-site use, but how breeding activity redistributes within small networks of alternative nesting sites, particularly in urban wetlands, is poorly documented. We quantified the spatial organisation of breeding Whiskered Terns Chlidonias hybrida across nine lakes of Vacaresti Natural Park, a spontaneously developed urban wetland in Bucharest, Romania, using nest counts repeated every one to three weeks over four breeding seasons (2022-2025). The number of occupied lakes fell from nine in 2022 to three in 2024, before a partial recovery to six in 2025, while the annual peak of simultaneously active nests ranged from 53 (2024) to 222 (2025). Spatial concentration at the annual peak survey was higher in 2024-2025 (dominant-lake proportion 0.55-0.56; Simpson's concentration index C = 0.42-0.50) than in 2022-2023 (0.41-0.46; C = 0.29-0.32), and the peak-survey distributions of 2024 and 2025 showed no overlap (Bray-Curtis dissimilarity = 1.0). The breeding season was long and asynchronous in 2022, when the largest colony shifted repeatedly among lakes, but compressed and synchronised in 2024-2025, when a single lake remained dominant throughout the season. Lake surface area (0.18-2.25 ha) was not detectably associated with occupancy or seasonal maximum nest numbers. These results indicate highly flexible colony-site use and rapid interannual reorganisation of breeding activity within a compact network of alternative nesting sites. Conservation and monitoring of urban wetland colonies should therefore encompass the entire network of potential nesting sites rather than only the currently dominant colony site.