Todorov, E. | bioRxiv Science

bioRxiv2026

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.

zoology↗

bioRxiv2017

Mapping critical habitat of waterbirds in the Arctic for risk management in respect of IFC PS6

O_LIEconomic development and energy exploration are increasing in the Arctic. Important breeding habitats for many waterbird species, which have previously been relatively undisturbed, are now being subjected to these anthropogenic pressures. The conservation of the habitats and the species they support is a significant challenge for sustainable development. Even if governments and corporates operating in this fragile environment are committed to sustainable development, there is little information available to avoid, mitigate and manage environmental risk and impacts. Taking a risk management perspective, we followed the International Finance Corporations (IFC) Performance Standard 6 (PS6) criteria on Environmental and Social Sustainability and developed an approach to identify \"critical habitat\", as defined in IFC PS6, for waterbird species breeding in the Arctic. While the range of these waterbirds is roughly known, more accuracy is needed for proper risk assessment.\nC_LIO_LIWe have therefore gone a step further by modelling suitable habitat within these ranges. Depending on the relevance of the species for IFC PS6 and the level of certainty we separated the classes likely and potential critical habitat. We tested the approach for Russian breeding populations of five Anatidae species (White-fronted Goose Anser albifrons, Lesser White-fronted Goose Anser erythropus, Brent Goose Branta bernicla, Redbreasted Goose Branta ruficollis and Bewicks Swan Cygnus columbianus bewicki). Likely critical habitats were identified through a review of literature and available data for these waterbird species and multi-species congregations. To address the information gap for most of the Russian Arctic a species distribution modelling approach was used. The outputs of this approach were labelled as potential critical habitat, indicating the lower level of certainty than likely critical habitat.\nC_LIO_LIBased on existing information the amount of likely critical habitat is estimated to be at least x,xxx,xxx km2. For the five Anatidae species, X,XXX,XXX km2 potential critical habitat was identified; 95% of these areas were outside of the area boundaries of likely critical habitat for the species.\nC_LIO_LIInsufficient data in the east of the study area did affect the results, as some areas known to support breeding populations were not identified as suitable. Conversely, species distributions may be overpredicted in other areas; It should also be recognized that the analyzed species currently have depressed populations and may therefore only utilize a proportion of suitable habitat available.\nC_LIO_LIFor risk assessment purposes however, it is better to predict false positives, rather than false negatives. The study indicates that there are large areas in the Arctic that are potentially important for each of the Anatidae species modelled, but are not yet recognised as key important areas. The results confirm that there is still much to learn about waterbird distribution and abundance in the Russian Arctic.\nC_LIO_LISynthesis and applications The critical habitat maps produced do not just provide a new source of information for the economic development sector, but provide it in a way that is relevant to the sector and directly applicable. The maps are useful for initial risk assessments of potential developments, to identify likely impacts and to consider mitigation options, in accordance with IFC PS6. Risk assessors should exercise caution and detailed surveys for any development in areas predicted to be suitable for each species should be carried out.\nC_LI

ecology↗