Understanding the links between breeding and wintering areas of migratory species has important ecological and conservation implications. Recently, stable isotope technology has been used to further our understanding. Stable isotope ratios vary geographically with a range of biogeochemical factors and isotope profiles in organisms reflect those in their food and environment. For inert tissues like feathers, isotope profiles reflect the environment in which they were formed. Following large-scale habitat destruction, the globally threatened aquatic warbler Acrocephalus paludicola has a fragmented breeding population across central Europe, largely in Belarus, Poland and Ukraine. The species' sub-Saharan African wintering grounds have not yet been discovered, and this significantly hampers conservation efforts. Aquatic warblers grow their flight feathers on their wintering grounds, and we analysed stable isotope ratios (delta(15)N, delta(13)C, delta D) in rectrices of adults from six main breeding sites (subpopulations) across Europe to determine whether different breeding subpopulations formed a single mixed population on the wintering grounds. delta(15)N varies considerably with dietary trophic level and environmental factors, and delta D with the delta D in rainfall; neither varied between aquatic warbler subpopulations. Uniform feather delta(15)N signatures suggest no major variation in dietary trophic level during feather formation. High variance and inter-annual differences in mean delta D values hinder interpretation of these data. Significant differences in mean delta(13)C ratios existed between subpopulations. We discuss possible interpretations of this result, and consider differences in moulting latitude of different subpopulations to be the most parsimonious. delta(13)C in plants and animals decreases with latitude, along a steep gradient in sub-Saharan Africa. Birds from the most northwesterly breeding subpopulation (Karsibor, Poland) had significantly lower variance in delta(13)C and delta(15)N than birds from all other sites, suggesting either that birds from Karsibor are less geographically dispersed during moult, or moult in an area with less isotopic heterogeneity. Mean delta(13)C signatures from winter-grown feathers of different subpopulations were positively correlated with the latitude and longitude of breeding sites, suggesting a strong relationship between European breeding and African winter moulting latitudes. The use of stable isotopes provides novel insights into migratory connectivity and migration patterns in this little-known threatened species.

Biodiversity monitoring is important as it allows to prioritize research into the causes of declines and assessing the efficacy of conservation measures. Regional assessments are valuable, because conservation policies and management are often implemented on national and sub-national level. We analyzed data from the German Common Bird Monitoring for 1990–2018. We derived indices of population size using standard log-linear models, based on point counts and route territory mapping at up to 1200 plots annually. We summarized species trends by ecological trait groups. Among the 93 common breeding birds, farmland birds declined strongly, birds of settlements declined. Forest birds initially declined, but recovered after ca. 2010. Wetland birds increased strongly, albeit the number of species with data was low. Consistent declines were found in ground-nesting birds, granivorous and invertebrate (other than insect)-feeding birds. Trends of insectivorous birds were stable on average, but farmland insectivores declined strongly since the year 2000. Long-distance migrants showed more negative trends compared to short-distance migrants and resident species. Species with narrow habitat niche declined disproportionally. Trends over the entire period were more negative in common species in the dataset (with a German breeding population of > 50 K and < 1 M pairs). On the opposite, short-term trends were more negative in less common species (< 50 K pairs). Cold-adapted species showed largely negative, warm-adapted largely positive trends. Multi-species indicators showed no directional change (i.e., a change from decline to increase or vice versa) conditional on the inclusion or omission of single species, but the magnitude of change was affected in groups with low sample size. This suggests that efforts should be made to develop robust monitoring schemes for rarer species that are not covered by the analyses here. We conclude that conservation policies in Germany should aim at halting the worrying declines in ground-nesting, often insectivorous, farmland birds. The recovery of forest and wetland birds is encouraging, but future trends need to be monitored. Ongoing climate change will affect species directly (via their thermal niche) and indirectly (e.g., through more forest disturbance). Conservation strategies will, therefore, need to consider species adaptation to environmental and climate change, e.g., in better protected area connectivity and management.

The Aquatic Warbler (Acrocephalus paludicola) is a song bird breeding in fen mires and similarly structured other wetlands with a water depth of 1-10 cm. Widespread in central-European wetlands at the beginning of the 20th century, the species is now globally threatened. The westernmost and genetically distinct Pomeranian population is even on the verge of extinction. The major challenge in the conservation of remaining habitat is the costefficient removal of biomass. About 50% of the Pomeranian population survives in a valley fen near Rozwarowo in Northwest Poland, where between 1993 and 2007 a conspicuous change in breeding habitat has taken place from summer grazed sedge meadows to commercial winter cut reed beds. We compared vegetation structure, site conditions, and potential prey abundance with the distribution and abundance of Aquatic Warblers in Rozwarowo Marshes and studied temporal changes and the compatibility of conservation and reed cutting interests. Aquatic Warblers now occur almost exclusively in sparsely growing, low reed with abundant Thelypteris palustris, Carex elata, and Lysimachia vulgaris. This vegetation type provides more potential prey for Aquatic Warblers than the higher productive tall reed, whereas the patches of sedge vegetation have become too small following succession after abandonment. Currently, commercial reed cutting maintains suitable Aquatic Warbler breeding habitat. Considering the impending changes in the reed market, there is a need for flexible agri-environmental schemes (AES) to ensure that stripes are left uncut and to prevent eutrophication by high and long flooding of the site.

The globally threatened Aquatic Warbler Acrocephalus paludicola is an umbrella species for fen mires and is at risk of extinction in its westernmost breeding population due to severe habitat loss. We used boosted regression trees to model Aquatic Warbler habitat selection in order to make recommendations for effective management of the last remnant habitats. Habitat data were collected in the years 2004–2006 in all remaining breeding sites in Pomerania (eastern Germany and western Poland) as well as in recently abandoned sites. Models were validated using data from similar Aquatic Warbler habitats in Lithuania. The probability of occurrence of Aquatic Warblers in late May/early June was positively associated with low isolation from other occupied sites, less eutrophic conditions, a high proportion of area mown early in the preceding year, high availability of vegetation 60–70 cm high, high prey abundance and high habitat heterogeneity. Early summer land management is needed in the more productive sites to prevent habitat deterioration by succession to higher and denser vegetation. As this also poses a serious threat to broods, management that creates a mosaic of early and late used patches is recommended to preserve and restore productive Aquatic Warbler sites. In less productive sites, winter mowing can maintain suitable habitat conditions. Aquatic Warbler‐friendly land use supports a variety of other threatened plant and animal species typical of fens and sedge meadows and can meet the economic interests of local land users.

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