The Hooded Crow Corvus comix, density as a predictor of wetland bird species richness on a large geographical scale in Poland

摘要

Evaluating wetland bird species richness on a national scale can be extremely hard as it is prohibitively expensive, expert knowledge of bird species determination is indispensable, and wet areas tend to be difficult to study. For these reasons we tested the usefulness of the Hooded Crow, a bird easy to detect, as a surrogate for wetland bird species richness in Poland, its primary habitats are situated in flooded river valleys and other wet areas where it is a nest predator of many water birds. What is more important, we found a significant correlation between this raw species density and our target group (r= 0.29, p ＜ 0.001). Therefore, we suspected that on the whole country the Hooded Crow could potentially reflect wetland bird species richness. We analyzed factors affecting the Hooded Crow's density and wetland bird species using data from 776 1 km × 1 km study plots and a set of 22 environmental variables. Next we developed analysis where apart environmental variables we also used density of the Hooded Crow as an additional predictor of wetland bird species. Habitat and climatological predictors were aggregated using Principal Components Analyses, then related to the Fourier estimator of the Hooded Crow's density and the Jackknife estimator of wetland bird species richness by applying General Additive Models. Finally, competing models were assessed according to information - theoretic criteria. The best-supported models for both the standardized Hooded Crow's density and wetland bird species richness covered 8 almost the same aggregated predictors. The models on wetland bird species could be further improved by including the standardized Hooded Crow density, showing the potential to use Hooded Crow as surrogate for wetland bird species richness (delta AIC=10.5). Both groups prefer lowland water areas surrounded by meadows, where rivers are distributed rather evenly, and where precipitation is relatively high. The greatest differences between these groups appear on areas where farmland mechanization is high. Such areas are used by Hooded Crows, but avoided by wetland birds. Besides, unlike wetland species, Hooded Crows definitely avoids the north of Poland which is covered by large dense forests interspersed with rivers. Therefore, the Hooded Crow reflects wetland species richness only in open lowland habitats, dominated by wet meadows, where the landscape is interspersed with rivers and lakes, and hydrological conditions are repeatable from year to year. The Hooded Crow cannot be used as an indicator of wetland birds on a large area of intensively used farmland or in large deciduous forest interspersed with rivers.