Can rivers be important movement corridor for waterbirds in urban areas?

The movement corridors for waterside animals, such as waterbirds, have been studied less than those of terrestrial animals. To clarify the role of rivers as movement corridors in urban areas, we investigated the movements of waterbirds in central Tokyo, Japan. This study was carried out around the Sumida River in the winter of 2019–2020. We divided the study site into grid cells of approximately 500 m in length and width and recorded the number of individual flying over each cell. Based on the results, we compared the abundance of moving waterbirds between cells with and without rivers. In addition, we constructed a linear model showing the relationship between the abundance of moving individuals and the four environmental variables across the 60 cells containing rivers. For species recorded abundantly (Black-headed Gulls, Herring Gulls, Black-tailed Gulls, and Great Cormorants), most of moving individuals were observed in cells with river. When only the cells, including rivers, were considered, the number of individuals of all the species was positively associated with the open river area and negatively associated with the proportion of areas covered by elevated roads running directly above rivers. Our findings show that rivers can function as waterbird movement corridors in urban areas. In urban planning, to maintain connectivity for waterbirds along river segments, it is recommended to avoid constructing obstructions over rivers, such as elevated roads and other man-made structures which may impede bird flight along rivers.