Distinctive river environments such as gravel bar are maintained by natural flow regime and provide essential habitats for many disturbance-dependent species. Today, however, anthropogenic impacts such as flow alteration by dams become more intensive, which results in woodlands expansions over the gravel bars and floodplains of Japanese rivers. In our research, we hypothesized that woodlands expansion alters the composition of riparian bird communities. Specifically, abundance of bird inhabiting gravel bars (open-land bird) decreases, while that of bird inhabiting forests (forest bird) increases.

We tested this hypothesis by comparing multiple rivers with and without dams, and the past and current states of riparian vegetation in those rivers. We conducted field survey of birds in the rivers, and constructed past and current vegetation maps using air photos. We built species distribution models which predict abundance of each bird group using vegetation areas. We estimated the past and current abundance of bird groups in each river based on the models.

The result showed that open-land birds have decreased and forest birds increased with the woodland expansion in some of the dam-impacted rivers. Meanwhile, there were very slightly changes in natural rivers. Thus, dam constructions could affect terrestrial bird community through changes in riparian vegetation.