Usage of Urban Rivers by Gulls and Cormorants as Movement Pathways in Winter

For the long-term survival of bird populations in urban areas, it is necessary to protect both bird habitats fragmented by urbanization and potential pathways for movement between them. However, urban pathways for waterbirds have rarely been studied. Suspecting that certain waterbird species that visit inland waters would tend to move along rivers, as movement pathways, we surveyed waterbird movement along the Kanda River in Tokyo, Japan in the winter of 2017/2018. We defined those species that very frequently (more than 95% of all flights) flew along the river as “river travelers”. Three species, Great Cormorant Phalacrocorax carbo, Black-headed Gull Larus ridibundus, and Herring Gull L. argentatus proved to be river-dependent movers that frequently used the Kanda River as a pathway. The gulls relied more heavily on the river as a movement pathway than the cormorant. In this sense, urban rivers may play a role for these species that is analogous to that of linear vegetated spaces for terrestrial birds. In addition, the distribution of river travelers (especially Black-headed Gull) may have been affected by the extent of riverside vegetation alongside, and highways covering, the river. To protect movement pathways for gulls and cormorants in urban areas, it is necessary to consider the differences among bird species in terms of their relative dependence on urban rivers as movement pathways and their comparative susceptibilities to the impact of manmade structures covering the rivers.