“Ukiya no Sato” in the central part of the Kanto Plain, Japan, contains a mix of agricultural lands, such as paddy fields, and semi-natural grasslands, such as wetlands, and harvest mice (Micromys minutus) are found in both habitats. In this study, we investigated the nesting habits of harvest mice and the formation of semi-natural grasslands in this region to examine the conservation status of each. We observed many harvest mouse ball nests in the paddy fields on the plains, which were a long distance from the hilly and mountainous areas. It was inferred that the wetlands helped to maintain the mouse population through geographical and hydrological factors, as well as historical factors related to rural development. It was noteworthy that harvest mice in the study area particularly used the wetland species Carex dispalata as a nesting material and that the timing of leaf growth overlapped with the harvest mouse breeding season, indicating that this species was positively used as a nesting plant. From the viewpoint of conservation of the harvest mouse in the study area, it was considered necessary to shift to various plant communities in the wetland environment by improving the site condition closer to the wet side.
