Abstract
The function of a multipurpose retarding basin in Kanagawa prefecture was investigated, particularly focused on the habitat for Herons. Populations of Egrets in the basin had more than the surrounding for an area, when water level in the basin. Egrets were recognized to use the basin as not only feeding place in the day-time, but also roosting place in the night-time. Egrets have been believed to make a communal roost on trees however, in this retarding basin area, they were observed to roost in the shallow water or on the marshy meadows in the basin. Even when the water level of the retarding basin was high caused by heavy rain, roosting in water or on the marshy meadows were continued.
