Onuma is a brackish marsh of approximately 10km in perimeter with a maximum depth of 1.5m. It is surrounded by wetland and approximately 5,000 whistling swans a day fly in during the migration seasons between March and May and October and November, enabling visitors to observe over 100 species of wild birds throughout the year. Seasonal flowers including skunk cabbages can also be appreciated.
Onuma Bird House has an observation room and an exhibition area within the building and visitors are also welcome to take a break here. The area around the facility is designated as a natural landscape protection district as well as a wild life sanctuary and harming wild birds in any way is strictly prohibited.