According to the Wildlife Trust, harvest mice live in long, tussocky grassland, reed beds, hedgerows and around woodland edges.
They struggle to live in thin or short grass, as the stems are too weak to support their spherical nests which they create from tightly woven grass, elevated from the ground in tall grasses.
A monitoring programme has now been set up with Yorkshire Mammal Group to understand the spread of the mouse population on the site.
Breary Marsh is a Local Nature Reserve and Site of Special Scientific Interest, which borders Golden Acre Park in north Leeds.