The small rodents, which weigh approximately the same as a tenner, make their nests on the ground in tall, tufted grass and reed beds.
Harvest mouse nests have been discovered in a second Northumberland nature reserve.
Some of the mice released at the East Chevington Conservation Area in 2021 are thought to have moved a mile away to the Druridge Pools.
The Northumberland Wildlife Trust called it a “great indicator” that the small animal population is growing.
The small rodents, which are listed as a near-threatened species, create circular grass nests a meter above the ground.
The trust’s Catch My Drift initiative included the 200 mice abandoned on the East Chevington Reservation two years ago.
Since then, 36 nests have been discovered on the 185-acre property, 13 of which were discovered last fall, more than a mile from the other nests.
The mice, Britain’s tiniest animals, were brought to the region for the second time after 240 were released in the same location 18 years earlier.
Earlier this year, volunteers undertook a “extensive search” in the region surrounding the East Chevington reservation to check if the mice had travelled outside the boundary.
They discovered two nests at Druridge Pools, which provide the same environment as harvest mice – tall tufted grass, reeds, decent scrubland, and nesting hedges.
The nests discovered at Druridge Pools are the first indication that harvest mice moved from East Chevington.
“Finding the East Chevington nests a mile distant from the others on the reserve moved us on to investigate further afield,” said Sophie Webster of the Wildlife Trust.
“Harvester mouse numbers have been falling across the UK over the past 40 years and are now relatively rare.
“The fact that we now have nests in two sites is a fantastic indication that they are not only reproducing but also utilising habitat corridors to grow and establish populations in the region.”