The harvest mouse is seldom seen in the North East, but a recent survey by the Durham Wildlife Trust found nine new records.
This was more than was seen in the whole of the 20th century and the first in Teesdale for 40 years.
Vivien Kent, conservation officer at Durham Wildlife Trust, said: “A national survey in the 1970s reported only a handful of harvest mouse records in the North East and there have been very few since.
“These new records represent more than were reported in County Durham during the whole of the 20th century.”
The tiny mammals are on the northern edge of their distribution in the North East and are mainly found from central Yorkshire southwards.
Their favoured habitats are tall grass in areas such as cereal farmlands, road side verges, hedgerows, reed beds, dykes and salt marshes where nests can be built.
Ian Bond, from Northumbria Mammal Group, who also took part in the survey, said: “Research by Northumbria Mammal Group over the past decade changed our view of harvest mice in the North East by indicating that they seem to be quite widely distributed across the Tees Valley.
“However, this survey has revised that picture again with new records showing that they are also present across East Durham, with isolated records in Teesdale and Upper Weardale suggesting that they might be much more widespread than anyone had imagined.”