Dormice are thriving in Lincolnshire thanks to a scheme to reintroduce the mammal into the wild.
Several pairs of dormice were released into a small area of Chambers Farm Wood, near Wragby, four years ago.
After breeding, they have moved into a wide area of coppiced woodland which is ideal for their needs.
Ecologist Anne Goodhall, who works for ecological consultancy ESL, said dormice were a good indicator of the environment's health.
"A woodland like this - that can support everything right down to dormice - has all the things that we're looking for in a wood," she said.
"We're delighted with the way our dormice are thriving. We have something like 1,000 acres here that they can spread into.
"As we go around trying to put biodiversity back, dormice are an indicator that we've got it right."
Common dormice may spend up to three quarters of their life asleep. They hibernate to conserve energy when food is scarce.
They are distinguished from other mouse-sized mammals by their thick, bushy tail.
They live in deciduous woodland with scrub, coppiced woodlands and hedgerows. A loss of suitable habitat in Britain has led to a steep decline in their numbers.