The pine marten - declared extinct in England a decade ago - is making a comeback in North Yorkshire, wildlife experts believe.
'Credible' sightings have prompted efforts to track the pine marten
New sightings on the edge of the North York Moors have prompted the Forestry Commission to set out feeding tubes to collect hair samples for DNA analysis.
They hope that bait - including the mammal's favourite jam sandwiches - will lure the animals into the tubes.
Once common in England, the animals were driven away by Victorian trappers.
The Forestry Commission has teamed up with the Moors National Park, Hull University and local conservationists to find conclusive evidence that pine martens have ventured south from their refuge in the Scottish highlands.
Brian Walker, Forestry Commission biodiversity officer, said: "My gut feeling is that we do have pine martens in this part of North Yorkshire.
"Over the years we've had many sightings, some cases of mistaken identity, but others very convincing.
"One of these came in July when an experienced ornithologist and wildlife photographer saw a creature matching the description of a pine marten."
If their presence in Yorkshire is confirmed, forestry workers will attempt to manage woodland to suit the ferret-like animals' needs.
As well as 100 feeding tubes, they have erected 10 den boxes in an area of forest near Osmotherley to encourage breeding.
The efforts have been prompted by what the Forestry Commission say are "highly credible recent sightings" by local wildlife experts.
It is believed the animals have returned because large areas of forest planted after the First World War have now reached maturity, providing new habitats which had been lost.
Johnny Birks, of the Vincent Wildlife Trust, said: "I'm reasonably confident we do have martens in the Moors, but it is like looking for a needle in a haystack.
"Clearly numbers are small and the creature may be clinging onto existence."