As an idealistic PhD student, Wendy Fail’s ambition was to reintroduce harvest mice to Northumberland. She painstakingly bred 240 mice in captivity and in 2004 released the elusive mammals on to a coastal nature reserve with plenty of reedbeds for them to hide in.

When not a single harvest mouse – Britain’s smallest rodent – was recaptured in subsequent trap surveys, Fail concluded that her efforts to reintroduce them had been unsuccessful.

Now, 15 years later, fresh harvest mouse nests have been found for the first time at East Chevington nature reserve, showing that the descendants of Fail’s original harvest mice are in fact thriving.

A team of volunteers found two distinctive, orb-shaped nests in dense reedbeds close to where Fail had originally released them. Sophie Webster, of Northumberland Wildlife Trust, who led the search, said: “It was really exciting for everybody. It’s such a big area to search we doubted we would actually find them so it was a real treat.”

The nests were woven into grasses a metre (3ft) above ground and contained green leaves showing they were freshly made. Webster is planning a more extensive search of the wider reedbeds, with camera traps and live mammal trapping to see if they can calculate the population.

Fail, who now works in outdoor education, was contacted by one of her original team members with the news. “To say I’m ecstatic with this positive result is an understatement,” said Fail. “I’m proud that my work, along with the support of volunteers and experts, has managed to secure the survival of one of Britain’s declining species.”

She also emailed her old PhD supervisor. “I told him, ‘This is the email I always wanted to send you’ and he replied, ‘Bloody hell, it worked!’ He was as surprised as I was.”

A priority species for conservation and protected by law, the harvest mouse is the weight of 10p piece and is Britain’s only mammal with a prehensile tail, which it uses for grip and balance as it lives in the stalk zone of tall plants.

Preyed upon by barn owls, cats and even pheasants, the mouse also suffers in cold, wet weather. Historically, it is more common in southern and eastern England than in the north. It is rarely found north of the Tyne, although recent surveys have discovered it on several sites in County Durham.

Fail’s original studies found harvest mice were vulnerable to being predated by wood mice when they were first released and so the 240 captive-bred harvest mice were first released into secure cages to acclimatise.

The mice were released incrementally into the wild over the course of eight weeks to avoid saturating the site or endangering all the mice at once.

Follow-up survey work using specially adapted traps, which capture small mammals without hurting them, caught plenty of wood mice, shrews and even an angry weasel, but failed to find any trace of harvest mice, despite successfully catching them in trials.

Fail said she hoped her phased return methodology could be potentially used to help other reintroduction projects, now it had been shown to be successful.

She added: “I’m not saying we’ve changed the world but I hope that what we’ve done gives other people faith and hope that it is possible to conserve a much-loved species with a bit of hard work and dedication.”

Mike Pratt, chief executive of Northumberland Wildlife Trust, said: “We’re really pleased and encouraged that in these days when we think we know everything about everything because we’ve got so much data, we can still find something that we didn’t know was there. We know wildlife is suffering terrible declines so this is refreshing to say the least.”

Last year, Northumberland also witnessed the return of the pine marten in the wild for the first time in 90 years.