A project to remove non-native stoats from Orkney has boosted threatened birds and a species of vole found only on the islands, conservationists said.

The Orkney Native Wildlife Project, which organisers say is the largest stoat removal from an inhabited landscape anywhere in the world, has delivered benefits to hen harriers, rare wading birds and the Orkney vole since it began in 2019, a report published by the RSPB has said.

The stoats – first recorded on the islands in 2010 – are an invasive predator which is not native to the islands, posing a threat to the Orkney vole, an endemic species found nowhere else, and to ground-nesting birds.

Since 2019, more than 6,500 stoats have been removed, using humane lethal traps, across the Orkney mainland and island sites, and monitoring has taken place to see how the removal has affected wildlife preyed on by the invasive species.

The report says 2023 was a “great year” for the Orkney vole, with signs of activity such as droppings and grass clippings by the elusive species, which spend most of their time concealed in grass tunnels, up 200% since 2019.

Nest successes for wading birds including red-listed curlews and lapwings, as well as oystercatchers, are all higher than they were in 2019, and more than three times higher for curlews and oystercatchers, although there was a slight dip in 2023.

And threatened hen harriers, which are also ground-nesting, have seen their most successful nesting year since the Orkney native wildlife project began, the report said.

Conservationists said there are several factors that affect the nest success rates for all species, but one constant over the past five years has been the removal of the stoats from the Orkney mainland and linked islands.

Looking over the five years of data between 2019 to 2023, they said they were “confident” the project was having a positive impact on the breeding success of native ground-nesting birds and Orkney voles.

Preparations are being made for the final phase of the project, which is working with the local community in partnership by RSPB Scotland, NatureScot and Orkney Islands Council, to completely remove stoats from the islands, and the team is calling for additional funding to finish the scheme.

The accidental introduction of stoats to the islands has had a devastating effect on our wildlife, especially smaller mammals and ground-nesting birds

Anne McCall, director of RSPB Scotland, said: “This is a success story for conservation against the backdrop of a nature crisis that is pushing more of Scotland’s wildlife to the brink.

“The accidental introduction of stoats to the islands has had a devastating effect on our wildlife, especially smaller mammals and ground-nesting birds.

“Today’s report is a strong indication that our work is making a difference.

“There will continue to be native predators, disease, bad weather and many other factors that influence the survival of our wildlife, but these are largely things that our wildlife has adapted to.

“Removing the invasive stoats from the island is our best chance at maintaining this balance and giving our special wildlife the best chance to build the resilience it needs to meet these threats.”

The final phase of the project is being funded by additional cash from EU Life, the National Lottery Heritage Foundation and the Scottish Government’s Nature Restoration Fund, but she said the team was looking for additional financial support to “finish the job”.

“Through the continued support of the National Lottery and NatureScot we are over halfway to securing the funding we need and are calling on individuals to help us protect and defend the wildlife of Orkney,” she said.

NatureScot’s head of biodiversity Dr Katherine Leys added: “It’s thanks to the Orkney Native Wildlife Project’s sustained efforts to remove stoats from the island that we are now seeing promising signs of recovery among ground-nesting birds and the Orkney vole population, and we hope that these trends will continue.”