Orkney stoat eradication project boosts rare voles, hen harriers and owls

Efforts to remove invasive stoats from Orkney have given a boost to the islands’ native vole species – and one of the UK’s rarest birds of prey.

A new report shows Orkney vole activity in spring 2025 was the highest since 2019 when a project to protect the islands’ native wildlife from the invasive non-native stoats began.

It also showed the number of breeding attempts by rare hen harriers – which rely on the voles as a food source – was the highest since 2019 when the scheme began, and breeding short-eared owl numbers were also high compared with six years ago.

Conservationists said it was “fantastic” to see the impact of removing the invasive stoats from Orkney, where they posed a serious risk to native wildlife.

Stoats, which are native to mainland Britain but not to Orkney, were first found on the islands in 2010.

Conservationists warn that without removal, they could devastate populations of wildlife including the native voles which are found nowhere else on Earth.

Individual stoat food stores containing as many as 100 Orkney voles have been discovered on the islands.

Since 2019, more than 8,500 stoats have been removed using humane lethal traps in what the team behind the scheme say is the largest stoat eradication on an inhabited landscape anywhere in the world.

A new monitoring report found that activity of Orkney voles – surveyed by looking for signs such as droppings and grass clippings – was at its highest recorded level in spring 2025 for any year since the surveying began in 2019.

Across 22 sites surveyed in Mainland, Orkney and the interconnected South Isles, the islands with stoats, signs of voles were found in a third, 358, of the 1,082 small survey squares searched.

Monitoring found autumn activity by the voles was also high.

Hen harriers, one of the UK’s rarest birds of prey and a species which has a significant breeding presence on the islands, have also benefited from the stoat removal, the team said.

In 2025, volunteers from the Orkney Raptor Study Group (ORSG), found hen harriers breeding in 74 locations on islands with stoats, making it the best year for the birds since 2012.

However, wet weather at a critical time for their chicks led to high numbers of deaths, although it is thought nearly 60 chicks still successfully fledged from monitored nests, an important contribution to the overall hen harrier breeding population of the UK of around 650 pairs.

While being hard to survey, short-eared owls were confirmed at 55 sites on islands with stoats during the breeding season, with 16 places where they definitely bred and nests found at a further six places.

The conservation team said the results showed a continued increase in breeding sites for short-eared owls since 2019, when the stoat removal began.

Both hen harriers and short-eared owls rely on Orkney voles to feed themselves and their chicks, and good vole numbers are strongly linked to breeding attempts and success for the birds.

The stoat eradication is being undertaken by the Orkney Native Wildlife Project – a partnership between RSPB Scotland, NatureScot and Orkney Islands Council.

It also aims to protect other species including curlews, a ground nesting bird with a nationally significant population in Orkney, and involves detection dogs to check for the presence of stoats, as well as biosecurity efforts to prevent the invasive mammals from colonising stoat-free Orkney islands.

Matt Marsh, monitoring officer for the Orkney Native Wildlife Project, said: “It’s fantastic to see the continued positive impact that removing stoats from Orkney is having on our native wildlife.

“Hen harriers are one of the most fascinating species we monitor, as well as being one of the rarest birds of prey in the UK, so it’s excellent to see that their numbers are increasing.

And he said: “Orkney voles are found nowhere else in the world and are the species that is probably most threatened by the arrival of stoats, so to see historically high rates of vole activity is very encouraging.”

Anne McCall, director of RSPB Scotland, said: “The Orkney Native Wildlife Project is showing that we can halt and reverse species declines if we take action to address the threats wildlife is facing.

“Invasive non-native predators are a significant threat to much of the iconic wildlife found on Scotland’s islands.

“There are many factors that influence survival.

“However, to give species like the Orkney vole their best chance, removing invasive non-native species like stoats from islands is essential,” she added.

It is the latest success for an island invasive species eradication project around the UK, after conservationists announced they had successfully removed feral ferrets from Rathlin Island off the Northern Irish coast, protecting thousands of breeding seabirds and other rare birds.