First harvest mouse found on Scottish islands 'may have been stowaway'

The abandoned nest of the creature, a critically endangered species in Scotland, was discovered by Anna Martlew, a volunteer with the Mammal Society, while walking on the isle of Mull last month.

The find is the first ever record of the species on any Scottish island.

Experts believe that harvest mice may have recently been introduced to Mull by accident. One theory is that the mouse was a stowaway in a delivery of hay from the mainland.

The harvest mouse (Micromys minutus) is Britain’s smallest native rodent, and it is known to be present across much of England and Wales, where it is considered to be an indicator of healthy ecosystems.

Though it was assumed in the past that harvest mice were not present further North than Southern Scotland, surveying in recent years has revealed unexpected populations present in the Highlands.

The harvest mouse was thought to be absent from all Scottish islands until this discovery.

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Rob Raynor, NatureScot Mammal Specialist, said: “Harvest mice are critically endangered in Scotland and so this discovery on Mull is very exciting for the species.

“It’s important we build up an accurate picture of how this rare and tiny creature is faring, so we encourage people get involved with this year’s national harvest mouse survey in October.”

Ms Martlew has extensive experience of surveying for harvest mouse nests.

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She was responsible for discovering evidence of harvest mouse presence in the Highlands of Scotland, when she found a nest in Glenfeshie in the Cairngorms in 2022.

Accurate data on where harvest mice are present is vitally important to inform conservationists and land managers looking to make informed decisions that support nature recovery.

Due to their small size and elusive nature, harvest mice are difficult to spot in the wild.

Their spherical nest made of woven grass - where female harvest mice give birth during the summer - are the best indicator of their presence.

By October, these nests are left behind and can be found through a careful search of potential habitat such as tussocky fields, reed beds and hedgerows in the winter and early spring, without fear of disturbing harvest mice families.

Ms Martlew’s find was confirmed by Derek Crawley, vice chair of the Mammal Society Council, who is also a verifier of mammal records on iRecord, the tool used for collecting records of wildlife sightings.

He noted key identifying features such as splits in the grass blades, which are cut by harvest mice lengthwise as they weave the tennis ball sized nest in which they will give birth to up to seven young.

While the stowaway theory is being considered, another possibility is the mouse was brought deliberately to Mull by someone who breeds them in captivity.

Releases of animals to the wild outside their native range in Scotland is illegal without a ‘non-native species licence’ from NatureScot.

Mr Crawley said: “Finding signs of harvest mice takes some expertise, as their nest blend into the surrounding vegetation.

“In recent years the Mammal Society has championed the search through its annual volunteer-led survey for harvest mice nests, and have been finding evidence of presence of the species across a wide area in Scotland, but this is the first record on one of the islands. We need to find out just how widespread they are on Mull, and this will be a focus for the next survey season starting in October 2026.”