Harvest mouse

Harvest mice are Britain’s smallest rodent, weighing only 4-6 grams. They create ball-shaped nests woven from grass to sleep and breed in and love munching on a variety of seeds, berries, and insects!

Taxonomy chart

Animalia – Chordata – Mammalia – Rodentia – Muridae – Micromys minutus 

Conservation status: UK Red List

GB: Near Threatened    

England: Least Concern  

Scotland: Critical 

Wales: Vulnerable  

Global: Least Concern 

Summary

Diet: Omnivorous. Seeds, berries, insects, moss, roots, fungi, and cereal heads. 

Habitat: Tall grass and other vegetation in grasslands, hedgerows, field margins, road verges, reed beds and ditches. 

Size: Weight: 4-6g Size: 50-70mm long with a tail of 60mm. 

Lifecycle: Live up to 18 months. Breed from late May to October, having 2-3 litters a year, which are weaned after about two weeks. 

Conservation concerns: Harvest mice are listed as a BAP (Biodiversity Action Plan) Species and conservation plans are needed to reverse their decline. Main threats are changes in habitat management and agricultural methods. 

Terminology 

Prehensile tail: A tail that can grasp and hold objects. 

Geography

In the British Isles, harvest mice are mainly found south of central Yorkshire, although they do also occur in Scotland and Wales. 

Harvest mice favour a wide range of complex habitats that have tall grasses and other vegetation, including tussocky grasslands, hedgerows, field margins, road verges, reed beds, salt marshes, and ditches, as long as nests can be built.

Biology

Harvest mice can grow to around 50-70mm and are Britain’s smallest rodent! Adults weigh 4-6g. Their tails are the same length as their body – 50-70mm. 

Covered by sandy-golden/russet fur, harvest mice have a blunt nose, small eyes, and small hairy ears. They are much smaller than our other mice species and are the only mammal in Britain to sport a prehensile tail, which means it can grasp or hold objects, such as grass stalks. Harvest mice have high energy requirements due to being warm-blooded and coping with a high surface-to-volume ratio. They also have acute hearing and can hear sounds of danger up to 7m away. 

Harvest mice live an average of 18 months. They usually have two or three litters a year of six young at a time in freshly built nests, between late May and October, with most being born in August. If the weather is mild, they can have litters into December. Young are born blind and hairless but grow extremely quickly and start to explore outside by their 11th day. At 16 days old, their mother leaves, though they keep access to their nest. Cold, wet weather is a major cause of mortality for harvest mice. 

Ecology

Harvest mice are extremely active climbers and feed in the stalk zone of long grasses and reeds, particularly around dusk and dawn. They eat a mixture of seeds, berries and insects, although may also take moss, roots and fungi. Harvest mice sometimes take grain from cereal heads, leaving characteristic sickle-shaped remains, though noticeable damage to cereal crops is extremely rare. Weasels, stoats, polecats, foxes, cats, owls, hawks, crows, and even pheasants are their main predators. Acting as both an important prey species for larger animals, and a major predator of insects which can be agricultural pests, harvest mice play a vital role in British ecosystems. They are also an important indicator species for arable habitat health, so understanding where they are found can help reinforce the case for responsible farming practices and support for enhanced protections that align with that. 

Harvest mice create sleeping nests as well as breeding nests of living woven grass and are the only British mammal to do so well above-ground. Nests tend to be found in dense vegetation such as grasses, rushes, cereals, grassy hedgerows, ditches and brambles. Nests are built from leaves that have been split lengthways and woven together to form the framework of the nest. These leaves are still attached to the plant, supporting the nest by keeping it attached to the surrounding vegetation. In autumn, most nests will be brown in colour, and the entrance hole can be visible, indicating that the nest is vacant. 

Conservation

Harvest mice are classified as ‘Critical’ in Scotland, ‘Vulnerable’ in Wales, and ‘Near Threatened’ in Great Britain overall. Harvest mice are listed as a BAP (Biodiversity Action Plan) species because they are thought to have become much scarcer in recent years, and they require conservation plans to reverse the decline.   

Harvest mice are thought to be ‘Critical’ in Scotland and ‘Vulnerable’ in Wales due to changes in habitat management and agricultural methods, such as through the use of stubble burning, combine harvesting, and pesticides. These are thought to be the main cause for the loss of populations from certain areas, although there have been no reliable studies to quantify this change. However, data has historically been lacking on harvest mice, making it difficult to assess their status or agree conservation approaches. Reintroductions and releases of captive-bred harvest mice have been undertaken in several locations, such as Ealing in 2024 and Cheshire between 2002-2004. In 2021, the Mammal Society launched the National Harvest Mouse Survey - a volunteer-led annual nest survey that aims to improve understanding of the status of harvest mice across the UK.

History

It is uncertain whether harvest mice have been in Britain since the end of the last ice age or were introduced later from Europe in human cargoes of hay and cereals. 

Prior to the 14th century, harvest mice appear in the Welsh ‘Four Branches of the Mabinogi.’ In Manawydan, the ‘Third Branch of the Mabinogi’, a sorcerer places the four main characters of the Mabinogi under a curse. The curse causes all the humans and domestic animals in an area in south-west Wales to disappear, and then two of the characters cannot move away from this area. Their two other companions try to plant three fields of wheat. However, a horde of harvest mice appear and eat all their crops. It is later revealed the mice were shape-shifted humans.  

Though there are many other tales of mice from the British Isles and further abroad, it is unsure whether these are based off harvest mice or other species. 

Although once extremely common and widespread, harvest mice are thought to have become much scarcer recently, due to changes in habitat management and agricultural methods. 

Identification and surveying

Body: Harvest mice are very small, weighing only 6g when adult. Their fur is a golden-sandy brown colour with a paler underside. They are only 5-7cm long. 

Field signs: Harvest mice sometimes take grain from cereal heads, leaving characteristic sickle-shaped remains. 

Nests: Breeding nests are the most obvious sign indicating the presence of harvest mice, as they are the only British mammal to build nests of living woven grass well above-ground. Nests are usually around 10cm in diameter and are found in dense vegetation such as grasses, rushes, cereals, grassy hedgerows, ditches and brambles. They are generally located on the stalk zone of grasses, at least 30cm above the ground in short grasses and up to a meter above ground in tall reeds. Harvest mice also produce solitary or non-breeding nests, which are smaller, with a diameter of around 5cm. Nests are built from leaves split lengthways (strips of roughly 1-2 mm wide) and woven together to form the nest's framework. These leaves are still attached to the plant, supporting the nest by keeping it attached to the surrounding vegetation. In autumn, most nests will be brown in colour, and the entrance hole can be visible, indicating that the nest is vacant. 

Surveying period: October to March. 

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