The Tiny Acrobat of the Margins: Britain’s Harvest Mouse
Britain’s smallest rodent, the harvest mouse, is relatively rare in England and is listed as “Near Threatened” in England on the UK Red List. However Bob Cowley's examination earlier this year of barn owl pellets from Long Mead has revealed they are present in Eynsham's floodplain meadows!
Weighing just 4-6 g and only 5-7 cm long, the harvest mouse (Micromys minutus) is the only British mammal with a prehensile tail, capable of wrapping around grass stems to climb like a miniature acrobat. Its soft sandy colour makes it perfectly adapted to the grassy habitats it depends on: field margins, hedgerows, reedbeds, and rough grasslands.
Largely nocturnal and most active at dusk, it feeds on seeds, berries, and insects, making it an important part of farmland ecosystems. Its presence indicates healthy, varied vegetation - a sign that land management is benefiting more than just crops.
Status and Threats
Despite its charm, the harvest mouse’s future is uncertain. It is listed as “Near Threatened” in England on Britain's Red List, and populations in parts of Scotland and Wales are now considered “Critical” or “Vulnerable.”
According to the Mammal Society’s National Harvest Mouse Survey, one of the biggest problems is the lack of reliable data - no one knows how many remain, or how rapidly numbers are changing. Modern farming practices have reduced tall, tussocky vegetation and hedgerows, while pesticides and mechanical harvesting threaten their fragile nests. Habitat loss is the main driver of decline.
The Signature Nest
The harvest mouse’s most distinctive feature isn’t its size - it’s its nest. These delicate, tennis-ball-sized spheres are woven from split grasses and suspended among stems 10-100 cm above the ground. They’re unique in Britain, making them the easiest way to detect the species without ever seeing a mouse.
From October to March, when vegetation dies back and nests are empty, volunteers can safely search and record them. A brown, weathered nest is usually abandoned; a fresh, green one should be left untouched.
Surveying for harvest mice is simple and rewarding - no specialist equipment is needed. As described by Hayley Beck, Berks, Bucks & Oxon Wildlife Trust (BBOWT) volunteer ecology wildlife trainee, the best sites are field margins, reedbeds, and grassy verges where the vegetation remains tall and uncut.
Hope in the Margins
Every nest recorded adds to a growing national picture. The National Harvest Mouse Survey has already revealed new populations across Britain, thanks to volunteers. Continued monitoring and wildlife-friendly land management - leaving uncut margins, planting hedgerows, and maintaining tall grass - will be vital to its recovery.
Expert Dr Amanda Lloyd will join us on Sunday 16th November to help survey Long Mead for more evidence of harvest mice, do join us to learn more!
(Sources: BBOWT – “Surveying Harvest Mice”, PTES – “Harvest Mouse Facts & Figures”, The Mammal Society – “National Harvest Mouse Survey”)