The mouse map
BBC Wildlife|August 2021
The harvest mouse is the smallest and least known of all British rodents, mainly because it is so difficult to find. A five-year citizen science survey in Kent is revealing valuable new information.
Amy Jane Beer

Ecologist Steve Kirk has a nose for harvest mice – or, more precisely, for the places they live. “You can be driving along with him,” says Suzanne Kynaston of the Wildwood Trust, “and he’ll suddenly shout ‘Stop the car!’ and jump out, and within minutes he’s found a nest in the verge.” It’s a remarkable skill: harvest mouse nests are notoriously difficult to find. Woven from living strips of leaf blade, they are beautifully camouflaged. But Steve insists there’s no trick to it, just a keen eye and insight honed by years of experience.

The popular image of the harvest mouse is of a tiny creature clinging to a stem of golden wheat, but the species’ natural habitat is long, grassy vegetation and reeds, such as might be found in rough pasture, scruffy margins, wetlands and ditches. As grass-stalk zone specialists, they spend their lives clambering from stem to stem – feeding, sleeping and breeding without ever needing to descend to the ground. In the days of less intensive agriculture, arable land was an extension of this natural habitat, and the mice were most often seen fleeing to the safety of field margins when crops were cut by hand – a scene described by 18th-century naturalist Gilbert White, who documented the natural history around his rural parish in Selborne.

Changes in agriculture mean crop fields are now seldom the haven they once were – and the mice that do venture into them are much less likely to survive the onslaught of a vast combine harvester. Meanwhile, many former wetland habitats have been converted to farmland or urban sprawl.

Pressure on land is particularly acute in Kent, one-time Garden of England, where intensification of agriculture and seemingly relentless pressure on land for housing, business and transport infrastructure have wrought a century of drastic change. Marshes have been drained, hedgerows removed, green space eaten away. Harvest mice were declared a Species of Principal Importance for UK biodiversity in 2006, but the designation offers no real protection and the animals are rarely given any consideration in development plans.

Historical sightings

Wondering what hope there might be for an old-fashioned mouse in a 21stcentury landscape, Steve began looking for harvest mouse data in the early 2000s. He discovered that while historical records were well-scattered, suggesting the species had been widespread, they were also incredibly thin on the ground. “Actual sightings were recorded by the county biological records centre at a rate of about two a year since 1961,” he told me. “There was a national survey in the 1970s to which Kent contributed a total of 12 records. What’s more, that survey only recorded at a scale of hectads. What can a conservationist in 2021 do with the information that one mouse was present in a 10 x 10km square nearly half a century ago? Nothing.”

To assess a species’ status, conservationists need to know where populations live and how they are faring in different landscapes. But wild harvest mice are tricky to spot and their nests hard to find; they also leave few obvious field signs. They don’t create runways through grass like ground-dwelling mice and voles, they don’t gnaw nutshells or create large caches of food, and their droppings are too small for even the most sharp-eyed ecologist to spot.

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