BBC Wildlife Magazine
The harvest mouse is the smallest and least known of all British rodents, mainly because it is so difficult to find. A fiveyear citizen science survey in Kent is revealing valuable new information.
“Woven from living strips of leaf blade, harvest mouse nests are beautifully camouflaged” says the naturalist. She finds out how the public are helping to record their wherabouts.
As grass-stalk zone specialists, they spend their lives feeding, sleeping and breeding without ever descending to the ground.
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.
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.
Steve began actively seeking harvest mouse nests across Kent in 2004, and recorded more than 500 of them in 10 years. He noticed that many were not in so-called ‘broad habitats’ such as fields or reedbeds, but strung out across networks of linear habitats, including field margins, ditches and road verges. It occurred to Steve that such in-between places might provide a solution to the data problem. Road verges had been largely overlooked in previous surveys, yet are – by definition – widespread across the country and relatively accessible. With the aid of Google Street View, Steve found he could scope an area from his desk and thus narrow down his search areas – and use the same method to direct other surveyors to likely locations.
He also realised that to be meaningful, the survey needed to generate a higher
resolution of data than previous efforts. The hectads often used in national species mapping are vast compared to the scale of harvest mouse home ranges and habitats. Much more useful is a tetrad, an area of 2 x 2km – four of the kilometre grid squares on a standard Ordnance Survey map.
There are 1,004 tetrads in Kent (1,100 if you count those that spill into other counties), so it became clear that monitoring the entire county at this resolution was going to require a lot of effort. And thus a citizen science project was born. With lottery funding and by harnessing the logistical and public engagement capacity of Wildwood Trust, Steve began to reach out. Over the next five years, he recruited and trained more than 600 citizen scientist surveyors, aged from 8 to 80.
Top search tactics
“We had to accept that you can’t fully standardise a volunteer, so the protocol asked simple questions, and we gave a lot of direction in terms of where and how to look,”says Steve. “The critical instructions were on how to give an accurate grid reference and the importance of a walking stick – not for health and safety reasons, but because without one you can’t part the vegetation to get a proper look without cutting your hands to ribbons on sedges and brambles.”
The volunteers were allocated survey sites in which they were asked to walk transects along suitable habitat features, such as verges, ditches and field margins, probing and looking for the tell-tale woven orbs. When nests or possible nests were found, the volunteers photographed them and sent pictures in for confirmation.
The project has transformed the harvest mouse map of Kent with a bonanza of precious data. Because volunteers were concentrated in areas of high population, the coverage was a little patchy, but results came in from half of all the tetrads in Kent and covered a wide range of landscape types. Nests were identified in 304 tetrads,
confirming harvest mouse presence in just over a third of the county. But Steve emphasised that the real figure will be much higher, because there will also be harvest mice in many unsurveyed tetrads.
On the face of it, this dramatic infilling of the map looks like good news, but Steve warns that the headline figures don’t tell us how precarious the harvest mouse populations are. “Take the North Kent Marshes, which flank the Thames Estuary. They probably represent our largest continuous area of harvest mouse habitat – but they are also the most threatened by proximity to London. The development and destruction there is just relentless, and it’s the same on the Essex side.”
Meanwhile, those marginal habitats that are so crucial elsewhere are also at risk. “An individual verge or ditch can disappear overnight without anyone even noticing.” Steve’s concern is borne out by a random repeated survey of the relatively few map squares with historic harvest mouse records. This suggests a 22 per cent decline in the species’ range, and it’s reasonable to assume that overall numbers are down too.
The results also provided valuable insights into other aspects of harvest mouse ecology. Of just over 1,000 nests identified in five years, the height range varied from 10 to 120cm above the ground, though most were found between 20 and 50cm. While many were woven from grasses, reeds and sedges, some were constructed entirely from the down of willow herb or thistle seeds, and in every year there were incidences of harvest
Nests were identified in 304 tetrads, confirming harvest mouse presence in just over a third of the county of Kent.
mice taking over and modifying bird nests. Some nests were still green into October, suggesting their inhabitants were breeding well into the autumn. Harvest mice were spotted only 19 times during the survey, reinforcing the decision to focus on nests rather than sightings of actual mice.
The five years devoted to the survey revealed a natural cycle of population boom and bust. One farm surveyed had 187 nests the first year, but on several later visits there was none, then suddenly they were back. “It’s encouraging to see how well harvest mice can respond to new and ephemeral habitat opportunities,” says Suzanne.
“But their resilience has to be dependent on connectivity. They can persist where there is a reservoir of population to recolonise cleared areas. That is where the marginal and linear habitats come in. There is an urgent need to consider the management of these superficially mundane features of the landscape.” The results have given the Kent team a clear idea of what needs to be done to shore up harvest mouse populations in the county. “It would be great to give the species some kind of protected status,” says Steve, “and also to designate its edge habitats.”
A place to call home
Protecting remaining marshlands is a priority for conservation, as is the creation of buffer zones in areas of development. Elsewhere, it is clear that management of ditches, field margins and road verges is vital in securing a network of connectivity that will make populations more resilient over a wider area. The team plans to publish advice to councils on managing verges and emphasizing the value in letting them become tussocky and rough, where it is safe to do so. This would be cheaper and much more beneficial in ecological terms.
Both Steve and Suzanne stress that further monitoring is also essential, both in Kent and elsewhere. There have been some efforts (a remarkable ecologist called John
Dobson has almost single-handedly covered the road verges of Essex, and a similar survey is ongoing in Devon), but the clearer picture emerging in Kent emphasizes the extent to which the national picture is very much unknown. Steve and Suzanne are now working with the Mammal Society and coordinating a regional mapping effort in southern England. Thousands of volunteers will be needed. Could you be one?
FIND OUT MORE
Read about harvest mice at discoverwildlife.com/harvest-mice. For details about becoming a harvest mouse volunteer, email harvestmouse@ wildwoodtrust.org. The Mammal Society is seeking volunteers for its National Harvest Mouse Survey: visit mammal.org.uk/scienceresearch/harvest-mouse-project.