BBC Wildlife Magazine

AMY-JANE BEER

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.

- By AmyJane Beer

“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.

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.

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.

Nest revelation­s

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.

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 ??  ?? The harvest mouse is the only British mammal with a prehensile tail, which comes in useful when climbing among grass stems.
The harvest mouse is the only British mammal with a prehensile tail, which comes in useful when climbing among grass stems.
 ??  ?? Above: the size of a harvest mouse nest can vary from 5cm wide to 10cm for breeding. Below: this species weighs in at less than a two-pence piece.
Above: the size of a harvest mouse nest can vary from 5cm wide to 10cm for breeding. Below: this species weighs in at less than a two-pence piece.
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 ??  ?? Clockwise from above: adult harvest mice are 5-7cm in length with a tail almost as long as their body. Their average lifespan is about one and a half years; Steve
Kirk has a knack for finding nests and advocates using a stick to help; with pale yellow or ginger fur and a white belly, this species is found across the UK, south of Yorkshire.
Clockwise from above: adult harvest mice are 5-7cm in length with a tail almost as long as their body. Their average lifespan is about one and a half years; Steve Kirk has a knack for finding nests and advocates using a stick to help; with pale yellow or ginger fur and a white belly, this species is found across the UK, south of Yorkshire.
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 ??  ?? Clockwise from above: the harvest mouse mainly feeds on fruits and seeds, along with the occasional invertebra­te; look out for this species in tussocky grasslands, wetlands and farmland; the Wildwood project has come to an end, but its work monitoring and safeguardi­ng the species in Kent is set to continue.
Clockwise from above: the harvest mouse mainly feeds on fruits and seeds, along with the occasional invertebra­te; look out for this species in tussocky grasslands, wetlands and farmland; the Wildwood project has come to an end, but its work monitoring and safeguardi­ng the species in Kent is set to continue.
 ??  ?? Leaving uncut vegetation provides a welcome refuge for harvest mice. Inset: there is evidence that they are settling in at Holnicote Estate.
Leaving uncut vegetation provides a welcome refuge for harvest mice. Inset: there is evidence that they are settling in at Holnicote Estate.
 ??  ?? AMY-JANE BEER is a naturalist, writer and author of more than 20 science and natural history titles.
AMY-JANE BEER is a naturalist, writer and author of more than 20 science and natural history titles.

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