Western Morning News (Saturday)
Playing detective in the secret world of the harvest mouse
This is the time of year to look for evidence of one of our most endearing mammals, the tiny harvest mouse. Charlie Elder joins the hunt for their well-camouflaged nests
Beneath the nodding seed heads, our smallest rodent lives out of sight amid a towering forest of grass stems. Yet this time of year, as foliage dies back, offers a glimpse into the secret world of the harvest mouse.
One is unlikely to spot one of these enigmatic thumb-length mammals, but they leave evidence of their whereabouts – intricately woven nests suspended above the ground in dense vegetation.
Surveying the nests helps to get some idea of their distribution, and I joined expert Sarah Butcher on a search in East Devon.
However, they’re not always easy to find.
Sarah organises the Devon Mammal Group Harvest Mouse Project, which runs from October to March – a time when the nests are not only more visible, but unlikely to be in use.
She showed me a couple of examples of previously-collected nests, used for training demonstrations and talks, before we set off, so I had some idea of what to look for.
Females build several nests from late spring until autumn in which they raise separate litters of offspring, shredding blades of grass between their teeth and weaving them to create a snug, hollow ball. These breeding chambers, small enough to be cupped between two hands, are used only once, stretching and drying out by the time the young leave after just over a fortnight. Those nests that are still green are potentially in use and should be left well alone.
Other smaller nests, just a few centimetres across and equally well camouflaged, are also created in which individuals can sleep out of sight of predators.
With backs bent, and spaced several metres apart, we set to work checking the edges of a field in Woodbury. The nests are typically hidden within thick tussocks, and the search technique involves checking around the outside of the grassy clumps first before parting strands from above to carefully examine the inner recesses.
We had no luck, despite this having been an area with previous records, and moved on to a promising patch beside heathland at Hawkerland, a common that is part of the East Devon Pebblebed Heaths.
The light was beginning to fade and we must have searched dozens of tussocks before Sarah struck lucky and called me over.
Concealed at knee height amid ranks of vertical stalks was a spherical mass resembling a ball of yarn. This disused nest, that once cradled life like a beating heart at the centre of the dense cluster of stems, was wonderful to find.
“Lovely to know they’re here,” said Sarah. “Though you’re unlikely to spot harvest mice in the wild – they’re really tiny, and too wary and quick.”
Harvest mice (which have the delightful scientific name Micromys minutus) are undeniably cute, with gingery-brown fur, a characterful expression and a prehensile tail which helps them climb through vegetation.
They live among crops, in reedbeds, beside ditches and along field margins in the southern half of Britain and are found across the Westcountry.
In Devon, the harvest mouse nest survey has, over the four years it has been running, revealed that they are present across roughly two-thirds of the county, though are so far believed to be absent from urban areas and inhospitable high ground on Dartmoor and Exmoor.
There are gaps in knowledge about their exact distribution and Sarah is keen to encourage more volunteers to sign up and get trained to take part in local surveys this winter. “Although people think harvest mice are common, we don’t know exactly what is happening to their population numbers. The last few years’ worth of surveys indicate they could be really struggling ,” she said.
“The idea of the survey is to find out where in Devon they are.”
We search on but fail to find another nest. “One would have expected more here, but their populations do fluctuate and most
Their populations do fluctuate and most don’t live much longer than a year SARAH BUTCHER
don’t live much longer than a year,” Sarah said.
“They’re incredibly susceptible to weather extremes, such as cold winters and spells of drought.”
Around 30 nests found in a single day at one small grassland site in mid-Devon plummeted to just one nest record the following year after 2018’s freezing ‘Beast from the East’.
Harvest mice have high energy requirements and need a plentiful supply of natural foods to sustain their populations through good times and bad, which includes seeds, fruit, berries and invertebrates. They also depend on suitable tussocky habitat, where grasses, reeds and vegetation grow tall enough to enable these active
climbers to clamber about safely off the ground and to support and conceal their nests.
Gone are the days when, as naturalist Rev Gilbert White wrote in the 1700s, they “abound in the harvest”. Changes in habitat management and agricultural methods are believed to have caused them to become much scarcer, and they were listed by the Mammal Society this year as having a ‘Near Threatened’ status in Great Britain.
They are still fairly widespread in Devon and scattered records paint much the same picture in Cornwall, where the Cornwall Mammal Group encourages people to submit sightings of all mammals and describes the harvest mouse as “poorly recorded”.
Joining Sarah in a search for nests has opened my eyes to mammal life at a small scale, to the clues amid the undergrowth.
I had previously learnt to identify the presence of dormice by examining discarded hazelnut shells scattered on the ground beneath trees and hedgerows – a hole with a smooth inner rim indicating it was gnawed open by a dormouse. Now I’ll never look at a patch of tussocky grass again on a winter’s walk without wondering whether a harvest mouse nest might be hidden within.
To take part in the Devon Harvest Mouse Project surveys from this month through winter, email harvestmouse@devonmammalgroup.org. Also visit the Facebook page Devon Harvest Mouse Project, or find updates on Twitter at: @HarvestMiceDVN.
For Cornwall records visit the Cornwall Mammal Group at: https://www.cornwallmammalgroup.org/