By Jacky Pearce, one of our wonderful volunteers

When you’re lucky enough to volunteer for SWLT, you have enviable access to some free training opportunities, one being the harvest mouse survey training in late February.  On a cold and wet day, a room of volunteers from different sites across the Trust enjoyed the view over Roadford Lake while being taught by Sarah, from Devon Mammal Trust.

Devon Mammal Group set up a survey project four years ago because, while a protected species, harvest mouse sightings don’t legally have to be reported.  There are a growing number of projects across the country and negative results also add to the overall picture.

The first surprising aspect of the training was that harvest mice aren’t automatically found in cereal crops, as I’d previously imagined.  In fact, it seems they’ve learned their lesson from seasonal crop harvesting and are more likely to be found in the field margins or in long, tussocky grass, phragmites (common reed) or molinia (moor grass).  They’ll build nests in brambles and crocosmia too.  Anywhere a tennis ball can be hidden in the undergrowth is somewhere you could potentially find a nest. 

As harvest mice mate from March to October, the survey season runs from October to March.  You’ll know a harvest mouse when you see one; as its scientific name of micromys minutus suggests, it’s the smallest British rodent with a body length of 5-6cm and total length up to 13cm, including the tail.  Adults can weigh as little as a 2p piece.  When you Google images, you’ll see its sparsely furred prehensile tail and they’re the only mammal to have one!  The harvest mouse has a ginger body, much like a dormouse, but has a white belly, which sets it apart.  It also has an opposable outer toe on its hind feet, so it really is built to climb.

Below: Dormouse (left), Harvest Mouse (right) ©Jacky Pearce


Green nests in a tennis ball size are likely to be active maternity nests, although the female can build up to 3 nests before deciding on which one to use, and brown tennis ball sized nests are no longer in use.  The female will build a new nest for every litter.  However, golf ball sized nests are built by both males and females and these are the year-round solitary nests, so could be in use no matter what colour they are. 

These gorgeous rodents only realisitcally live for around 9 months, although can survive for twice as long.  As they don’t hibernate in winter, the cold, wet weather and starvation are what really harm them and their winter mortality rate is high.  Their range of foods such as seeds, fruit, berries, some invertebrates, butterflies and some fungi and mosses aren’t as plentiful as at other times of year either.  They also have natural predators, such as owls, stoats, weasels, corvids, foxes and cats.  Let’s not forget loss of habitat from their greatest threat: humans.

Luckily, gestation takes only approximately 19 days and they can breed several times in one year, having litters of between 3 – 8 young.  It only takes 4 weeks for a harvest mouse to reach maturity and to start breeding and these short time frames are what allow the population to continue, despite the short life span.

While a few records exist in Scotland, harvest mice prefer to remain a little further south, but not at all in Ireland.  Maybe they rode out on the backs of the snakes with St Patrick!

To make a nest, a harvest mouse will start by weaving a cup shape into upright stems, roughly around a foot above ground height (although this is changeable), bringing grass in if the nest is in bramble or crocosmia, etc.  The nest will be securely fastened and woven around the uprights so it can’t roll out, whatever the weather conditions.  If made in grass, the blades are shredded lengthways to a narrower width than the surrounding grass and this long but narrow grass is then intricately woven into a ball shape. 

In fact, the ball weaving is so good that if you pinch one piece of the nest and hold it up, the rest will still stay intact; if it falls apart when you pinch the top of it, it’s not a harvest mouse nest and the short pieces of grass that come apart easily suggest a nest more indicative of a field vole.  If the nest is the size of a tennis ball and does not squeeze without a fair bit of pressure, it could be a dormouse nest.  Nest construction takes approximately 6-8 hours in total and will be in the centre of a tussock.  They access the nest from the side and might leave very tapered droppings on top! 

Below: breeding and solitary nests (with thumb holes poked in them!) and top view of nest in grass ©Jacky Pearce


Unfortunately, efforts to help harvest mice by providing nesting boxes have failed; they will happily go inside them, but will not use them to nest.  A group in Arundel tried using Wimbledon’s old tennis balls with a hole drilled in them but these were also not deemed suitable nesting sites for the fussy harvest mice!

Looking for nests is relatively easy.  Harvest mice hide, so it needs to be in dense vegetation; if you can see through it, it’s not a good nesting site.  Can a tennis ball or golf ball fit in it?  With gardening gloves on to protect you, use both hands to part the grass/reeds/vegetation as far down as possible and look through the whole clump for spherical nests.  Try to remember which tussocks have already been searched! 

After all this theory, which was very interestingly delivered by Sarah, the majority of the group then braved the weather to do a small survey to see what they could find.  They were lucky and found a nest! 

Emma Scotney, SWLT Ecologist, on a site survey 


I’m actually stunned that, in all my years of being a conservation volunteer, I’ve never accidentally (or intentionally) strimmed a harvest mouse nest.  In future, I’ll certainly be looking more closely before I clear archaeological remains of their annual growth.

SWLT now has a survey training document based on the training given by Devon Mammal Group so if you wish to be involved in searching for this lovely little species, please do contact our Ecologist, Emma Scotney, for more informaiton. 

If you do find a nest, please email Sarah at the Devon Mammal Group ([email protected]) as well as the local site Warden and Emma ([email protected]).