Making room for Micromys: Advice for helping Harvest Mice on your land

Image: Derek Crawley

I’m coming towards the close of the harvest mouse survey season, and by far one of the best parts of the job has been seeing the sheer enthusiasm for the species in the volunteers and colleagues I’ve worked with. Whether it has been meeting our captive ambassador mice (the first they have seen the animal for many, and a surprising realisation of how small they are), or delight at uncovering a nest in a place they were least expected, there’s no doubt that it’s very difficult for harvest mice not to win people over.

As a result, this has lead to a common recurring question, particularly among those who own or manage land; “What can I do to help harvest mice?” More than once during this job, I have met people who had a sudden pang of guilt when they realised they had or were going to start cutting away a prime piece of harvest mouse habitat under their stewardship.

Unlike the far more strictly protected dormouse, there is little published work or legislative impetus for managing land with harvest mice in mind. In addition, some of the best breeding habitat for harvest mice is commonly regarded as ‘messy’ by land managers and is earmarked for cutting back or grazing off. One volunteer notably remarked at a training day that cock’s foot grass, one of the best species for harvest mouse nest building, was regarded by their colleagues as ‘the enemy’!

Unfortunately, management beneficial for other target species is not always great for harvest mice. Butterflies are notorious for this, and what is regarded as brilliant for rare species that love short swards can be anathema to the mice!

Luckily, we have made some of the people we work with think twice about cutting after the discovery of harvest mouse nests, and pushing this educational-advisory work is something we’d like to look at in the future. For now however, here is my key advice for all land managers who want to keep the little guys in mind. It is entirely unofficial and based on what relatively little we know about harvest mice and my own observations rather than any peer-reviewed research, but every little helps!

Learn the key nest species

As long as it is dense enough to weave a nest around themselves and the stem is relatively stiff but can be split lengthways, harvest mice don’t seem to be too fussy about which grasses or reeds they weave breeding nests from. However there are a few key species that are far more likely to be used. Cocks foot, reed canary, purple moor, reed sweet-grass and common reed are all fairly typical, so ensuring you have thick, continuous clusters of these species will increase your chances.

Examples of nests in (clockwise order) Reed Canary, Purple Moor Grass (Pete Cooper) and Cock’s Foot (George Hyde).

 

Cut as late as you can

In a triumph of common sense, harvest mice are generally breeding when warm temperatures and plentiful food means it’s sensible to do. Generally this means April to late September/early October, but in very mild years they can go all the way through to the start of December.

This means that by the traditional start date for cutting of 1st September, successful harvest mouse mothers might still be able to push out a litter or two, while mice born earlier in the season will be getting on with rearing the next generation already. Holding back until mid-Autumn once mice have dropped down closer to the ground may leave some with itchy feet, but the critters will thank you for it.

Spare and rotate areas of land for cutting

Nature is a dynamic process, with different species occupying unique niches that are created through variation in the landscape. This is true even on very small scales, and ensuring that some grass is left uncut, or is only done so every other year, can go some way towards this.

Theoretically at least, by adopting this instead of an ‘all or nothing’ policy, you can have both your harvest mouse and pearl-bordered fritillary cakes and eat them! (Actually, don’t do that. PB Frits are protected for a start.)

Steady on the grazing

In places where I’ve not found harvest mice, over-grazing seems to have been a significant factor. Purple moor grass tussocks, normally perfect for nests, are nibbled down right to the stump by ponies. At one wildlife trust reserve managed particularly for rare butterflies (them again), I may as well have given up when I saw it was grazed over its entire area by sheep, which are essentially a living lawnmower.

Purple moor grass tussocks that have been overgrazed to the stump, and no longer have any use as breeding habitat. (Pete Cooper)

While a full-blown harvest mouse recovery project would be recommended not to utilise grazing, that’s not to say they’re entirely incompatible. It is however a very finely tuned balance of stocking, timing and scale. As few animals as possible, over a large area and a limited window of time will give the tussocks for nest plants the best chance of survival.

Much like in the previous step, ensuring there are areas where stock are fenced off will allow the dense vegetation required by harvest mice to grow, so long as these aren’t isolated island-like within a wider grazing zone. Sometimes natural exclusion zones are created by thorny vegetation. In one wet heathland site I surveyed grazed by ponies, a thick cluster of gorse protected non-grazed purple moor grass that were full of nests.

Create new habitats

Of course one of the best things to do is to create entirely new harvest mouse friendly habitats. Bigger is better, so if you have a field or several with no idea what to do with it, why not allow it to grow wild? Provided it is well connected to the surrounding landscape (such as through hedgerows or stream banks), harvest mice are likely to find it quickly enough, and these large swathes of habitat are increasingly rare for an animal often limited to field margins and verges. Harvest mice will also prosper in reedbeds, culm grassland and heathland to name a few examples, so if you have the right kind of ground for these habitats they are all worth trying.

One of the easiest things to do is maintaining field margins that are rich in dense, tall tussocky grass. Ideally these should be at least six feet in width, and while these are cheaper to keep in arable situations, can be used in grazing situations by fencing off the required section.

While we are discussing management for harvest mice, in nature nothing should be considered in independence, and of course the proper, ‘messy’ grassland harvest mice is appreciated by so many other flora and fauna trying to hang onto our countryside. Some of the best field margins for harvest mice I’ve seen was created as a wild food source for the nationally rare Cirl bunting. Likewise, if you decide to create, spare or alter the way you look after your land, rest assured it’ll be more than just harvest mice thanking you.

Pete Cooper

Devon Harvest Mouse Project Officer

Image: Pete Cooper

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