The Harvest Mouse

That Wonderful Procreant Cradle

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Introduction

1 Discovery and Context

2 Micromys - Description

3 That Wonderful Procreant Cradle

4 Where Are They Now?

5 Deaths and Entrances

6 Keeping Harvest Mice as Pets

Appendix A

I found a ball of grass among the hay
And progged it as I passed and went away;
And when I looked I fancied something stirred,
And turned again and hoped to catch the bird—
When out an old mouse bolted in the wheats
With all her young ones hanging at her teats;
She looked so odd and so grotesque to me,
I ran and wondered what the thing could be,
And pushed the knapweed bunches where I stood;
Then the mouse hurried from the craking brood.
The young ones squeaked, and as I went away
She found her nest again among the hay.
The water o'er the pebbles scarce could run
And broad old cesspools glittered in the sun.

Mouse's Nest by John Clare (1793-1864)

The nest.
Construction.
Location.

The Nest

Pickling mice in brandy and rolling a nest full of babies around a table might be frowned upon today, but research must start somewhere and Gilbert White’s description of the durability of a Harvest Mouse’s nest quoted in chapter one is highly memorable.

The breeding nest is not built at or below the ground as in other mice and voles (Dormice excepted) but generally at least 300mm above the ground and made principally from live grasses. It is built by the pregnant female alone and solely for the purposes of housing and raising her litter of young though occasionally a disused nest is used for shelter.

The completed nest is a sphere and generally a bit larger than a tennis ball – about 8 cm diameter but sometimes up to 12cm. It is quite unlike any birds’ nest; Wrens and Long-tailed Tits build an enclosed nest but theirs are not spherical but shaped more like a rugby ball on end and are considerably bigger than a tennis ball. Although Wrens build theirs from grasses, there are lots of leaves combined into it and it has an obvious entrance hole. Long-tailed Tits’ nests are similar in size and shape (to a Wren’s) but composed of copious quantities of moss, and downy feathers and are generally high up in a thorny bush. Neither of these birds’ nests would be built amongst growing grasses. Many birds do build low down, close to the ground but those that do build cup-shaped nests.

Construction

The female Harvest Mouse build its nest mostly from live grass blades which it tears linearly with its incisors into several strips, whilst holding the blade with its front feet. The mouse weaves these into a loose ball once a basic structure has been achieved it works from the inside, drawing further live grasses into the nest and weaving the structure together. Another crucial difference between these and birds’ nests is that the mice incorporate the stiff, erect grass stems into the structure.

Harvest Mouse Nest

The mice need a strong vertical component in the form of grass stems (culm) or other rigid material that will support the nest. They can’t build where the grass is purely of leaf blades unless these are growing up amongst thistles or brambles.

Once the supply of live grass blades within reach of the growing nest are exhausted, the mouse will sever some from nearby and carry them up to weave them in. The effect of using mainly live grass is that the nest remains green and camouflaged for a prolonged period. The used nest on the right shows how the live grasses remain green and the severed ones have turned brown. There is sometimes a very basic lining – perhaps some thistle-down or finely chewed grass or, as in one case I found, some scraps of blue polythene. The whole process takes two to three days (though the work is usually undertaken nocturnally) or longer in poor weather. In captivity a nest can be made in one day.

Adjacent nests

The pregnant female builds a new nest for each litter and occasionally, nests can be found within close proximity indicating multiple litters by the same female. I once found a nest immediately above another - see photo on right. Some nests show no evidence of having been used for breeding; there is no lining and no hair or faeces present. This may be due to the dam becoming disturbed but probably only relates to the first litter of the year since there is little time between subsequent litters for repeat building to take place. Certainly, in captivity at least, the mother will move her family, one by one, to a new location if she feels they are threatened. Sometimes if her litter is disturbed she will eat the babies.

Location

Finding a nest from scratch in the height of summer is a challenge rarely accomplished since not only are they very hard to see at this time but there may be no certainty that a wild population exists in the search area. But in the late autumn, when the grasses are dying back and the brambles are shedding their leaves, the now brown and abandoned nest becomes much easier to find and with practice and concentration a good proportion of nests in a search zone can be located if they exist. Finding abandoned nests is by far the best way of establishing the presence or absence of Harvest Mice in the preceding season and for this reason I give some detail as to how this can best be accomplished.

Since I wrote this I have been involved with a study undertaken at a site near Nottingham in which raised traps found Harvest Mice living in reed beds throughout the winter of 2014-2015. In November 2014, I attempted to find used nests at the same sites and I failed rather miserably; I searched intensively and managed to find the nests of several intact Reed Warbler's but only two bedraggled and fallen nests of Harvest Mice which were barely recognisable.

The conclusion might be that live trapping is a better method of establishing the distribution of Harvest Mice but I would like to see whether or not Harvest Mice actually bred in the reed beds, or rather, moved in there for the winter. Another explanation might be that strong winds blowing through an exposed reed bed are more likely to dislodge a nest than in a more sheltered grassland site - though the warblers' nest survived.

Nevertheless, I am convinced that the belief initiated by Gilbert White, and perpetuated by every writer since (including me) that Harvest Mice live at ground level in winter, is clearly questionable or downright wrong.

The best time to search, varies a little from year to year: At the end of an Indian summer, when all is calm and sublime it may well be December or January that will prove best. But, if left too long and the Indian summer is followed by a deluge of rain and stormy winds, then December will be too late and October or November would turn out to be a better choice as otherwise the nest will be battered, soggy and bedraggled - though with experience, a nest finder can declare with confidence that a lump of shapeless grass was indeed, once a Harvest Mouse’s nest. If the weather remains fine and or the nest is sheltered, it may remain a perfect sphere for many months.

Harvest Mouse

Sunny days are not good for nest-finding as the searcher's pupils are dilated and won’t be able to see clearly into the shadowy depths of bramble bushes so well. Choose cloudy, still days. It is sometimes useful to have a stick (one with a hooked handle is ideal) to part the vegetation now and then, but it is not necessary to damage the habitat severely.

Having said that, grassland (the Harvest Mouse’s habitat) is prone to ecological succession and without management, scrub will invade and the habitat will be lost. So occasional management is recommended where this is an option and strimming or brushcutting invasive scrub is often an effective way of finding nests as the intact ball will be easy to spot amongst the debris. I would recommend undertaking such management late in the season as it will dislodge mice as well as nests. I've heard of conservation work parties seeing Harvest Mice in the open during their activities.

The height of the nest above ground level is largely determined by the height of the vegetation and it can be greater than one metre high in reed beds. In south Nottinghamshire, a landscape of rolling, mainly arable agriculture, I find their nests anywhere that there is dense grassland growing up amongst brambles, thorny scrub or low shrubs and at these sites the nests are generally around 30 to 50cm above ground. Such habitat exists in field margins, lane verges, inaccessible corners of rough-grazing fields, on the edge of woodlands and in country parks; almost anywhere that is not severely manicured or otherwise intensively managed. I’ve found them in Charnwood Forest, Leicestershire at about 200 metres above sea level and others have been found at up to 350m altitude, but they are more likely to find lowland vegetation to their liking and are the commonest mammal in marshy wetland habitats because of their ability to clamber around in the stalk zone. Common Reed Phragmites australis and Reed Canary Grass Phalaris arundinaceus (grasses of wetland habitats) are highly suitable for nest sites being stiff stemmed and leafy. Common Reed is sometimes unsuitable as in some conditions the reeds are too far apart. I’ve found them in quite small stands of reed on the edge of a canalised brook with only a narrow uncultivated headland amidst intensive arable fields and in corners of arable fields where building rubble has been dumped and grass and thistles have grown through. In Suffolk recently, nests were found in game cover crops (especially where these included more than one variety of millet). I am yet to find a nest in their archetypal habitat of wheat and I discuss this habitat in chapter three.

Known Distribution of Harvest Mice in Notts.

Do not expect to find them every year in the same place at the same abundance. In common with other small mammals, Harvest Mouse populations fluctuate markedly from year to year and from place to place. I have thoroughly searched sites in successive autumns without finding a single nest and then, perhaps at the third attempt, I have found them in abundance. I cannot predict which will have been good years and which will prove to be bad years. One of my study areas is Keyworth Meadow Local Nature Reserve in south Nottinghamshire and here, 2007 was an atrociously wet summer, which I expected to be disastrous for Harvest Mouse breeding success. Yet I found several nests that autumn, two of which were immediately adjacent to the access path that is regularly walked by people and dogs. The table below shows data from this site. At another site, Bingham Linear Park, I found many nests in 2005, just one very bedraggled nest in 2008 and more than 10 (with assistance) in November 2009 despite preceding storms battering the habitat.

Year Number of nests found
2001
1
2002
5
2003
None
2004
None
2005
None
2006
None
2007
5
2008
None
2009
4
2010
None
2011
1

Table 1. Nests found at Keyworth Meadow LNR 2001 - 2011

The nest is effective in keeping the young litter warm and dry, but only up to a point. Whilst light rain runs off, continuous rain will eventually saturate the nest and wet babies will chill rapidly and die. Usually the nest is in a sheltered location such as just beneath brambles and these will provide additional shelter. Clearly the very wet summer of 2007 did not wipe out the Harvest Mice at Keyworth Meadow as I had feared.

There is more about their population dynamics in Chapter 4.

Gilbert White was disbelieving that five or six well-grown babies could be attended to by their mother in the nest. Well, this does happen and yes, it must be very congested, but one of the beauties of the nest is its flexibility and its pliancy. A lot of descriptions of a Harvest Mouse nest describe the entrance hole. There isn’t one! The adult pushes her way through the wall of the nest anywhere she pleases and the “door” closes behind her. What there often is in a used nest is an exit hole; where the babies have all left by the same route when the nest is getting on a bit and the springiness of the grass is past.

Harvest Mice also build non-breeding nests, at or below ground level. These look similar to breeding nests but are much smaller at around 3 - 4 cm diameter and are generally more flimsy and without a lining. They are temporary sleeping quarters and built in crevices, deep down in grass or sedge tussocks, in old vole tunnels, amongst bales and at one time in cereal ricks. Captive mice, where live grasses are unavailable, build loose balls of chewed hay under the litter of hay and wood shavings. These resemble vole nests (which are of finely chewed grass and made at ground level – often under shelter of some kind. Refugia of felt squares or corrugated steel put out to survey for reptiles, frequently attracts nesting voles.