The harvest mouse (Micromys minutus) has never been the most prominent
of British mammals. In fact, there is evidence to suggest that it has
been overlooked by naturalists for centuries. Gilbert White first identified
the harvest mouse in 1767, when he stated its apparent 'abundance at
harvest'. Historically, this species has been associated with traditional
agricultural landscapes and linear landscape features, such as ancient
hedgerows. Changes in habitat management and agricultural methods are
thought to have caused a reduction in abundance, although there have
been no studies to quantify this change.
Tall vegetation
Whilst the status of the harvest mouse in Britain remains largely unknown,
recent studies have drawn attention to a dramatic reduction in range
in the last 25 years. They inhabit numerous habitats, although they
are known to favour areas of tall vegetation and grass where they can
make their nests.
Harvest mice are thought to be more prevalent in southern and eastern
Britain, with populations becoming more sparse in the northern reaches
of England, with no recorded remaining populations in Scotland. In Suffolk,
records are thinly scattered across the county. It is therefore vitally
important that information is collected on where this species currently
occurs so that we can help to protect them effectively in the future.
A relatively easy and effective way of doing this is to record the location
of harvest mouse nests.
Distinguishing features
The harvest mouse is one of Britain's smallest mammals, weighing an
average of only
4-6g and having a body length of only 50-75mmm. Its blunt nose, small
hairy ears and small size distinguishes it from other British species
of mice, as does its colouration of russet-orange fur and white underside.
Harvest mice usually have two or three litters a year between late
May and October and even December if the weather is mild - although
most litters are born in August. Cold wet weather is a major cause of
mortality. There are usually around six young in a litter, which are
born in the carefully woven grass nests. The young are born blind and
hairless but grow extremely quickly and start to explore outside the
nest by the eleventh day. A fresh nest is built for each litter.
Harvest mice usually feed around dusk and dawn in the stalks of long
grasses and reeds, for which they need to be extremely agile climbers.
Their small front feet have toes adapted for gripping and their tail
is used to wrap around stalks for stability. They have many predators
including weasels, stoats, foxes, cats, owls, hawks, crows and even
pheasants. Understanding more about harvest mouse distribution in Suffolk,
as well as increasing grassy habitat and habitat links, should help
to ensure this endearing little mammal doesn't disappear without trace.
How do you identify a harvest mouse nest?
Nests are generally found on stalks at least 30cm above ground in short
grasses and up to a metre in tall reeds. The two species of grass most
favoured are cocksfoot (Dactylis glomerata) and reed grass (Phragmites
arundinacea), but any stiff leafy grass may be used for nest building.
Harvest mice do not favour thin grasses (where there is not enough cover
for their nests), short grasses, or those where the stems are too weak
to support their nests. If the supporting plants form tussocks of leaves,
the nest usually sits on top of the tussock between the flowering stems,
or under the overhanging leaves. In this case, the nest will only be
about 10cm above the ground
The nest can vary from 10cm in diameter for breeding nests to only
5cm for non-breeding nests and tend to be found in dense vegetation
including areas such as grasses, rushes, cereals, salt marshes, grassy
hedgerows, bogs, marshes, scrub, ditches and brambles.
Part of the key to identifying the nest is to know how it is built.
Nests are built by pregnant females, usually at night, who shred lengths
of grass which are still attached to the stalk with her incisors, whilst
sitting on another stalk, supported by her hind feet and tail. Having
shredded a large number of leaves this way, they are woven together
to form an outer framework for the nest. The nest is then lined by pulling
further grasses through the wall until it is densely woven without leaving
an obvious entrance. Nests tend to stand out more in winter when the
surrounding vegetation dies back.
Breeding nests are the most obvious sign indicating the presence of
harvest mice. The harvest mouse is the only British mouse to build nests
of woven grass well above ground.
Ways you can help the harvest mouse
If you own land, try to provide green corridors that link habitats.
Recent surveys indicate that harvest mice are more likely to occur in
habitats that are interconnected and not isolated from other suitable
habitats by distances of over 50m. Wide field margins are beneficial
and areas of long vegetation in suitable areas should be left uncut
where possible.
