Harvest Mice at Ogston Reservoir

The harvest mouse is Europe's smallest rodent, weighing only 4-6g (about
the same as a 20p piece) and measuring only 5-7cm in length. Its fur is a
reddish-golden colour on the upperside and white on the underside with a
sharp division between the two areas. In contrast to other mice it has a
much more rounded nose and smaller, more hairy ears. It also has
characteristic bright beady eyes and a unique prehensile tail, which it can
use to wrap around objects such as grass stalks in order to improve
balance.

Largely nocturnal, harvest mice are extremely active climbers, living and
feeding in the stalk zone of long grasses and reeds. Although typically
associated with cereal fields, they are certainly not restricted to this
habitat and can in fact be found almost anywhere where tall grass, reed or
sedge species exist throughout the year.

Nests, which are round and usually 5-10cm in diameter, are used for both
shelter and breeding purposes. They are built from the leaves of plants,
which are shredded lengthways and woven together to form a tight ball. The
nest remains attached to the stalks of the vegetation from which it is made
and is lined with finely shredded grass or thistle down. Whilst in use
there is no obvious entrance to the nest however once abandoned, a visible
entrance hole may be left.

Harvest mice usually have two or three litters a year in the wild, between
late May and October (or even December if the weather is mild). Most
litters are born in August. There are usually around six young in a litter,
which are born blind and hairless but grow extremely quickly. They become
independent after about 16 days but continue using the nest. A fresh nest
is built for each litter.

In autumn and winter the mice descend from the dying stalks of tall
vegetation to live at ground level. During this time hedgerows become
valuable sources of food and shelter, with nests of grass being built near
ground level.

Based on the apparent decline of harvest mouse populations across the
country, its vulnerability (particularly to adverse weather conditions and
human intervention) and on the general lack of information about the
current distribution and status of harvest mice in Derbyshire, the harvest
mouse was highlighted as a locally important species in the Lowland
Derbyshire Biodiversity Action Plan.

Derbyshire Mammal Group have been working over the past couple of years to
increase information on the harvest mouse in Derbyshire through undertaking
surveys which concentrate on looking for abandoned summer nests. Thus it
was that with the kind permission of Severn Trent Water, a group of
Derbyshire Mammal Group members descended onto Ogston Reservoir on a crisp,
clear but cold morning in November.

On the west bank of the reservoir, to the south of the Ogston Bird club
hide, there is a large area of Reed Canary Grass, which is a favourite nest
building material of harvest mice. Feeling confident we set to work,
parting the stalks to look for the characteristic nests. It wasn't long
before the first nest was found, perhaps appropriately by Ian Wildbur, a
STW volunteer ranger and Ogston Bird Club member! The nest was sitting,
perfectly formed, about 35cm above the ground and judging by its size, it
was almost certainly a breeding nest.

Things just got better from there and in total 15 nests were found, many of
which were breeding nests, all within a 100 metre stretch of the bankside
vegetation. This indicates a good breeding population on site and further
highlights the importance of the site for a wide range of wildlife.

These records are the first from this part of the county and therefore
provide valuable information in the quest to improve our knowledge of this
diminutive mammal in Derbyshire. Monitoring of the status of the population
at Ogston over the next few years will also provide important information
on population trends and the impacts of climatic variations on harvest
mouse populations.

For more information on the harvest mouse and other Derbyshire Mammals,
visit the Derbyshire Mammal Group website at
http://www.derbyshiremammalgroup.com