The harvest mouse (Micromys minitus) is Britain’s smallest
rodent, weighing only six grams when fully grown and measuring no more than 70
millimetres in length (excluding the tail). By comparison, the house mouse is
usually around 70-90 millimetres long, and the wood mouse is 80-130
millimetres.
It is therefore not surprising that the harvest mouse is rarely
seen, but loss of habitat in recent years has made this even more difficult.
Although the species is not officially threatened, active steps have been
taken, for example in Somerset,
to reintroduce harvest mice in areas where their numbers have declined
markedly. Their distribution within the British Isles is confined to England, with few being found in the far north
of England.
Apart from their size, harvest mice are easily distinguished
from other mice. They are golden brown in colour and white underneath, with
blunt muzzles and hairy ears. They are the only British mice with prehensile
tails, which are about the same length as the head and body combined.
Their breeding habits also set them apart from other British
mice in that they build nests well above ground level, among the stalks of
vegetation such as cereals, rushes or brambles, at a height of 30-60
centimetres. The nest looks like a ball of grass about ten centimetres in
diameter. It is not easy to spot during summer, but old nests stand out when
the grass has withered to brown and surrounding vegetation has died back. The
nests are built by female mice when already well into pregnancy.
Breeding takes place from late May until October, or even
later if the weather is mild, with litters varying in size up to as many as
eight. A female can produce as many as seven litters during a season, which
might be her only chance to breed because the harvest mouse is short-lived with
many only living for six months. The maximum lifespan in the wild is around 18
months.
The young, who are usually born at night, are suckled for
about nine days and then fed on seeds that have been chewed by their mother
(the male is usually ejected by the female before the litter is born). It is
common for the female to become pregnant again within days of raising a litter,
so the young mice may be ejected within two to three weeks of birth. They will
themselves be sexually mature within six weeks.
Harvest mice feed on seeds, berries, grass shoots and
insects. They are themselves preyed upon by many larger animals, such as
stoats, weasels and foxes, and by birds such as hawks, crows and especially
owls. Harvest mice tend to be nocturnal during the summer and less so during
the winter. During summer months they are therefore very susceptible to being
taken by barn owls, and studies have shown that, where harvest mice are common,
they can account for as much as 65% of the diet of barn owls.
Harvest mouse populations can suffer considerably during
adverse weather, such as a severe winter, although February, which can be both
cold and wet, is the month during which mortality is highest.
However, changes in farming methods are also a serious
threat. The use of combine harvesting and pesticide spraying has reduced
harvest mouse numbers, as have changes in the pattern of crop growing and the
introduction of short-stemmed cereal varieties that prevent harvest mice from
building their nests at a safe height above the ground.
© John Welford
