Harvest mouse

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