Harvest mice have bouts of activity throughout the 24-hour period, but they tend to be more active during the evening and night (5). In summer they become increasingly nocturnal, whereas during the winter they are more active in the day (5). They are adept climbers, and typically feed up in the stalk-zone of long reeds and grasses (2). Depending on the time of year, harvest mice feed on grass seeds, cereals, berries, insects, fruits and the young shoots of grasses (5).
The nests of harvest mice are the most complex structures made by any British mammal (3). These spherical nests, constructed by pregnant females, are made of woven grasses and may measure up to 10cm in diameter (2). They are located up to 1 meter above ground in grasses or reeds (2). Breeding takes place between May and October, and when the weather conditions are suitable, they may even continue to breed until December (5). Between 3 and 7 litters are produced a year, each consisting of 1-8 young (5).Births usually occur at night. The female suckles the young until they reach around 9 days of age, at which time they are given their first solid food in the form of chewed seeds (3). When the young reach around 18 days of age, the female may become aggressive towards them, ejecting them from the nest. Upon reaching 6 weeks of age, the young will be able to breed (5). Very few harvest mice live beyond 6 months, although the maximum recorded lifespan is 18 months (5). Main causes of mortality are cold or wet weather, sudden frosts (5), and predation by weasels, stoats, foxes, cats, owls and crows (2).