Harvest Mouse survey
Description
The Harvest Mouse – Micromys minutus (Pallas) is the smallest rodent to be found in Britain and Europe. Adults usually weigh around 6g (unless pregnant) with an average body length of 5-7cm and a similar sized tail.
They differ in appearance from other mice by their vole-like facial features, e.g. small hairy ears and a blunt muzzle; however they are generally much more agile and mouse-like in other ways.
Adult animals are orange-brown above, with pure white undersides. Juveniles and sub-adults are recognisable by their grey-brown coats.
The small size and light weight of the harvest mouse are just two of several adaptations for life in the ‘stalk zone’.
The stalk zone is a vertical extension of the ‘field layer’, composed of tall grasses and herbaceous vegetation that fluctuates annually in height, density, flexibility, structural integrity and availability.
Other adaptations include an opposable big toe, a tendon-locking mechanism in the toes (i.e. an automatic grip) and perhaps most strikingly, a semi-prehensile tail. Using just one limb in combination with its tail, the mouse can anchor itself securely, leaving three other limbs available to move it through the habitat with ease.
Breeding ecology
Harvest mice build a spherical breeding nest that is suspended above the ground in their favoured grassy habitats. Grass leaves have parallel veins and this enables the mouse to split the terminal ends into multiple fine threads that can be woven into the fabric of the nest while the leaf base stays intact and securely attached to the stems.
The nests are found from 10cm off the ground to over a metre up in tall grasses, such as reeds (Phragmites). Most are located between 20cm and 60cm high and are typically 8-10cm in diameter.
They also build smaller, temporary shelters and winter quarters in a similar manner. These nests can persist for a long time after use and thereby provide a very useful field sign indicating their presence in an area.
The need for the grass to grow sufficiently tall to hang a nest from means that the mice start breeding later in the year than many other rodents – May or June-December with a peak in August- October.
To compensate for this and their fast metabolism, harvest mice become sexually mature by the time they are 45 days old, even sooner in captivity. Other breeding adaptations include a rapid turnaround of young. Gestation is 18 days; the young can venture out of nest after 10 days and become independent at a mere 15 days old.
It may be that regurgitation feeding of chewed seeds that has been observed in some captive mice helps speed up the weaning process. In a good year three litters of 4-5 young may be produced.
Harvest mice can be very aggressive to their own kind and will inflict damage upon each other. In captivity they may bite another’s tail off. Nests therefore, are socially distanced to minimise interaction between two females or their offspring. This distance apart varies depending on the population density in any given year.
Food and behaviour
Harvest mice are active both during the day and at night, with an emphasis on dusk and dawn. The small body mass of the harvest mouse gives a high surface area to volume ratio. In order to maintain body temperature, it has a high metabolic rate and needs to consume one third of its body weight per day.
The mouse eats a wide range of food that tends to change with the season. In spring shoots are taken; in summer, invertebrates and hedgerow fruits, e.g. hips, haws and blackberries; while small seeds, such as sedges, grasses, docks and nettles become available from late summer through autumn and winter.
Harvest mice are short-lived, with a maximum wild life-expectancy of a year and a half; however the majority fails to make it through the first few months. They are predated upon by a wide range of animals and birds, particularly raptors, and consequently harvest mouse skeletal remains are often found within regurgitated owl pellets. They can also succumb to diseases, parasites and sudden chills.
Unlike the dormouse, the harvest mouse does not hibernate but remains active all year, even in the snow. In some places a three-dimensional habitat may persist through the winter and the mice might be expected to make use of it. Where the vegetation dies down drastically or is mown away, harvest mice will re-locate and of necessity spend more time lower down, in the tunnels and runways of other rodents.
In the winter, harvest mouse numbers decline considerably. There is evidence that harvest mice are more tolerant of each other’s company in the winter and may even share nests for warmth.
Habitats
A wide range of both wet and dry grassy habitats are selected for locating breeding nests. These include crop margins, some crops, unmowed meadows, reedbeds, grassy hedgerows, ditches, grassy bramble patches, farm woodland plantations and to a far lesser extent than historically, cereal fields.
Road verges and arable field margins, especially in lowland agricultural England, including Kent and its neighbouring counties, are particularly important.
It is becoming increasingly evident that harvest mice, like many other animals, require a habitat mosaic to meet all their ecological requirements.
Foraging areas may not be the same as nesting habitats, and overwintering may occur in yet a different location.
Status and distribution
Over its global range, which extends in a broad band across temperate Eurasia, North to the Arctic Circle in Finland, as far South as northern Spain and most of Italy, and east into China with outposts in SE Asia, the harvest mouse is classified as being of Least Concern in the IUCN red list. In the UK, however, it is currently classified as near threatened (NT) in England, vulnerable (VU) in Wales and critical (CR) in Scotland. This mainly reflects a lack of good, clear data from the past 45 years. Following Wildwood’s Heritage Lottery Funded five year Kent Harvest Mouse Survey, which concluded in 2020 its status, distribution and habitat preferences are well understood in this county. You can read a copy of the report here.
A National Harvest Mouse Survey is currently ongoing, organised by the Mammal Society and co-ordinated at a local level. Volunteers can participate through Wildwood Courses by clicking here. It is hoped that at the end of this 5 year survey (2025), if sufficient geographical coverage has been achieved, a definitive statement on the contemporary distribution and status of the harvest mouse will be possible, until then most historical information should be considered outdated. The results of the Mammal Society surveys so far can be accessed on their website, here.
Threats
The greatest threat to harvest mice also looms large over many other varieties of wildlife – habitat loss and fragmentation through development.
Rapid growth of housing around the outskirts of towns, or the creation of new towns or garden villages is erasing important harvest mouse habitat in the south-east and elsewhere.
The perception that marshes – key habitats for harvest mice – are low-grade landscapes has led to them being undervalued and becoming prime targets for development.
There is still a strong link between arable farming and harvest mice even if it is no longer the original one. Harvest mice seem to have adapted to modern farming and in Kent, at least, may be in some sort of equilibrium. However, farming methods can evolve quickly and further rapid changes could easily prove catastrophic for the mice.
At a local level, inappropriately timed mowing of verges, hedge-cutting and ditch dredging has serious impacts on harvest mouse populations. With this in mind, Wildwood has produced a leaflet entitled Harvest Mice and Road Verges - Information and Guidance for Councils and Land Managers. A downloadable pdf copy is available here.
Kent Harvest Mouse Survey
Despite being a stronghold for harvest mice there are still gaps in our knowledge about the species in Kent. We continue to work with the Kent Mammal Group to discover more. In addition, our surveys contribute to the Mammal Society’s developing bigger picture in England, Scotland and Wales. Find out how you can get involved in the Kent and National Harvest Mouse Survey by clicking here.
For a high resolution version of the 2020 Kent Harvest Mouse survey full report, please click here.
Survey Aims
Given the widely disjunct nature both in space and time of Kent’s historical records, our aim was to determine the presence or absence of the harvest mouse across the county, and in doing so discover continuity of spread, as well as the mouse’s landscape use and habitat preferences in the wider countryside. By discovering the mouse in novel areas we will garner a sense of whether the species’ presence is robust, threatened or in need of reintroduction. This survey will establish an effective baseline with which to raise awareness of the mouse’s presence so that it may be incorporated into landscape and reserve management plans, environmental reports and be accounted for across the county wherever development may occur or its habitat be threatened.
