A quick spot check

Suffolk is a stronghold for the harvest mouse and by adopting a few simple management techniques you can help keep it that way.

• Leave areas of rough grass and cut them on a 3-5 year rotation so that there is always suitable habitat for breeding, feeding and overwintering

• If you own any land close to ponds, ditches, rivers, dykes or any other wet areas leave the surrounding vegetation to grow taller to provide nesting places

• Try to provide corridors of long or tussocky grass to link together suitable habitats

• If you are planting trees, allow the grass to become long to provide a new habitat as the trees grow

• Plant wild bird or cover crops with millet as part of the mix, it is ideal for harvest mice to nest and feed in
 

Harvest mouse

Harvest mouse by Amy Lewis

Suffolk, with its extensive lowland arable landscape and river valleys, has become a stronghold for the harvest mouse. Despite populations of the species declining by about 70% in other parts of the UK due to intensive farming, a recent Suffolk Wildlife Trust study revealed that the harvest mouse is still widespread in the county.

But despite its success, Europe's smallest rodent is still vulnerable. By careful management it's possible to give the harvest mouse  a helping hand and ensure there is suitable habitat for breeding, feeding and overwintering.

Habitat and lifestyle

Most people, when thinking of the harvest mouse, picture the tiny red-brown animal clinging to a ripe ear of wheat. Even the name ‘harvest mouse’, is an undeniable link to lowland arable landscapes. But a study done by the Trust from 2009-2011 has now shown that this classic image is probably no longer true and you would be much more likely to find the mouse in a reedbed or a clump of cocksfoot grass in an arable field margin.

The harvest mouse is the smallest rodent in Europe, weighing an average of only 5-8g which is about the same as a 2 pence piece. Its blunt nose, small hairy ears and small size distinguishes it from other British species of mice, as does its coloration of russet-orange fur and white underside. It is adapted to climb, with a very light skeleton, sensitive forepaws and a prehensile tail, which can be wrapped around plant stems to keep the mouse from falling.

Cold winter weather, along with lack of food and predation after harvest, naturally causes populations to crash in winter. It is thought that cold and wet weather causes the most harvest mouse mortality as they cannot keep warm due to their tiny size. The mice make up for this by breeding rapidly in late summer when the weather is warm, food is abundant and the vegetation thick for nest building.

Harvest mice can have two or three litters a year between late May and October and even into December if the weather is mild – although most litters are born in August and early September. There are usually around six young.

The young are born blind and hairless but grow extremely quickly and start to explore outside the nest by the eleventh day. Harvest mice usually feed around dusk and dawn in the stalks of long grasses and reeds, for which they need to be extremely agile climbers.

In winter they retreat to ground level, feeding on fallen grass seeds and sheltering in the bases of thick tussocky grasses.

Harvest mice are thought to be more prevalent in southern and eastern Britain, with populations becoming sparser in the northern reaches of England. Past studies have suggested that the harvest mouse has undergone a rapid decline of 70% since the 1970s due to changes in farming practices across the UK. This may be true in counties outside of East Anglia but following the 2009-11 Suffolk study, the harvest mouse appears to be widespread across Suffolk and possibly also the surrounding counties of Essex, Bedfordshire and South Norfolk. It is very possible that along with the barn owl, East Anglia is providing a stronghold for the species due to its extensive lowland arable landscape and river valleys.

How do you identify a harvest mouse nest?

Harvest mice will build their nests in many different habitats, the most common being rough grass often arable field margins and wet areas such as ditches, dykes, reed beds and pond or river edges. In grasses they are normally found within the tussock as little as 10cm off the ground whereas in wet habitats such as reed beds, the nests can be as much as 150cm of the ground.

The nest is built ‘in-situ’ in one night and is created by splitting and weaving leaves from the surrounding plants. A good way of identifying a nest for certain is to look for split leaves that have been pulled in to weave the nest but are still fastened to the surrounding plants. Breeding nests are the most obvious sign indicating the presence of harvest mice. The harvest mouse is the only British mouse to build nests of woven grass well above ground.

The species of grass most favoured are cocksfoot in dry areas and reed canary grass, reed sweet grass and reed  in wet areas. Although these species are commonly used to build nests, any stiff stemmed plant with leaves that can be split lengthways can be used. Harvest mice cannot breed in thin vegetation (generally where it has been cut in the last 2 years), short grasses, or those where the stems are too weak to support their nests. The nest can vary from 10cm in diameter for breeding nests to only 5cm for non-breeding nests.

Nests tend to stand out more in winter when the surrounding vegetation dies back. They can be found from October (but watch out for late litters from Oct-Dec) right through to March when the new growth makes it difficult to find the nests.

Shelter nests are also built which are generally smaller and looser. Watch out for field vole nests which are looser and built on the ground under tussocks of grass – if in doubt take a picture and send it to Suffolk Wildlife Trust for identification.

How you can help the harvest mouse

• Leave areas of rough grass and cut them on a 3-5 year rotation so that there is always suitable habitat for breeding, feeding and overwintering. Margins of arable fields, paddocks, wildflower meadows and even large gardens can provide good habitat. Aim for a margin that is at least 2m wide or ideally even wider.

• If you own any land close to ponds, ditches, rivers, dykes or any other wet areas leave the surrounding vegetation to grow taller to provide nesting places for harvest mice.

• Try to provide corridors of long or tussocky grass to link together suitable habitats so populations do not become isolated.

• If you are planting trees allow the grass to become long to provide a new habitat as the trees grow. Plan into your area clearings or margins where the trees will not shade out the grasses and keep it as a long grass habitat.

• Plant wild bird or cover crops with millet as part of the mix, as well as benefitting farmland birds, it is ideal for harvest mice to nest and feed in.