The winter months provide an ideal opportunity for collecting information on the distribution of Britain’s smallest species of rodent. Weighing just six grams when fully grown, the diminutive harvest mouse is rarely seen alive and many of the records that I receive, as county mammal recorder, relate to individuals taken by pet cats. The harvest mouse is the only British mouse to build a nest of woven grass leaves well above ground level and it is these nests that can be found by careful searching during the winter months. Two types of nest are built: a breeding nest (up to ten centimetres in diameter) and a smaller non-breeding one. Both are spherical in shape and composed of grass leaves, carefully woven together from living material by the female. There is no obvious entrance hole in a fresh nest and the female may enter and leave by forcing her way through the weave. After the female has abandoned the young, usually at around 15 days of age, they will continue to use the nest for several more days. During this time, obvious entrance holes may develop as the nest receives something of a battering from the young.
As the vegetation dies back in winter, harvest mice abandon the stalk zone in which they have spent the summer searching for insects and seeds, and move down to ground level where they use runs made by other small mammal species. Here they may build temporary non-breeding nests in the base of tussocks. A few years ago, I spent time studying small mammals in an area of wet meadow and it was only during the winter months that I tended to catch harvest mice in my live-traps, positioned at ground level. These russet-coloured mice are ideally suited to a life in the stalk zone and are equipped with a semi-prehensile tail, which they wrap around stalks to steady themselves as they move through the vegetation.
Because harvest mice need long vegetation, it is worth searching for their nests in areas of rough grassland, alongside the base of hedgerows or ditches and in reed beds. The well-constructed nests often last well into the winter and careful searching of clumps of cocksfoot grass can prove rewarding. Anne Brewster, who has been recording the presence of harvest mice around her village since 1976, has now found the species to be widespread locally. The species is probably more widespread within Norfolk than the records gathered to date would suggest. So, why not check out a few likely sites in your area and let me know if you find any abandoned nests. As county mammal recorder I am keen to improve our understanding of this often overlooked species.