Harvest mouse remains overlooked

The other day a friend was telling me about a recent encounter with some harvest mice, his first sighting of these overlooked mammals in thirty-odd years of watching wildlife. Two of these tiny creatures were clinging to some reeds in an area that had been inundated with floodwater and he was struck by their small size and by the delicate way in which they used their tails to secure their hold on the reeds.

The harvest mouse is a mammal that I know well, having once kept and successfully bred them in captivity. They are amazing creatures, far more delicate than their larger relatives (like wood mouse, which I have also kept in captivity) and with a lot of charisma. It is difficult to know how common they are across the county, their small size and arboreal habits make them difficult to trap using conventional live traps, and I suspect that they are easily overlooked. Most records will either come from cat owners, whose pet has delivered an unfortunate harvest mouse to its owner, or from those who encounter the cricket ball-sized nest in a field margin or reedbed setting. Other records come from the pellets cast by hunting owls.

At this time of the year harvest mice may still be giving birth, the first births beginning in late June and the last extending into October or beyond. Cold wet weather at this time of the year can be a particular problem. The nest into which the young are born is up to 10cm across and woven into the vegetation. The young mice will use the nest for 2-3 weeks, the female abandoning them at 16 days if she is pregnant with her next litter, and the nest then becomes increasingly battered as the youngsters explore their growing world and gain independence.

I used to find harvest mice would enter my live traps, set for field voles on a wet meadow, from November through into March, suggesting that at this time of the year they spent more time on the ground. It was always a thrill to see the russet-orange of a harvest mouse drop into the polythene bag into which I emptied the contents of any trap that had been triggered. Once released on a stem their agility could be appreciated and, if I sat very still, they would sometimes sit and groom, balanced by tail and a single foot alongside the vertical tube of plant stem. It was these encounters that probably made the harvest mouse my favourite mammal, topping even the dormice that I had studied in Sussex several decades ago. It’s been a while since I saw a harvest mouse so maybe I should go looking for one.

Keep an eye out for our smallest mouse

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.