Spotlight on... Harvest Mouse

article written by Ellen Lee, TVERC Biodiversity Data Manager

Try Googling “harvest mouse” and what do you get? The answer is a whole heap of cute photos, most of which are set up using captive animals. Given their obvious popularity, it’s a bit surprising then how little is known about the life of a harvest mouse, its population and distribution in Britain. There are a number of reasons for this, linked to their lifestyle and the difficulty of observing them in the wild. I trained up to do harvest mice surveys in 2017 and since then I have seen precisely one live animal, and that was a sickly specimen with a large, well-fed tick on its head. Presumably, it was not long for this world. What I have seen are plenty of nests. Searching for nests is the best way of recording  their presence. This can be done from late autumn through the winter and involves manually checking through their preferred habitats: hedgerows, rough grassland, arable fields, reed beds etc. I’ve been involved (in my spare time) since 2017 in organising a group of volunteers on the RSPB reserve at Otmoor to carry out annual surveys, mainly concentrating on the area around the reed bed. I find it interesting and fun, but it does take a certain kind of person to spend several hours poking long grass with a stick with no certainty of success. On average, we find one nest every 1.2 pph (person poking hours!). However, when you do find a nest, it’s a magical experience, parting some grass to find this small, miracle of animal engineering nestling within.

So, what is known about harvest mice? For a start they are the UK’s smallest rodent. An adult weighs in at about 8g and is roughly 5.7cm from nose to base of the tail. The tail is about the same length again. Its fur is usually described as a russet colour and it has a clearly demarked white chest patch. During the summer it lives mainly in the stalk zone, the female building breeding nests which are typically 6-10cm in diameter (tennis ball to grapefruit sized), woven dextrously from live grass and typically around 30cm off the ground. In the winter both sexes retreat to ground level and build smaller, winter nests which they may wait out particularly cold spells, emerging to feed when the weather improves.

They have several interesting physical adaptations, making this kind of existence possible. Firstly (and most noticeably) they have a long, prehensile tail which they use to balance when climbing and as a brake when descending. Less obviously they have outer toes on each foot that are more or less opposable, so they can grasp, making weaving nests (for example) possible. Their ears are thought to be adapted to hearing low frequencies that carry over a distance, assisting predictor avoidance. They also have one very important behavioural adaptation which is in their food selection. They preferentially eat seeds which are high in nutrition and low in water, meaning that they need to eat both fewer calories and a lot less weight of food than a similar sized insectivore, a common shrew. The latter needs to eat its entire weight in food every day, whereas a harvest mouse only consumes 30% of its bodyweight. Calorie wise, a harvest mouse consumes around the same number of calories in a day as a wood mouse which is over twice its weight. All this means that it doesn’t need to feed constantly and can sit out bad weather in its nest, conserving energy further.

Despite this, harvest mouse populations are thought to have declined some 70%  or more in recent years, mainly due to habitat loss. As they fare best in dry summers, the fact that climate change will probably, on average, lead to wetter summers in the UK is an ever-increasing threat. So, monitoring their population has never been more important. Indeed, The Mammal Society have launched a national survey this year and you can read more about this here.

TVERC’s database bear’s out the story of under recording. Below is a distribution map of all TVERC harvest mouse records by 1km square. The square with the most records includes the BBOWT reserve of Chimney Meadows which has had a long running recording programme. Last summer, TVERC shared the harvest mouse records it holds with the Mammal Society to help give them a baseline prior to their national survey and we look forward to receiving records for Berkshire and Oxfordshire at the end of the survey in exchange. Hopefully then our map won’t look quite so sparse and there will be a better understanding of the distribution at a national level.