Dormice Dozing (Part 1)

Whilst the trees in the woods appear to be standing still, waiting for spring to arrive, buried within the leaf litter one of our most iconic native mammals has also put its life on hold, until spring arrives.

The size of a thumb, our native hazel dormouse, Muscardinus avellanarius, is a mostly arboreal woodland species that hibernates at ground level in a nest made from local materials during the winter months. Photoperiod – the ratio of light to dark in the day cycle is thought to trigger hibernation, although individuals have periods of torpor at other times and may wake during hibernation to feed and move to a new nest location. The winter nests are very difficult to locate – different methods have been tried and the skill and the experience of the surveyor is the most important factor.

Dormice are only monitored during their ‘active’ period – this is a sleeping dormouse found in a nest box during the monitoring season from April to October

The UK’s hazel dormice population is distributed to the south of a rough line between Shropshire and Suffolk (except for a population in the Lancashire area) and are completely absent from Ireland and Scotland. It is a flagship species for habitat fragmentation, meaning it is used as an ambassador, to draw attention to the conservation issues surrounding fragmentation of habitats. There is a responsibility within the National Nature Reserve to maintain and enhance the dormice population, as the species are noted within the reserve’s Special Site of Scientific Interest designation.  The dormice population within the reserve have subsequently been monitored in different ways, and for differing reasons over the years, by both Natural England and the Woodland Trust.

Dormouse being returned to box after physical checks

The National Dormouse Monitoring Programme (NDMP), coordinated by the People’s Trust for Endangered Species (PTES), is one of the largest running national programmes with 400 registered national dormouse nest-box schemes across the UK and has been monitoring dormouse populations for 29 years. This monitoring records populations during the dormice’s active period, from April through to October, using a series of nest-boxes in permanent locations that allow long term data to be collected for each site. Monitoring is carried out by surveyors licensed by Natural England in England, who once registered (after training with an established surveyor) can handle dormice.   

Dormouse nest inside a nest box – they use strips of honeysuckle bark and leaves to form their woven nests

Dormouse nest-boxes are artificial substitutes for tree hollows and are used for resting, breeding and feeding during the active season. They differ from bird boxes as the entrance is at the rear of the box, but dormice will use bird boxes.  Dormouse boxes were originally created to try to dissuade dormice from using bird boxes, but no-one has systematically investigated preference of bird boxes to dormouse boxes. The scheme provides national data and has highlighted a population decline of approximately 50% since 2000 – although the decline has slowed in recent years and 29% of monitored sites have increased their population.

Habitat quality is thought to be the main influence in improving populations. Devon is a stronghold for the species, so it is easy to forget the species is classed as ‘Vulnerable’ on the International Union for Conservation of Nature (IUCN) Red Data List, which uses precise criteria to evaluate the extinction risk of species. The State of Britain’s Dormice 2019 provides a fuller analysis of national data.

Within Yarner Wood, the National Dormouse Monitoring Programme has historically shown dormice present in low numbers, and individuals regularly used the bird boxes. Within the Bovey Valley woodlands, the population data from the nest boxes (and tubes) that are in place, are not fed into the national programme, as the location of boxes and tubes are moved, and the data is being used for a different purpose. (This will be described in more detail in our next blogDormice Dozing – Part 2).

During the active season, a licensed surveyor will record and check information on individuals using their nest box series.  The information collected includes:

  • Age:
    • Hair colour changes as the individual ages – from grey/yellow, to more golden after each moult and orange in females, tail fur length and size.
    • New-borns – known as pinkies
    • Juveniles
    • Sub adults: after first moult at 2-3 months
    • Adult
  • Sex (M/F) and if lactating indicating females are nursing young
  • Weight: this fluctuates over the year; 17-19 grams is usual for an adult but can be up to 30 grams prior to hibernation. Individual dormice may continue to eat late into the autumn if weather conditions allow to gain the necessary weight for hibernation.
  • Individual features: white tail tips are a genetic variation. Surveyors will also note other signs of dormice including dormouse nests and chewed nuts.
Dormouse asleep in a nest tube

Dormouse nest tubes are another surveying method commonly used by ecologists to assess presence of dormice as a one-off check (although it cannot be assumed that dormice are not present at a site, if tubes remain unused).  Tube series have been used annually across Yarner Wood, since 2011, as an unlicensed method of surveying the distribution of dormice across the woodland. The results show that dormice are widespread, but appear to be in low numbers.  The tubes are installed in winter and left completely undisturbed during the active season and then taken down the following winter to check for summer nests and chewed nuts (leaving tubes untouched if there is any sign of activity). It has been an opportunity to involve, both, community groups and the Moor than meets the eye Conservation Assistant trainees, to participate in monitoring dormouse presence at Yarner and to promote an understanding of hazel dormice ecology and conservation issues.

Dormouse nest tube in Yarner Wood

Wider data, from non-licensed activities that demonstrate dormice presence, provide valuable information that can give additional valuable data from outside the long-term monitoring schemes (which are often placed within protected sites, such as the National Nature Reserve).  One of the easiest ways to do this, is through checking for chewed nuts at potentially suitable sites. Dormice open nuts, including their favoured hazelnuts, in a way that is distinctive from other small mammals. Dormouse chewed hazelnuts have a smooth inner rim, with toothmarks at an angle to the hole on the nut surface and look a little like a clog. The illustration (see link to PTES fact sheet below), shows the distinctive teeth markings associated with dormice.

Now, when the trees are bare of leaves, is an ideal time of the year to look for chewed nuts – the nuts are usually eaten when green in the shrub layer  so fall onto the leaf litter  under the feeding site and can be found scattered across the woodland  floor.

In our next blog Dormice Dozing (Part 2) we will be describing some of the innovative methods that are being used to learn more about this elusive woodland mammal – including getting high up into the tree canopy to search for dormice using climbing equipment!

You can more information on how to undertake a dormouse nut hunt in this PTES Fact SheetHelp us find hazel dormice)

You can also download our East Dartmoor NNR Tracks and Signs of Woodland Mammals trail leaflet to find out more about dormice and other mammals that can be found living on the reserve.

Written by Linda Corkerton

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