Dormouse Surveys

The dormouse is a strictly protected species

The dormouse

The hazel dormouse (Muscardinus avellanarius) is reputedly the sleepiest of the British mammals. They are a strictly nocturnal species found mainly in southern counties of England and Eastern counties of Wales. There are some scattered locations in West Wales. Hazel dormice are predominantly found in deciduous woodlands and hedgerows but despite their name, they are not exclusively associated with Hazel trees (Corylus avellana) for their habitat and food source. The ideal habitat would be a semi-natural ancient woodland with mixed hazel coppice.

Their favoured food source is hazel nuts, but will eat flowers, pollen, nuts and insects. A key indicator of the presence of dormice in a woodland or hedgerow is the discarded shell of a hazel nut. Whilst other mammals will leave discarded shells after feeding, the key identifier for a dormouse is the neatly nibbled round hole in the top of the shell. The hole will have a smooth inner rim with tooth marks that are angled around the hole.

Where to find dormice

Being nocturnal these animals are rarely seen, spending most daylight hours asleep, coming out at night to feed. They have large black eyes with a brown- gingery furry coat and a fluffy tail. During the winter months (October-April) they will come down from the trees and hedgerows to hibernate in tightly woven nests the size of a tennis ball on the ground, in leaf litter and around tree roots.

Dormice build nests in dense, thorny scrub, in hedgerows and in holes in trees. Their nests are woven structures of grass and leaves and found within low growing shrubs measuring between 100-150mm in diameter. Their typical home range is approximately 50 metres.

Protected under law

Dormice are strictly protected by law and may not be intentionally killed, injured or disturbed in their nests, collected, trapped or sold except under licence.

They are protected in the UK under the Wildlife and Countryside Act, 1981. Priority Species under the UK Post-2010 Biodiversity Framework. Listed as a European Protected Species under Annex IV of the European Habitats Directive. Dormice are afforded this protection as their population numbers and distributional range has halved in the past 100 years.

Dormouse surveys

Dormouse surveys are undertaken by licensed and experienced ecologists to establish their likely absence or presence using different techniques depending on the site being surveyed and time of year. There are restricted periods when certain surveys are possible.

Dormouse survey techniques range from simple nut searches and nest tube surveys, through to nest box checks and footprint tunnels. The objective is to confirm how dormouse uses the site and make an estimate on the population size, so proportionate mitigation or compensation measures can be implemented under a mitigation licence.

Frequently Asked Questions

Why might I need a dormouse survey?
What would be considered to be breaking the law surrounding dormice?
You are breaking the law if you conduct certain acts, including
- deliberately capture, injure or kill hazel dormice
- damage or destroy a dormouse resting place or breeding site
- deliberately or recklessly disturb a hazel dormouse while it’s in a structure or place of shelter or protection
- block access to structures or places of shelter or protection
- possess, sell, control or transport live or dead hazel dormice, or parts of hazel dormice
If found guilty of any offence you could get:
- An unlimited fine
- Sent to prison for up to 6 months
Dormice can also be harmed by:
- handling
- disturbance, eg noise and light, woodland and hedgerow management
- habitat removal, clearing woodland and removing hedgerows
- habitat fragmentation and isolation
What methods are used to survey dormice?
1. Hazel nut checks – these can be used if there is only a limited amount of work to be carried out, such as losing small amounts of hedgerows or scrub. For larger projects then this survey should be used alongside one other survey method.
2. Nest tubes – to be used where large areas of habitat are to be removed /lost. At least 50 tubes should be deployed spaced 15-20 metres apart.
3. Nest boxes – to be used where large areas of habitat are to be removed /lost. Boxes should be placed 20 metres apart.
When should dormice surveys be carried out?

Area searches for nuts should be carried out between September and December in areas dominated by hazel trees. This method of surveying should complement another survey or to update older survey data (over 3 years – if the habitat has not significantly changed). A dormouse licence is not required for this survey.

To determine presence/ absence of dormouse within a suitable habitat then you can deploy nest tubes or nest boxes between April and November.

If evidence of dormice is found then it should be assumed that they are present in all onsite, suitable habitats.
How long should dormice surveys take?

Checking for nuts is not determined by any length of time. The search for nuts should be done on a regular basis, across the effected habitat (hedgerow/woodland) between September and December when a new crop of hazel nuts is present. However, the nuts will still be present on the ground outside of these months.

Ideally, to save effort this would be done at the same time as checking nest tubes and nest boxes.

Nest box and nest tube surveys require a more time-consuming effort. These need to be set out between April and May and left in place for the whole season (April- November) and be checked at least every 2 months.
What kind of mitigation measures might be implemented?
The first consideration when confirming the presence of dormice is whether the habitat can be retained or if redesigning the project is required. If this is not possible, mitigation measures will need to be implemented to reduce the impact on dormice. If there is still a negative impact after the mitigation measures have been employed, then compensation measures will be needed.

Measure for mitigation can include:
- Persuading the dormice to clear the habitat to be affected. This method should only be used if the habitat loss is less than 100m and the remaining habitat is connected to a larger dormouse suitable habitat
- If a strip of woodland is to be removed, it should be less than 50m wide and remaining habitat is connected to a larger dormouse suitable habitat
Persuasion can consist of:
- the removal of narrow strips of vegetation no larger than the dormouse home range of 50 m. This method is only appropriate if the land lost is part of a large, continuous area of habitat.
- Clear the area carefully by hand
- Use nest boxes to make the adjacent area of habitat more attractive
- Clear ground level vegetation during the winter months (November – March). In doing so it presents a less favourable habitat when they come out of hibernation. Continue to clear the vegetation, when they are expected to have emerged from hibernation (by the end of May).
Translocation should only be used:
- As a last resort
- The scale of the project does not allow for persuasion
- There is no suitable adjoining habitat to used persuasion
Build a bridge- dormouse bridges have been used for some sites to allow dormice to cross between habitats that have been fragmented. The use of a bridge is only suitable if the gap between habitats is less than 100m
How much do dormice surveys cost?

The cost of a survey will depend on the type required and the size of the project. For example, where a small project requires 20m of hedgerow to be removed, then a nut check may suffice and will require an ecologist to search the area prior to any work being conducted. This may cost a few hours of an ecologist’s time.

For larger scale projects, for example the removal of 100 m² of woodland/hedgerow in an area known for dormice or records are present, then nest boxes and nest tubes will need to be put in place within the habitat and monitored.

For this example, a minimum of 50 tubes will be required and nest boxes placed 20m apart. These have individual costs depending on the supplier. These monitoring devices should be checked by a licenced ecologist at least every 2 months, throughout the season. The cost of this will be determined by the area covered, the number of boxes, the accessibility of the habitat and time taken to check each monitoring box/tube.
Where can I find out more information?