Guidance

Hazel or common dormice: surveys and mitigation for development projects

Standing advice for local planning authorities to assess the impacts of development on hazel dormice.

Survey reports and mitigation plans are required for development projects that could affect protected species, as part of getting planning permission or a mitigation licence. Surveys need to show whether protected species are present in the area or nearby, and how they use the site. Mitigation plans show how you’ll avoid, reduce or manage any negative effects to protected species.

This is Natural England’s standing advice for local planning authorities who need to assess planning applications that affect hazel or common dormice.

This information should be used to decide what is needed for surveys and planning mitigation measures for hazel or common dormice. Ecologists need to decide which survey and mitigations methods are right for the project being worked on. If this guidance can’t be followed, they’ll have to include a statement with the planning application explaining why.

Where this guide says ‘you’ it means the ecologist.

Get more detail on:

Decide if you need to survey

Survey for dormice if:

  • distribution and historical records suggest dormice may be present
  • the development will affect an area of woodland, hedgerow or scrub suitable for dormice habitat

Don’t survey for dormice if it’s an area of woodland, hedgerow or scrub unsuitable for dormice habitat and development is unlikely to affect dormice.

Survey methods

You need to be a suitably experienced ecologist with a dormice licence to survey for dormice. You do not need a licence just to search for evidence of dormice such as hazelnut marks.

You can limit surveys to visual searches for nests and opened nuts if the work only involves losing a small amount of habitat, for example:

  • gaps in hedgerows
  • removing a small amount of bramble scrub

For more damaging projects and licence applications, acceptable methods for surveying dormice are:

  • using nest tubes
  • using nest boxes

You can combine using either nest tubes or nest boxes with checking hazelnuts for dormouse marks.

Once you’ve found dormice, assume they’re present in all suitable habitat on the site.

Nest tubes

For this method:

  • use at least 50 tubes spaced 15 to 20 metres apart
  • leave nest tubes in place from April or May for the whole season
  • check each tube regularly, at least once every 2 months

Use this table to work out your score. Add up the index values for all the months the nest tubes will be in place. Base this on using 50 tubes - double the scores for 100 tubes and halve for 25.

Your score needs to be over 20 for your survey to judge presence or likely absence.

Month Index of probability (if using 50 nest tubes)
April 1
May 4
June 2
July 2
August 5
September 7
October 2
November 2

Example: 50 nest tubes were left out from April to September, scoring 21.

If you’re using a different number of nest tubes, adjust the index value. For example you might need fewer tubes because the area is small and you can’t access neighbouring land.

Example: 25 nest tubes were left out from April to September, scoring 10.5 - this score isn’t high enough to assume dormice aren’t present.

Don’t use a very high number of nest tubes for a short time just to get a high index score.

Nest boxes

You can use nest boxes alongside a nest tube survey. You should:

  • place the boxes 20 metres apart
  • use a grid or line formation through the site

Nest boxes can’t be included in the scoring system, but they can do both of the following:

  • increase the probability of finding dormice
  • be useful for post development monitoring

Check hazelnuts for dormouse marks

Only use this method:

  • alongside another method or to update an older survey if the development was delayed
  • in areas dominated by fruiting hazel trees
  • between September and December

For this method you can either:

  • systematically search 5 quadrats of 10 by 10 metres
  • check 100 nuts that have been opened by small rodents for signs of dormice

Don’t use this method as evidence that dormice are absent from the site.

Effort required

You’ll need to decide exactly how many survey visits to make according to the survey objective and local conditions.

If you need to deviate from the following minimum standards, include a statement with your licence application explaining why.

You could be asked for more surveys if your planning or licence applications are based on poor data, unless you can show the area is of low importance to hazel dormice.

Area to survey

Survey the land that will be affected by the project, as a minimum. For developments that are part of a larger scheme, cover the entire site in the survey and not just the first phase.

You’ll also have to survey suitable habitat next to the site. This includes woody areas, even if the habitats are fragmented.

Age of survey data

The survey should be from the current or previous active season. Surveys up to 3 years old are acceptable if the habitats haven’t significantly changed.

Estimate population density

It’s not possible to give a totally accurate assessment of population size for dormice, but you can make an estimate. For a mitigation licence application for one or more of the following you need to provide an estimate of the minimum pre-breeding population density:

  • large-scale schemes
  • where a significant amount of dormouse habitat will be lost

Use the following method:

  1. Take the number of dormice found in boxes in May.
  2. Divide by the area in hectares covered by your nest box scheme.

If the development will have a big impact, you should also state whether dormice are breeding on the site.

For a planning application, you only need to provide an estimate of population density if significant areas of dormice habitat will be lost (for road and rail schemes for example). Otherwise, you’ll usually only need to provide numbers caught through surveys.

To find out more about this survey methodology and estimating population density, email enquiries@naturalengland.org.uk.

Influences on survey results

Mention and explain the implications of anything that has affected your survey counts, for example:

  • you needed to start the surveys late in the season
  • unusual weather
  • nest tubes or boxes were interfered with

You may need additional surveys to deal with these problems and make sure you’ve got enough survey data.

Assess the site’s importance

Unless the development will have a low impact on dormice, provide an assessment of the importance of the site on a local and national scale with your planning or licence application. Factors you should consider when assessing the site’s importance include:

  • the presence of other dormouse populations in the area
  • whether dormice are widespread or limited to a few sites
  • location of the population relative to dormouse range
  • habitat composition
  • predicted densities

For low impact work, survey data based on a visual inspection of habitats that will be lost is enough.

Assess the impacts

Assess the impacts this development would have on dormice if no mitigation measures were planned, and submit the assessment with your planning or licence application. Explain how these impacts would affect the dormouse population, and the status of dormice locally and nationally.

Short-term impacts during development to consider include temporarily:

  • disturbing dormice, for example with increased construction lighting
  • increasing the risk from predators
  • losing foraging habitat

Long-term impacts include:

  • losing or changing habitats in a way that affects the dormouse population
  • losing hedges or other linear habitat that links populations
  • habitats becoming fragmented or isolated, for example by positioning a road through woodland

Impacts after the development include:

  • increased disturbance from people, for example from a new housing development
  • increasing the risk from predators like domestic cats

Mitigation and compensation methods

Address the potential impacts you’ve identified on dormice with by creating mitigation plans.

  1. Aim to avoid negative effects, such as by redesigning the scheme.
  2. If this isn’t possible, use mitigation measures to reduce the impacts.
  3. Use compensation measures if there are still negative impacts for dormice.

You’ll need to decide exactly which methods to use for the site and local conditions.

Persuasion as a mitigation measure

Use persuasion if either of the following apply:

  • less than 100 metres of hedgerow will be removed - as long as the remaining habitat is linked to a larger dormouse habitat
  • less than a 50 metre wide strip of woodland will be removed - as long as the remaining habitat is linked to a larger dormouse habitat

If the land to be lost is part of a large, continuous area of habitat, persuade the dormice to leave the area by progressively clearing narrow strips of habitat. Each strip you remove should be narrower than the radius of the dormice’s typical home range (an average of 50 metres).

You should:

  • clear the area carefully by hand
  • use nest boxes to make the adjacent habitat more attractive to the dormice
  • make sure the dormouse population doesn’t become unnaturally high when they’ve moved to the new area

Clear ground-level vegetation during winter (November to March) for areas up to the range of one dormouse home, for example:

  • 1 to 1.5 hectares of woodland
  • 300 metres of hedge

Clear enough vegetation in winter to persuade dormice to move when they come out of hibernation. When they have all emerged, by the end of May, continue clearing the site fully. Cut down trees and shrubs between November and March. Repeat this over more than one winter if the area to be cleared for development is large. Don’t remove tree stumps or remove earth until the following summer when the dormice have left the area.

Only clear the vegetation in summer (May to late September) if the habitat is:

  • smaller than 50 square metres for high quality woodland
  • a large area of low quality habitat
  • a short length of hedge

Take out a small amount of vegetation every day, when the dormice are active and can respond immediately. Be careful not to disturb nesting birds between May and September.

Moving dormice (translocation) as a mitigation measure

Only use translocation if:

  • there’s no other option - it should only be a last resort
  • the scale of what you’re trying to do doesn’t allow for persuasion
  • there’s no suitable habitat next to the site so persuasion wouldn’t be suitable

You should do both of the following:

  • identify a suitable receptor site well before translocation
  • not translocate dormice into a site where there’s a dormice population already

Keep dormice within their home range.

Create new habitat as compensation

You may need to:

  • plant new areas of hedgerow, scrub or woodland
  • use dormouse boxes

Dormouse bridges as mitigation

If you create bridges for dormice to connect habitats don’t try to bridge a gap of more than 100 metres.

Improve retained habitat as compensation

Improvements can include:

  • thinning to prevent continuous dense shade and promote a diverse understorey
  • coppicing - as a long-term management technique to improve habitat variety (but can disturb dormice in the short term)
  • new planting - this can be a foraging resource but can take a long time to provide nesting sites so translocate coppice stools and hedgerows from the cleared site if possible

Management and site maintenance after development

You’ll need to consider:

  • who owns the site in the long term
  • who’ll be responsible for managing and maintaining the dormouse population
  • how long the maintenance can be guaranteed for
  • who’ll fund it

For large sites, make sure that ways to manage the dormouse population are included in any landscape or ecological management plan.

Population monitoring

For all but very small or low impact schemes, put a monitoring plan in place to assess the dormouse population after mitigation. Your licence will tell you how to report this information.

Use the same counting methods as before the development so you can compare population trends.

Published 28 March 2015
Last updated 29 July 2015 + show all updates
  1. Factual errors corrected and changes made based on expert and customer feedback.
  2. First published.