Hazel dormouse surveys: nest tubes, nut searches, licences

A dormouse survey for development is a seasonal commitment, and the number that matters is 20. Under the index of probability set out in the Dormouse Conservation Handbook, each survey month carries a score, and a total of 20 or more is treated as giving reasonable confidence in a negative result. You cannot reach it in a fortnight, and you cannot reach it at all if the site is only walked in June. That single arithmetic fact drives more programme problems on hedgerow and woodland edge sites than any other ecological constraint.

The hazel dormouse is listed on Schedule 2 of the Conservation of Habitats and Species Regulations 2017 and Schedule 5 of the Wildlife and Countryside Act 1981. Survey work, and any mitigation, needs a suitably licensed ecologist. A drafted report is a starting point for that professional, never a substitute for the survey itself.

Habitat that triggers a survey

The screening question is whether the site holds connected, structurally diverse scrub, hedgerow or woodland with a fruiting or flowering understorey. Dormice feed arboreally through the season, so they need a succession of food sources: hawthorn and bramble flowers, honeysuckle, hazel, oak, and later berries and nuts.

The features that raise the risk are old species rich hedgerows with a dense base and a continuous canopy, woodland edge and coppice with hazel, bramble thickets linking blocks of scrub, and railway or road verges that connect otherwise isolated habitat. A closely flailed, gappy hedge on its own, surrounded by arable, is a lower prospect, though it may still function as a link.

A Preliminary Ecological Appraisal should reach a clear view: no further survey, further survey required, or presence assumed with mitigation designed accordingly. That third option is worth considering seriously on small sites, because a full season of nest tube survey can cost more than designing around the hedge in the first place.

Nest tube effort and the index of probability

The standard method is nest tubes, deployed along the habitat features rather than scattered across the site, checked at monthly intervals through the active season. Fifty tubes is the usual starting point for a small site, spaced along hedgerow and woodland edge and tied into the understorey at around chest height, and the effort scales with the length and area of suitable habitat.

The index of probability weights each month by how likely dormice are to use a tube then. The weightings in common use are April 1, May 4, June 2, July 2, August 5, September 7, October 2 and November 2.

Two things follow. September carries the highest single weighting, so a survey that stops in August throws away the most productive month. And because the maximum for a full season is 25, missing September and August together makes 20 unreachable in that year.

Reviewers should check that the tube count, the spacing and the check dates are all reported alongside the conclusion. A survey of 20 tubes over four months is a different piece of evidence from 50 tubes over six, even where the stated index score looks similar.

What a nut search can and cannot tell you

Where hazel is present and fruiting, a nut search is quick, low cost and can confirm presence in a single visit. Dormice open a hazelnut by gnawing a smooth, round hole with the tooth marks angled around the inner rim, which is distinguishable from the ragged edge left by a bank vole or wood mouse and from the split shell left by a squirrel.

The limitation is one sided. A positive find is good evidence of presence, and searching the litter layer under hazel from early autumn is worth doing on any site with a hazel component. A negative find proves very little. Nuts may not have set, the litter may have been cleared, and the search area may simply be away from where the animals forage. The method also depends entirely on hazel being there, which rules it out on bramble and hawthorn dominated sites.

Treat it as a screening tool that can shortcut to presence, and keep the nest tube programme running unless it does.

The season and why a late start costs a year

Dormice hibernate at or near ground level through the winter, and the survey season runs from April to November. A scheme that instructs an ecologist in October has two realistic options: run a nut search and a partial tube survey, or assume presence and design mitigation.

That second route is often the commercially better decision. Assumed presence means you design a scheme that retains and buffers the habitat, then apply for a licence on the basis that dormice may be affected. The programme risk moves from survey seasons to the licence determination period.

An illustrative sequence for a small site where dormice are found: tubes deployed in April, monthly checks to October, index of 22 reached, a nest recorded in September, mitigation designed over the winter, licence application submitted after permission is granted, and supervised habitat works the following April. That is 12 months from instruction to first cut, and a start in August rather than April pushes almost all of it into the next year.

Licensing and the derogation tests

Where development is likely to kill, injure, capture or disturb a dormouse, or to damage or destroy a breeding site or resting place, a licence from Natural England is needed. The licence cannot be granted unless the three derogation tests are met: the work is for imperative reasons of overriding public interest or another permitted purpose, there is no satisfactory alternative, and the action will not be detrimental to maintaining the population at a favourable conservation status.

Planning permission normally comes first, because the licensing body expects to see the consented scheme and the conditions attached to it. Councils are also required to have regard to the derogation tests when they decide the application, so the ecology chapter should address them rather than leaving them to the licence stage.

Mitigation for dormice is habitat based. Retention of the linking features, generous buffers, new planting that will function within a few seasons rather than a decade, phased vegetation clearance by hand under ecological supervision during the active season, and a management plan that stops the new hedge being flailed to a stump in year three. Receptor sites and translocation are a last resort and hard to justify.

Hedgerow connectivity and severance

The impact that gets underplayed in reports is severance. A scheme can retain 90 per cent of a hedgerow by length and still cut the population off from the wood it depends on, because dormice are reluctant to cross open ground and a gap for an access road is a real barrier.

Assess connectivity as a network, not as a tally of metres retained. Show the existing links on a plan, show what the layout does to each of them, and set out how continuity is maintained: a retained canopy over a footpath, a green bridge on a larger scheme, or planting that will knit together before the gap matters. Where a link is lost, say so plainly and deal with it in the mitigation, because a reviewer will spot a diagram that shows a severed network described as retained.

Habitat retention and enhancement will also feed the Biodiversity Net Gain Assessment, where hedgerow units are counted separately from area habitats. The statutory metric has to be completed by a competent person under the Environment Act 2021 regime, and the biodiversity gain plan is approved by the local planning authority. Dormouse mitigation and net gain planting should be the same scheme, designed once.

Reviewing a dormouse chapter

If you are signing off someone else's work, five checks catch most problems. Is the index score stated and does the month by month arithmetic add up. Are the tube numbers, spacing and check dates reported. Does the survey area cover all habitat that the development or its access will affect, including off-site hedgerows. Is severance addressed as well as loss. And does the mitigation have a timetable tied to the licence and to the Protected Species Survey findings, rather than a general commitment to good practice.

This is general guidance, not legal advice. Survey design and licensing for a European protected species must be settled with a suitably licensed ecologist for the specific site.

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