Dormouse survey seasons and nest tube programmes

The hazel dormouse survey season runs from April to November, and the effort required is measured by a weighted index rather than by a number of visits. Nest tubes are installed, left to weather and then checked monthly, and each month's check carries a different weight depending on how likely dormice are to be found in a tube then. The scores accumulate, and a total of 20 is the conventional threshold for concluding likely absence. Start in April and a single season can reach it. Start in late summer and it cannot, which is the single fact that decides whether a hedgerow removal happens this year or next.

Dormouse survey work and any licence application rest on a suitably experienced ecologist, and handling the animals requires a licence.

Where dormouse presence is plausible

The species is patchily distributed and largely confined to southern England and the Welsh borders, with scattered populations further north. A desk study against the local record centre and the national distribution picture is the first filter, and it rules out a great many sites straight away.

Habitat is the second filter. Dormice need connected, structurally diverse woody habitat with a succession of food sources through the season:

• Species-rich hedgerows with mature standards, bramble and honeysuckle, connected to other habitat rather than isolated
• Coppice woodland and woodland edge with a developed understorey, particularly where hazel is present
• Scrub mosaics, bramble thickets and overgrown railway and road verges
• Continuity of connection, because dormice are reluctant to cross open ground

A single hedge running between two arable fields with no connection to woodland is a much weaker prospect than the same hedge meeting a copse. Say which you have, with a habitat plan, before anyone orders tubes.

Nest tubes and the index of probability

The standard method places nest tubes in the shrub layer and checks them monthly. Dormice use them for shelter and for nesting, and a woven nest of honeysuckle bark, leaves and grass inside a tube is diagnostic.

The index of probability assigns a score to each month, set by how likely a check is to detect dormice at that point in the season. The later summer and early autumn months carry the highest weights, and April carries the lowest, which reflects behaviour rather than convenience: animals are active and numerous in autumn, including young of the year, and they are torpid and scarce in early spring.

Two consequences follow:

A survey that runs from April through to November accumulates enough score to reach the threshold within one season, provided the checks are done every month and the results are negative throughout. A survey that begins in August has only the highest-weighted months available and still falls short of 20 by November, so a second season follows.

The scores for each month are published in the recognised dormouse conservation guidance and should be taken from it rather than from memory, because getting the arithmetic wrong is the commonest reason a survey report is rejected by a local authority ecologist.

Tube density and the effort involved

Density is set by habitat rather than by a single rule. Along a hedgerow, tubes are spaced at regular intervals over the length being assessed; in woodland, they are distributed across the area at a density per hectare that the guidance sets out. Either way the number rises quickly on a site of any size, and each tube is visited every month.

Practical points that change the cost:

1. Tubes need to weather for a few weeks before the first meaningful check, so installation happens ahead of the survey start
2. Checking is slow work in dense scrub, and a large site needs more than one surveyor to complete a round in a day
3. Tubes are lost to weather, livestock and people, so an allowance for replacement is realistic
4. Checking tubes does not itself require a licence, though handling a dormouse or disturbing a nest does, so the surveyor's competence and licence position need to be clear before the first round
5. Access agreements for third party land along a hedgerow have to be in place for the whole season, not for the first visit

Budget the survey as eight monthly rounds plus installation and reporting, and hold the cost across a year end where the season straddles one, as our note on planning a survey budget across two financial years sets out.

Nest and nut searches as supplementary methods

Two other methods add evidence and neither replaces tubes on its own.

Searching for dormouse nests in the shrub layer works where the habitat is accessible, and the nests are distinctive once you have seen one. Detection rates are low, so a negative result carries little weight.

Nut searches are more useful where hazel is present. Dormice open hazel nuts in a characteristic way, leaving a smooth, round inner rim with tooth marks angled across the surface, quite unlike the ragged edge left by a squirrel or the small round hole made by a weevil. Nuts are searched for on the ground in autumn, from around September to November. A positive nut find is strong evidence of presence and can shorten a survey programme considerably, because once presence is established the question moves from detection to mitigation.

That is the practical use of nut searches: they can confirm presence early and let the design and licensing work start, rather than waiting out a season to prove a negative.

When a second season becomes unavoidable

Three situations produce a second year, and all of them are foreseeable:

A late start, where the available months cannot reach the threshold. Incomplete rounds, where weather, access or lost tubes mean a month's check is missing or partial, which removes that month's score. And equivocal results, where field signs suggest presence but tubes stay empty, leaving an authority's ecologist unwilling to accept absence.

The way to avoid all three is to commission the habitat appraisal in winter. A Preliminary Ecological Appraisal in January tells you whether dormice are a possibility, which means tubes can go up in March and the first check can happen in April.

Illustrative example: a scheme requiring removal of 180 m of species-rich hedgerow, where tubes are installed in March at regular spacing along the hedge and in the adjoining woodland edge, checked monthly from April, with a positive result in June that ends the detection phase and starts the licence work. Details here are illustrative only.

Licensing and the timing of habitat manipulation

Dormice are European protected species, protected under the Conservation of Habitats and Species Regulations 2017 and listed under the Wildlife and Countryside Act 1981. Where development cannot avoid killing, injuring, capturing or disturbing them, or damaging a breeding site or resting place, a licence from Natural England is required, and the application has to satisfy the derogation tests: an acceptable purpose, no satisfactory alternative, and maintenance of favourable conservation status.

The timing here runs against intuition. Dormice hibernate at ground level, in nests among leaf litter and the base of hedgerows, from roughly November until April. So winter, which is the safe season for birds, is the dangerous season for dormice, because ground-level clearance in January destroys hibernation nests that cannot be detected.

Habitat manipulation is therefore done in the active season, when animals are in the canopy and able to move. It proceeds in stages, by hand, over a period of days: the woody vegetation is reduced progressively to encourage animals to move into retained habitat, then the stumps are removed under supervision. Breeding peaks are avoided within that window, which narrows it further.

What this means for a hedgerow removal programme

Sequence the whole thing backwards from the clearance window:

• Winter: desk study and habitat appraisal
• March: install tubes and allow them to weather
• April to November: monthly checks, or an earlier stop if presence is confirmed
• Autumn and winter: design mitigation, agree replacement planting and connectivity, prepare the licence application with the survey evidence
• Spring: licence determined
• Active season: staged habitat manipulation under licence, then clearance under supervision
• Following seasons: replacement planting established, with monitoring

Retained and replacement connectivity is the substance of the mitigation, and it interacts with the biodiversity net gain position on the same site, because hedgerows are assessed as a separate module in the statutory metric. A Protected Species Survey prepared with that programme in mind is what turns a two-year constraint into a one-year one, and our note on discharging pre-start conditions for a spring start covers the rest of the sequence.

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This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.