Bat roost potential in trees before felling

Assessing bat roost potential before tree felling is not optional good manners. Bat roosts are protected whether or not a bat is in residence at the time, under the Conservation of Habitats and Species Regulations 2017 and the Wildlife and Countryside Act 1981, so felling a tree that contains a roost without a licence is a criminal offence. The sequence is always the same: a ground level assessment of the tree, further inspection or survey where features are found, and a European protected species licence from Natural England before any work that would damage or destroy a roost.

A licensed ecologist has to carry out that work. This is one of the report types where a competent, licensed professional is legally required, and an AI-assisted draft is a starting point for that person rather than a substitute for them.

Why a tree can be a roost

Bats use trees for different purposes through the year: day roosts in summer, maternity roosts where females gather to give birth, mating and swarming sites in autumn, and hibernation sites in winter where the cavity is stable and humid. A single tree can serve more than one of those functions in different seasons.

The law protects the place a bat uses for shelter or protection, so protection attaches to the feature rather than to the animal's presence. A cavity used by a maternity colony in July is still protected in January. That is the point most people miss when they assume an empty tree can be felled in winter.

Trees in gardens, hedgerows and field boundaries are as likely to hold roosts as woodland trees, and mature exotic species in parks and churchyards are often better than native ones because of the way they decay.

Features that indicate roost potential

Surveyors look for potential roost features rather than for bats. The usual list runs:

• woodpecker holes, knot holes and old pruning wounds that have occluded around a cavity
• splits and lifted bark, particularly on dead limbs and storm damaged stems
• cracks following a shear or lightning strike
• cankers, hazard beams and the hollows behind old branch tears
• dense ivy stems on the trunk, which create sheltered crevices
• compression forks and included bark where water tracks into the stem

Each feature is judged on its shape, depth, orientation, dryness and the presence of staining, droppings or scratch marks at the entrance. Potential is then categorised, broadly as negligible, low, moderate or high, under the Bat Conservation Trust good practice guidelines. That category sets the survey effort for everything that follows, so it carries real consequences for a programme.

Ivy deserves a note. It is routinely stripped before inspection, and stripping ivy from a tree with roost potential can itself destroy a roost. Inspect first.

Ground level assessment and where it stops

A preliminary ground level roost assessment uses binoculars, a high powered torch and a careful walk around the stem at different angles. It is quick, inexpensive and can be done in any month, which makes it the sensible first step on any site where trees are to be removed.

It has limits that matter. Features on the upper surface of a limb are invisible from below. A cavity entrance facing away from the only accessible viewpoint will be missed. And a dense summer canopy hides a great deal, which is why winter assessment has advantages for tree inspection even though it rules out emergence surveys. Our note on the winter inspection limits on a tree survey sets out the same trade-off for arboricultural work.

Where the ground level assessment finds negligible potential and the surveyor can see the whole stem and crown, that can be the end of it. Record the date, the weather, the equipment and the viewpoints, because an officer or a reviewer will want to see that the assessment was capable of reaching its conclusion.

Aerial inspection and endoscope work

Where features are found, the next step is to get to them. An aerial inspection is carried out by a climbing arborist working with, or trained as, a bat surveyor, using an endoscope and a borescope camera to look inside each feature.

Climbing near a potential roost is itself capable of disturbing bats, so the inspection needs to be done by someone holding the appropriate Natural England licence, or under the supervision of someone who does. The inspection records the internal dimensions, the dryness, evidence of use and whether the feature connects to a larger void. A feature that looks promising from the ground is often 40mm deep and full of water.

Aerial inspection can downgrade a tree, and that is its value. It can also upgrade one, which is why it belongs before the programme is fixed rather than after the felling date is booked.

Emergence and re-entry surveys and their season

Where inspection cannot rule out use, the tree is watched. Emergence surveys start before sunset and record bats leaving; re-entry surveys run before dawn and record bats returning. Surveyors use infrared or thermal equipment and position observers so every face of the feature is covered.

The season is fixed by bat behaviour. The optimal window runs from May to August, with May to September treated as the acceptable period in most circumstances, and the number of visits scaled to the assessed potential under the current good practice guidelines. A tree assessed as high potential needs more visits than one assessed as low, and those visits must be spread across the season rather than crammed into one week.

That is the scheduling risk on nearly every small site. A tree assessed in October with moderate potential cannot be cleared through survey until the following May. Commission the ground level assessment at the feasibility stage, not after the application is submitted. The same point applies to the wider Bat Survey work on buildings, which runs to the same calendar.

Illustrative example: a self-build plot with four mature ash trees, two of which carry old woodpecker holes. A ground level assessment in November puts both at moderate potential. Aerial inspection in December finds droppings in one cavity. The scheme cannot fell that tree until emergence surveys have run in the following summer and a licence has been obtained, which adds roughly nine months to the programme. The applicant redesigns the access to retain the tree instead.

When a licence is needed before felling

A European protected species licence is required where the works would damage or destroy a breeding site or resting place, or would deliberately disturb bats. Natural England issues those licences, and the application needs a survey report establishing the baseline, a mitigation and compensation strategy, and evidence that the three statutory tests are met.

Where the impact is low, registered consultants can work under a class licence instead of a bespoke licence, which shortens the process considerably. That route is only open where the circumstances fit the terms of the class licence, and the decision is the ecologist's to make rather than the client's.

Two things a licence is not. It is not planning permission, and in most cases the planning authority will want the ecological evidence before it determines the application rather than leaving the licence to be sorted out afterwards. And it is not a route around a Tree Preservation Order, which needs its own consent for felling or lopping.

Soft felling and sectional dismantling

Where a licence permits removal, the method is specified. Soft felling, sometimes called sectional felling, means taking the tree down in sections with the roost features cut last, lowered under control rather than dropped, and left overnight on soft ground so any bat present can leave of its own accord.

Timing is prescribed too, usually outside the maternity season and outside hibernation, which in practice narrows the window to spring and early autumn. An ecologist is normally present on site to supervise. Replacement roost provision, often bat boxes on retained trees, is a standard licence condition, with a monitoring period attached.

Who is competent to advise

The person who assesses potential roost features and designs mitigation should hold a Natural England survey licence for bats and have experience of tree roosts specifically, which is a narrower skill than surveying buildings. Membership of the Chartered Institute of Ecology and Environmental Management at a suitable grade is the usual marker of standing.

Arboricultural and ecological work has to be joined up, because the Tree Survey prepared to BS 5837 and the bat assessment look at the same trees for different reasons and should not reach contradictory conclusions. Where a draft report needs checking by a licensed specialist before it goes to the council, the professional directory is the route to that review.

This is general guidance, not legal advice, and it does not substitute for the judgement of a licensed ecologist. For specialist technical reports, qualified professional input may be required.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.