Bats in trees: roost features and aerial inspection

Potential roost features in trees are assessed from the ground first, and only a minority of trees ever need a climber. The judgement that matters is which features carry enough suitability to justify aerial inspection, emergence surveys, or both, because a tree recorded as having moderate potential and then felled without further work is the route to an offence under the Conservation of Habitats and Species Regulations 2017. Aerial tree inspection for bats is a targeted follow up to a ground assessment, never a substitute for one.

What counts as a potential roost feature

A potential roost feature is any space in or on a tree that a bat could use. The common ones are woodpecker holes, knot holes, tear outs and limb fractures, lifting or loose bark, hazard beams and splits along the grain, compression forks with a cavity behind them, dense ivy on a mature stem, and rot holes at branch junctions.

Size is a poor guide. A crevice a few millimetres wide can hold a crevice dwelling species, while an obvious open cavity may be too draughty or too wet to be used. What the assessor is looking for is shelter, a dry interior, and a clean entrance that does not flood.

Dead and dying trees carry more features than healthy ones, which puts bat interest and arboricultural risk in the same place more often than anyone would like.

Assessing from the ground and when that is enough

A ground level assessment is done with close focusing binoculars and a torch, working around the whole stem where access allows, and it takes longer than people expect on a mature tree. The assessor records each feature, its height, aspect, apparent depth and any staining, droppings or scratch marks below it.

Season affects what can be seen. A full summer canopy hides features that are obvious in February, so a ground assessment made in leaf should say so. Several appraisals arrive with a line about dense foliage limiting the inspection and then conclude negligible potential, which does not follow.

Where every tree on a site is young, uniform and free of defects, a ground assessment properly recorded is the end of the matter. Say that clearly in the Preliminary Ecological Appraisal rather than recommending further survey as a reflex.

When a climbing inspection is justified

Aerial inspection earns its place when a feature cannot be resolved from the ground and the works would affect it. Typical triggers are a cavity whose depth cannot be seen, a feature more than about 6 m up, evidence below a feature that needs checking, or a tree scheduled for felling rather than light pruning.

It is not justified where the works do not touch the feature. Pollarding a stem on the opposite side of a tree from a high knot hole may not require anyone to climb, and a proportionate recommendation carries more weight with a council ecologist than a blanket one.

Climbing has its own competence requirement. The inspection is carried out under rope access by someone holding the relevant climbing and aerial rescue qualifications who is also a bat ecologist licensed to disturb bats, or working directly with one. Councils will ask for the licence number.

Endoscopes, evidence and what gets recorded

Inside the feature, the surveyor uses a torch, a mirror and an endoscope. Evidence takes several forms. Live bats, obviously. Then droppings adhering to the cavity wall, feeding remains, staining and polishing at the entrance where fur has worn the wood smooth, and a characteristic smell.

The record for each feature should include a reference number, a photograph, the measurement of the entrance, how far the endoscope reached, whether the base of the cavity was visible, and the conclusion. The last of these is the one that gets fudged. There is a real difference between no evidence of bats and no bats present, and the report should use the wording that matches what was actually established.

A single aerial inspection cannot rule out use. Bats move between roosts through the year, so a negative inspection in March says little about a maternity period. Where the feature is suitable and the works are destructive, the honest recommendation is emergence and re-entry surveys in the active season, which for tree roosts is usually taken as May to September with the most productive months earlier in that window. That is a Protected Species Survey with a season attached, and it is the reason tree works so often sit on the programme critical path.

Categorising trees and what each category commits you to

Trees are usually sorted into four bands of roost suitability: high, moderate, low and negligible. The bands are useful only if the consequences attached to them are stated in the same table.

Write the viewing conditions into the negligible rows. A negligible call made from one side of a stem in July is not the same assessment as one made from three positions in winter.

Sectional felling under supervision and its limits

Where a low suitability feature has to be removed, the usual control is sectional dismantling under the supervision of an ecologist, with the section containing the feature lowered rather than dropped, left supported overnight where practical, and inspected before it is processed.

This is a precautionary method for low risk situations. It is not a way of dealing with a confirmed or likely roost, and presenting it as one in a method statement invites a refusal or a condition that stops the works. If a roost is confirmed, the legal route is a licence, not a careful chainsaw.

Timing helps. Removing a feature outside the hibernation period and outside the maternity period reduces risk, but it does not remove the need for the method statement or the supervision.

Licensing once a roost is confirmed

Bats and their roosts are protected whether or not a bat is present at the time. Damaging or destroying a breeding site or resting place is an offence, and the lawful route is a licence from Natural England, obtained by a suitably licensed ecologist.

Two routes exist in practice. A registered consultant can use a class licence for certain low conservation significance roosts where the qualifying conditions are met. Everything else goes through an individual mitigation licence, which needs a survey record, a mitigation design and usually planning permission in place first. Programme the licence application after consent, not alongside it.

Keeping the arboricultural and bat surveys in step

The same trees appear in two documents, and they should carry the same reference numbers. A Tree Survey to BS 5837:2012 numbers every tree for arboricultural purposes, and it is worth agreeing at the outset that the ecologist adopts those numbers rather than inventing a parallel set.

Coordinate the recommendations too. A tree the arboriculturist wants reduced for safety reasons may hold the feature the ecologist wants retained, and the resolution is a design decision rather than a clash discovered on site. Where a tree must go, the mitigation usually pairs bat boxes with a replacement planting commitment, and both belong in the conditions schedule.

Councils expect this work from an ecologist holding relevant professional membership, and for licensed activity the competence requirement is a legal one rather than a preference. Browse all 66 PlanScope report types to see which ecology and arboricultural documents a site is likely to need before you book a climber.

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