Bat surveys

All bat species and their roosts are protected by law. Bats may roost in cavities within trees and within man-made structures including, but not limited to, houses, churches, bridges, tunnels and mines.

If bats are reasonably likely to be present in a structure, tree, feature, site or area under construction and may be affected by a proposed activity, then surveys are required to determine their presence or likely absence, to identify the species, and how and when they are using the affected areas.

A commonly used method to establish presence or likely absence of roosting bats is a thorough internal and external inspection, searching for evidence of bat use and assigning a level of potential to support roosting bats. In the case of buildings, the exterior is searched for potential roosting opportunities such as gaps, loose or missing roof tiles and flashing, and potential access points to internal cavities. Internally, roof voids and any other potential cavities are searched for evidence of bat use in the form of droppings, feeding remains and scratch marks. A survey licence is required to enter a known roost or handle a bat.

For trees, a Ground Level Tree Assessment (GLTA) is usually undertaken searching for potential roosting opportunities such as crevices and holes, loose bark, splits and cracks, dense ivy and dense epicormic growth and evidence of bat use in the form of droppings and scratch marks, staining and/or flies around entry points.

Bat surveys – to look for signs of bats – can be carried out at any time of year, but bats are most likely to be seen or heard in roofs during the summer or autumn, or seen in subterranean areas during the winter. If evidence of bat use is found, or a potential to support roosting bats identified, then further surveys are required to identify species and type of roost. This is assessed by dusk emergence and dawn return-to-roost surveys, often in conjunction with activity surveys using bat detectors to record foraging and commuting activity and the use of the wider site by bats. These surveys can be undertaken from May to August, with September being a sub-optimal month. Depending on the level of potential assigned to a feature, multiple surveys may be required, to be undertaken at least one month apart.

Our bat surveys and mitigation strategies are guided by best practice including the Bat Conservation Trust’s Good Practice Guidelines (Collins, 2016) and Natural England Standing Advice, and our surveyors meet the competencies set out by the Chartered Institute of Ecology and Environmental Management (CIEEM). Where bats or their roosts may be affected by a development, our specialists can provide advice on suitable mitigation options and guidance on minimising ecological risk. To allow works to proceed lawfully, we can apply for the development licences necessary on your behalf.

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Further reading and related articles

Thomson Ecology Handbook

This online version of the Thomson Ecology Handbook provides a general overview of current wildlife legislation* and is aimed at helping project managers understand and plan for ecology from the start.

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