All bats and their roosts are fully protected under Schedule 5 of the Wildlife & Countryside Act 1981 (as amended) and the Conservation of Habitats and Species Regulations 2017 (as amended), with further protection granted through the Countryside and Rights of Way Act 2000. Combined, this legislation makes it illegal to deliberately disturb, kill, injure, or capture bats, as well as damage, destroy or obstruct any bat roosts.
The aim of a dusk survey is to determine whether a building or structure is currently being used by roosting bats. The survey also determines which species of bat are present, the number of bats present, and which specific roosting features are being used by bats. The results from the survey inform an assessment of the population size of roosting bats (if present) and the level of bat activity around the site. In turn, the ecologist will determine whether the development is likely to cause a significant adverse impact on the roost or the bats themselves. If it has been assessed that the roost or the bats will be significantly impacted by the development, avoidance and mitigation measures will be provided for the works to legally process. These measures may include obtaining a European Protected Species Licence for roosting bats.
The number of dusk surveys required for a structure or building depends on the outcomes from a Preliminary Roost Assessment (PRA), which assesses the building/structure for its suitability for roosting bats. In a PRA, a building/structure can be assessed as having negligible, low, moderate, or high suitability, or a confirmed roost (if evidence of bats is recorded). Buildings assessed as having negligible suitability for roosting bats do not necessitate any further bat surveys. For buildings with low suitability, one dusk survey is required; moderate suitability necessitates two; and high suitability and confirmed roosts require three dusk surveys. If bats are found to be roosting in the building during the dusk survey(s), then three dusk surveys in total will be required, regardless of how it was assessed for suitability during the initial PRA.
Dusk surveys are undertaken to inform a planning application, and it is illegal for councils to condition these surveys. All dusk surveys must take place between May to August (inclusive), with one survey potentially being carried out in September. Surveyors will not require access inside the building to undergo dusk surveys. Permission from the landowner and a full risk assessment are required prior to a dusk survey.
A dusk survey begins 15 minutes prior to sunset and continues for at least 1 hour and 30 minutes after sunset. Two or more surveyors will position themselves around the building, each equipped with a night-vision camera, an infrared light, and a bat detector. Each aspect of the building will be observed by a surveyor. Where this is not possible, a night-vision camera will be used for that position, and the footage will be watched by an ecologist as soon as possible after the survey.
During the survey, the surveyor will observe their aspect of the building for any bats emerging from or entering the structure. If any roosting bats are present, the surveyor will record the times, species, and emergence points of the bats, alongside any additional information that may be useful. The night-vision camera, with the aid of the infrared light, is another record of the times and emergence points of emerging bats, which can be used to affirm a surveyor’s observations, in the office after the survey has been completed.
During the dusk survey, the bat detector plays back calls of any nearby emerging, foraging or commuting bats, at a frequency appropriate for human ears. This helps the surveyor identify the species of any emerging bats at the time of the survey. The bat detector also records what it detects, which means all bat calls can be analysed at the office, after the survey. Once the number and species of all calls has been analysed for all dusk visits, the ecologist is able to assess the bat activity around the site. At aLyne Ecology Ltd., we aim to get the finalised report to our clients within 2-3 working days after the completion of the last survey visit.
aLyne Ecology Ltd. undertakes dusk surveys in line with the most up-to-date bat survey guidelines (Collins, J., 2023). As stated in this guidance, multiple dusk surveys for one building or structure must be completed three weeks apart from one another. Weather conditions that are likely to affect the behaviour or activity of bats should be avoided for dusk surveys, which includes heavy rain and high winds. At aLyne Ecology Ltd., we always check weather conditions before the survey, to reduce the risk of having to stop a survey part-way through. If the forecast appears poor, with reference to the bat survey guidelines, then it is likely that the dusk survey will need to be rescheduled for a different evening.
