Bat Survey

Bats are typically found in a broad range of habitats throughout Britain, though certain species have restricted geographic ranges. Eighteen species of bat are found in Britain. Though many species are still relatively common, their populations declined dramatically during the 20th century, due to a combination of losses and degradation to habitat and roosting resources. They are fully protected under UK and European law, making it an offence of kill, capture, or disturb them, or to damage their habitat. Appropriate survey licences are required to work with these species.

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The appraisal is used to inform the emerging scheme design, enabling significant ecological effects within the zone of influence to be avoided, mitigated, or compensated for, where necessary. This ensures the continued favourable conservation status of bats, post development, and ultimately compliance with nature conservation legislation and planning policy. 

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There are several types of bat survey, comprising general habitat quality appraisals, activity surveys, Preliminary Roost Assessments (PRAs), Ground-Level Tree Assessments (GLTAs), Aerial Inspection Surveys (AISs), dusk emergence surveys, and hibernation surveys. Habitat quality appraisals involve assessing the potential of habitats to be used by foraging and dispersing bats, generally based on three indices: habitat composition (species diversity); habitat structure; and ecological connectivity to the wider landscape. Where ecologically valuable bat habitat is deemed likely to be affected by project proposals, activity surveys may be required, to establish species composition, abundance and distribution on site. These surveys entail a combination of night-time bat walkovers, and the deployment of static detectors, left to record for consecutive nights. The site is surveyed multiple times over the course of the bat activity season.

 

PRAs comprise a systematic visual and physical inspection of a structure, searching for evidence of bats, for features that bats could use for ingress/ egress, or for features potentially suitable for supporting roosting or hibernating bats. The results of the PRA inform the cope for any further recommended work. If the structure has negligible suitability to support bats, it can be scoped out of the study. However, if the structure is assessed as having greater than negligible suitability, or if evidence of bat utilisation is found, further survey utilising a different methodology may be required. GLTAs effectively entail the same process, though for trees, undertaken at ground-level.

 

Dusk emergence surveys involve the deployment of surveyors or strategically placed night-vision aids around a structure at dusk, to record bats emerging from any roosts within it, thereby establishing presence/ likely absence, and appraising roost characterisation - neither of which can be achieved from a PRA alone. Ultrasound bat detectors are used during this survey to aid with species identification. Hibernation surveys perform a similar function, though with a different methodology. They are undertaken either via physical inspections of the structure, or the deployment of static detectors, left to record for consecutive nights over the course of the core hibernation season.

 

AISs involve climbing trees to physically inspect potential roost features, searching for evidence of bat utilisation and appraising suitability. If the features are deemed to be unsuitable, they can be scoped out of the study. Sub-optimal features that are likely to be affected by project proposals can be subject to mitigation measures, to reduce the scope for adverse impacts. Suitable features may require two additional AISs or dusk emergence surveys to be undertaken, to determine the presence/ likely absence of roosting bats.

 

Requirements for any necessary avoidance, mitigation, or compensation measures will be identified, and opportunities for ecological enhancement will be outlined, to integrate on-site biodiversity net gain into the scheme at an early stage, where possible.​ If the proposals are deemed likely to adversely affect roosting bats, a European protected species mitigation licence may be required, prior to the commencement of works.

 

Bat surveys are seasonally constrained and may be protracted, making it prudent to plan ahead and consider ecological issues early on in the planning process. A robust bat survey at an early stage in project planning and design can therefore deliver significant benefits, avoiding potentially costly and time-consuming delays.​

 

When

 

Habitat quality appraisals, PRAs and GLTAs can be undertaken throughout the year, though the optimal season for GLTAs lies between December-March inclusive (when foliage does not preclude observations). Dusk surveys and AISs must be undertaken between May-September inclusive. Hibernation surveys can be undertaken between late October-February inclusive, while activity surveys typically extend throughout April-October inclusive.