What is a bat survey?
There are several types of bat surveys, different surveys are used to gather different types of information. Our ecologist specialise in delivering and designing Bat Surveys inline with the best practice guidelines
If your project impacts a building or structure, the two most common bat surveys are:
- An initial assessment – can often be called a Scoping Survey, Preliminary Roost Assessment (PRA) or Internal & External Visual Assessment.
- A nocturnal survey – is often called a Bat Dusk/Dawn Survey, or Bat Emergence/Re-Entry Survey.
If your project impacts a tree:
- A Ground-Based Potential Roost Feature (PRF) survey is required
The process
The first phase of the process is to carry out a Preliminary Roost Assessment (PRA) on the building or structure, or a Ground-Based Potential Roost Feature (PRF) survey on the site’s trees.
Then if the building or structure offers suitability for roosting bats or if evidence of roosting bats is recorded during the PRA, then Nocturnal Presence/Absence surveys are often required. Likewise, if the tree PRF survey records features that are suitable for roosting bats, Aerial Tree PRF surveys or Nocturnal Presence/Absence surveys are required.
When do you need a bat survey?
A bat survey is often requested by a planning authority as part of a planning application process if the application involves activities that could injure, kill or disturb the resting places of bats (roosts) or damage or impact foraging and commuting habitats. These include but not be limited to:
The conversion, modification, demolition or removal of buildings (including hotels, schools, hospitals, churches, commercial premises and derelict buildings) which are:
- agricultural buildings (e.g. farmhouses, barns and outbuildings) of traditional brick or stone construction and/or with exposed wooden beams;
- buildings with weatherboarding and/or hanging tiles that are within 200m of woodland and/or water;
- pre-1960 detached buildings and structures within 200m of woodland and/or water;
- pre-1914 buildings within 400m of woodland and/or water;
- pre-1914 buildings with gable ends or slate roofs, regardless of location;
- located within, or immediately adjacent to woodland and/or immediately adjacent to water;
- Dutch barns or livestock buildings with a single skin roof and board-and-gap or Yorkshire boarding if, following a Preliminary Roost Assessment, the site appears to be particularly suitable for bats
Preliminary Roost Assessment
A Preliminary Roost Assessment is a visual assessment of buildings or structures to search for Potential Roosting Features (PRFs) and evidence of roosting bats.
The building or structure is searched for potential roosting features such as small gaps in barge boards, soffits, and fascia boards, raised or missing ridge or roof tiles, and gaps in association with wall-plates or gables. These types of features have the potential to be used as access and exit points for bat roosts.
Evidence that potential access points are actively used by bats includes grease marks, presence of bat droppings or urine staining, lack of cobwebs or other detritus. Indicators that potential access points are likely to be inactive include the presence of cobwebs and general detritus within the feature’s access point or damp or wet conditions.
The interior and exterior of the building, including roof voids, are visually assessed for evidence of bat activity, where possible. The building or structure is also classified by its overall suitability for roosting bats.
Evidence of a roost is determined as the presence of a dead or live bat, concentrated piles or scattered droppings, food remains such as insect wing fragments, as well as scratch marks and/or staining.
When a roost is positively identified during a visual PRF Assessment, the building or structure within which the roost is located is classified within the category roost present. Other buildings are classified as having high, moderate, low or negligible suitability for use by roosting bats based upon the number and quality of PRFs present and the building’s position in relation to the surrounding habitats.
Following current good practice guidelines, buildings or structures with high suitability require three nocturnal surveys to adequately confirm the presence or likely absence. Likewise, buildings with moderate suitability require two surveys and low suitability buildings require one survey.
Nocturnal Presence/Absence Surveys
Nocturnal bat surveys are to determine the presence or likely absence of a bat roost in a building, tree, or other structure (bridges, tunnels, caves, etc.). These surveys take place at dusk and/or dawn and involve surveyors observing the building, tree, or structure from vantage points. Any bats seen to be entering or leaving the building or structure are recorded. In addition, surveyors record any other bat activity detectable from their survey position. Where possible, the time of recording, species, the number of bats, type of activity, and flight path of observed bats are also recorded. Bats entering or leaving a building or tree are considered evidence of bat roost presence within that entity.
Surveyors are equipped with broad-band bat detectors to allow calls to be recorded and analysed at a later date if required.
Our in-house bat ecologists have over 10 years of experience undertaking nocturnal surveys on trees as well as a range of building and structure types.
Nocturnal Bat Surveys can be undertaken from May to September, with the optimal period being May to August. The survey needs to be undertaken in weather conducive to bat activity, i.e. little or no rain, low wind, and temperatures above 10°C. Emergence (dusk) surveys commence 15-30 minutes prior to sunset and continue until at least 1.5 hours after sunset. Return (dawn) surveys commence at least 1.5 hours prior to sunrise and continue until 15-30 minutes after sunrise.
Ground-Based Potential Roost Feature (PRF) Survey
A ground-based PRF survey classifies the bat roost suitability of any trees within the predefined survey area. Features that are typically used by roosting bats are natural holes, woodpecker holes, cracks and splits in major limbs or stem, loose bark, dense thick-stemmed ivy, hollows/cavities, and bird, bat, or other mammal boxes. Signs indicating the use of tree features by roosting bats include scratches or staining around entry points, bat droppings in or around or below the entrance, audible squeaking at dusk or in warm weather, and flies around entry points in addition to smoothed surfaces and the absence of cobwebs and other detritus.
When a roost is positively identified during a visual PRF Assessment, the tree within which the roost is located is classified within the category roost present. Other trees are classified as having high, moderate, low, or negligible suitability for use by roosting bats based upon the number and quality of PRFs present and the tree’s position in relation to the surrounding habitats.
Aerial PRF Surveys
After a Ground-Based Survey, any features within the tree which offer high or moderate suitability or are classified as roost present can be reassessed from height. The licensed surveyor will use either a mobile elevated platform or ladders and ropes to undertake a close visual inspection of the Potential Roost Feature. This close visual inspection allows further evidence to be collected to either adjust (usually downgrade) the suitability of the feature or to inform further surveys.
