Bat Surveys

The first stage of a bat survey in the UK is the Preliminary Roost Assessment, commonly referred to as the 'Phase 1'. At this stage, one of our experienced ecologists will examine the site to determine the presence or potential existence of bats within the building. This assessment involves conducting a detailed inspection of structures, such as trees, buildings and other roosting features. The ecologist will look for visual clues such as droppings, markings, and signs of entry points, as well as evidence of any previous or current use by bats. They also consider the habitat surrounding the site, including the availability of foraging opportunities and suitable commuting routes for bats. Once completed, this assessment provides valuable insights into the need for further surveys and informs the subsequent steps of the bat survey process.

If there is no evidence or suspected potential for bats to roost in the building, no further surveys will be required.

Following the Preliminary Roost Assessment, if evidence suggests the presence of bats, Emergence Surveys are conducted. These surveys aim to gather additional data on the bat species, their population size and activity patterns. Emergence Surveys are carried out during the summer months, when the bats are most active at dawn or dusk. Ecologists strategically position themselves at vantage points near known or suspected roost sites and use specialist equipment, such as bat detectors, to record the ultrasonic echolocation calls emitted by bats as they leave their roosts to forage. These calls can be recorded and monitored to identify species and estimate population numbers. By repeating the survey over multiple nights, ecologists can build a comprehensive picture of bat activity and seasonal variations, further informing conservation efforts and any necessary mitigation measures that may need to be implemented.

In-depth data on the bat species presence and their use of a site can be recorded using advanced licenced bat survey techniques. These are recommended in areas where very rare or endangered bat species may be roosting and where the development may have severe impacts. For example, if a site falls within a certain distance from a Special Area of Conservation (SAC) that has been designated for a rare species such as Barbastelle bat (Barbastella barbastellus). These survey methods carry a much higher risk to the health and welfare of the bats, so a project licence from Natural England is required to undertake them. They are typically reserved for situations where the data required for a development exceeds the capacity of less intrusive.

Bat trapping surveys are not typically conducted in the maternity period (May to July inclusive), as trapping bats with dependent young can be seriously harmful.

During a bat trapping survey, the landscape is assessed for suitable bat habitat, and survey effort is focussed on features that may be used by bats for foraging or commuting. Just before sunset a combination of harp traps, mist nets, and lures are set up among these features and are checked periodically. Any bats that fly into the traps are removed, checked over for their health and have their biometrics taken (for example weight, forearm length, life stage) before being released to carry on with their night. These surveys carry on until the peak activity drops, at which point the traps are packed down.

The results of bat trapping surveys can inform developers on which bat species are using the landscape and in what capacity. For example, if lots of post-lactating females are trapped it indicates there is a maternity roost nearby. Trapping surveys can also be paired with radiotracking, during which a radio transmitter tag is humanely attached to a bat and surveyors can follow the radio signals back to a roost. This is quicker and more cost effective for finding roosts in woodlands than climbing surveys undertaken on every tree.

Bat Activity Surveys (or Night Time Bat Walks), are sometimes required where developments may impact suitable bat habitat on a landscape scale. Examples of these include:

• Temporary removal of hedgerow to accommodate a cable route for an offshore windfarm;

• Removal of hedgerows or trees for a new housing development; or

• Installation of new lighting (e.g. road lighting).

The aim of these night-time transects or fixed-point surveys is to determine the use of the landscape by bats, and how they may be affected by the proposal. Important flight lines will be identified and can be mitigated for with the design of the development to ensure bats will still be able to commute and forage in the landscape.

During an activity survey, surveyors will walk a set route around the identified area from sunset for 2 to 3 hours, documenting any bats heard or seen. Using this data, maps will be drawn of where the bats are flying to and from, and how frequently. This can also help identify bat roosts or swarming sites. Night-time bat walks are also paired with static bat detectors, which are left at important features in the landscape to record nightly bat activity for several nights. This data is combined to help build a landscape-scale picture of bat activity in the area.

Bat surveys can only be conducted between April and October inclusive, and surveys must be spread out evenly over this period, as well as conducted in fair weather.

The survey results will help determine where appropriate mitigation is needed to minimise the impact of the development on bats. The main aim of these surveys is to inform the design of a development to minimise the impact of bats on the scheme.

Bat hibernation surveys may be required when a building or feature is identified as having hibernation roost potential. This will have been identified during the initial scoping assessment, and is still required even if emergence surveys are also taking place.

During a hibernation survey, the identified feature or building is inspected for the presence of bats. This may include the use of an endoscope for deep features that cannot be seen into properly, for example a deep crack in a wooden beam in a barn. Any bats found will be noted on a map with their species and numbers, and then they will be left to remain in situ. Temperature readings are taken outside as well as inside the roost.

These surveys can only be undertaken during the hibernation period in winter, with the best time for this being January or February, as temperatures need to be consistently low to allow for hibernation behaviour.

If hibernating bats are found, the planned works will need to attain a European Protected Species License (EPSL) from Natural England to enable the works to proceed, which will detail the timings of the works as well as mitigation and enhancements to encourage bats back to the site. A development will need to mitigate for any impact to the roost. For example, bat boxes can be added to a property where re-roofing works may require a loft void to be impacted. The presence of a hibernation roost will help to inform when works can be conducted; i.e. outside the hibernation period. 

The final stage involves compiling a detailed report to summarise the findings and propose suitable mitigation measures. These may include incorporating bat-friendly design elements into the development, such as bat boxes or specially-designed roosting features.