Bat Mitigation Guidelines: Preserving a Hibernaculum
For updates on an active project we have worked on, visit this write up by the Landmark Trust!
Mine-workings are often used by bats as hibernacula, and Middleton Upper Quarry in Midlothian has been the winter home to Daubenton’s, Natterer’s and brown long-eared bats since the 1980s.
When a local authority enforcement notice meant the quarry was to be ‘reinstated’ as grassland we were commissioned to protect the hibernaculum. It is now 10 years since this project was completed, and with its inclusion in the 2023 Bat Mitigation Guidelines, we wanted to share a summary.
Middleton Upper Quarry in 1945
What Was done
The most used of the three adits was reinforced with a 9m high, steeply sloping gabion basket funnel. This acted as a retaining wall when the quarry was filled in with 750,000 tonnes of inert spoil.
Helping Bats find the entrance
To ensure bats could still find the entrance, 25m of well-connected hedgerow was maintained leading them to the retained adit. The cliff face above the adit was retained as a familiar feature, making up one side of the protected entrance.
Managing the Temperature
Bats require suitable temperatures to facilitate hibernation. As the mine sloped towards the retained adit, warm air was still able to vent and an additional vent was installed at another former entrance.
Between these two vents, the temperature of the mine has remained between 4°C and 8°C, with a greater area around 6°C increasing the area of the mine used by hibernating bats.
Middleton Upper Quarry before infilling with 750,000 tonnes of inert spoil.
Monitoring & Access
As the gabion is 9m deep, fall arrest blocks and safety harnesses are used to reach the adit, with a new steel ladder inside the adit allowing safe access to the mine floor 6m below the entrance. To keep the site safe, there is security fencing with a locked gate around the entrance, and we make regular inspection and survey visits to ensure all is well.
The gabion basket funnel and original cliff face above the adit.
Surveys & Monitoring
Prior to filling in the quarry, three adits were acoustically monitored to identify which were most used by bats entering and leaving the mine. Acoustic monitoring was accompanied by surveys of the hibernacula and in the three years’ before infilling began, a mean of 4.9 bats were observed across 7 surveys.
In the 8 years’ after infilling a mean of 6.0 bats were found across 11 surveys, with all three species still well represented. Comparing the visual survey data with acoustic monitoring suggests this is a small part of the population of bats using the mine.
This represents a successful mitigation strategy to preserve the function on the quarry, while allowing the owner’s to meet the requirements of the local authority enforcement notice.
If you require protected species surveys or advice, please contact our team of specialist ecologists today.