ANHIB Maternity Colony Bat Fence Mitigation

Introduction

Project description

A three species maternity colony identified within a disturbance distance of bridge replacement works (2019). Impacts assessed as vehicle collision mortality, severance of commuting routes and disturbance (noise vibration, light) of during maternity season potentially leading to roost abandonment. Main mitigation included bat fencing and timing of works.

Ecologist’s name and contact details

Erin Grieve BSc (Hons) CEcol MCIEEM, Jacobs.

Client’s name

Transport Scotland (via BEAR Scotland)

Planning authority

Highland Council

Brief site description

The site is located in a rural location on the A887 road in north-west Scotland. A minor branched watercourse traverses the site and flows into the River Moriston (an SAC for Freshwater pearl mussel (Margaritifera margaritifera) and Atlantic salmon (Salmo salar) 150m north. The surrounding habitat is good for foraging bats. In addition to watercourses, the site is surrounded by predominantly broadleaved woodland (primarily birch stands) to the north, east and west with extensive conifer plantations to the south. Within the wider landscape this woodland connects to other woodland within a valley system surrounded by mountainous areas with limited suitable habitat for bats.

Pre-works roost structure

Type of structure

Building

Use

Domestic Dwelling

Condition

In Use

Approx. age

Cottage unknown 50+ years with extension age unknown but within 20 years. Shed unknown but likely within 20 years.

Main construction material of walls

Stone

Roof design

Various

Roof material

Slate

Internal roof structure

Timber Frame

Lighting present on site and its proximity to the roost

only home security (private land only accessed by residents) lighting triggered by activity in very quite location (very little activation).

Photos or annotated figures of roost structure

Pre-works roost description

Species

Soprano pipistrelle

Number of bats max count

600+

Type of roost

Maternity Roost

Evidence of bats

Bats Recorded Emerging/Re-entering

Roost location

Gable Wall Tops

Aspect of roost

Various

Height of roost entrance (m)

Minimum height of 2m (guttering) to top of roof (lead flashing approximately 5m from ground level).

Roost material(s)

• Slate Tiles
• Timber Roof Frame

Nearest commuting feature

Treeline

Distance to nearest commuting feature (m)

0-10m

Nearest artificial light source to roost

5m - but facing away from bats

Photos or annotated figures of roost

Proposed works

Description of works

Bridge replacement scheme including installation of temporary road with culverts and realignment of minor watercourse; undertaken during summer 2021. Tree felling undertaken in winter 2020. Duration of main works was seven consecutive months, and due to other sensitive ecology on site (e.g. fish passage and dewatering), were required to be undertaken between April and October. This required consultation with fisheries board and NatureScot.

Type of impact upon the roost

Disturbance

Relevant annotated figures

Proposed mitigations

Type of mitigation

Avoidance

Specific technical detail of measure

• All works were undertaken under a derogation licence and in consultation with NatureScot.
• A pre-start briefing as part of the site induction and toolbox talk was delivered to all site staff.
• Key activities throughout works were supervision by a suitable qualified and licenced Ecological Clerk of Works. As site visits were reduced due to Covid-19 restrictions (see monitoring section below), constant communication was maintained between the ecology team and contractors on site (minimum aiming for weekly updates).
• A no works exclusion zone within 30m of BB2 and BB3 was implemented during the peak maternity period (June, July and August inclusive).
• Sequential phasing of works (e.g. earthworks, temporary road construction in segments within area within 30m of cottage and shed built in early maternity season) both within and outwith the 30m exclusion zone were advised on by the bat licenced ecologist to reduce impact on the main roosts.
• During April and May, a 52m long and 4m high bat fence was installed between the temporary road (already raised 1-1.5m from ground level) and the roost locations. This fencing aimed to: discourage bats from using a previous commuting route which would mean crossing the temporary road and guide them to safer routes; to shield the main roost points from light spill and noise from the cars from the temporary road; and to maintain some of the structure of the surrounding habitat lost from tree felling located around BB2 and BB3 (identified as important socialising area). To achieve these aims the lower 2m of fence consisted of wood paneling with the upper 2m of narrow wire mesh. The bat fence included a ‘kick-back’ (end of the fencing partially turning back on itself) at the most easterly extent to encourage bats away from the temporary diversion road where the fencing ends.
• The temporary road was subject to speed restrictions (40mph) both for the safety of road users and to reduce potential vehicle collision mortality of bats.
• At key points throughout works, activity surveys were undertaken to monitor the population size and composition of all roosts on site as well as the disrupted commuting route. The aim was to allow ecologists to monitor the effectiveness of the implemented mitigation and to amend or introduce further mitigation, if required.
• Night works were minimised where possible and any night lighting was angled away from remaining commuting routes and roost locations.
• Evidence of collision mortality to be reported to the ecologist.
• After works, the habitat (specifically the felled trees either side of the existing A887 and those connecting BB2 and BB3 to the new bridge) was reinstated with new trees planted.

Roost location

Under Fascia Boards

Aspect of roost

N

Roost material(s)

Nearest commuting feature

Hedge

Monitoring data

Length of monitoring proposed

Due to restrictions of the number of people on site during Covid-19, monitoring site visits were limited to the duration of the main works (April to September 2021 inclusive).

Frequency of monitoring

One site visit to undertake an array of surveys and daytime inspections approximately every two weeks coinciding with key work activities. See Table 1 below.

Type of monitoring

Roost Inspection

Additional species mitigation undertaken

• Emergence and re-entry surveys on known roosts to monitor population dynamics during works. Due to Covid-19 restrictions on number of surveyors, surveys focused on the front side of BB2 and BB3 (side closest to works).
• Bespoke vantage point/ crossing point surveys of bat fencing and commuting routes to evaluate effectiveness of mitigation.
• Activity surveys on BB3/BB2 were paired with crossing point surveys on the bat fencing to provide context to results.
• Static detector deployments at BB3 to monitor roost activity and population dynamics.
• Daytime endoscope inspections of known roost points, where necessary.
• All activity surveys utilised infrared and/or thermal cameras and full spectrum bat detectors.


Bat roosts
Records of peak roosts counts are provided in Table 2 below. Overall, there were relatively consistent numbers of bats recorded throughout both baseline years (2019 & 2020) compared with the year of works (2021) and so no negative impact on the bat populations on site were identified within this time period.

Bat fence and commuting routes
No dead or injured bats were identified on site and all bat species were recorded utilising alternative commuting routes encouraged by the fence. Observations indicated the majority of bats seen interacting with the fence were guided away from crossing over the road and towards safer crossing points.

Pipistrelles were observed using the woodland side of the fence for foraging. In addition, pipistrelles were observed flying over the fence and across the temporary road; however, the fence encouraged these bats to fly at a height above almost all traffic (except lorries). The ‘kick back’ was not shown to have any impact on pipistrelle sp. and a small number of pipistrelle sp. were observed flying beyond the limits of the fencing and crossing over the road at car level.

Brown long-eared bats and Daubenton’s bats were also observed interacting with the bat fencing. At least one bat from both species were observed flying along the fencing, entering the ‘kick back’ and then turning back. A few individuals were observed flying beyond the ‘kick back’ but then turning back the way they had come and away from the open road habitat. These species were not observed flying over the fencing.

Remedial action was required to amend the site boundary Heras fencing (located along one of the alternative commuting routes; see photo below). During the crossing point surveys a small number of bats were observed colliding with this fencing (no injuries observed) and as such the fencing was replaced with half height fencing. No further collisions were recorded.

Photos or annotated figures

Final details

Lessons learned

This was a highly complex project both from a biodiversity and engineering standpoint. As such, a clear understanding of the ecology of the biodiversity involved and key potential impacts as well as engineering and design constraints were essential. A clear and cooperative communication was required with iterative and responsive survey and mitigation design, especially with changing Covid-19 restrictions. Bespoke survey design (paired surveys, amended crossing point surveys, floating surveyors on activity surveys) was crucial to provide the relevant information to inform the impact assessment and mitigation design.

Evidence from literature was important to inform the bespoke mitigation design, support conclusions and increase likelihood of success. Consideration of the long-term impacts (e.g. not just from works happening at the time, but also impacts on the socialising area for success in future years) was an important aim with such a high conservation value roost. Similarly, understanding how bats were using the area (rather than just roosts) made designing mitigation for long-term impacts more likely to be successful.