Introduction

Project description
Autumn swarming bat behaviour was confirmed in 2009 at the east and west ferneries (internally and externally) of the South Terrace at Cliveden. This National Trust site underwent major restorative works between 2012-2017, planned to avoid affecting bat use of the site. Post-development monitoring confirms continued bat swarming activity onsite.
Ecologist’s name and contact details
Name: Chris Damant. Company: Bernwood ECS Ltd. Address: Hensmans Farmhouse, Nearton End, Swanbourne, MK17 0SL. Email: chris@bernwood.net. Mobile: 07817131683.
Client’s name
Client: The National Trust. Site: Cliveden. Contact: Jo Hodgkins, Wildlife Advisor – London and South East. Email: joanne.hodgkins@nationaltrust.org.uk.
Site postcode
SL1 8NS
Planning authority
Chiltern District Council, now Buckinghamshire Council.
Brief site description
The National Trust’s property at Cliveden is situated on an elevated wooded escarpment high above the river Thames, set within a mixed historic landscape of semi-natural woodland grassland, garden parterre, and avenues. The South Terrace at Cliveden is thought to be the oldest surviving structure of the original house. Built in the late 1600’s, the South Terrace is a unique architectural feature, altered to include two ferneries (one east and one west) and four cement-lined water tanks (two at each end). Within the ferneries are brick vaulted ceilings with voids between them and the terrace surface. Blind window vaults provide additional void space for bats, as do the cavity walls of the ferneries and water tanks. Over 300 years of exposure to the elements, water damage from faulty drainage, and natural crumbling of the masonry found the structure in dire need of repairs. The National Trust commenced a £6 million pound restoration project on the Terrace in 2012. The major repair proposals required the complete renovation of the South Terrace: removal of render, re-laying of surface paving, repair of the central staircase, repair of brick work damaged by metal spalling, and ironmongery, including gilded grills and gates. Initially, the rebuilding of the ferneries was programmed to take two years of continuous construction activity. Bat swarming activity at the Terrace was first suspected during a Biodiversity Evaluation in 2008. Bernwood secured a specific Natural England project licence to conduct six detailed swarming surveys in 2009 with the assistance of both the Berkshire and South Buckinghamshire and the North Bucks Bat Groups. The surveys found the swarming activity confined to the ferneries and water tanks at each end of the Terrace, and identified a total of 113 individuals of four different species swarming: four Daubenton’s bats Myotis daubentonii, one Bechstein’s bat Myotis bechsteinii, two brown long-eared bats Plecotus auritus, and 106 Natterer’s bats Myotis nattereri. It also identified that the bat activity is dominated by males, a characteristic of bat swarming activity, with 93 males and 20 females caught in total during the six surveys (87 male and 19 female Natterer’s bats). In 2012, hibernation surveys were carried out at the Terrace, observing a single Natterer’s bat inside. Repeated every year by Bernwood, observation of bat hibernation is limited to small numbers of brown long-eared and Natterer’s bats, partly due to restricted surveyor access both to the water tanks (confined spaces requiring emergency recovery support) and to inaccessible hibernation areas (i.e. wall cavities). In the absence of bat mitigation measures, the initial two-year period of continuous construction activity would have resulted in the loss of the site for autumn bat swarming and hibernation use. By working with the National Trust and their appointed architects, the restoration proposals evolved into a more nuanced approach to avoid, minimise, and compensate for any effects of the works on bats. For brevity, this form will focus on the Natterer’s bat swarming survey data, but all works and measures affected all species on site.

Pre-works roost structure

Type of structure
Building
Use
Other
Condition
Not In Use
Approx. age
Approximately 350 years, as originally built around 1670; subject to alterations since.
Main construction material of walls
Brick
Roof design
Flat Roof
Roof material
Other
Internal roof structure
Not Applicable
Photos or annotated figures of roost structure

Pre-works roost description

Species
Natterer’s bat
Number of bats max count
Totals: 106 (87 males, 19 females) bats over six surveys in 2009; 12 (5 males, 7 females) over five surveys in 2010; 83 (50 males, 30 females, 3 undetermined) over two surveys in 2011; 17 (15 males, 2 females) bats over three surveys in 2012.
Type of roost
Swarming Site
Evidence of bats
Bats Identified in Hand
Roost location
In Cavity Wall
Aspect of roost
Various
Height of roost entrance (m)
The south terrace is approximately 9m in height, although bats utilise the whole structure for swarming, including external areas.
Roost material(s)
  • Brickwork
Nearest commuting feature
Other
Distance to nearest commuting feature (m)
0 m; situated within the complex historic landscape of semi-natural woodland grassland, garden parterre and avenues on an elevated wooded escarpment high above the Thames River.
Photos or annotated figures of roost

Proposed works

Description of works
Initial proposals to complete the restoration over two years would have resulted in the site’s loss of bat use for the duration. Through collaboration with the National Trust and their appointed architects (Julian Harrap Architects LLP and Rodney Melville & Partners), the restoration proposals evolved to protect and maintain the bat use of the South Terrace. The proposals took on a more nuanced approach, considering the significant autumn swarming bat activity at either end of the terrace. This shift strictly adhered to the ecological mitigation hierarchy: avoidance, minimisation, mitigation, compensation, and enhancement; and the proposed actions resulted from consultation with the National Trust, their architects and specialist advisor John Altringham, together with Natural England. - Avoidance was proposed through the implementation of construction phases, i.e., scheduling works outside of the autumn swarming and hibernation seasons with - Permit to Work systems; as well as through any repairs that could be completed off-site, e.g. ironmongery of gilded grates. - Minimisation of the repairs meant restricting works to those essential to maintain the structure’s fabric, leading to the retention of the ferneries, water tanks, and voids. - Mitigation in the form of pre-start briefings and toolbox talks for all involved in the project, as well as supervision by an Ecological Clerk of Works. - Compensation for the total loss of the water tanks as a result of potentially drying out from the newly restored Terrace drainage system was prevented through the provision of water troughs and securing of a water supply, allowing humidity and temperature control in the water tanks if necessary in the future. - Enhancement of the site for increased future roost use included the creation of new bat access points into the ferneries’ cavity walls and ceiling voids, the installation of bat tiles and bat bricks within the water tanks.
Type of impact upon the roost
Disturbance
Relevant annotated figures

Proposed mitigations

Type of mitigation
Avoidance
Specific technical detail of measure
Works were restricted to those essential for the structure’s maintenance, timed to avoid the autumn swarming and hibernation periods, and all contract specifications included obligations to the biodiversity interest of the site. This included: - Extension of the contract period from two to five years to allow for the phasing of works. - Supervision by a suitably qualified Ecological Clerk of Works. - Contracts let with specific clauses to control the phasing of works, including a Permit to Work system for ferneries and water tanks. - Limiting scaffold to the central areas only (away from the water tanks/ ferneries) during these periods. - Sequential phasing of the terrace surface repairs at the east and western ends to take place during the summer months. - Off-site repairs to grills and grates with gilding taking place during summer months. - Pre-start briefing and toolbox talks included in site inductions for all staff, contractors and sub-contractors working on the project. - Provision of bat tiles and bat bricks within the water tanks to increase the availability of hibernation points. - Creation of new bat access points to the bricked-up blind windows inside the western water tanks. - Creation of small access points to the ferneries’ cavity walls. - Leaving a brick out of west fernery’s barrel-vaulted ceiling to allow for bat access to void above, thought to be previously inaccessible from the fernery. - In case the water tanks show signs of long-term drying out (below 70% relative humidity) as a consequence of the drainage works, a water supply and water troughs were installed in the water tanks to allow for future humidity control.
Roost location
Under Fascia Boards
Aspect of roost
N
Roost material(s)
Nearest commuting feature
Hedge

Monitoring data

Length of monitoring proposed
During works and 5 years post-development; for a total of 10 years.
Frequency of monitoring
Annual, 2012-2022/2023.
Type of monitoring
Dusk and Dawn Survey
Date and time
8th September 2012 01:25
Evidence recorded
Over three bat catching surveys: 23 Natterer’s bats, 1 Daubenton’s bat, 1 brown long-eared bat, and 1 common pipistrelle.
Interventions made
None.
Type of monitoring
Dusk and Dawn Survey
Date and time
23rd September 2013 00:28
Evidence recorded
Over two bat catching surveys: 88 Natterer’s bats, 6 brown long-eared bats, 3 Daubenton’s bats, 1 common and 1 soprano pipistrelles.
Interventions made
None.
Type of monitoring
Dusk and Dawn Survey
Date and time
17th September 2014 00:07
Evidence recorded
Over two bat catching surveys: 54 Natterer’s bats, 6 brown long-eared bats, and 1 Bechstein’s bat.
Interventions made
None.
Type of monitoring
Dusk and Dawn Survey
Date and time
23rd November 2015 22:36
Evidence recorded
Over three bat catching surveys: 25 Natterer’s bats, 2 Daubenton’s bats, 2 brown long-eared bats, 2 soprano pipistrelles, and 1 barbastelle.
Internal temperature and humidity recorded
Average specific humidity was 7.86 g/kg in December 2015 (first year measured), and average temperature was 10.52°C.
External temperature and humidity recorded
Average specific humidity was 7.65 g/kg in December 2015 (first year measured), and average temperature was 10.66°C.
Interventions made
None.
Type of monitoring
Dusk and Dawn Survey
Date and time
22nd November 2016 21:50
Evidence recorded
Over three bat catching surveys: 52 Natterer’s bats, 4 brown long-eared bats, and 1 common pipistrelle.
Interventions made
None.
Type of monitoring
Dusk and Dawn Survey
Date and time
24th September 2017 00:30
Evidence recorded
Over three bat catching surveys: 47 Natterer’s bats, 11 brown long-eared bats, 4 Daubenton’s bats, and 1 common pipistrelle.
Internal temperature and humidity recorded
Average specific humidity was 6.28 g/kg in December 2017 – February 2018 (after works completion), and average temperature was 7.52°C.
External temperature and humidity recorded
Average specific humidity was 5.22 g/kg in December 2017 - February 2018 (after works completion), and average temperature was 5.43°C.
Interventions made
None.
Type of monitoring
Dusk and Dawn Survey
Date and time
6th October 2018 00:15
Evidence recorded
Over three bat catching surveys: 104 Natterer’s bats, 9 brown long-eared bats, and 1 common pipistrelle.
Interventions made
None.
Type of monitoring
Dusk and Dawn Survey
Date and time
26th September 2019 00:20
Evidence recorded
Over one bat catching survey: 42 Natterer’s bats, 1 Bechstein’s bat, 1 Daubenton’s bat, 1 brown long-eared bat, 1 common and 1 soprano pipistrelles.
Interventions made
None.

Final details

Lessons learned
Flexibility and commitment to biodiversity was important, as well as clear communication channels with all parties involved in drafting the design proposals, to maintain continued bat use of the site during and after works. This prioritisation of biodiversity was at the heart of all decision-making, and the results that simple avoidance, the first step of the ecological mitigation hierarchy, can produce were emphasised. To date, no practical interventions have been made as a consequence of monitoring data. It remains too early to say whether the South Terrace repairs have had either a positive or negative impact on the environmental conditions within the ferneries or water tanks. However, continued bat use over the years has been well-documented through bat catching, hibernation checks and acoustic surveys. The COVID-19 crisis has impeded autumn swarming surveys in 2020 due to the risk of handling bats and the impact on the financial resources of the National Trust. The intention is to resume surveys in 2021. In response to the crisis, Bernwood Ecology have been experimenting with alternative monitoring methods that are less invasive and potentially less costly, such as using infra-red break beam bat counters, as well as coupling infra-red and thermal imaging footage as an alternative to mist-netting. We hope to submit updates on these techniques to the ROOST Awards next year. Project Partners Client: National Trust; Jo Hodgkins, Vicky Marshall, Janet Gough, Rob Brind, and property staff Ecologist: Bernwood Ecology Architects: Rodney Melville & Partners and Julian Harrap Architects Contractors: Daedalus Conservation