two mile bottom bat hibernaculum from folly to fantasy

1 TWO-MILE BOTTOM BAT HIBERNACULUM

Trans. Suffolk Nat. Soc. 49 (2013)

TWO MILE BOTTOM BAT HIBERNACULUM FROM FOLLY TO FANTASY

NICK GIBBONS

Introduction At some time in 2002 John Goldsmith suggested to me the idea of a new artificial bat hibernaculum for the Thetford Forest, a pipe-dream he had harboured for several years and, in a weak moment, I appear to have agreed with him. The site he had in mind, having considered most of the forest area, was at Two Mile Bottom where a small, shallow valley, possibly a site of old sand-diggings, ran down almost to the River Ouse. The bottom of the shallow valley had no trees and those at the top of the slopes were relatively immature at 22 years of age. It fulfilled several key requirements: near a river; existing bat usage along the water-course that had been confirmed by detectors; a north-facing entrance; shaded; away from existing used tracks and away from areas of high human usage; no archaeological sites present to disturb, while the existing valley shape meant much less soil to move, so a less expensive project.

A plan for a 100 metre-long ‘Y’ shaped tunnel was put together by John Goldsmith and myself based on a similar construction to the very successful High Lodge hibernaculum that had been built in 1991 to a ‘Goldsmith/Tilford’ design although the general design and approach had been used at several sites. The High Lodge hibernaculum had a wide variety of hibernating bat species, including Barbastelle, At that point it was not thought that the new tunnel would be too expensive and the three quotes received for the work were a bit of a shock at over £40,000.

With money from Forestry Commission and a grant from WREN supported by Forest Heath District Council it was decided that it would be possible to build a smaller, ‘Y’ shaped tunnel at around £20000, with provision to extend it at a later date if funds became available. In the autumn of 2003 the Friends of Thetford Forest (FoTF) volunteer group carried out clearance of obstructing scrub, and all was set to go. At the last minute additional funding became available from the Environment Agency, and Suffolk County Council also sponsored the interpretation and access facilities, thus allowing the tunnel to be built to the planned and full size.

Construction

Although FC is technically exempt from the requirement to apply for planning permission from the local authority there is agreement that normal planning requirements are followed. A hibernaculum is something of a moot point as it does not neatly fit into any category such as a dwelling, agricultural building etc. Having got planning permission construction commenced in February 2004.

The hibernaculum was built by a cut and back-fill process to produce a Y shaped tunnel some 95m long. The walls are concrete blocks whilst the ceiling was designed to be of floated reinforced concrete with bat bricks set into the ceiling. Due to its length manholes were fitted at the end of each arm


Suffolk Natural History, Vol. 49 2

of the tunnel as emergency outlets, and two 100 mm plastic pipes were set into the end wall of each arm for ventilation. The entrance was fitted with a grill and a concrete apron and bund installed at the front (Plate 1).

There were a number issues encountered during the construction. The first was with the depth of the trench required at the far end of the short arm during construction as the site was found to be ‘running sand’, even at a depth of three metres. Consequently the proposed slope of the main tunnel had to be reduced as, in order to allow safe working, an extra three rows of trees would need to be removed and many hundred of extra tons of sand taken out. The final result was that the short arm of the tunnel did slope downwards 1:100 but the long arm was level. The running sand also meant that additional foundation support was required between the walls to cope with any inward pressure after backfilling.

The ability to put a poured concrete roof on the tunnel was also found to be problematical, due to access issues for lorries, while pumping the concrete to the site. This would have sent the costs of the project even higher. Instead



a beam and block roof, which had been successfully used at a site at Bayfield Hall, was incorporated in its place. This change in construction methods led to other issues as the supporting beams were reinforced with high tensile steel wire and the steel ends would be open to soil, water and acids that would cause the steel to slowly rust inside the concrete and weaken the roof. In addition, greater strength had to be engineered into the abutment between the beams and the walls as the “L” shaped steel bars could not be inserted in the way that they would have been with a poured concrete system of construction. The agreed compromise was that the steel ends were treated to seal them and, once the backfilling around the tunnel was near to roof level, the ends of the beams would be encased in concrete to produce extra strength and help reduce any corrosion problems.

One issue that did not raise its head until some time after construction was that the door and frame went right down to the level of the floor concrete. This resulted in the lower lock and bolt system being constantly inundated with water during wet weather and sand and grit getting into the lower bolt slider making it often very difficult to move. There should have been a gap of 5cm plus between the bottom of the door and the tunnel floor.

Volunteers finally fenced the site to prevent accidental damage from forestry machinery, although the roof of the tunnel was constructed in such a way that it would withstand a vehicle crossing.

The tunnel was completed in the summer of 2004 and officially opened by John Goldsmith

Fittings During construction 90 Norfolk bat bricks that had been incorporated at approximately 1m intervals, mainly in the ceiling, but with a few in the top of the side walls. In addition to these a number of ‘holed’ engineering bricks were incorporated into the structure at changes of angle of the tunnel.

During the summer of 2004 other fittings such as hanging planks and large logs with slots cut in by chain saw were added, scattered along the length of the tunnel by the FoTF volunteers. A number of spare concrete blocks were also stacked at various points with spacers between to create additional niches.

Monitoring The hibernaculum has been regularly monitored for bats during the winter months by Forestry Commission staff and members of the Suffolk Bat Group. Bill Landells has monitored the temperature using digital temperature recorders.

Bat usage During the first winter no bats were found, although some Brown Long-eared droppings were deposited near the entrance. However, it was well used by Herald moths – up to 28 being counted. This species is always a good indicator species, suggesting that any construction is not too far away from being environmentally suitable for bats, as the two species typically co-occur in hibernation sites.



Then, in January 2007, a single Brown Long-eared Bat was found hibernating behind an engineering brick in the hibernaculum. This increased to seven individuals during the autumn of 2007, when they were recorded flying around inside the tunnel. In January 2008 two Brown Long-eared bats were found hibernating. Things seemed promising at this point, in that the hibernaculum was at least registering on the local bats’ ‘radar’. Whilst this was a success, the target species of Natterer’s, Daubenton’s and Barbastelle were still missing.

In January 2009 Arthur Rivett and myself visited the hibernaculum during a very cold spell of weather (a ground temperature of −16oC was recorded at Santon Downham) and sixteen bats were recorded. These were of three species: Daubenton’s (9), Natterer’s (4) and Brown Long-eared (3). These bats were recorded in a variety of situations within the tunnel, including behind wooden boards, in structural holes, in the slotted wooden blocks and the bespoke bat bricks.

Two Mile Bottom Total bats present

0

10

20

30

40

50

60

70

Ja

n 2

00

7

Se

pt 2

00

7

Ja

n 2

00

8

Ja

n 2

00

9

Fe

b 2

00

9

Ja

n 2

01

0

Fe

b 2

01

0

No

v 2

01

0

De

c 2

01

0

Fe

b 2

01

1

De

c 2

01

1

Ja

n 2

01

2

Fe

b 2

01

2

No

v 2

01

2

De

c 2

01

2

Ja

n 2

01

3

Fe

b 2

01

3

Ma

r 20

13

Ma

r 20

13

Date

No

:

Figure 1 Total bat usage of Two Mile Bottom.

A second visit on 6 February by Arthur Rivett, Nick Woods and myself during a very cold snap recorded a total of 13 bats: 6 Daubenton’s, 5 Natterer’s and 2 Brown Long-eared. The two Brown Long-eared are likely to be the same ones as found in the January check as their position, one in a log and one hanging free on a roof beam, were the same. All of the Daubenton’s in February were in the bat bricks and none were found behind the hanging boards as two had been at the January check. One of the Natterer’s in the February check was located by Nick Woods who heard it squeaking. It was eventually located with the aid of a mirror, being tucked behind one of the horizontal bat bricks (Plate 3) where there was a small gap in the concrete. One Natterer’s was again located as before, in a block in the roof.

Numbers continue to rise and in December 2011 a total of 31 bats were already present and in February 2013 this had increased to 62 bats. Based on previous years peak counts this is likely to be the peak time for bat usage.



As well as constructing the tunnel, a number of bat boxes were placed on the trees around the site, both wooden and woodcrete. These proved very successful. Within the first year Leisler’s, Pipistrelle and Brown Long-eared bats were using them and Noctule was recorded in 2012.

Temperature and ventilation The temperatures recorded in the tunnel during the first winter were not as low as expected and several modifications were made to try and ‘tweak’ the microclimate in the summer of 2005. The earth bund outside the entrance of the hibernaculum was slightly re-shaped and lowered by around one metre to try and capture more cool air. The four ventilation pipes at the far end of the two tunnels tunnel were modified to increase the dimensions from 25 mm to 50 mm to try and get a better air flow through the structure. This was later increased the following year to the full 100 mm that corresponded with fittings in the roof of the tunnel.

This has been calibrated using the information gathered from the three data loggers that are installed in the tunnel, one by the entrance and one at each end of the far ends of the tunnels. Temperatures have fluctuated rather more widely and more quickly than were predicted.

The temperature in the entrance clearly fluctuates quite markedly with the ambient outside temperature in all of the data. With the smaller vents in 2008 to 2010 the variation in both the short arm and the long arm are much more stable and also were remarkably similar, following one another very closely.

In 2010 a larger vent (30 cm diameter) was fitted into the manhole cover to the short arm of the site.

0

10

20

30

40

50

60

70

No:

Jan 2

007

Sept 2

007

Jan 2

008

Jan 2

009

Feb 2

009

Jan 2

010

Feb 2

010

Nov 2

010

Dec 2

010

Feb 2

011

Dec 2

011

Jan 2

012

Feb 2

012

Nov 2

012

Dec 2

012

Jan 2

013

Feb 2

013

Date

Two Mile Bottom Bat Usage by Species

Natterer's

Daubenton's

Brow n Long-eared

Figure 2. Bat usage by species.

0

10

20

30

40

50

60

70

No:

Jan 2

007

Sept 2

007

Jan 2

008

Jan 2

009

Feb 2

009

Jan 2

010

Feb 2

010

Nov 2

010

Dec 2

010

Feb 2

011

Dec 2

011

Jan 2

012

Feb 2

012

Nov 2

012

Dec 2

012

Jan 2

013

Feb 2

013

Date

Two Mile Bottom Bat Usage by Species

Natterer's

Daubenton's

Brow n Long-eared



Figure 3. Temperature records for the winter 2008/9.

Figure 4. Temperature records for the winter of 2010/11.

After fitting the first of the large 30 cm vents in autumn 2010 the temperatures of the short arm that winter became much more intermediate to those at the entrance and in the long arm. This definitely lowered the temperature and increased the air flow in this section and bats were found more frequently in the short arm.

As a result of the above a 30 cm square vent was also added to the end of the long arm in August 2012.



Figure. 5 Temperature records for the winter of 2012/13.

Discussion It is difficult to correlate bat usage with the changes that have been made in the ventilation system as the site is new and a steady increase in bat usage was clearly hoped for.

There was some evidence after the 2010 vent changes that more bats were hibernating in the entrance and short arm of the tunnel with the increased air flow. This was particularly so for Daubenton’s bats whilst more Natterer’s hibernated in the longer arm.

Results from the winter of 2012–2013 indicate that the bats are much more evenly spread through the tunnel with the two larger vents present. This cannot be directly correlated with bat numbers as the numbers are still on the increase due to the recent age of the site. It does indicate that in general there are more suitable conditions throughout the tunnel system.

In terms of the type of site that was used for hibernation by bats the Norfolk bat bricks were the favourite. The slotted logs are very well used although some have suffered from fungal growth that has made them unusable and will need refurbishing/replacing. After the first year it was decided that the concrete block piles should be abandoned as there was concern that hibernating bats in this area were more prone to predation from such animals as Wood mice. To make the logs easier to survey the logs were raised up off the ground using the blocks.

Wall planks have been installed but are clearly not a favourite site and are very prone to fungal attack in the moist conditions and will probably not be replaced when they are no longer maintainable.

Bats have also been recorded in the engineering bricks, small cracks and crevices in the concrete at the top of the wall and even between two of the concrete wall blocks at a turning point and, as normal, free hanging from the ceiling beams.



The Future Continued monitoring of both bat usage and temperatures will be carried out by the Suffolk Bat group and adjustments made to the ventilation system as necessary.

Vegetation growth on the hibernaculum needs to be controlled. Grass roots had penetrated through the blocks of the roof of the hibernaculum and these have been trimmed back annually by hand as it was thought that these might obstruct bat flight. Bracken and birch have also started to grow on the site. This will need to be controlled to prevent root damage to the roof. Treatment of the birch was carried out in 2012 (Plate 2) and bracken will be treated in 2014. This will clearly be an ongoing issue.

Great interest has been shown in the site by other groups and there are plans in the pipeline for similar constructions elsewhere.

Natterer's bat

0

5

10

15

20

Jan 2

007

Sept 2

007

Jan 2

008

Jan 2

009

Feb 2

009

Jan 2

010

Feb 2

010

Nov 2

010

Dec 2

010

Feb 2

011

Dec 2

011

Jan 2

012

Feb 2

012

Nov 2

012

Dec 2

012

Jan 2

013

Feb 2

013

Date

No

:

Entrance

Short arm

Long arm

Figure 6a. Natterer’s bat usage by location within the hibernaculum.

Natterer's bat

0

5

10

15

20

Jan 2

007

Sept 2

007

Jan 2

008

Jan 2

009

Feb 2

009

Jan 2

010

Feb 2

010

Nov 2

010

Dec 2

010

Feb 2

011

Dec 2

011

Jan 2

012

Feb 2

012

Nov 2

012

Dec 2

012

Jan 2

013

Feb 2

013

Date

No

:

Entrance

Short arm

Long arm

Daubentons bat

02468

10121416

Jan 2

007

Sept 2

007

Jan 2

008

Jan 2

009

Feb 2

009

Jan 2

010

Feb 2

010

Nov 2

010

Dec 2

010

Feb 2

011

Dec 2

011

Jan 2

012

Feb 2

012

Nov 2

012

Dec 2

012

Jan 2

013

Feb 2

013

Date

No

:

Entrance

Short arm

Long Arm

Daubentons bat

02468

10121416

Jan 2

007

Sept 2

007

Jan 2

008

Jan 2

009

Feb 2

009

Jan 2

010

Feb 2

010

Nov 2

010

Dec 2

010

Feb 2

011

Dec 2

011

Jan 2

012

Feb 2

012

Nov 2

012

Dec 2

012

Jan 2

013

Feb 2

013

Date

No

:

Entrance

Short arm

Long Arm

Figure 6. Daubenton’s bat usage by location within the hibernaculum.



Conclusion These findings make this artificial hibernaculum a great success and that the design appears to be a good general recipe for success. Although some ten artificial hibernation sites, of any validity, have been recorded in the recent bat literature as having been constructed, only two have seen more than a few Brown Long-eared bats using them. This is why the general UK bat conservation advice has been to add to and improve, or reinstate, existing sites, not to produce brand new constructions, which, up to now, have had a low probability of success. The site at Two-Mile Bottom shows that an understanding of what bats really need, by the serious application of several experienced “bat brains” and an appropriate amount of money, can produce the results that have so far been elusive, even if it takes several winters for bats to find the site and use it.

Acknowledgements Sincere thanks to FC, WREN, SCC, FHDC and EA for putting up the funds for what could have been seen as a very speculative project based on historical success of artificial hibernacula.

Thanks also to all those who have supported the project in a voluntary capacity, clearing the site, erecting fences, fitting it out and maintaining the fittings and structure.

Thanks to FC Conservation and Engineering staff for continuing support in all the modifications that have been made to such items as the ventilation ducts and on site maintenance and support.

A special thanks to Bill Landell for carrying out the temperature logging, and Arthur Rivett and the Suffolk Bat Group for organising the monitoring and also with general advice.

Thanks also to John Goldsmith for having such a brilliant idea and his constant help in putting this project together both in terms of both its construction and ongoing management and publicity. It would surely not have happened without his perpetual enthusiasm.

Nick Gibbons 4 Mackenzie Road Thetford Norfolk IP24 3NQ

Plate 1: Bat hibernaculum at Two-Mile Bottom, Thetford under construction in May 2004 (p. 2).

N. G

ibb

on

s

Plate 2: The same site in January 2012 (p. 8).

N. G

ibb

on

s

Plate 3: Natterer’s Bat using a bat brick in the hibernaculum April, 2009 (p. 4.)

N. G

ibb

on

s

two mile bottom bat hibernaculum from folly to fantasy

Documents