Over a year on from its restoration the old ice house at Ketteringham Hall is receiving guests!
Jane Harris our local bat expert explains:
Ketteringham Ice House Bat Hibernaculum
The Grade II listed icehouse has been a known bat hibernation site since the 1970s and the recent restoration had to be undertaken carefully to ensure that the law protecting bats and their roosting sites was complied with.
All UK bat species are insectivorous feeding on flying insects but, in the UK’s temperate climate, there are few flying insects around in winter. To survive the winter period when there is insufficient food, the bats hibernate as a means of conserving energy until the spring when insect populations start to increase. In September and October, bats concentrate on feeding to build up body fat for the winter months, and when temperatures fall in October, they seek out suitable hibernation sites and begin periods of torpor. They enter a state of torpor by lowering their body temperature to reduce the rate at which stored fat is used. In deep torpor, a bat may breathe only once every hour and its heart rate may be 10-60 beats/minute compared to 900-1,000 in flight. By using only a few milligrams of fat each day, a bat can stay in hibernation for the winter months. December, January and February are usually the coldest months of the year during which bats are in deep torpor and hibernating. By the end of February they have little fat left and in March, they may begin to emerge and search for food and water to drink. By April, bats have mainly come out of hibernation and are hungry and active, feeding on most nights, but in years when spring is late and cold, they may still be hibernating during this month.
Where to hibernate?
Successful hibernation depends on choosing the right roosting site which should be undisturbed, frost-free and cool (2 to 10oC) with stable temperatures and relatively humid (damp). Bats are very vulnerable to dehydration and need to drink at intervals. High humidity reduces the amount of water lost to the air so that the bats do not have to arouse and drink so often. Most British bats hibernate in caves, or artificial (man-made) structures that meet these requirements such as tunnels, ice houses, lime/brick kilns and cellars. These structures are typically well-insulated against fluctuations in external temperatures, usually above freezing and humid.
Several species show a marked preference for roosting in cracks and crevices and hibernation sites can be made more attractive by creating artificial crevices. This is achieved by installing purpose-built bat bricks (normal sized bricks with narrow slits) at multiple locations in hibernation sites to provide artificial crevices with a range of roosting conditions. Bat bricks are very successful in increasing the number of hibernating bats in man-made structures.
Enhancing the hibernation site
The restoration work on the ice house was carried out in the summer months, outside the winter hibernation period, when the bats were not present. Since the walls of the ice house were smooth with few crevices, eighteen bat bricks donated by East Carleton and Ketteringham parishioners were installed to create crevices at different locations in the vault. The safety grille erected at the entrance to the vault was designed with gaps which bats could easily fly through to access their hibernation sites. A solid locked outer door was installed for security, both to prevent disturbance of the bats and to stop vandalism. This door also has an access hole for bats to fly through.
Monitoring the bats
The ice house is now surveyed for bats by a licensed bat worker twice every winter as part of the voluntary National Bat Monitoring Programme run by the Bat Conservation Trust. This programme collects and analyses the survey data to establish trends in the populations of different British bat species Before restoration, the maximum number recorded was four Daubenton’s bats, a species which feeds over the Ketteringham Hall lakes and usually roosts in tree holes in the summer. These bats were often seen hanging precariously on the walls and roof of the vault. In winter 2014, the first year after restoration, four Daubenton’s bats were recorded again, with one individual using a bat brick. This confirmed that the bats were entering through the access hole in the new outer door and through the inner grille. In January 2015, the measures taken to improve the ice house for hibernation were beginning to show further signs of success. Three of the four Daubentons bats using the ice house were in bat bricks, and a single Natterer’s bat, a species not recorded previously in the ice house, was also found in a bat brick.
It is hoped that more bats of both species, and possibly other species such as brown long-eared bats, will find and use the ice house in future years, and that the project will be a success for bat conservation as well as restoring a building of historical interest.