Counting Bats

Counting Bats

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Bat Pro
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Incubator Creche

Populations of winter Horseshoe bats using the mines were assessed quite easily, as they hang freely from the walls like black plums or pears.

They were counted by walking around enabled routes and using powerful lamps to spot them against the nearly-white limestone mine walls. Vespertilionid bats winter numbers could not be found this way, as they often crawl into cracks in the walls, or gaps in the piles of rubble. Hence another method had to be devised. Automated bat detector systems were used to record their ultrasonic calls, produced when they periodically aroused from hibernation. Arousals occur at frequencies up to every 30 days.

Bat Detector System

The first detector system tried did not produce usable data due to frequent water droplets dripping from the roof, which generated explosions of ultrasound. To solve this problem a low-sound engineer designed and built an automated detector that could reject water splashes, but record bat calls. The result was the Eco Mega bat detector run on 12 volt, 40 amp hour leisure batteries that lasted for up to two weeks monitoring. Up to 6 detectors were simultaneously used to monitor various parts of the mines, either to discover the parts of the mines used by all types of bats, or to monitor for Vespertilionid bats as specified by the bat licences. They were used underground for over 9 years more or less continuously.

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Data Collected

Eco Mega data confirmed that some areas of inner Byfield mine and most of Firs mine were not used by any type of bats through the winter. This allowed engineering works to proceed in many areas without the need to abide by the bat protocols required by the licence in bat-occupied areas. The monitoring has confirmed that Vespertilionid bat populations, as judged by their call levels, had stayed at similar levels throughout the works programme until 2007, after which they were excluded to allow bat mitigation works in Byfield bat areas.

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Discovery of the second roosting site

In the summer of 2000, the roosting site of the small maternity colony of Greater Horseshoe bats was not identified. Counts of emerging bats from the Byfield Mine that summer showed that small numbers of adults were living somewhere in either of the two mines. It was important to be able to remove the colony from Firs, if it was there, and into Byfield to permit unhindered works in Firs. This was achieved after Bat Pro designed an underground structure called an incubator chamber. It consisted of a small underground room with a roof lined by marine plywood and covered by mesh for bats to hang from. The upper part of the space was heated, and kept at about 27°C. It was built by Hydrock, in June 2001, within a steel set safety shield for use by bats. After about one month, Lesser Horseshoe bats began to use it, followed by Greater Horseshoe bats as shown by droppings collected from trays placed in the room. The following summer, the first baby Greater Horseshoe bats were seen in the chamber. By 2007, the numbers of babies and adults had nearly tripled. In late September 2007, all bats were excluded under licence to allow permanent bat habitats to be built.

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