3.4.5 Identifying and counting bats in hibernation sites Ideally, hibernating bats should be counted when outside temperatures are both low and most stable. This is when the numbers of bats visible in underground sites will be highest and also most stable. In Britain, January and February are generally the months with the most stable cold temperatures. Because of the negative relationship between temperature and the number of visible bats, it is recommended that temperature be recorded when making a count and notes made about the previous night’s weather conditions. As the size and complexity of the hibernation site increase, the probability that all bats will be observed and counted decreases, thus it is also good practice to record some details about the site. Horseshoe bats hang in the open from ceilings or walls but vespertilionid species often tuck themselves away in cracks and crevices and may not be immediately obvious. Horseshoe bats can also hang CHAPTER 3 SURVEY AND MONITORING 34 from rocks and boulders close to the floor and other species may hibernate in loose rubble on the floor so tread carefully. Careful searching with alight suited to the circumstance is essential. Bats select a variety of places to roost but most vespertilionid bats are found within a few 100 m of entrances, which are associated with the lowest temperatures. Lesser horseshoe bats can also be found in such places but are commonly found much deeper underground where temperatures are warmer and more stable. In all survey work great care must betaken to avoid disturbing bats unnecessarily. Do not make excess noise or stay near bats longer than is necessary and do not shine bright lights on bats for longer than is required to identify them. Arousing hibernating bats can affect their ability to survive through to the spring, so the aim of surveyors should be to count and identify bats without handling them. If identification is difficult, as with whiskered and Brandt’s bats for example, make a note of the uncertainty rather than arousing the bat. Identification of the Myotis species without handling them takes practice and experience and the best way of learning is to accompany an experienced bat surveyor. Further guidance is given by Greenaway & Hutson (1990). No licence is required to search sites where bats have not previously been found, but unlicensed surveyors must withdraw if bats are found so that bats are not intentionally disturbed. Many bat hibernacula or potential hibernacula can be improved for bats by appropriate management such as grilling or altering the airflow (see Chapter. The scientific evaluation of the effects of such practices is an important part of these projects and as much information as possible should be collected both before and after any changes have been made. The value of such work is greatly increased if numbers of bats at the site can be monitored for one or two winters before any works take place or if a comparable control site can be monitored at the same time. Examples of such studies are given by Stebbings (1965, 1992) and Vote & Lina (1986).