Non-acoustic Detection
Visual observation is the obvious means of detecting bats, but of course this can only be done in daylight or crepuscular conditions. (dusk and dawn). Emergence counts are done visually at dusk, using a bat detector for confirmation of species. In lower light conditions a night vision device can be used but the more affordable generation 1 type has a lag time which fails to provide a suitable image of a flying bat.
Infrared (IR) cameras and camcorders are used with an IR illuminator to observe bat emergences and bat behaviour inside and outside roosts. The problem with this method is that deriving a count from a recording is tedious and time consuming, but camcorders can be useful as a backup in roost emergence counts to observe bats re-entering the roost. Many Sony camcorders are sensitive to infrared.
Infrared beam devices usually consist of a dual array of invisible IR beams. The size of the roost entrance determines the number of beams necessary and thus the power required and potential for off-mains use. Single beam DIY systems are available for bat boxes but these do not log the direction of transit. Almost all the systems in use today are non-commercial or DIY. A system in use in some mines in Wisconsin uses two arrays of beams however they are spaced quite far apart and consequently only log approximately 50% of the bats although extrapolated figures are achieved through correlation of time stamped video and beam break data. The Countryside Council for Wales (CCW) uses two similar systems with beams spaced close enough together that every bat transiting the entrance is logged along with the temperature. These systems require either mains power or 12 V deep cycle batteries. They can be used in conjunction with an Anabat Zcaim installed in a 6" soil pipe and pointed across the roost entrance to discriminate between species by correlating the time stamp data from the IR array and filtered Anabat Zcaim data for horsehoe bats (relatively easy due to their easily identifiable CF echolocation which can be filtered automatically using Anabat software).
Data from beam break systems must be carefully analysed to eliminate "light sampling behaviour" (environment sampling) where the bats repeatedly leave the roost and return immediately if the conditions are not suitable. Some systems discriminate for bat sized animals; they determine if the beams are broken by a bat sized animal and ignore all other transits. It is important that data is analysed using a methodology which takes light sampling behaviour into account. The method which seems to give the most accurate results is as follows: "out" transit assigned 1, "in" transit assigned -1. Start count is set to zero at 4 p.m. daily. Using a spreadsheet, the counts are added cumulatively from 4 p.m. each day until 9 a.m. the next day. The maximum "positive" count can easily be found for each day. Since every transit is time stamped, the exact time of the maximum daily count is also known. Light sampling counts are eliminated from the data since an "out" 1 is cancelled by an "in" -1, resulting in a cumulative count of zero for light sampling bats.
Thermal imagers which are of a high enough definition to register bats at over 30 metres range are expensive, but have been used to assess the dangers of wind turbines to birds and bats. "Affordable" thermal imagers have a bat detecting range about the same order of acoustic bat detectors due to the small size and the low heat emissions of bats.
Passive infrared sensors are slow with a response speed of the order of a tenth of a second and will normally not detect a small fast mammal like a bat.
Radar has been used to detect bats beyond the acoustic limit, but is very costly in equipment and man hours. Bird Aircraft Strike Hazard (BASH) installations are capable of detecting bats, but are usually situated where few bats fly. There are very few suitable mobile terrestrial radars available anywhere. Hand-held doppler radar modules have been used in the field to allow researchers to compensate for the doppler shift imposed on recordings of bat signals due to their flight speed. This allows the researchers to tell whether the bats are changing the pitch of their calls in flight.
Read more about this topic: Bat Detector