2013: Advent Biology: One bat, two bats, three bats, four…

by on December 2, 2013

So yesterday I started with bat transect surveys. While relatively quick and able to be carried out with minimum experience, in most environments and with fairly basic kit-though you can of course use more exciting and expensive equipment, like the EM3 which I’ll take a look at later on-it has quite a bias in the data collected this way. Field identification can be quite tricky even with the higher end detectors and some species such as the Brown Long Eared have weak or low echolocation calls, so end up being under-recorded. Identification visually relies upon actually seeing the creatures as well, which tends to be pretty hard. Bats fly fast. Of course, experience plays a large factor, but at the end of the day what you will often end up with is a recording device full of sound files which needs to be analysed for the calls you’ve noted down, and to see if you missed any…

So how else do we gather information on bats? Many methods still involve the use of echolocation detection, and have different purposes.

Emergence and re-entry surveys are activity surveys like transects but in one location: a suspected or confirmed roost site can be monitored, and populations of colonies assessed fairly easily by simply counting individual bats as they leave the roost or come back after a night’s hunting.  Roosts can range from crevices and hollow trees to roof spaces in buildings, under bridges, caves, barns or any outdoor building, so assessing the potential of any structures for bat roosts or any signs of bats (such as droppings, insect remains etc.) can be a very detailed job, even before you get started with sitting outside a barn or tree for two hours in the dark with only a red torch and a thermos flask for company. Climbing likely trees or exploring lofts are all a part of these initial surveys. In winter, bat presence in roost surveys will be limited to hibernacula, which licensed surveyors are allowed to enter and identify the species using the roost. At any time of the year, unlicensed persons are not allowed in known roosts, and disturbing bats you know are there without someone with a license at your side to make sure it’s done properly is going to get you into trouble.

Static monitoring is rather more technology intensive. Recording devices such as the Anabat or SM2, designed to be out for a long time, set to turn on for the hours of likely bat activity night after night, and fairly unobtrusively too, allowing a study of the species present in the area without the potential disturbance and people-hours of activity surveys. Just replace the batteries and sound card, and then take your reams of data off to be analysed-that’s where the time has to be factored in.

Probably the most intrusive methods involve bat capture: this gives you a huge amount of information on species identification, reproductive and overall condition as well as the ability to tag the individual. Radio tagging allows you to keep tracks on foraging habits and roost sites, flight patterns and activity levels. Catching the bats to tag might be with the aid of recorded social calls to lure bats in, or just trying to predict where they will be flying and putting out mist nets or harp traps nearby. This is especially useful in woodlands, as the species that frequent these tend to have quieter echolocation and often similar calls. Besides, inspecting every tree in the woodland for bat roosts can be quite an overwhelming task!

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