Woodland bats are difficult to find
Some of Britain’s rarest bats are woodland specialists: small, fast-moving, nocturnal animals that regularly switch between tree roosts.
Tree-climbing inspections provide valuable information, but repeated visits are labour intensive. BCT’s standard approach for features suitable for maternity colonies involves three aerial inspections between May and September, normally at least three weeks apart.
Widely spaced inspections can repeatedly coincide with bats being absent. Andrews & Gardener, writing in In Practice 88 (2015) and drawing on Dietz & Pir (2011), describe a 5% encounter probability for each separate visit during May and June. In their modelling, monitoring a single potential roost feature on 14 consecutive days was needed to reach a 50% probability of encountering bats. Repeated climbs therefore require considerable effort and provide little certainty. They can provide other evidence, e.g. droppings, staining, fly cases or DNA, but this evidence can be short-lived in the living ecosystem of a tree.
Radio-tracking is an effective way of locating tree roosts, but requires specialist staff and equipment and can be costly. It follows a sample of tagged bats rather than the whole colony and covers approximately two weeks, so may not reveal every roost. Trapping, handling and tagging also require a clear survey justification, careful welfare consideration and appropriate licensing.
Camera trapping provides a means to survey over entire seasons with only two survey visits: deployment and collection. Cameras require an equipment outlay, suitable power, storage and video review time, but can be cheaper and more effective than other methods or form part of a wider survey strategy.
What conventional trail cameras can achieve
Conventional trail cameras can record bats at tree roosts. BATS Research & Training has investigated SpyPoint Force Dark cameras, sometimes positioned around 40 cm from an entrance to improve detection.
Their work demonstrates the value of camera trapping using relatively inexpensive equipment, while highlighting the difficulties of monitoring small, fast-moving animals with cameras designed mainly for larger mammals.
Most trail cameras use a passive infrared (PIR) sensor to trigger capture. PIR sensors are designed to detect subtle changes in heat from large mammals; they are the same sensors used in bathrooms to trigger an air freshener. Ordinary trail cameras are usually in a low-power sleep mode until the PIR is triggered. They then turn on their internal components and start recording video or take a photograph. This power switch takes time, and a bat can trigger the PIR and leave the camera’s field of view before the photograph or video begins. Moving the camera closer to the feature or looking into open features where bats sit before emergence can help, but this reduces the video coverage, and bats can still be missed depending on the circumstances. Empty recordings can result from missed bat movements as well as false triggers caused by vegetation and environmental changes.
Long deployments also test reliability. BATS Research & Training used external 18650 batteries in plastic wrap to extend operating time and reported some cameras taking photos for an entire year, but others suffering moisture-related failures after a few weeks. Detection, weather protection, mounting, power and storage all matter.
How Reflex cameras capture brief events
Detect movement in the image
Reflex detects movement without a PIR sensor, by looking for changes in pixels in its live video stream. This means it can be extremely sensitive. Adjustable detection zones can be set to monitor changes around a tree hole and surrounding flight space while excluding areas likely to generate unnecessary recordings (moving leaves or cobwebs).
Retain the moments before the trigger
With circular recording enabled, Reflex holds the preceding two seconds of video and adds them to the saved clip when motion is detected. This preserves the lead-up to a brief emergence or re-entry, ensuring no bats are missed.
Examine fast movement in detail
Reflex Standard and Lynx cameras offer 30, 60 and 120 fps recording. At 120 fps, more frames are available to examine fast movements, behaviour and often diagnostic features for species identification. Focus, exposure, illumination and viewing angle determine the detail captured.
Lower frame rates can reduce power consumption for longer deployments.
Frame the whole feature
A potential tree roost feature does not always have a small open entrance that can be covered by placing a camera directly in front of it.
Lightning strikes can create openings running metres down a trunk, while hazard beams may extend along a limb. Thick plates of loose or flaking bark are also important, particularly for barbastelle bats (Barbastella barbastellus). These features can provide roosting opportunities, including maternity roosts, for rare woodland bats such as barbastelle and Bechstein’s bat.
The task may therefore be to discover where along a feature bats emerge, rather than monitor a single small hole.
Fragile bark benefits from monitoring at a distance, without placing equipment against or behind it. A conventional trail camera positioned close to one suspected entrance may cover only a small part of the feature.
Reflex’s image-based detection allows broader coverage of the tree and surrounding flight space:
- Standard: close monitoring, with broader coverage than a conventional trail camera positioned within 4 m of an entrance.
- Lynx: brighter infrared imagery with less illumination, suitable for larger features or greater working distances.
- Thermal Pit Viper: thermal monitoring of larger areas without relying on infrared illumination.
Choose the camera, distance and framing to retain enough detail for the survey question.
Check the setup before leaving
Unlike traditional trail cameras, Reflex cameras stream the video output over Wi-Fi so users can check framing, focus and illumination whilst mounting them in a tree. Users can stream the video on a phone, which is useful for climbers.
Position detection zones around the area being monitored and likely flight paths, excluding moving vegetation where possible.
Use higher frame rates and suitable illumination when fine detail or behaviour matters. For presence monitoring over longer periods, balance image detail with power consumption and storage.
The camera, mounting and power arrangement should suit the feature without damaging it.
Bats may be absent on the night of a survey
A night when bats are absent does not mean a tree is unused. Bats moving between a network of roosts may return after several unoccupied nights.
A Reflex camera extends the question from “Are bats present tonight?” to “When and how do bats use this feature?”
Reflex cameras allow you to monitor for a night, week, month, season or longer, according to the survey objective. External 12 V batteries, solar systems and scheduled operation support extended deployments. Reflex cameras can be scheduled to shut down in the daytime when monitoring is not required and power up before sunset for the next recording period.
Where no bats are recorded, consider the camera’s operating time, coverage and settings before interpreting the result as absence.
From presence to behaviour
At Tortworth, a Reflex Standard camera approximately 15 m up a tree has monitored a male noctule bat (Nyctalus noctula) lekking roost for more than two years. Recordings include advertising behaviour, investigation flights, scent marking and interactions with squirrels, woodpeckers and Bechstein’s bats.
Video reveals how bats approach, enter, leave and interact around a roost. Close, detailed imagery can support species identification; wider views can be paired with acoustic monitoring for additional evidence.
Each motion-triggered recording has a summary image showing where movement occurred. Scan these to locate activity, then open the original video to examine the event or review it frame by frame. This allows really fast review of months’ worth of video clips. As the camera is not recording constantly, a month’s worth of monitoring typically produces between a couple of GB and 10 GB of video.