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HOW TO “FREEZE” A BAT IN FLIGHT: WHY IT IS DONE AND WHAT IT CAN TELL US

  /  News   /  HOW TO “FREEZE” A BAT IN FLIGHT: WHY IT IS DONE AND WHAT IT CAN TELL US

HOW TO “FREEZE” A BAT IN FLIGHT: WHY IT IS DONE AND WHAT IT CAN TELL US

Teresa Kearney, Curator: Vertebrate Department, Small Mammals, DITSONG: National Museum of Natural History (DNMNH)

Bats are primarily nocturnal (active at night), and their behaviour is largely unseen by us given it mostly happens in the dark. Ernest Seamark of AfricanBats NPC has equipment that is used to image bats in flight. Here are some images of bats in flight taken in South Africa, showing them as you have probably never seen them before.

Who knew Cloeotis percivali (Percival’s Short-eared Trident Bat) almost flips onto its back as it flies around a tree close to the cave opening where it roosts (Figure 1)? Besides being a view into flight behaviours not previously observed, these images may be used to passively record easily identifiable species, and document moult patterns and sex ratios. These images are also important for promoting bats to a wider public, to foster an appreciation for these mammals that provide such important ecosystem services (see https://onlinelibrary.wiley.com/doi/abs/10.1111/1749-4877.12552 for more on this topic).

The kit to capture these images consists of a series of laser beams that are linked to a flash, which in turn is linked to another series of flashes (Figure 2). The lasers are placed in a position that bats are likely to fly over or past. For instance, near the entrance of a roost the bats will be emerging from or returning to (Figure 3), or close to the water that bats will be drinking from (Figure 4). When a bat flies through the laser beam it breaks the beam for a short period, which in turn triggers the flashes. The flashes are positioned around the area the bat should fly through to trigger the beam, to light the bat for the desired effect (with lots or few shadows).

To capture the image of bats “frozen” in flight you use a camera that can be left on bulb mode, which keeps the shutter open. If you are not familiar with how a camera works, an image is formed by light reaching film or a digital sensor. The lens, aperture (the blades of which can be modified to change the size of the opening through which light passes), and the shutter (like a door that when open would let light through, or when closed allows no light through) are integral parts of getting light to film or the sensor to form the image. Allowing too little light through can make an image dark (under-exposed), in some cases to the point the image is completely black. Conversely, allowing too much light through lightens an image (over-exposed) (Figure 5), in some cases to the point the image is entirely white.

When a bat flies through the laser beam and breaks the beam the flashes are triggered, and you close the camera shutter to capture the image of the bat “frozen” in the moment it passed through the laser beam. You then reopen the camera shutter ready for the next time a bat breaks the beam and triggers the flashes. If this process starts at dusk before it is completely dark the camera shutter needs to be closed more frequently before the flash is triggered to avoid over-exposure of the image (Figure 6). “Ghosting” can occur if a single image captures information from the flashes being triggered two or more times. This happens if the laser is broken in quick succession, and one doesn’t close the shutter fast enough to capture each flash trigger as an individual image. Instead, the information from each flash is overlaid into a single image, so anything appearing in the image on the first flash will be lit again on the second or subsequent flashes, and any bats that pass through on the second or later flashes may overlap bats captured in earlier flashes (Figure 7).

The area that is being covered for the image depends on the camera you have and what detail you want. Trying to fill the frame with an individual bat will reduce the area covered and so the odds of getting perfect images decreases, as not every bat will fly through the exact space your camera might be focused on, so the number of images with only parts of bats in the frame increases (Figure 8). Also, not every bat will be flying toward the camera (Figure 9). However, when everything goes to plan in the set-up, and you don’t forget to take the lens cap off, you can get some impressive images (Figures 10-12).

If this intrigues you, you might want to read this open access publication: Rydell, J., Russo, D., Sewell, P. et al. Bat selfies: photographic surveys of flying bats. Mamm Biol (2022). https://doi.org/10.1007/s42991-022-00233-7.

Figure 1. Cloeotis percivali in an upside-down position on its flight path around a tree near its cave roost

Figure 2. A view of the lasers sensors and flashes set up at a cave entrance. The flashes are circled in red and the bank of laser sensors that are facing upward are circled in yellow.

Figure 3. Cloeotis percivali (Percival’s short-eared trident bat) in flight near the entrance to a cave where it roosts.

Figure 4. Hipposideros caffer (Sundevall’s Leaf-nosed Bat) about to drink from a raised water reservoir.

Figure 5. The flashes were set incorrectly over-exposing the image with too much light in some places and too much shadow in others. However, this does show the structure of the left wing of a Rhinolophus sp. (Horseshoe Bat) beautifully. Including the elongated hand and finger bones and thin muscles within the wing membrane, as well as parasites (next to the fourth and fifth finger bones).

Figure 6. The bat appears very faintly against the cave entrance surroundings that have lightened more because the camera shutter was left open for too long while it was still light.

Figure 7. A Cloeotis percivali (Percival’s Short-eared Trident Bat) and Rhinolophus simulator (Bushveld Horseshoe Bat) passed through the laser beam in quick enough succession that the camera shutter was not closed between the flashes. You can tell these bats weren’t flying that close together and were imaged by the flashes being triggered twice, due to the “ghosting” circled in yellow where the wings overlap that allows you to see both wings. If they had been together with only a single trigger of the flash, one wing would be visible and the other would be hidden.

Figure 8. Not all of this Rhinolophus simulator (Bushveld Horseshoe Bat) was captured in the image, as it was flying partly outside the field of view of the camera when it broke the laser beam with its right wing dipping into the water. It’s also not well lit by the flashes.

Figure 9. Bats aren’t always flying toward the camera. Although one can often sex individuals from this angle.

Figure 10. Nycteris thebaica (Egyptian Slit-faced Bat) in flight near its cave roost.

Figure 11. Miniopterus natalensis (Natal Long-Fingered Bat) in flight near its cave roost. Its mouth is open as this species emits sound through the mouth for echolocation.

Figure 12. Rhinolophus simulator (Bushveld Horseshoe Bat) in flight near its cave roost.

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