Roost types
Bat Boxes
What's unique
Bat boxes and multi-chamber "condos" are common across many species. Most have a single exit at the bottom, though some have several, and this guide is for boxes mounted on a pole; a box mounted flush against a building behaves more like the Buildings guide. Pole boxes are usually one of the cleaner setups: the camera looks up at the box against open sky, so bats stand out at high contrast, and the small, predictable exit concentrates the action in one part of the frame.
In the field
- Put the camera below or to the side, looking up at the bottom opening against open sky. That's your cleanest, highest-contrast background.
- A moving background creates false detections, especially rustling deciduous trees on a visible camera, so be mindful of the background when placing the box. A stationary background that contrasts with the bats is best.
- Frame the opening and the drop-out path with room around them. Don't let the exit sit right at the edge of the frame.
- Set the distance or zoom so a single bat is a reasonable size on screen, while still having sufficient buffer around the roost.
Analysis
- Gate: just below the opening, across the drop-out path, draw the gate sufficiently above the area where bats exit.
- Use exclusion size filters to ignore smaller free-flying bats in the distance.
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Buildings
What's unique
Bats emerge from a large structure like a building or barn. You often don't know exactly where they'll come out, since a building has many possible gaps, vents, and eaves, and the exit may not be obvious until you watch an emergence. Contrast also varies a lot with the exit: a dark doorway or window, a sunlit exterior wall, and the building against the sky each look very different, so how well the bats stand out depends on where you film.
In the field
- Capture the whole likely exit area. Since you may not know the exact exit, frame broadly enough to catch wherever the bats come out, with the most uniform stretch of wall you can manage behind them.
Analysis
- Because the background changes with the exit, the bats may read lighter or darker than it, so set polarity to "Both" as a starting point.
- Because you don't know where the bats exit, draw a very large gate over the whole likely area and do a default run to find the activity (see Finding the action in long, low-activity recordings), then tighten the gate onto the real exit once you've spotted it. The run also produces a composite image of the tracks, showing all the movement at once, which makes the emergence points obvious.
Related guides. Finding the action in long, low-activity recordings.
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Cave Entrances
What's unique
A cave or rock opening, usually filmed with the camera right up close, so the bats are large targets. They often circle for a while before they actually exit, so there's a lot of swirling action that makes manual counts really challenging.
In the field
- Get the whole exit in frame with a little room around it. Ideally the bats cross perpendicular to the camera, exiting left or right, with the entrance to one side of the frame and more room in the direction they travel.
- You can film a cave head-on, but it's more challenging: bats change size as they approach the camera, and their movement is radial, spreading in several directions at once.
- Favor a clean background. Bats stand out better against a clear stretch of rock than against foliage, and framing the entrance from below, looking up, can sometimes give an even cleaner backdrop.
Analysis
- Gate placement makes a big difference with caves. The rock entrance gives a gradient of contrast, so some areas are higher contrast and easier to detect than others, and bats bunch up, spread out, or swirl in different spots. Choose a spot where, in priority order: (1) contrast is high, (2) bats are spaced enough to read as individual targets, and (3) it is past the swirling range. Contrast and spacing matter more than clearing the swirl.
- Keep the gate off the frame edge, leaving a buffer at least big enough to detect a bat, so bats crossing there are seen on both sides and counted. The exception is when the frame does not capture the whole entrance and it runs off the edge; then some edge crossings are unavoidable and a few bats go uncounted.
- Looping bats are not a problem. If one crosses out and back in, it gets both an exit and an entry, so it cancels out of the net count.
Related guides. Zero or Negative Gate Counts (gate placement).
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Trees
What's unique
Cavities, exfoliating bark, and snags. In most respects a tree roost films very much like a bat box: a small exit part-way up a trunk or limb, best captured from below against open sky. Two things make it harder. You often don't know exactly where the exit is until the emergence starts, and foliage works against you twice over, moving in the background and blocking the view of bats that have just dropped out.
In the field
- Start from the bat box setup: camera below or off to the side, looking up at the exit against open sky for the cleanest, highest-contrast background.
- If you aren't sure which cavity or crevice they're using, frame broadly enough to cover every candidate on that trunk, the same way you would with a building, then tighten up on later nights once you've seen where they actually come out.
- Work the sightline around the foliage. Leaves in the background move and generate false detections, and branches between the camera and the exit hide bats outright. Look for an angle that puts sky, not canopy, behind the drop-out path.
- Wind matters more here than at any other roost type, since a breeze sets the whole background moving. Film on the calmest nights you can.
- Expect only a short visible window. Bats often drop out and are into the canopy within a second, so place the camera to catch that first stretch below the exit and keep the gate close in.
- Be ready for more than one exit. A single tree can have several, and you may not know until the emergence starts, so film from more than one angle where you can, or bring a second camera.
Analysis
- Run a test over a section of the video before you commit to a gate. The run produces a composite image of the tracks, so the emergence shows up as a concentration of flight paths and you can see where the bats are really coming out (see Finding the action in long, low-activity recordings).
- Gate: just below the exit that test shows you, across the drop-out path, in the cleanest patch of sky you have, as with a bat box.
- Mask the moving foliage that isn't on the flight path. It is the main source of false detections at a tree roost, and masking it costs you nothing as long as the gate stays clear.
- Don't assume last night's framing and gate still apply. Tree-roosting bats switch roosts and exits readily, so recheck both each night.
Related guides. False detections (moving background), Finding the action in long, low-activity recordings.
Walkthrough video coming ↑ Back to roost types
Bridges
What's unique
Bridges are a hard one. Bats roost in crevices, expansion joints, and the spaces between girders, so a single structure can have exits spread along its whole length and bats leaving in more than one direction at once. On a big or complex site, complete coverage may simply not be possible. Mexican free-tailed bats are common bridge roosters, so a bridge colony can also be a very large emergence, with all the small, fast, overlapping targets that brings.
In the field
- Film the bats dropping down, with the camera placed below the bridge looking up at the cold underside if possible. That shaded concrete is the best background the site has to offer.
- If bats leave in a column, anticipate where that column will be and get as perpendicular to the bats as possible.
- If bats leave in multiple directions, you may need multiple cameras.
- Consider various placements to optimize for the background before you settle on one.
- On a complex site, focus on capturing the largest emergences, and note from concurrent visual observations to what extent the cameras captured the bats leaving. That note is what makes a partial count interpretable later.
Related guides. Very Large Emergences.
Walkthrough video coming ↑ Back to roost types
Very Large Emergences
What's unique
A very large emergence, where huge numbers pour out at once, typically at a large cave mouth or rock opening (Mexican free-tailed bat and Grey bat colonies). The camera is far from a big opening, so each bat is only a few pixels across, and a scene this large usually has varied contrast, so in some areas the small, faint bats are hard to pick out. The sheer numbers also crowd the frame with overlapping bats.
In the field
- For cave species, the framing is much the same as a cave entrance.
- You don't need the whole entrance, just a section where the bats are passing without overlapping too much, ideally against high contrast. If you've watched the emergence before, use what you know about where most of the flow is to pick that spot.
- Stay no further back than you have to; every extra bit of distance shrinks the already-small bats and makes them fainter.
- A higher-resolution camera helps here. With each bat only a few pixels, more resolution makes them easier to detect and to tell apart.
- A very stable camera matters here. The analysis is already challenging with such small, faint targets, and camera shake makes it worse.
Analysis
- Noise is hard to filter out here. The bats sit at the smallest end of the target-size range, so the exclusion size filter can't cleanly separate noise from real detections, and brightness exclusion is limited too since valid bats are faint.
- That leaves background subtraction as your main defense against noise, so keep it on where you can. Setting Max Density to Extreme (20+) turns background subtraction off to keep up with the heaviest activity, which can flood the frame with false detections, so use it only as a last resort, when the numbers are so high that nothing else works.
- Mask areas of swirling bats that aren't crossing the gate to lighten processing; only the bats crossing the gate need to be counted.
- You may need to increase the sensitivity to pick up the lower-contrast, fainter bats.
Species profiles
Mexican long-nosed bat
Leptonycteris nivalis · LENI
Roosts
Caves and abandoned mines, often vertical entrance shafts.
Cameras
Thermal Near-IR
In the field
- They emerge late, so visible cameras are not useful.
Analysis
- Likely need to use Variable Size Tracking with near-IR video, because changes in the wingbeat cycle can shift target size substantially.
Related guides. Cave entrance.
Brazilian free-tailed bat
Tadarida brasiliensis · TABR
Roosts
Caves, bridges, buildings, and bat condos. Colonies can be huge, 100,000s to millions, and they emerge before dark as a dense stream that can last several hours.
Cameras
Thermal Near-IR Visible
Push resolution and frame rate here. The emergence is huge, fast, and dense, and each bat can be only a few pixels, so higher resolution helps pick them apart and a higher frame rate keeps fast bats from streaking together.
In the field
- Frame the column for clean counts. Keep bats crossing perpendicular to the camera, picture where the gate will sit, and keep that zone free of clutter.
- A good vantage is often near the entrance looking out toward open sky: you get a clean backdrop and the bats move away from the camera, instead of filming the entrance head-on with bats coming toward you.
- Scout where the column tends to emerge, and how that might change with the weather. You may need multiple cameras to cover it.
Analysis
- A long emergence can span changing thermal and lighting conditions, so the scene may not look the same at the end as at the start. Plan for that when you set up.
- The biggest emergences may need Max Density at Extreme, which turns off background subtraction, so an empty, uncluttered background is especially useful.
- May need the higher speed option in Advanced settings.
Related guides. Caves, Very large emergences, Buildings, Bat boxes.
Gray bat
Myotis grisescens · MYGR
Roosts
Cave obligate in both summer and winter. The caves they use are cold, which gives strong thermal contrast and makes them easy to detect on thermal.
Cameras
Thermal
In the field
- Filming from inside the cave is sometimes an option, when there's a spot that's easier to film than the exit. They tend to fly near the ceiling, so a camera looking up can work well.
- Protect the camera from humidity transitions, especially when counting from inside a cave.
Analysis
- Very small targets, so select the smallest target size.
- Use masking for dense swirling areas.
Related guides. Very large emergences, Reflections over water, Cave entrance.
Myotis spp.
Various Myotis species · MYSP — including the Indiana bat (Myotis sodalis), little brown bat (M. lucifugus), and Yuma myotis (M. yumanensis).
Roosts
Mostly summer roosts in buildings and bat boxes.
Cameras
Thermal Near-IR Visible
These often trickle out of boxes and buildings only a few at a time, so you can get away with far less camera.
In the field
- Exits from buildings and trees can be unknown or shifting, so scout first and expect roost-switching.
Analysis
- Can be one of the most straightforward exits to count because they trickle out consecutively and don’t overlap much.
- Draw the gate sufficiently below the exit so each bat is counted both inside and outside the gate.