By Dr. Karen Shaw Becker
As bats fly, they make high-frequency calls (too high-pitched for human ears to hear) that bounce off of physical objects in their surroundings. By listening to the returning echoes, bats can form an acoustical "map" of their surroundings, navigating their world in a way that's profoundly different than the way you or I do.1 In many ways, echolocation is incredibly advanced, allowing bats to catch thousands of tiny insects — in the dark and while moving high speeds — on any given night.
Their echolocation makes it possible for bats to not only detect insects in their path but also gather information about their size, shape and even which direction they're heading.2 "Building a sufficient three-dimensional perception of their environment on a lower-dimensional sensory input than human vision," bats perform a complex task, notes Peter Stilz of the department of animal physiology, Institute for Neurobiology at the University of Tübingen, Germany.
But in so doing, he explains, "They are thus forced to apply a high degree of processing and interpretation to the sensory input, making them prone to sensory deceptions."3 One "sensory trap" to bats appears to be glass windows, mirrored surfaces on buildings and other smooth vertical surfaces.
While bats can successfully navigate around natural obstacles in their path, like trees, man-made buildings covered in glass may pose a unique, and highly hazardous, risk. The bats have trouble noticing that they're there, instead believing their flight path to be clear, leading to regular collisions.
Windows, Mirrored Glass Pose a 'Sensory Trap' for Bats
Writing in the journal Science, researchers pointed out that bats rely primarily on echolocation for foraging, orientation and navigation. However, they can mistake "smooth, vertical surfaces as clear flight paths, repeatedly colliding with them, likely as a result of their acoustic mirror properties."4 They carried out a study on 21 mouse-eared bats, which flew in a dark flight tunnel in which a metal plate had been placed, either horizontally or vertically
While no bats collided with the horizontal plate (bats are known to view smooth horizontal surfaces as water, and as such try to drink from them), nearly all (19) of the bats collided with the vertical plate. None were injured during the experiment, but this isn't likely to be the case in the wild.5 "In our experiments we didn't see any injuries," study author Stefan Greif told Popular Science.6 "We had them in a small flight room, in a narrow tunnel where they're flying relatively slowly compared to the speeds at which they might fly outside."
Indeed, collisions also occurred in the wild, among three different bat species, when vertical metal plates were placed near bat caves. A number of factors seem to influence the likelihood of collision, including the number of echolocation calls and how long the bat spends in front of the surface. Specifically, bats were more likely to collide with the plate when they were making fewer calls, approaching at a more acute angle and flying at higher speeds.
"[I]t is necessary to more closely monitor potentially dangerous locations with acoustic mirror properties (such as glass fronts) to assess the true frequency of fatalities around these sensory traps," the researchers noted, raising concerns about the future planning of cities to work in concert with nature — not against it. Dead or injured bats are sometimes found near the base of buildings with large windows, and this study may explain why.
Bats Are Also Known to Collide With Wind Turbines
Many man-made objects pose threats to bats in the wild, including wind turbines. While wind energy is viewed as an environmentally friendly energy source, it comes at the expense of wildlife. It's been estimated that hundreds of thousands of bats may be killed by U.S. wind turbines annually. According to one study in the journal BioScience:7
"Dead bats are being found underneath wind turbines across North America, and bat fatalities have been documented at almost all of the wind facilities at which thorough bat surveys have been conducted. The results of those surveys suggest that thousands of bats may be killed annually at some wind facilities, and recent estimates suggest that hundreds of thousands of bats may be killed annually in the contiguous United States."
According to the U.S. Geological Survey's Fort Collins Science Center, most bat fatalities at wind turbines occur among bats that migrate long distances and roost in trees.
The most deaths occur during late summer and autumn, which coincides with autumn migration and mating behavior. "Seasonal involvement of species with shared behaviors indicates that behavior plays a key role in the susceptibility of bats to wind turbines, and that migratory tree bats might actually be attracted to turbines," they write.8
As is the case with building collisions, the first step to protecting bats trying to navigate through man-made environments is identifying and documenting the extent of the problem. "Only through further research," USGS noted, "will we make progress toward minimizing the impact of this new form of sustainable energy on our nation's wildlife."9
Protecting Bats Is Crucial
Bats are incredibly important to the environment, offering significant benefits via pollination and pest reduction. They're facing numerous threats, not only from collisions but also due to habitat loss and fragmentation and white-nose syndrome. Unfortunately, 1 in every 5 bat species is at risk of extinction in the next five decades. If you'd like to get involved, the following steps may help protect bats worldwide:10,11
- Avoid caves and mines where bats are hibernating during winter.
- Encourage natural bat habitats around your home by reducing outdoor lighting, minimizing tree clearing and protecting streams and wetlands. Install a bat house.
- Adhere to cave closures. Check with your state and federal agencies or a local chapter of the National Speleological Society for the status of caves and caving in your area.