Why Windows Pose a Threat to Bats Using Echolocation

Story at-a-glance -

  • Twenty-one mouse-eared bats 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, 19 of the bats collided with the vertical plate, either flying directly into it or attempting to swerve, albeit too late to avoid impact
  • When vertical metal plates were placed near bat caves in the wild, collisions also occurred
  • The smooth vertical surfaces, such as windows, may act like “acoustic mirrors” that prove harder for bats to navigate than rough surfaces

By Dr. Karen Shaw Becker

Bats use echolocation to form acoustical “maps” of their surroundings. When they emit their high-frequency calls, they bounce off objects around them, allowing bats to complete impressive feats, like catching tiny flying mosquitoes in pitch-black darkness, all while moving at high rates of speed.

Yet, the system is not without its faults, particularly because, as Peter Stilz of the department of animal physiology, Institute for Neurobiology at the University of Tübingen, Germany, explained, bats are “forced to apply a high degree of processing and interpretation to the sensory input, making them prone to sensory deceptions.”1 In research published in the journal Science, 21 mouse-eared bats 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, 19 of the bats collided with the vertical plate, either flying directly into it or attempting to swerve, albeit too late to avoid impact. None were injured during the experiment, but this isn’t likely to be the case in the wild.2 In fact, when vertical metal plates were placed near bat caves, collisions also occurred.

The likelihood of collision seemed to be influenced by 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.

As noted in Science News, “Smooth surfaces act as acoustic mirrors, which could present a problem for a bat: They reflect sound at an angle away from the bat, producing fuzzier, harder-to-read echoes than rough surfaces.”3

Bats May Collide With More Objects When They Rely on Vision

While echolocation appears to include some blind spots for bats on the move, bats that rely on their vision may fare even worse. Although the saying “blind as a bat” suggests bats have poor vision, in reality they can see with varying degrees of acuity, depending on species. Their vision works best in low ambient light, such as at dusk or dawn, and gets worse during daylight hours.

Further, their eyes are adapted for long-distance use in order to detect things beyond their echolocation range, whereas their visual capabilities in shorter distances are largely unknown according to researchers writing in PLOS One.4

However, when researchers set up an obstacle course of sorts for wild bats in Ontario, Canada, and observed bats flying through it in different lighting (dark, dim and brightly lit), the bats relied primarily on their vision when navigating the brightly lit obstacles, even though it made them more likely to crash. According to National Geographic:5

“In the obstacle course, the team used fabrics of three different visibilities — a clear fabric, an opaque fabric, and a reflective fabric. If the bats were mostly using their sonar, they should have detected all three. But the bats did not sense some fabrics — such as the clear one — suggesting the animals were depending more on their vision.”

In light of the Science study, it’s also possible the bats missed the clear fabric due to a “blind spot” on their echolocation. That being said, the study also turned up an interesting phenomenon: the bats crashed more in the light than the dark in early August, but this changed in mid- to late August, when they started crashing more in the dark.

The researchers believe this is due to hormonal fluctuations that occur during different phases of bats’ preparations for hibernation — and shows there’s still much to be learned about how bats interact with their visual environments.

Bats View Smooth Horizontal Surfaces as Water

Another interesting factoid about bats is that they perceive any “echo-acoustically smooth” extended horizontal surface to be water, even if there is conflicting information present from other sensory inputs like taste and touch. Being able to recognize bodies of water is clearly crucial to bat survival, not only for drinking but also for orientation, and in one study bats attempted to drink not only from water surfaces but also from smooth plates.

Young bats that had never been exposed to a body of water also tried to drink from the smooth plates, which suggests they have some innate knowledge of how to recognize this important part of their habitat. “The high number of consecutive drinking attempts that the bats showed within a short time, despite being unsuccessful, indicates a hardwired neural processing of echoacoustic water recognition,” researchers explained.6

How Common Are Bat Collisions in the Wild?

Many man-made objects pose threats to bats in the wild, including vehicles, roads7 and wind turbines on windfarms. In the latter case, ecological impact assessments to gauge the risk of wind turbines to bats in the U.K. are required before their construction, and while some estimated the impact to be minimal, the outcome turned out to be quite different.

For instance, University of Exeter scientists revealed that 18 of 29 U.K. sites with an ecological impact assessment available reported that the windfarm would not likely affect protected species like bats. In reality, virtually all of them were in areas with bat activity and half of them had collisions between bats and turbines, some with upwards of 64 bat deaths a month.8

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.9 As for bat collisions with windows, the statistics are less clear. Building windows are estimated to kill about 1 billion birds a year in the U.S.,10 but it’s unknown how bats are affected.

One thing’s for certain, however, and that is bats deserve every protection we can give them. A single brown bat may consume up to 7,000 mosquitoes in one night,11 serving as a valuable form of pest control. They also act as important pollinators. You can encourage natural bat habitats around your home by reducing outdoor lighting, minimizing tree clearing, and protecting streams and wetlands, while installing a bat house in a spot high off the ground (12 to 20 feet up is ideal) and in direct sunlight for six to eight hours a day.12

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