Why bats crash into windows

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Walls can get the best of clumsy TV sitcom characters and bats alike.

New lab tests suggest that smooth, vertical surfaces fool some bats into thinking their flight path is clear, leading to collisions and near misses.

The furry fliers famously use sound to navigate — emitting calls and tracking the echoes to hunt for prey and locate obstacles. But some surfaces, especially human-made ones, could mess with echolocation. Bats interpret flat, horizontal surfaces as water (attempting to drink from them) and have been observed colliding with glass windows.

Stefan Greif of the Max Planck Institute for Ornithology in Seewiesen, Germany, and his colleagues put bats to the test in a flight tunnel. Of 21 greater mouse-eared bats (Myotis myotis), 19 crashed into a vertical metal plate at least once, the researchers report September 7 in Science. In some crashes, bats face-planted without even trying to avoid the plate while others swerved, but too late. Bats involved in head-on collisions emitted fewer calls than those that narrowly avoided crashing.

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.

Infrared camera footage of wild bat colonies inhabited by M. myotis and two other bat species showed that vertical plastic plates trick bats in natural settings, as well. Whether lots of bats are similarly stymied in the wild is unknown.

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Crash reel

This video shows three experiments into how smooth surfaces affect bat flight. In one lab test, a vertical metal plate gave a bat the illusion of a clear flight path, causing it to crash into the barrier. In a second lab test, a horizontal metal plate created the illusion of water; the bat dips to surface to take a sip. Finally, near a natural bat colony, a bat collides with a vertically hung plastic plate, showing that smooth surfaces could impact bats in the wild, as well.