Bats are well known for their sophisticated use of echolocation to navigate through areas riddled with obstacles, but now a new study reveals that this useful ability is hindered in the face of smooth vertical surfaces - those that are particularly likely to be manmade. The results help explain why injured or dead bats are often found near buildings, and underscore the negative impact of human development on wildlife. To navigate through the dark, bats rely heavily on echolocation, where they emit high-frequency sounds and use the returning echoes to detect, classify, and localize objects in their environment. However, several observations of bats colliding with smooth vertical surfaces (such as glass windows) suggest that bats have problems recognizing them. To explore this issue in greater detail, Stefan Greif and colleagues monitored greater mouse-eared bats (Myotis myotis) as they flew through a continuous, rectangular flight tunnel in the dark. In the corner of the dark tunnel, the researchers placed a metal plate either vertically or horizontally. In natural habitats, smooth vertical surfaces are rare, yet bats encounter smooth horizontal surfaces in the form of water. Of 21 individual bats, 19 collided with the vertical plate at least once (on average 23% of passes) but never with the horizontal plate. The researchers found that when the bats collided with the vertical plate, they were producing fewer calls, spending less time in front of the plate, approaching the plate at a more acute angle, and had higher flight speeds relative to the bats that avoided collision. The authors report similar findings in field experiments outside of caves of three different bat species. These results are discussed in a Perspective by Peter Stilz.