Why bats crash into buildings

People of an anthropomorphising and Gothic bent often rhapsodise about the probable joys of being a bat.

They fondly imagine flying, batlike, through the blackness of a romantic night, dipping and diving and turning, zooming between obstacles with grace and mystery.

Such fantasies, however, rarely include encountering a vertical mirror or plate-glass window, and this is a good thing. Research published in the journal Science shows that when encountering such objects many bats are very likely to face-plant straight into them, injuring themselves, sometimes fatally.

Stefan Greif, from the Max Planck Institute for Ornithology in Germany, and colleagues established that smooth vertical mirror-like surfaces confound bats’ echolocation abilities, causing them to interpret them as open flyways – with catastrophic results.

Greif and his colleagues were prompted to examine the phenomenon after reviewing other research, some published in the early 1960s, that reported bats uncharacteristically flying into things. In 2010, for instance, Dara Orbach from the University of Western Ontario in Canada, and colleagues, observed bats colliding “with stationary objects they should easily detect by echolocation and avoid”.

Orbach’s team suggested that light reflecting off these objects might be interfering with the bats’ ability to augment echolocation with visual inputs.

After setting up controlled experiments using greater mouse-eared bats (Myotis myotis), Grief’s team concluded that, indeed, reflection was the problem, not of light but of acoustic signals.

The researchers set bats to fly through a tunnel, in which was placed a reflective plate, sometimes on a horizontal plane and sometimes on a vertical one. No bats collided with the horizontal object, but on repeated flights the bats smacked into the vertical one (attempting to avoid it at the last moment, or not at all) in 22% of cases. (No bats were injured, by the way.)

As a further experiment, the team then placed vertical plates near, but not directly in, the flightpaths of three wild bat colonies. Sure enough, several of the animals collided with them.

Analysing the results, the scientists suggest that shiny upright surfaces disrupt echolocation signals, which in other circumstances bounce back from potential obstacles allowing bat brains to form a “picture” of their position and shape.

“This is likely attributable to the acoustic mirror properties of smooth surfaces, where echolocation calls are being reflected away from the bat and no echoes return from the position of the plate,” they write.

Greif and his co-authors conclude by calling for more research, and for regulations discouraging the installation of large reflective surfaces on buildings that are on bat “migratory highways”.

The concern is echoed in an editorial in the same journal, written by Peter Stilz of the Institute for Neurobiology in Germany’s University of Tubingen.

“Given the abun¬dance of vertical mirroring structures like windows or glass-front buildings built by humans into the bat environment in recent decades,” he writes, “it would be desirable to determine whether these structures represent a rel¬evant ecological threat, how well bats can learn to deal with this threat, and whether potential casualties can be avoided.”