Lights Are Driving Bats From Their Belfries

The trend of pointing floodlights at churches in Sweden has driven some long-eared bat colonies out of their historic roosts

An unlit church in Sweden Johan Möllerberg / Alamy Stock Photo

For decades, European bats went in steep decline due to habitat loss and a long tradition of indiscriminately killing the flying mammals. But in recent years, the bats have begun to bounce back. Now, Mary Beth Griggs at Popular Science reports that a recent study has found another potential culprit in the bat drop—and a potential solution. It turns out that proud villagers in Sweden have installed floodlights to show off their quaint, historic churches, which are literally driving bats out of the belfries.

According to Griggs, Swedish biologist Jens Rydell conducted a census of brown long-eared bats, Plecotus auritus, in 61 church attics and steeples in southwest Sweden during the 1980s. Since then, the trend of lighting up churches has taken off, so he decided to investigate the potential impact, revisiting those same churches as well as 50 others last year.

Fred Pearce at New Scientist reports that Rydell found that the 14 churches that never installed floodlights all still had long-eared bat colonies. Of the churches that were partially lit, half had lost their bat colonies. The houses of worship that were bathed fully in light on all four sides all night long had no reported bats.

“Generally bats are faring quite well in this area,” Rydell tells Griggs. “But the massive introduction of lights can clearly change that. A 38 percent reduction of colonies, as we found, is a lot.”

It’s not that the lights keep the bats up all night. Instead, the bats prefer darkness to protect them from predators like tawny owls and kestrels that nab them when they leave their roosts. At Brunn church, which is only lit on three sides, Rydell noticed that the bats stuck to the shadows as much as possible. “We counted 68 bats leaving the attic in the evening. Every one emerged from a narrow dark corridor at the eastern end of the church near to a big oak tree that almost reached the church wall,” he tells Pearce. In essence, they used the dark spot as an escape route to avoid potential predators. 

Bob Yirka at points out that all bats are protected in Sweden and that harming them or disturbing their roosts is illegal. It’s likely that villagers put up the lights to highlight their architecture, not shoo away bats. Yirka reports that the researchers suggest a compromise, saying that the churches could leave one side, preferably a side facing trees, dark while lighting up the other three sides.

Since Sweden has very few caves, bats have been using churches as roostes for over 1,000 years, reports Griggs. In fact, Rydell says, attics and belfries make excellent roosts for the flying mammals. “Church attics are large enough and have piles of hibernating insects such as blow flies, butterflies (tortoiseshells), and also, of course, potentially harmful insects to the wood.” Rydell tells Griggs. “The bats feed on these and can thus be active even [when] weather is unfeasible outside. They can even grab a meal if the get hungry during daytime or in the middle of the winter.”

Artificial lights in general have had a wide impact on bats. Many bat species are light shy, and will avoid outdoor lighting like streetlights or floodlit buildings, while others are drawn to the insects those lights attract. The conservation group EUROBATS is currently putting together guidelines for the bat-friendly use of nighttime outdoor lighting.