Billions of birds die each year from collisions with tall glass buildings, communication towers and power lines – a gobsmacking toll that's expected to increase as cities grow outwards and upwards.
A recent study suggests there could be a way to fix our deadly mistakes – by installing 'acoustic lighthouses' that blast white noise in short bursts, stopping migratory birds on a collision course with towering metal structures, tall buildings, and possibly even wind turbines.
Field trials testing two types of sound signals reduced bird activity around communication towers by up to 16 percent, and researchers think these acoustic lighthouses could reduce the risk of birds colliding with wind turbines, too.
Wind turbines are built to harness energy from the same wind streams migratory birds travel along, explains conservation biologist Timothy Boycott of William & Mary College, who led the study.
"It's an area of high potential conflict between migrating birds and human development," Boycott says.
And that development is only set to increase at a staggering rate.
"We're building structures at a faster rate than pretty much anytime in human history," wildlife ecologist Jared Elmore, who was not involved in the study, told Audubon.
"Glass is seen as a really attractive and cheap building material. And that's not good for the birds."
Conservationists have devised all sorts of strategies in an effort to reduce the death toll of bird collisions, from patterned glass to rope curtains and laser lights. But these strategies aren't always effective, likely because birds view the world quite differently from humans.
Most birds have eyes on either side of their heads, facing outwards, which means birds flying heads down and wings up have a gaping blind spot right in front of them. Wind turbine blades that cut a sweeping path through the air also pose a moving hazard different to reflective glass buildings and steel towers.
Given all this, Boycott and colleagues thought sound might be a better safety warning for birds fast-approaching a human-made structure; an idea some researchers in the group first tested with captive zebra finches.
In these latest field trials, conducted during the North American fall migration, speakers pumped out white noise in 30-minute bursts around two communication towers on the Delmarva Peninsula, on the east coast of the United States.
"It's a geographic area that sees an incredibly high abundance of birds moving through," Boycott says. "And these birds are moving southward along the Atlantic migratory flyway, headed as far south as the very tip of South America in some cases."
With the speakers set up at the base of the towers, two different types of white noise were tested over six days.
"They were both broad-spectrum, a kind of staticky, hissy type sound," explains Boycott, with one to match the hearing range of many birds and another, at higher frequencies, chosen to stand out against background noise.
Using cameras to capture flight behaviors of more than 1,500 birds passing within 100 meters (328 ft) of two communications towers allowed the researchers to tally potential birds saved, as opposed to counting birds lost, as other studies have done.
Boycott and colleagues recorded the birds in full flight from multiple angles, analyzed the footage, and found both sounds tested deterred birds from flying too close to the towers. But more birds steered clear of the towers sooner when the lower frequency sound (4-6 kHz) was projected, compared to normal conditions.
"[Birds] stayed farther away from the towers and they angled their flight trajectories away from the towers more," Boycott notes, which might indicate that lower frequencies are more clearly audible to birds – though it may vary species to species.
Small birds also veered away from the communication towers quicker than larger birds could, but relying on video footage meant the study did not track individual species of birds. Some species are more prone to collisions, especially migratory birds that fly in large groups at night.
"Future studies would be really important to see how those differences in flight behavior actually translate to mortality on the ground," Boycott adds.
Earlier testing also showed that sound alone might not divert birds but rather alert them to hazards ahead, so additional visual cues may still be needed. For example, a study in Norway recently found that painting one wind turbine blade black to increase visibility could help reduce collision risk.
As for acoustic lighthouses, frequencies may need to be tweaked for native birds on other continents as well. It would also be wise to consider the impacts of artificial noise in general, since humans encroaching on the natural world is what created this predicament in the first place.
"Acoustic warning signals could also present challenges to other wildlife, masking of communication signals or increasing stress," Boycott and colleagues note. "To reduce unintended negative consequences of acoustic warning signals, context-dependent intermittent use may be a compelling solution."
As an initial proof-of-concept, however, these audible warning signals could be a promising step in the right direction.
"There would be great value in extending this test to other times of year, to other landscapes and seascapes, and to other structures that present collision hazards," the team writes.
The study was published in PLOS One.