Projecting targeted sound into the airspace surrounding buildings, communication towers and wind turbines could give birds a heads-up before they fatally strike such obstacles, researchers say.
A study published Wednesday in PLOS One details a concept known as an acoustic lighthouse, a novel approach to diverting avian air traffic away from large structures built by humans. "That has an essentially additive effect," Timothy J. Boycott, a conservation biologist for the Commonwealth of Massachusetts and corresponding author of the study, told The Academic Times. "You have what is still a visually conspicuous structure, you add a conspicuous acoustic cue, and they combine for a multimodal signal that has a high chance of grabbing birds' attention."
North America's bird populations are in a steep decline, as nearly 30% of birds have vanished in the U.S. and Canada since 1970, according to a 2019 study by the Cornell Lab of Ornithology that analyzed 529 bird species. That study's lead researcher, conservationist Ken Rosenberg, has said the loss of billions of birds is "a strong signal that our human-altered landscapes are losing their ability to support birdlife."
Boycott was drawn to the "immediate, tangible conservation applications" of the acoustic lighthouse concept during his time as a graduate researcher at the Institute for Integrative Conservation at the College of William and Mary in Virginia. Most previous efforts to reduce the number of birds striking buildings have been based on making structures more visually obvious. "What motivated our study was seeing this body of work taking that approach and asking, 'Is there still a lot of evidence to suggest that it's not working?' There's a high degree of perceptual blindness for these birds and they're still colliding with what seem to be pretty obvious structures," he said.
Why can't many birds seem to avoid, say, a cell tower directly in their flight path? Generalizing broadly, many birds' eyes are positioned on the sides of their heads, providing exceptional peripheral vision, while their binocular vision is more associated with near-sighted tasks such as feeding and landing on a perch, Boycott said. In other words, many birds have a blind spot right in front of their beaks. What's more, they keep their heads tilted downward during flight to become more aerodynamic — and don't anticipate anything at all being in their way.
"When they're moving through airspace at high altitudes — like they are when they interact with communication towers and wind turbines — their attention is very much focused on terrestrial features," he said. "They're using landscape markers to guide their movements."
"Another aspect is evolutionary," Boycott continued. "These structures are very new. These birds used to move through what used to be a highly predictable open airspace, and these structures standing hundreds of feet into that airspace are evolutionarily novel. Before contemporary times, there was no reason for a bird to be paying attention to a potential collision hazard."
The researchers found hazards to monitor during their field study on the eastern side of the Chesapeake Bay on the coast of Virginia, an area navigated by migratory birds flying south in the fall, Boycott said: "There's a huge increase in the abundance of birds flying through this peninsula that get funneled through a fairly narrow stretch of land. We found communication towers that were high enough to be of concern and were isolated enough for us to test out this acoustic lighthouse technique."
Angling highly directional speakers upward at the base of the towers, the researchers played sounds at 4 to 6 kilohertz and slightly higher frequencies between 6 and 8 kilohertz high into the surrounding airspace. "Both were white noise stimuli, kind of staticky broad-spectrum sounds," Boycott said. "The high one ends up sounds a bit like a gas leak and the lower one sounds something like a running mountain stream."
Both frequencies showed the potential to reduce the presence of birds around the towers, with the higher frequencies reducing their presence by about 12%, compared with a roughly 16% reduction at lower frequencies, according to the study. The researchers also tracked 145 "at-risk" flights, finding that birds were more likely to reduce their flight speed and angle away from the towers during sound treatment. The 4 to 6 kilohertz signal proved the most effective, causing birds to alter their flight paths earlier and at a larger distance from the towers.
Though the findings are promising for the potential real-world application of acoustic lighthouses, the concept has practical limitations. Most significantly, the frequencies used in the study were audible to humans, meaning that the technique would be limited to remote settings and very tall structures.
"Unfortunately, bird hearing and human hearing overlaps quite a lot," Boycott said. "Our solution to that was to go to remote locations and use highly directional speakers that contain the sound field in the airspace hundreds of meters off the ground. So, it is not a solution you'd hang on the side of your house to protect against bird collisions, but it could potentially be used in high-rise buildings where you're projecting your signal hundreds of meters off the ground and it wouldn't reach street level."
The next step of the project is studying a wider range of frequencies and how they're perceived by specific bird species. Preliminary research indicates that sweeping the pitch of the acoustic signals — like an ambulance's rising and falling siren — can generate "robust behavioral responses," Boycott said. As counterintuitive as it seems, a growing body of evidence suggests that using disruptive sound could safely redirect avian air traffic. "Excess anthropogenic noise is often a problem for naturally occurring wildlife," he said, "so it was an adjustment to think of this as a beneficial tool for conservation."
The study, "Field testing an 'acoustic lighthouse': Combined acoustic and visual cues provide a multimodal solution that reduces avian collision risk with tall human-made structures," published April 28 in PLOS One, was authored by Timothy J. Boycott, Sally M. Mullis and John P. Swaddle, William & Mary; and Brandon E. Jackson, Longwood University.