Skyscrapers, communication towers and wind turbines are a regular sight in our daily lives, but if you’re a bird, they can be a dangerous obstacle course.
Each year, millions of birds fly straight into these towering structures, making it one of the leading causes of human-related bird deaths worldwide.
There has been no shortage of attempted solutions, from marking windows to placing special lights on top of buildings and towers.
However, while bright markings and lights stand out to the human eye, they have done little to prevent birds from crashing into them.
Now, a team of researchers in the US have used sound waves to prevent birds from slamming into tall communication towers.
These “acoustic lighthouses” reduced the number of birds flying close to the towers by up to 16 per cent, helping to cut the risk of fatal collisions.
The researchers’ findings were published today in PLOS One.
“When [birds] are flying, they can have a visual blind spot in front of them,” said study co-author Timothy Boycott, a conservation biologist for the Massachusetts state government.
“We know birds use sound to communicate, so we thought adding another signal could increase the chances of them noticing these structures.”
Taking a bird’s-eye view
Birds are known to have super-sharp eyesight, so why do they keep crashing into tall objects even when they are clearly marked?
Apart from owls, birds generally have their eyes on the sides of their skulls, which is great for perceiving the world peripherally, but not so good for seeing what’s coming up ahead mid-air.
As many birds use landmarks on the ground to help them navigate while flying, they often look down, rather than ahead, while travelling.
“They keep their head down to be as aerodynamically streamlined as possible,” said study co-author John Swaddle, a behavioural ecologist at the College of William & Mary.
“It’s like texting while driving.”
Mr Boycott added that birds evolved to fly through open spaces, which means they are not naturally accustomed to dodging human-built structures that block their path.
“They need not have expected a structure like a wind turbine to have existed,” he said.
Using sound as a safety warning
With these visual challenges in mind, the researchers emitted sound waves from speakers mounted in front of two tall communication towers on the Delmarva Peninsula in Virginia – a hotspot for annual bird migrations.
They used white noise cropped to two frequency ranges: 4-6 kilohertz, which sounds a bit like a fast-running stream, and 6-8 kilohertz, which is comparable to the hiss of a large gas leak.
These frequency ranges are perceptible to both humans and birds, and are distinct enough to stand out against background noise.
Using video cameras, the team recorded more than 1500 birds flying within 200 metres of the two towers, with less than 100 metres being flagged as the “danger zone”.
Both frequency ranges appeared to be effective bird deterrents, resulting in an average of 12-16 per cent fewer birds flying close to the towers compared to normal conditions.
The birds changed their flight movements faster and more drastically when exposed to the 4-6 kilohertz sound treatment, flying about five metres further away from the towers on average.
They also put the brakes on their flying speed and veered away from the towers sooner than birds that heard the higher frequency sounds, suggesting that their hearing is most sensitive to noise within the 4-6 kilohertz range.
While the researchers did not track individual species of birds in their study, they noticed that smaller birds responded more quickly to the noise than larger ones.
“It takes a little while longer for large birds to make a turn,” Professor Swaddle said.
“They’re a bit like oil tankers.”
Noise without the complaints
Mr Boycott said the new approach could be used in a range of settings, from wind turbines in regional areas to tall glass-windowed buildings in the middle of the city.
To avoid creating more noise pollution, Mr Boycott said speakers could be mounted on top of buildings and towers rather than on the ground, where the noise would reach human ears.
“This certainly isn’t something you would attach to the side of your house tomorrow,” Mr Boycott said.
Alessandro Ossola, an urban ecologist at Macquarie University who was not involved in the study, agreed that the approach could be scaled up to work in big cities.
“Wouldn’t it be ironic if artificial noise, a plague of our modern cities, turned out to be a great ally for preserving urban birds?”
Aussie birds may need a different frequency
Gisela Kaplan, a behavioural ecologist at the University of New England who was also not involved in the study, said the new technique offers a simple but promising approach to reducing bird collisions.
“It’s a very welcome and indeed much-overdue response to millions of birds dying annually as a result of large human structures,” Professor Kaplan said.
But because Australian bird alarm calls generally differ from northern hemisphere species, the frequency ranges would need to be tweaked for it to work on our native communities, Professor Kaplan added.
“It would be important to adjust the acoustic parameters accordingly.”
Next, the team will finetune their technique by exploring which sounds work best in different scenarios.
Professor Swaddle and his students are already testing out the use of oscillating sounds in the 4-6 kilohertz range, which resemble a police car siren.
“If birds learn that this sound means there’s danger ahead, then that’s a good thing,” Professor Swaddle said.