Projecting white noise in the direction of oncoming birds could stop them from colliding with buildings or wind turbines.
Birds keep their heads down to streamline their bodies as they fly, says John Swaddle at the College of William & Mary in Virginia. So, visual cues may not be enough to warn them of oncoming structures. That is why billions of birds around the world, particularly those that migrate long distances, die in collisions with manufactured structures each year.
“Birds fly a bit like texting while driving,” says Swaddle. He his team used white noise at around 70 decibels – about as loud as a vacuum cleaner – to try to get their attention when they are near tall structures. “These acoustic stimuli are like someone honking at them, making them more aware of their surroundings,” he says.
The team used directional speakers around two communication towers along the Delmarva peninsula in Virginia, an area that millions of birds pass through going south during the North American autumn migration. The speakers were angled to only be heard by oncoming birds travelling from the north, cameras recorded the flight paths of birds within a 500-metre radius.
The team broadcast two sounds within the frequency range that most birds can hear, playing them for 30 minutes at a time with 30 minutes of silence between them over a total of 3 hours. One sound fell between 4 6 kilohertz, the other between 6 8 kilohertz. The team played these sounds for six days between September November 2019.
Compared with the periods of silence, bird activity decreased roughly 16 per cent around the towers when the 4-6 kilohertz sound was played, 12 per cent while the sound at 6-8 kilohertz played.
When birds flew within 100 metres of the tower, they were considered at risk for a collision. But the lower sound frequencies caused them to slow down more divert their trajectories further around the tower. Swaddle says this may be because birds hear frequencies between 4 6 kilohertz more clearly.
Swaddle says this suggests that certain frequencies of sounds may be more efficient at making birds aware of structures like communication towers wind turbines. “The technology is already out there to do this so the implementation shouldn’t be that difficult,” he says.
Journal reference: PLOS ONE, DOI: 10.1371/journal.pone.0249826
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