People like to build airports on or very close to locations that have been used by migratory birds for rest and refueling stopovers for thousands of years. This will cause a lot of conflicts. The American Bird Strike Commission estimates that, due to wildlife and bird strikes, American civilian and military aircraft lose nearly $1 billion a year. Since 1988, these strikes have also caused more than 250 deaths.

However, since the scarecrow is a joke, is it possible to reduce the chance of bird strikes? Airport operations managers think so: They have taken a variety of methods to persuade birds to leave the airport, from pesticides and pyrotechnics to guns and falcons, and even recordings of bird distress calls. But there is evidence that these primitive methods will not be effective in the long term.

"You set up the propane cannon and they will get used to it. You broadcast the phone calls of predators and they will learn to ignore them. They will even get used to hordes of angry Wieners", Professor of Applied Science at the College of William and Mary Mark Sind Si said that he is also the co-author (reference) of a paper recently published in the journal Ecological Applications.

Professor Henders said in a statement: "The only way to exclude birds is nets, guns and poison." "But these are expensive and/or impolite."

This exclusion method may also be illegal.

This situation becomes more complicated because finding suitable migratory bird resting and refueling places is exhausting, so many migratory bird species are very faithful to their ancient stopovers (reference). This is especially true for larger birds such as ducks, geese, and swans, which arrive on time every year, even if their stopovers are paved and crowded with people, cars, and airplanes.

This question inspired Professor Hinders and John Swaddle, a professor of biology at the College of William and Mary, to collaborate on a project to develop an effective, non-lethal bird deterrent. To this end, the professors rely on their knowledge of the natural historical characteristics shared by birds. Basically, with few exceptions, birds are highly hearing and vocal creatures. For example, songbirds will sing to attract potential mates and claim territory. Birds are also very social, usually in groups when carrying out daily tasks. These flocks of birds talk to each other while feeding and usually warn their companions of danger.

Birds are very sensitive to certain noises and specific sound frequencies. For example, Professor Swadel has previously documented that the eastern blue bird Sialia sialis is disturbed by certain frequencies of noise generated by traffic and machinery (reference). An aviary study of the common (European) starling Sturnus vulgaris showed similar results.

With this information in mind, Professor Hinders and Professor Swaddle decided to design a device to short-circuit the dialogue between these birds. This device, which they call a "sonic net", is composed of traditional and directional (parametric) speakers that can produce a narrow beam of "colored noise".

If you are like me, you may want to know what colored noise is. Well, we have all heard of white noise-this is the sound you hear when you listen to an unused radio or TV channel. Basically, white noise is a steady flow of all or almost all auditory frequencies at the same energy level. Compared with white noise, the energy of colored noise is more concentrated on the high or low end of the sound spectrum, which changes the signal. For example, pink noise has stronger bass than white noise, and it has fluctuations in pitch and loudness, so it rumbles like a train.

The end result is that colored noise is annoying noise. When they conduct business, colored noise will interfere with normal bird chatter.

"It's like a cocktail party problem," Professor Hinders said. "You are in a room where many people are talking, and it’s hard to keep up with one person’s conversation. It doesn’t even have to be particularly loud. It’s just that all other people’s words fill the gap. So you go to a quiet room so you’re I can hear it."

But really, can pink noise stop birds?

To test the effectiveness of pink noise as a bird deterrent, Professor Hinders and Professor Swaddle conducted research at three locations in an active airport near Newport News, Virginia. All fields are covered with a single short cut grass, close to the runway and service roads, and all fields are roughly the same size (each half a hectare, about half the size of an American football field.)

Long-term sound processing was performed in the middle of the three sites, while the other two were used as reference sites (Figure 1):

At the beginning of the experiment, Professor Hinders, Professor Swaddle and their team installed a large outdoor speaker and amplifier at the central research site, and erected sound walls on three sides to prevent sound from spreading to two reference locations.

"We surveyed bird activities at three locations in the airport four times a week for eight weeks," Professor Swadel said in a statement. During the entire eight-week study period, the team conducted a 30-minute census of all the birds that appeared or flew over at each site. They calculated abundance and species richness. 2 times in the morning and 2 times in the evening, excluding heavy rain. In the first four weeks of the experiment, the speaker just stood on the test site and said nothing.

"The first four weeks provided us with a baseline for bird activity," said Professor Swadel. Then, after four weeks have passed, the speakers are turned on and play 2-10 kHz pink noise 24 hours a day from the end of the 4th week to the end of the 8th week.

The response was dramatic (Figure 2):

In the two reference locations where there is no broadcast sound, the number of birds remained constant during the study. However, you can clearly see that the test site processed with Sonic Net showed that the number of birds within 50 meters of the speaker dropped by 82%, and the noise level exceeded 80 decibels (this volume is similar to a passing freight train). And it drops by 65% ​​in farther places, where the noise level is between 65-80 decibels. In addition, this effect was still as strong at the end of the study period after 4 weeks as at the beginning (Figure 2d), indicating that the birds are not used to rackets.

Why do birds leave the Sonic Net area?

Researchers believe that the noise makes the area near the Sonic Net loudspeakers too risky, and the birds cannot stay for a long time because they cannot hear the siren from their companions. Therefore, sonic nets are a promising non-lethal technology that can persuade birds to leave places where their presence would endanger them or where they are not needed.

This technology can not only be used to keep birds and wild animals away from airports, but it can also keep them away from high-speed train tracks and possibly even highways. In addition, this technology can keep birds and wild animals away from farmland, orchards, salmon farms, garbage dumps, and golf courses. It can also be used to warn birds and bats away from wind and solar farms and glass buildings.

"The new football stadium in Minneapolis happens to be on the main flight path of many birds. Obviously they are building this stadium with a lot of glass. They must take measures to prevent the plague of bird deaths. A technology like this may be correct. It's the technology the NFL needs to avoid negative news again," said Professor Hinders.

It seems that the development and use of applications of this technology is only limited by the user's imagination.

"It may be pigeons that pooped on your car in the parking lot. It may be seagulls that pooped on your yacht," Professor Hinders said helpfully.

In addition, the technology can be modified to target specific species and regions. The team is already studying special speaker designs and is determining the precise sound frequencies that can be targeted to specific species while allowing other species to be undisturbed-this situation can greatly help, for example, farmers who wish to encourage beneficial species (more it's here).

But, of course, noise pollution is always a problem, so it is important to target specific areas.

Professor Hinders said: "If you do it right, you can make the narrow sound cancel itself after spreading a certain distance." "This control over the direction of the sound allows us to cover a specific area with the sound. In that area, The birds cannot communicate, so they left. Outside that area, there is nothing-so we won't produce any noise pollution."

Correction: An earlier version of this article indicated that since 1988, more than 250 people have died from bird strikes each year. Since 1988, the correct number is more than 250 people.

John P. Swaddle, Dana L. Moseley, Mark K. Hinders, and Elizabeth P. Smith (2016). Sonic Net Excludes Birds from Airports: Impact on Reducing Bird Strikes and Crop Loss, Ecological Applications, 26(2):339–345 | doi:10.1890/15-0829.1

Paulo Catry, Vitor Encarnação, António Araújo, Peter Fearon, Anne Fearon, Michael Armelin and Peter Delaloye (2004). Are the long-distance migrating birds loyal to their stopover locations? Journal of Bird Biology, 35(2):170–181 | doi:10.1111/j.0908-8857.2004.03112.x

Caitlin R. Kight, Margaret S. Saha, and John P. Swaddle (2012). Man-made noise is related to the reduction of productivity in breeding the Eastern Bluebird (Sialia sialis), Ecological Application, 22(7): 1989–1996 | doi:10.1890/12-0133.1 [$£€] (free PDF)

Acoustic Scarecrow: A humane, non-lethal way to reduce bird strikes? | @Grrl Scientist