Sonic scarecrow

BIRDS were an inspiration for early aviators, but they are a pilot’s nightmare. Collisions with birds are reckoned to cost America’s airlines almost $1 billion in repairs and flight delays annually. When a multiple bird-strike knocked out both engines of a US Airways Airbus A320 taking off from LaGuardia Airport, New York, in 2009 all 155 passengers and crew survived only after the pilots carefully glided the aircraft to a splashdown on the Hudson River. Airports have tried all sorts of ways to shoo away birds but few work for any length of time. Now an ornithologist has come up with a bird scarer that might be the answer.

John Swaddle and his colleagues at the College of William and Mary in Williamsburg, Virginia, think the trick is to blanket the area around a runway with a noise that makes it difficult for birds to communicate. Sound is already used to scare off birds: examples include the boom of propane cannons and recordings of predatory hawks. But birds soon get used to such ruses.

Dr Swaddle knew that noise from traffic and machinery at a certain frequency could disturb birds. To explore this, the researchers decided to investigate how European starlings, a species notorious for ending up in aircraft engines, responded when feeding areas in an aviary were placed next to speakers playing a noise that overlapped with the frequency of their calls. The noise reduced foraging by half over several days.

The group then tested the idea at an airstrip near Newport News, Virginia. They placed speakers on a patch of grass and for four weeks before the speakers were turned on carried out regular bird counts on the site and on two others nearby without any speakers present. Bird numbers were much the same.

Then the speakers started pumping out a noise of 2-10kHz, which overlapped with the birds’ call frequencies. Just as with the aviary experiment, the result was dramatic. As the team report in Ecological Applications, bird numbers plummeted by 82% within a zone 50 metres from the speakers—where noise levels were over 80 decibels, a similar volume to a passing freight train—and by 65% farther out, at noise levels of 65-80 decibels. Crucially, the reductions within the zone persisted for the four-week period the speakers were on, suggesting that the birds were not becoming accustomed to the racket. At the sites with no noise, bird numbers remained constant.

The researchers believe the noise pattern may make birds believe an area is too risky to inhabit because their ability to detect alarm calls is compromised. If the system can be effective over even longer periods and is capable of being scaled up to cover an entire airport, it could save a lot of money and lives.