91av

CONCORDE may be to blame for the mysterious disappearance of thousands of
racing pigeons. A geophysicist in California has found evidence that shock waves
from supersonic aircraft stop homing pigeons hearing the low-frequency sound
that may help them find their way home.

Pigeon racing has become a worldwide sport thanks to the birds’ ability to
find their way home over thousands of kilometres. Researchers have shown that
pigeons have an inbuilt “compass” that allows them to navigate using the Earth’s
magnetic field and the position of the Sun. But to reach their destination the
birds also need to have a “map sense”—a mental chart linking their
starting position and destination so they know which way to go. “Nobody has been
able to understand how they do that,” says Jon Hagstrum of the US Geological
Survey in Menlo Park.

Some scientists have suggested that the key to the avian map sense is
infrasound—low-frequency sound waves that birds can hear, but not people.
Waves in the oceans continually exert pressure on the seabed, making the land
shake. These seismic shivers radiate infrasound up into the air. According to
Hagstrum, steep hillsides reflect the sound waves horizontally, forming unique
infrasound beacons whose signals can travel for hundreds of kilometres, similar
to the homing range of birds.

Hagstrum now has evidence that pigeons use these beacons to “hear” their way
home. He was intrigued by the fact that occasionally most of the birds in some
races are delayed, or fail to return home at all. In June 1997, for instance, 60
000 English pigeons were released in Nantes, France, but around a third did not
return to England, and the rest were late.

Hagstrum suspected that the four races he studied had one thing in common:
the birds had crossed the path of infrasonic shock waves generated by Concorde’s
sonic boom during supersonic flight. To find out, he compared the birds’
predicted routes home with Concorde’s flight paths and departure times on the
days the races took place. The calculations showed that in all the races, the
hapless pigeons would have entered the cone-shaped shock wave. This could have
temporarily or permanently deafened the birds to infrasound.

Hagstrum says this supports the idea that infrasound is the key to the birds’
map sense. Bob Beason, a bird navigation expert at the State University of New
York in Geneseo, says the work is intriguing. “The idea that pigeons might use
infrasound for orientation has been around for a while.” However, he thinks
Hagstrum has more work to do to show that Concorde confused the birds. For
example, would Concorde be loud enough to deafen them?