Birds may not have big brains, but they know how to navigate. They wing around town and across continents with amazing accuracy, while we watch and wonder. Biologists believe that sight, smell, and an internal compass all contribute to avian orienteering. But none of these skills completely explains how birds fly long distances or return home from places they've never been. A new study proposes that the animals use infrasound—low-level background noise in our atmosphere—to fly by "images" they hear. These acoustical maps may also explain how other creatures steer.
Scientists have long considered infrasound as a navigational cue for birds. But until U.S. Geological Survey geophysicist Jonathan Hagstrum in Menlo Park, California, became intrigued by the unexplained loss of almost 60,000 pigeons during a race from France to England in 1997, no one pinpointed how the process worked. The race went bust when the birds' flight route crossed that of a Concorde jet, and Hagstrum wanted to know why. "When I realized the birds in that race were on the same flight path as the Concorde, I knew it had to be infrasound," he says. The supersonic plane laid down a sonic boom when most of the animals were flying across the English Channel.
Normally, infrasound is generated when deep ocean waves send pressure waves reverberating into the land and atmosphere. Infrasound can come from other natural causes, such as earthquakes, or humanmade events, such as the acceleration of the Concorde. The long, slow waves move across vast distances. Although humans can't hear them, birds and other animals are able to tune in.
When sound moves through the atmosphere, the waves get bounced upward by steep-sided terrain and bent downward by wind and cold air. The topography and prevailing weather conditions give places a sound signature. Hagstrum hypothesized that birds create maps with these unique acoustics, and he returned to homing pigeons to test the idea.
Hagstrum gathered information from release-site journals for pigeons from a Cornell University loft that were sent flying from three sites near their home in Ithaca, New York: Jersey Hill, Castor Hill, and Weedsport. Pigeons from this loft had a peculiar history of getting lost every time they flew away from the Jersey Hill fire tower, located 119 kilometers west of Ithaca. Inexplicably, the only day all the pigeons came home was 13 August 1969.
Release-site data from 1968 to 1987 also showed birds consistently left the Castor Hill tower at 50° off the bearing of their home loft, whereas they always left Weedsport 15° off course. Homing pigeons from other New York lofts didn't have that problem. "But year after year, they lost Cornell Loft birds at Jersey Hill," Hagstrum says. "It had to be missing sound."
To make his case, Hagstrum modeled atmospheric infrasound around the three release sites on the days the pigeons flew astray. The program factored in the effects of terrain and atmospheric conditions to create three-dimensional diagrams of sound waves bouncing through the atmosphere and back to the ground.
In a paper published today in The Journal of Experimental Biology, Hagstrum correlates the trajectory of sound waves at release sites with the pigeons' flight performance. He shows that the terrain at Jersey Hill interrupted sound transmission, which interfered with the birds' ability to navigate.
"Jersey Hill was a bad spot for Cornell birds," Hagstrum says. "The geometry of the area conspired to create a sound shadow." On the single day in August 1969 that the birds returned home, there was a temperature inversion that bounced sound back to the release site, allowing the pigeons to navigate.
"I think it's very convincing evidence that infrasound is a component of information birds use," says Alfred Bedard, a physicist at the Cooperative Institute for Research in Environmental Sciences in Boulder, Colorado, who wasn't involved in the study. "The open area is what infrasound they find most useful." Still, "these results aren't surprising," he says. "If creatures have information in their environment that's important to their survival, they would sense it."
Birds aren't the only animals attuned to infrasound. Whales and elephants sing at super-low frequencies to communicate across great distances. Tigers, giraffes, and okapi sense it, too. Perhaps the real question is why can't we see what the animals hear?