Homing in Pigeons
Homing is best known in the birds, especially the breed known as carrier or homing pigeons. Even when taken hundreds of kilometers away, they reliably return straight home. Homing pigeons were historically used to carry messages from military expeditions. Today they are bred for sport and studied by animal behaviorists and the occasional geophysicist.
Their homing ability is uncanny. They do it in daylight or darkness, in cloudy weather and clear. They get better with practice, but the capability is inborn. Homing pigeons usually set out in the right direction quickly, without a lot of casting around. Transporting them blindfolded or in darkness or over circuitous routes doesn't affect them.
Compasses, Maps and Beacons
Because it works at such great distances, the homing ability of birds must be deeply connected to the Earth's environment. Three different navigation methods have been seriously proposed.
Homing pigeons clearly detect the Earth's magnetic field direction and can use it as a compass, just as they use the position of the sun (and just as we do). That's well and good, but a compass is not the same as a map. And homing pigeons, which can fly home over terrain they've never seen before, clearly don't use a map. Maybe a map is the wrong metaphor.
That leaves the beacon metaphor, that birds somehow detect home and simply head in its direction. It's been proposed that birds can smell their way home, the way that lost pets can. The trouble is, birds don't smell well, and moreover they can find their way home even when it's downwind. That leaves the third proposed method: infrasound.
Just as the solid Earth is a place of constant seismic activity, the surface world is bathed in infrasound. And geophysicist Jon Hagstrum holds that birds can detect the infrasonic signature of home even at great distances. It's an attractive idea.
Attractions of Infrasound
Infrasound is special in that it travels much farther than ordinary sound; in fact we use it today to detect nuclear explosions and meteorite impacts around the world. And it comes from many different natural sources, including ocean waves, surf, mountain winds, storms, earthquakes and other geologic events. If you can hear infrasound, you can listen to the whole world.
People who explore the outdoors learn to stay oriented by recognizing the framework of landmarks around them. The mountains near and far, the prevailing winds and corresponding weather patterns, and other aspects of the landscape combine in a mental map of the territory. This sense of location is a valuable backup to paper maps and mechanical compasses, and it surely was relied upon by our most remote ancestors.
Perhaps this is why it's easy for a geologist, like Hagstrum, to imagine infrasound as a worldwide sonic environment with its own large-scale landmarks and patterns. Humans can detect it only with clumsy instruments, but the experience of blind people is suggestive. They learn to attune themselves to the audible environment to detect the presence of walls and the size of spaces. A wide-ranging animal could do the same in the much larger spaces of the great outdoors.
Evidence for Infrasound
Hagstrum cites several observations that are consistent with the infrasound model.
- In the laboratory, homing pigeons can detect infrasound of extremely low frequency, down to 0.05 Hz or three cycles per minute.
- Large pigeon races have been shown to be seriously disrupted when the birds were exposed to sonic booms. This suggests that (1) the birds use their sensitivity to infrasound in homing and (2) the boom damages that sense.
- Birds perform better in conditions that favor infrasound reception, such as avoiding flying over water and flying beneath rather than above temperature inversions in the atmosphere. Young birds, not yet expert navigators, perform worse when infrasound background noise is higher in winter.
- When infrasound from home is bent by topography or weather conditions, birds have a "site-release bias," setting off in a consistently wrong direction that matches how the sound is bent.
Hagstrum's 2000 paper "Infrasound and the Avian Navigational Map" goes into these in greater detail and is good reading. A large body of homing-pigeon data exists from experiments conducted at Cornell University that can help take these observations farther. Hagstrum has been exploring that data, along with weather records and atmospheric modeling software, in the years since as his time permits. As progress continues in infrasound research and as pigeon fanciers persist, our database will only grow.