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Are Waves the Answer?

We've all been lost at one time or another. And the farther we venture from home, the greater the chances that we'll need help finding our way back. Fortunately, we humans have maps, gas-station attendants, and now GPS devices to guide us. But what tools do birds use to point them in the right direction? And can interference from supersonic jets cause these tools to malfunction? One scientist believes he has the answers.

First, a distinction: In order for a bird to navigate accurately from one point to another, it must possess both compass sense and map sense. The former allows the bird to orient itself relative to the Earth's poles -- that is, fly north rather than west -- while the latter allows it to chart a course to its specific destination. For the most part, scientists have come to believe that the compass sense in birds is innate and, like a real compass, attuned to the Earth's magnetic field. As for the map sense, theories abound, but there's little consensus.

Enter Jon Hagstrum, a geophysicist based in Menlo Park, California. Hagstrum believes that it's low-frequency sound waves that provide birds with their detailed map information. In a study published this month in the Journal of Experimental Biology, he traces the formation of these sound waves to the ocean, where the pressure that water continually exerts on adjacent land causes the land to shake. These vibrations radiate into the air as sound waves and provide birds with valuable topographic cues. A steep cliff, for example, radiates a distinctive sound pattern that the bird then plots as a feature on its mental map.

To prove his theory, Hagstrum studied homing pigeons in Europe and the northeastern United States. These birds -- Rock Doves that have been trained by being released from sites progressively farther from home -- are known for their remarkable navigational skills. But what happens to homing pigeons when supersonic jets make the natural low-frequency sound waves inaudible? According to Hagstrum, the Concorde's sonic boom temporarily deafens the birds and renders their maps useless. The result is that the birds become lost.

Of course, Hagstrum's report is by no means the final word on the subject. Like previous attempts to explain what makes birds such expert navigators -- one theory speculated that the position of the sun guides them, while another proposed that odors play a key role -- it will no doubt have its critics. And until birds can speak for themselves, we may never know for certain.

 

 

 

 

 

2007 eNature.com