Take a pigeon hundreds of kilometers from its home, and it has no problem finding its way back. For years, scientists have suspected that the bird's stellar navigation has to do with its ability to read Earth's magnetic fields. Now, thanks to a geomagnetic anomaly in New Zealand, researchers have the strongest evidence yet that this is indeed the case.
Until now, support for a pigeon's internal compass has been mostly anecdotal. The birds tend to fly in erratic patterns during electrical storms, for example. The first hard evidence for the geomagnetic theory came from a study showing that pigeons could detect a magnetic field in a wind tunnel (ScienceNOW, 24 November 2004), but that field was many times more intense than Earth's. Also, because the field was either completely on or off, it left the question open of how exactly pigeons might use subtle magnetic differences in the wild to correct their trajectories.
Taking a more natural approach, a team led by Todd Dennis, a behavioral ecologist at the University of Auckland in New Zealand, released pigeons close to a place called the Auckland Junction Magnetic Anomaly. Here, a cluster of massive rock slabs deep below the surface causes a detectable spike in the geomagnetic field. Dennis reasoned that if the pigeons were released here, they would reveal how they were using geomagnetic information as they struggled to get clear of the anomaly. To keep track of their trajectories, the researchers strapped global positioning system (GPS) devices to the birds' backs.
The geomagnetic anomaly threw the pigeons for a loop. Of the 92 pigeons released around the anomaly, 59 clearly flew relative to the direction of the local field--not Earth's field. As soon as they got beyond the anomaly, however, the pigeons corrected their direction and headed right home. Dennis concludes that the birds are keeping track of gradients in the field to navigate.
The study is "fantastic," says Joe Kirschvink, a biophysicist at the California Institute of Technology in Pasadena. "Now we have the data to convert the speculation" about pigeon navigation "into reality." The next key experiment, says Cordula Mora, a biologist at Duke University in Durham, North Carolina, who led the magnetic wind tunnel study, "will be to see whether this observation holds true at other sites in Europe and North America" and to find out how the pigeon's magnetic organ--thought to reside in the beak--is wired to its brain.