Workings of Bird Navigation

How do birds find their way when migrating long distances? Scientists would like to know—they’ve been studying the way birds sense the Earth’s magnetic field for 50 years without many results, says Ed Yong in Discover:

We’ve known that birds can do it since the 1960s, but every new attempt to understand this ability—known as magnetoreception—just seems to complicate matters even further.

The New York Times reports that:

Navigating by magnetism includes several steps. Birds have to have a way to detect a magnetic field, and some part of the brain has to register that information; it seems likely that another part of the brain then compares the incoming information to a stored map.

One study this month failed to find the way birds detect a magnetic field (the first step), but another study brought us closer to understanding how the brain takes in the information (the second step).

The first study, published in Nature, tested a theory that the magnetic field receptors reside in birds’ beaks. Researchers sampled about 250,000 thin slices of tissue from beaks, and as lead author David Keays says, “We had hoped to find magnetic nerve cells, but unexpectedly we found thousands of macrophages, each filled with tiny balls of iron.”

The macrophages probably don’t play a role in magnetic sensing because they cannot produce the electrical signals required to cause neurons to fire. If the neurons don’t fire, there’s nothing to influence the birds’ behavior.

“The mystery of how animals detect magnetic fields has just got more mysterious,” Keays admits.

The second study doesn’t get us much closer to solving that mystery, but demonstrates how the magnetic fields are processed in the brain. Baylor College of Medicine researchers tested pigeons in a lab where the scientists could create and manipulate the magnetic field. They then recorded the activity in the brainstems of the birds.

The scientists found 53 neurons that responded to the magnetic field changes, co-author J. David Dickman says. “The cells responded to the angle and intensity of the magnetic field. Some cells were more sensitive depending on what direction we aimed the magnetic field around the bird’s head.”

The location of these neurons leads the researchers to believe that the inner ear could be involved in detecting the magnetic field, but more research is needed.

This study, published in Science, might also give us clues about the way human brains work. “Birds give us a unique opportunity to study how the brain develops these spatial maps and the receptors that feed into it because they have such a great ability to navigate,” Dickman says. “Birds actually have more similarities to the human brain than not, so understanding these characteristics could eventually lend itself to understanding how we create spatial maps and those disorders that affect these areas of the brain.”

New Scientist makes an exciting, specific prediction:

The new findings may one day lead to technology to restore a sense of direction in people with dementia.

Image: Alan D. Wilson/www.naturespicsonline.com