New study overturns prevailing theory of how birds navigate

Article Shared From: Bird Watching Daily

Posted Wed, Apr 11 2012 11:47 AM by Matt Mendenhall

Scientists have thrown cold water on the theory that iron-rich nerve cells in birds’ bills help them navigate using Earth’s magnetic field.

Researchers from Austria, France, Australia, and England, writing in a new study published today in Nature, report that iron-rich cells in the bills of pigeons are in fact specialized white blood cells called macrophages. Macrophages play a vital role in defending against infection and recycling iron from red blood cells, but they’re unlikely to be involved in magnetic sensing, the scientists say. That’s because they are not excitable cells and cannot produce electrical signals that could be registered by neurons and therefore influence a bird’s behavior.

The finding overturns a theory that has stood for more than a decade. Past studies, including a 2000 paper from the journal Biometals and this 2007 report from PLoS One, claimed that magnetite in beak tissue helps birds navigate.

We described birds’ reliance on the magnetic field in past articles in BirdWatching/Birder’s World. In “Amazing Birds” in our April issue, for example, Founding Editor Eldon Greij wrote that magnetite in birds’ bills helps them process information from the magnetic field. And Paul Kerlinger wrote in “On the Move” about birds’ abilities to sense the magnetic field and magnetite’s role in Bobolink migration.

“The mystery of how animals detect magnetic fields has just got more mysterious,” said study leader David Keays of the Institute of Molecular Pathology in Vienna. “We had hoped to find magnetic nerve cells, but unexpectedly we found thousands of macrophages, each filled with tiny balls of iron.”

High-resolution magnetic resonance imaging (MRI) of pigeons’ beaks allowed the researchers to find the balls of iron instead of magnetic neurons. (Researchers who investigate birds’ navigational abilities often study pigeons because their magnetic sensing systems are common among other species.)

Right: An MRI of a pigeon’s head and bill. Courtesy Johannes Riegler

“Our work necessitates a renewed search for the true magnetite-dependent magnetoreceptor in birds,” the scientists write.

Perhaps the answer will come from fish. The researchers conclude their paper by saying the undiscovered cells that govern magnetic sensation “may reside in the olfactory epithelium, a sensory structure that has been implicated in magnetoreception in the rainbow trout.” —Matt Mendenhall, Associate Editor

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