for National Geographic News
To find north, humans look to a compass. But birds may just need to open their eyes, a new study says.
Scientists already suspected birds' eyes contain molecules that are thought to sense Earth's magnetic field. In a new study, German researchers found that these molecules are linked to an area of the brain known to process visual information.
In that sense, "birds may see the magnetic field," said study lead author Dominik Heyers, a biologist at the University of Oldenburg.
Human-made compasses work by using Earth as an enormous magnet and orienting a tiny magnet attached to a needle to the planet's north and south poles.
Scientists have thought for years that migratory birds may use an internal compass to navigate between their nesting areas and wintering grounds, which can be separated by thousands of miles. (Related news: "Migrating Birds Reset 'Compasses' at Sunset, Study Says" [April 15, 2004].)
The new research helps explain how this natural compass may work.
Heyers and his colleagues injected migratory garden warblers with a special dye that can be traced as it travels along nerve fibers.
The team put one type of tracer dye into the eyes and another in a region of the brain called Cluster N, which is most active when birds orient themselves.
When the birds got their bearings, both tracers traveled to and met in the thalamus, a region in the middle of the brain responsible for vision.
"That shows there is direct linkage between the eye and Cluster N," Heyers said.
The finding strongly supports the hypothesis that migratory birds use their visual system to navigate using the magnetic field.
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