How Do Birds Navigate?
Birds use the Earth's magnetic field to navigate but scientists are still unsure of how it works.
Credit: BenTheWikiMan, Public Domain

About 50 animal species, ranging from birds and mammals to reptiles and insects, use Earth's magnetic field for navigation.

Yet Earth's magnetic field is very weak. It ranges from approximately 30 to 60 millionths of one tesla. By comparison, magnetic resonance imaging, or MRI, uses magnetic fields from 1.5 to 3.0 tesla.

So scientists unsure exactly how birds do it.

New research finds that a photochemical compass may simulate how migrating birds use the magnetic field, along with light, to navigate.

One theory for how it all works has been that photoreceptors in a bird's retina absorb light, which causes a chemical reaction that, in turn, produces a short-lived photochemical species whose lifetime is sensitive to the magnitude and direction of a weak magnetic field.

The idea is supported by the fact that blue light photoreceptors have been detected in retinas of migratory birds when they perform magnetic orientation. However, it has not been confirmed that a magnetic field as weak as Earth's can produce detectable changes within a photochemical molecule; nor, has a photochemical molecule been shown to respond to the direction of such a magnetic field.

Until now.

A new study, funded by the National Science Foundation and detailed online in the April 30, 2008 issue of the journal Nature, shows that the photochemical model becomes sensitive to the magnitude and direction of weak magnetic fields similar to Earth's when exposed to light.

A synthesized photochemical molecule composed of linked carotenoid (C), porphyrin (P) and fullerene (F) units can act as a magnetic compass, the researchers found. When excited with light, CPF forms a short-lived charge-separated state with a negative charge on the ball-like fullerene unit and a positive charge on the rod-like carotenoid unit. The charge-separated state lasts only as long as the magnitude and direction the field stays constant.

Why do scientists care about all this complex stuff?

Power lines and communications equipment also generate weak magnetic fields that can disrupt animal navigation, so "it is essential for humans to understand how animals navigate using Earth's weak magnetic field and the effects of human activity on animal navigation," said Devens Gust, professor of chemistry and biochemistry at Arizona State University.

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