Birds Can See A World Invisible To Us

It's not a new theory by any stretch, but scientists now believe they have proof that birds can find their way across vast distances of our planet's skies because they can see Earth's magnetic field :

Klaus Schulten of the University of Illinois, proposed forty years ago that some animals – including migratory birds – must have molecules in their eyes or brains which respond to magnetism.

The problem has been that no one has been able to find a chemical sensitive enough to be influenced by Earth's weak geomagnetic field. Now Peter Hore and colleagues at the University of Oxford have found one.

Cryptochromes are a class of light-sensitive proteins found in plants and animals, and are thought to play a role in the circadian clock, in regulating plant growth, and timing coral sex.

A few years ago, Henrik Mouritsen of the University of Oldenburg in Germany showed that they were present in the retinal neurons of migratory garden warblers, and that these cells were active at dusk, when the warblers were performing magnetic orientation.

Birds appear to orientate at dusk, and cryptochromes form their pair of free radicals when "activated" by the blue light typical of dusk.

Hore suggests that dusk might activate the birds' magnetic sense, producing the radical pair. The concentrations of each free radical would be controlled by the Earth's magnetic field, which is known to vary with latitude. As a result, he speculates, the radicals would bind in varying degrees with other signalling molecules, depending on how far north or south the animal is.

How birds decode their "magnetic sense" is another topic of debate. Mouritsen believes they have an additional layer to their vision, which when switched on allows them to visually "see" the Earth's magnetic field. The situation would be similar to "head-up displays" in fighter jets and some cars, where transparent screens displaying information are built into windscreens.

"Having that on all the time would be distracting, so you can see why it would be desirable for the system to switch on and off," says Hore.