It has been debated for nearly four decades but no one has yet been able to prove it is chemically possible. Now good evidence suggests that birds can actually "see" the lines of the 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

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.