Clear thinking is the hallmark of good research. I think we’re all in agreement here.

Researchers have known, in the special way that researchers do,that built-in biological compasses tell migrating birds which way to fly, but the details of how birds detect magnetic fields has been unclear.

Some argued that it was a magnetic sensor in the beak, others preferred the hypothesis that light-sensing cells in birds’ eyes sense the magnetic field.

There’s only one way to sort this out, as Harry Hill might say; Surgery!

A study published in the Oct 29 edition of Nature by Henrik Mouritsen of the University of Oldenburg in Germany has clarified the position.

Up to a point.

To find the location that houses the magnetic compass, Mouritsen and his colleagues caught 36 migratory European robins and made sure that the birds could all orient correctly under both natural and induced magnetic fields.

They then performed surgeries on the birds either to sever the nerve in the beak or to damage the brain cells that  decode the signals from the light-sensing cells.

Result? Birds with the severed beak-to-brain nerve — called the trigeminal nerve — still oriented perfectly, but those with the damaged light-sensing brain cells could not.

So, according to John Phillips, a neuro-ecologist at Virginia Tech in Blacksburg, the study “confirms that the trigeminal nerve is not involved in this direction sensing”.

Fine. Except that Mouritsen thinks the cells in the beak may play a different role in magnetic sensing, such as picking up changes in magnetic strength.

Didn’t do the surgery for that, though, did he?

However, now that we almost know how birds migrate, Mouritsen and his colleagues are confident that conservationists can trick birds into staying where it is safe, instead of flying back to their original migratory grounds, which may not be good for them.

After all, if we, as custodians of the planet, can’t tell the birds where and when they should fly, who can?

Original article posted  by Laura Sanders :http://www.wired.com/wiredscience/2009/10/bird-migration-light/