Mystery of bird navigation system still unsolved

When it comes to following routes without getting lost, birds come out on top. Some birds perform the most jaw-dropping migrations ? the Arctic tern, for instance, makes an annual roundtrip of 81,600 kilometres between the poles. Exactly how it does this remains a mystery, especially as new evidence shows that we have misunderstood the way birds sense the Earth's magnetic field.

There are two theories as to how birds can sense the Earth's magnetic field. One theory involves the spin of electrons, which is known to be affected by magnetic fields. Because this process requires light, molecules in birds' eyes may be receptive to magnetism.

A second theory claims that iron-rich cells are responsible. Some marine bacteria contain an iron mineral called magnetite, which appears to influence the direction in which the bugs swim. Therefore iron-containing cells in the birds' beaks may affect their flight path in a similar way. Studies have found that severing the nerve that supplies the birds' beak appears to disrupt their magnetic-sensing abilities, which suggests that the cells are neurons. In 2007, a research group claimed that a pigeon's beak contains six patches of iron-rich cells for navigating (Naturwissenschaften, DOI: 10.1007/s00114-007-0236-0).

To further investigate this second theory, David Keays, Christoph Treiber, Marion Salzer and their colleagues at the Institute of Molecular Pathology in Vienna, Austria, looked at pigeons' beaks in closer detail. The team started by collecting more than 200 pigeons from several European countries. They then removed the creatures' beaks and sliced each one into as many as 2,500 sections.

Keays's team then searched for magnetite using a stain that turns blue in the presence of iron.

"Much to our disappointment, we didn't find iron-rich cells in the six regions previously reported," says Keays. In fact, the cells appeared to be cropping up all over the place. There was also a huge variation in the number of iron-rich cells between birds. For example, one bird contained about 200 of the cells, while another's beak had almost 109,000.

The team are not even convinced that the cells are neurons. In one bird's infected beak, there appeared to be a cluster of iron-rich cells around the injury site. When the group examined the cells under an electron microscope, they didn't look like neurons at all. "Some had big tentacles and were engulfing other cells," says Keays, who reckons the cells are actually white blood cells known as macrophages.

It is not surprising that macrophages contain iron as they are known to collect the substance when recycling it from red blood cells. "Our argument is that these cells have nothing to do with magnetoreception," says Keays.

The findings suggest that any previous work based on identifying magnetic-sensing cells using the same staining technique may have to be reassessed.

"It's a pretty provocative paper," says Steven Reppert at the University of Massachusetts in Worcester. While it is still possible that magnetic-sensing cells exist in the birds (? and possiblyerhaps located in their brains) they will differ from the ones the research community has been focusing on, Reppert says.

"This is a very important study because it suggests that the iron-oxide structures that were discovered in the beak of birds a few years ago might be a red herring and have nothing to do with magnetoreception," agrees Thorsten Ritz, a biophysicist at the University of California, Irvine. "It looks like we need to hunt anew."

"We've certainly put the cat amongst the pigeons," says Keays.

Journal reference: Nature, DOI: 10.1038/nature11046

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