Warblers' eyes connect up to the magnetic navigation system in the brain.

How do migrating birds perceive which way is north? Research now points to the idea that they actually 'see' the Earth's magnetic fields, rather than feeling or sensing them in some other way.

Previous work has suggested that the Earth's magnetic field might act on the sensitivity of a migratory bird's eye, so that sight might be involved in finding magnetic north. Now researchers have firmed that up with evidence that molecules in the eyes of migratory birds are connected to the part of the brain that guides their direction of flight.

Dominik Heyers, at the University of Oldenburg, Germany, and colleagues injected migratory garden warblers (Sylvia borin) with a tracer capable of travelling along neuronal fibres along with nerve signals. They injected one tracer into the part of the forebrain known to be the only active area when birds orient themselves (known as Cluster N), and a different tracer into the retina.

After a bird experienced a desire to migrate, both tracers ended up in the same place, the researchers report in the Public Library of Science One¹ — a part of the thalamus responsible for vision.

This anatomical link strongly supports the notion that the birds probably experience magnetic fields as a visual sensation, say the researchers.

Northern black spot

It has previously been suggested that proteins called cryptochromes in the eyes of migratory birds might play a role in their compass-like ability.

The idea is that these cryptochromes might be sensitive to the electronic state of radical pairs. These pairs can exist as singlet or triplet states, and the relative proportions of these states is in turn influenced by the orientation of the molecules in the eye relative to the Earth's magnetic field (or any other magnetic field that the birds are exposed to).

"This means that if a bird looks in a certain direction, the magnetic north might be seen as a dark spot," says Heyers, although he adds that the precise way the birds see that magnetic field is subject to a bit of guess work: "we cannot ask [the birds] how they see it."

Beaks and eyes

Heyers's work showing a connection between the retina and Cluster N is a "great result", says Miriam Liedvogel, who studies migration at the University of Oxford, UK. But in her opinion it isn't enough to prove the hypothesis that birds can 'see' magnetic fields, she adds. She'd like to see experiments where changing the magnetic field is conclusively shown to change neuronal activity in the thalamus, she says.

And this will not to be the end of the story of how birds find their way. Other work has shown that migratory birds also have magnetic crystals in their beaks that are involved in navigation. Heyers thinks that the two systems probably exist to complement each other, with the beak being used to measure the strength of magnetic field as a kind of map, and the cryptochromes in the eyes acting as a compass.

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• References

  1. Heyers, D., Manns, M., Luksch, H., Güntürkün, O. & Mouritsen, H. PLoS One (2007).