• Kyodo

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Scientists have long been puzzled over the mechanism that enables birds and fish to return to their nests and home waters without fail.

But according to Hiroshima University President Yasuo Harada, an experiment conducted earlier this week proves his theory that traces of iron found in the otolith organs located in the inner ear of birds enables them to find their way back to their nests.

Otolith organs give animals the sense of horizontal and vertical. While animals all have two types of otolith organs, fish, birds and amphibians have a third version that helps them sense their position in the air or in water.

In March, Harada, who specializes in otolaryngology, detailed his theory that birds and fish use these iron traces in this third organ as a compass to sense terrestrial magnetism and thus get their bearings.

He said he discovered that the inner ear organ of birds had iron content ranging from 0.7 percent to 4.3 percent, while the corresponding figures for fish were 0.05 percent to 0.1 percent.

Researchers released 31 pigeons Thursday from a field in the city of Higashi Hiroshima, Hiroshima Prefecture, to see whether they could return to their nests in Hiroshima, about 17 km away.

Of these birds, 21 were normal, while nine had undergone surgery so that any activity in the inner ear organ would not reach their brain, such as by cutting the nerve linking the two organs or by implanting metallic fragments into their inner ears. The remaining pigeon was operated on but did not have this link severed.

According to Harada, all the normal birds, as well as the one with the inner organ still connected with its brain, returned to their nests in about 30 minutes.

Of the remaining nine, one returned to its nest about three hours later but the rest did not, he said.

“This experiment has proven that the otolith organ (particular to birds and fish) is a sensor that picks up terrestrial magnetism, and it may even be that catfish tend to thrash about before an earthquake because they sense shifts in the earth’s magnetism, too,” Harada said.

It is widely believed that the homing instinct of birds is the result of their gaining a sense of direction using the sun, stars and magnetic fields, but it has been unclear how they pick up the magnetic signals.

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