Bird migration: Key explanation skewered!

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Beautiful bird-navigation theory skewered by ugly fact!

Scientists have thought for a decade that iron-bearing structures in the homing pigeon’s beak help the bird find its location by “reading” Earth’s magnetic field. Now, it turns out that this iron occupies cells that battle infection, rather than nerve cells.

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Two white birds stand on wood planks

Courtesy David Keays
Homing pigeons “read” Earth’s magnetic field to fly 1,000 kilometers back home — from an unknown location. How?

Oops!

The new results leave a chasm in our understanding of bird navigation, says Charles Walcott, an expert on the subject at Cornell University, who was not involved in the study. “It’s astonishing that we have what seems like a terribly simple-minded problem. Take a homing pigeon any direction, and after circling a couple of times, it heads for home … and we don’t understand how these animals do this?”

Study leader David Keays, of the Institute for Molecular Pathology in Vienna, did not set out to debunk a beautiful theory, but rather to explore the nerve cells in the beak that supposedly register magnetism. “My background is in molecular biology and genetics, and I thought there must be some incredible biology involved. I wanted to get a handle on the molecules and create an artificial receptor.”

Because the “magnetic neurons” in the beak contained iron, Keays applied a blue stain that gloms onto iron. Christoph Treiber and Marion Salzer generated one-quarter million slices for microscope slides, each one-hundredth of a millimeter thick.

(Makes us dizzy … Didn’t they outlaw slavery?)

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Cross section of a cell: blue round blobs surround oval pink, all within a translucent tube

Courtesy David Keays
Iron in cells in the pigeon’s beak are stained blue; cell nuclei are pink. These cells, previously thought to be nerve cells, are actually macrophages, a type of immune cell.

A fly in the ointment!

Although the magnetic neurons were said to number just six, iron-rich cells showed up all over the beak. One beak had about 108,000 blue-stained cells while another had just 200, Keays says. “This did not make sense. If these were magnetoreceptors, we would expect a similar number in birds of the same age and sex.”

When the scientists treated the samples with stains that attach to neurons, there was almost no overlap with the iron-bearing areas.

As questions accumulated, the researchers got a lucky break. One bird’s infected beak attracted blue cells that resembled macrophages, immune cells that fight infection (and also process iron). “You could see the cells’ tentacles engulfing other cells,” Keays says.

Stains that attach to immune cells overlapped heavily with the iron stain, Keays says; further evidence that the iron was inside macrophages, not neurons.

The study is “quite interesting and convincing,” says Walcott, and it explains why scientists have found no connection between the iron crystals and the nervous system. “If this is going to be seen as a sense organ, I think the two ought to be connected.”

Arctic tern: Lindsay Robinson, Map: L. Shyamal
An Arctic tern flies the equivalent of three round-trips to the moon in its lifetime 1. Roll over to see several avian mega-migrations.

Paradigm paranoia

Although the new study overthrows the accepted explanation for the pigeon’s magnetic mastery, Walcott says magnetism isn’t the whole story in navigation; birds also use vision, memory and smell.

Looking at the sun can help the bird figure out direction, but magnetic methods are needed to find a location on the globe.

ENLARGE

Black and white photo of men in uniform standing around a bird-carrying bus.

Photo: Unknown
The amazing homing ability of the homing pigeon found use in World War I, when the British Army drafted a London bus as a pigeon loft. Pigeons carried messages from the front to the loft in the rear.

Confusingly, birds seem to have a mechanism in the eye that detects Earth’s magnetic field. But because this works only when the sun is shining, it’s unlikely to explain nighttime navigation.

Keays says attitudes have changed since he “released a cat among the pigeons” at a conference a year ago. “Half of the audience wanted to hug me, they had been very skeptical, but the other half wanted to kill me.”

Since then, however, “We were able to persuade some big players in the field that the original reports were wrong. I think the great thing about science is that it is a self-correcting enterprise. If we get it wrong, somebody is going to come along and work out what the truth is.”

At this point, though, mystery rules. “It’s absolutely clear that birds, pigeons, can detect magnetic fields,” Keays says, “but the way they do that is the mystery.”

– David J. Tenenbaum

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Terry Devitt, editor; S.V. Medaris, designer/illustrator; Molly Simis, project assistant; David J. Tenenbaum, feature writer; Amy Toburen, content development executive

Bibliography

  1. BBC Nature Watch: The Arctic Tern
  2. Clusters of iron-rich cells in the upper beak of pigeons are macrophages not magnetosensitive neurons, Christoph Daniel Treiber et al, Nature, published online, ahead of print, 11 Apr. 2012.
  3. Migration of Birds: A USGS Overview
  4. Neurobiology of Magnetoreception (ignore the part of birds…)
  5. More about macrophages
  6. Much more about macrophages
  7. Pigeon Messengers: “More reliable than radios on the battlefield.”
  8. About electron microscopes
  9. Homing pigeons following the roads
  10. Racing pigeons: A popular hobby
  11. Pigeons ‘intelligence’: Comparable to that a three-year-old child