More on Magnetic Navigation
Numerous experiments have shown that birds and bees use the geomagnetic field as a backup system when overcast skies prevent navigation by the sun or the stars. Also, pods of magnetic grains have been found inside the skulls of homing pigeons and in the abdomens of bees. Scientists suspect this is no coincidence. But even if these pods are the sensory organs that might detect both the strength and direction of the geomagnetic field, there remains the puzzle of how the information is transmitted to the animal's brain.
The answer may be in a suggestion by James L. Gould, writing in the May/June 1980 issue of American Scientist. In essence, his idea is that if the magnetic grains--each of which acts as a microscopic bar magnet--are properly arranged in linear chains, then the animal's ability to sense pressure changes could convey messages to the brain.
To see how this idea could work, suppose a pigeon lines up its head so that the magnetic chains are parallel to the magnetic field. Then in each chain the little bar magnets line up parallel to the chain so that the north pole of each is next to the south pole of the next magnet in the chain. Consequently, the magnets attract each other, causing the whole chain to contract. But when the pigeon turns at right angles to the magnetic field, all the little magnets try to turn also until they are crossways to the chain. The result is that all the north poles are adjacent to north poles and all the south poles are similarly adjacent to south poles, hence the little magnets repel each other. This causes the chain to expand.
Thus if a bee or a pigeon were to fly in a circle, the magnetic sensing pod would swell and shrink, pressing on pressure-sensitive sensory cells or not according to how the pod is oriented relative to the geomagnetic field.
So if you ever see a sad-looking pigeon holding its head in its wings, it's probably because it has a headache caused by the pressure of sitting in one position too long with its head crossways to the magnetic field.
