Science: The Physics of Pigeons

How does a homing pigeon navigate, often over hundreds of miles of territory it has never seen before? Professor (of physics) Henry L. Yeagley of Pennsylvania State College thinks he has found the pigeony secret. Last week, on General Electric’s Science Forum broadcast, he described the pigeon experiments he has been doing for the Army Signal Corps.

Yeagley began with the idea, not original with him, that a homing pigeon is equipped with some sort of “magnetic compasses,” i.e , some sensitivity to the earth’s magnetism. Yeagley tested this notion by fastening small magnets to the wings of well-trained pigeons. Confused by their own magnetism, most of the birds never got home. Others, carrying equal wing weights of nonmagnetic copper, made the home roost without trouble. The experiment indicated that the earth’s magnetism is a factor in pigeon navigation.

But a pigeon’s own magnetic compass could not, by itself, bring him back to his roost. Like a ship, a pigeon needs some other instrument too. Many places on the earth’s surface have identical magnetic conditions. A pigeon guided by his compass alone might end up almost anywhere.

Magnetism Plus. What was the other guiding factor? It could not be the sun and stars, for pigeons can navigate under clouds. Professor Yeagley considered all the possibilities. While looking at a map which had lines representing the intensity of the earth’s magnetism, he noted that the lines were crossed at varying angles by the parallels of latitude. The two sets of lines formed an irregular grid, something like the crossing lines on a sheet of graph paper. Used together, the lines served as a “frame of reference.” If pigeons, he reasoned, are sensitive to some factor connected with the lines of latitude, they have all they need to find their way home. Steering by a latitude-magnetism frame of reference, they would navigate almost as surely as ships guided by latitude and longitude.

Yeagley’s next problem was to find some physical force—something that pigeons might be able to detect—related to the lines of latitude. This was easy. The effect of the earth’s turning varies directly with latitude; objects near the equator are carried daily around the earth’s whole circumference, moving at over 1,000 m.p.h. Objects near the poles are carried around more slowly in smaller, tighter circles. The direction and variation of this circling can be felt by various man-made instruments, such as the gyrocompass. Why shouldn’t pigeons feel it, too? If they could, they would have, along with their “magnetic compass,” a satisfactory navigating instrument.

Away from Home. To test this theory, Yeagley trained hundreds of pigeons to return to their home roosts at State College, Pennsylvania. When they were thoroughly indoctrinated, he took them to a part of Nebraska where the lines representing the earth’s magnetism happen to cross the parallels of latitude at the same angle as at State College. When he released the pigeons to the east of this “false home,” they headed away from their Pennsylvania home. Thrown off their course by their navigating instruments, the pigeons all flew west. Professor Yeagley believes his experiment proves that pigeons are guided by both the earth’s magnetism and by its turning.

Migrating birds, especially those that fly unerringly to small islands in the Pacific, may use the same navigating trick. Just where they keep their instruments is still unknown, but Professor Yeagley says that birds have a mysterious organ in their eyes, at the end of the optic nerves. This organ may contain the delicate electrical nerve fibers that pick up variations of magnetism, and the even more delicate sense that measures the earth’s slow turning.