Magnetic Bird Navigators

A sun compass for migration during the day, a star compass for nocturnal migration -- life would be much simpler had this been the end of the story. For a long time there has been a popular theory that birds have a magnetic compass guiding their navigatory behavior. There is a good reason for that speculation -- it would be a convenient system to use during overcast days or nights. But many respected biologists assured ornithologists that for birds to sense such force was almost impossible because the earth's magnetic field is such a weak force. A group of German ornithologists conducted research that provided evidence to the contrary.

In the study, night migrants that were placed in orientation cages indoors in closed rooms showed weak but consistent and seasonally appropriate hopping directions, which suggested that they did not need the sun or stars to determine direction. By placing the cages within sets of wire coils through which a weak electric current was passed, it was possible to change the configuration of the magnetic field surrounding the birds to determine if they respond in a predicable way.

Although their responses continued to be weak, the birds did respond, and their orientation could be shifted by changing the magnetic directions. It therefore appears that is addition to sun and star compasses, at least several kinds of birds possess a magnetic compass, though as yet none of the field studies has found a clear indication of its influence on migratory birds in the wild.