Biological Bird Navigators

After about three decades of experiments on homing pigeons, scientists currently have two viable hypotheses concerning the "mapping" ability of birds. Although only homing pigeons have been studied, there is good reason to believe that migratory birds also rely on some sort of biological map to find their way back to traditional nesting or wintering sites.

The first hypothesis, conceived and tested primarily by a group of Italian scientists, involves an "odor map." The scientists propose that young pigeons learn this map by smelling different odors that reach their home loft on winds from varying directions. They would, for example, learn that a certain odor arrives on winds blowing from the east. If a pigeon is transported eastward from its loft it should smell that odor more strongly either on the way to or at the release site. This should tell the pigeon that it needs to fly westward to return home. Although it may sound preposterous to some, there is a large amount of evidence supporting this hypothesis. However, even its strongest proponents do not extend the idea to include long-distance migrants.

The second hypothesis proposes that birds may be able to extract latitude and longitude from the earth's magnetic field. Unlike the compasses that are thought to help birds determine direction, this map is believed to help birds determine location. The main support for this hypothesis comes from observations of pigeons released in areas of "magnetic anomalies" (places where the earth's magnetic field is distributed due to large iron deposits near the surface). When pigeons are first released in these areas, they depart in random directions, but after their initial confusion, most birds are able to correct their course and return home once they escape the influence of the anomaly. Because, in theory, magnetic anomalies are not a strong enough force to affect any of the birds' compasses --- magnetic, sun, or star -- proponents argue that the fact that the pigeons are initially affected indicates the existence of a different aspect of navigation that is being affected -- hence, the map.

Neither hypothesis has been proven to the satisfaction of all the experts, so new and different experiments continue to be performed. It may turn out that neither of these alternatives is correct, or a synthesis of the two may emerge.