Pigeon homing: observations, experiments and confusions
Homing pigeons can return from distant, unfamiliar release points. Experienced pigeons can do so even if they are transported anesthetized and deprived of outward journey information. Airplane tracking has shown that they make relatively straight tracks on their homeward journey; therefore, pigeons must have some way of determining the home direction at the release site. Manipulating the pigeon's internal clock causes predictable deviations in their flight direction relative to home. When the sun is not visible, such clock shifts have no effect. This result implies a two-step system: the determination of the home direction and the use of a sun compass to fly in that direction. When pigeons cannot see the sun they use a magnetic compass. The use of compass cues to select and maintain a direction of flight is well understood compared with the uncertainty surrounding the nature of the cues used to determine the home direction when pigeons are released at an unfamiliar site. Because they generally home successfully from any direction and distance from the loft, without requiring information gathered on the outward journey, it seems likely that they use some form of coordinate system. Presumably, a displaced pigeon compares the values of some factor at the release site with its remembered value at the home loft. This factor might be olfactory, it might be some feature of the earth's magnetic field or it might be something else. There is some evidence that pigeons may use several cues and that pigeons raised in different lofts under different environmental conditions may prefer to use one cue over another. I believe that it is this flexible use of multiple cues that has led to so much confusion in experiments on pigeon homing.