Homing pigeons can be taken from their lofts and transported hundreds of kilometers in covered cages to unfamiliar sites and yet, when released, be able to choose fairly accurate homeward bearings within a minute and fly home. Aside from reading the minds of the experimenters (a possibility that has not escaped investigation), there are two basic explanations for the remarkable ability of pigeons to "home": the birds might keep track of their outward displacement (the system of many short-range species such as honeybees); or they might have some sense, known as a "map sense," that would permit them to construct an internal image of their environment and then "place" themselves with respect to home on some internalized coordinate system.

The first alternative seems unlikely. One possible model for such an inertial system might involve an internal magnetic compass to measure the directional leg of each journey. Birds transported to the release site wearing magnets or otherwise subjected to an artificial magnetic field, however, are only occasionally affected. Alternately, if pigeons measure their displacement by consciously keeping track of the direction and degree of acceleration and deceleration of the various turns, and timing the individual legs of the journey, simply transporting them in the dark, with constant rotations, or under complete anesthesia ought to impair or eliminate their ability to orient. These treatments, however, have no effect. Unfortunately, no one has yet performed the crucial experiment of transporting pigeons in total darkness, anesthetized, rotating, and with the magnetic field reversed all at the same time.

The other alternative, that pigeons have a "map sense," seems more promising, yet the nature of this sense remains mysterious. Papi has posited that the map sense is olfactory: that birds come to associate odors borne on the wind with the direction in which the wind is blowing, and so slowly build up an olfactory map of their surroundings. When transported to the release site, then, they only have to sniff the air en route and/or at the site to know the direction of home. Papi conducted a series of experiments showing that pigeons whose nostrils have been plugged are poorly oriented at release and home slowly.

One problem with the hypothesis is that Schmidt-Koenig and Phillips failed to detect any ability in pigeons to distinguish natural air (presumably laden with olfactory map information) from pure, filtered air. Papi's experimental results, moreover, admit of simpler, nonolfactory explanations. It seems likely that the behavior of nostril-plugged birds results from the distracting and traumatic nature of the experiment. When nasal tubes are used to bypass the olfactory chamber but allow for comfortable breathing, no disorientation is evident. Likewise, when the olfactory epithelium is sprayed with anesthetic to block smell-detection but not breathing, orientation is normal.