Intrinsically, pigeon homing behavior is well suited to neuroethological study. Because the natural environment of the homer is a domestic one, it is easier to observe and manipulate pigeons in their normal surroundings without drastically affecting their natural behavior and setting. As a result, homing pigeons have become a favorite subject for the study of avian spatial navigation. Many ingenious methods for investigating the use and salience of various external cues in pigeon homing have been developed. From these experiments, we have gained no definitive explanation for the ability of homers to navigate back to their home loft, but rather we have learned that the mechanism underlying this behavior is extremely complex. Many experiments have yielded different and often contradictory results which make it difficult to develop a universal understanding of the homing mechanism. It is clear, however, that pigeons are able to utilize several sources of environmental information in order to home efficiently under variable conditions. Of these cues, the most well studied are the pigeon's use of the sun, environmental odors, the magnetic field and visual landmarks.

Gould (1982) describes the behavior of a homing pigeon as it is released from a site distant from its home loft. Initially, the pigeon circles around the release site several times. It then orients itself in a particular direction and flies in a relatively strait path away from the release site. The original bearing in which the pigeon flies is usually quite close to the actual direction of the home loft. Although the approximate direction is maintained during flight, the homeward journey of the pigeon varies. A pigeon, even when it is released from the same site on numerous occasions, does not fly an identical route each trip. The original flight direction is often referred to as the vanishing bearing or bearing at which the bird disappears from visual or radio contact. Because it is a reliable measure of the overall flight direction, the vanishing bearing is often used in the experimental literature to represent the homeward orientation. When many birds are released at a particular site, the mean vanishing bearing can be calculated by averaging the vanishing bearings of the individual pigeons. Interestingly, the mean vanishing bearing at a particular site often deviates slightly from the actual homeward direction. At each site, this deviation, known as release site bias, is reproducible and always occurs in the same direction (i.e. left or right) of the home loft bearing. In many instances, a mean vector length is calculated. The direction of the vector corresponds to the mean vanishing bearing and its length indicates the significance of the directional measure. If most pigeons are oriented in roughly the same direction, the mean vector length will be longer than when the vanishing bearings of the pigeons are highly variable. Most pigeon homing experiments manipulate the pigeons' ability to utilize one or more external cues and subsequently release the birds at a site distant from the home loft. Using the measures described above as well as others, experimenters can determine the effect, if any, of the experimental manipulations on homing behavior.

Gould (1982) describes three theories that have been proposed to explain the navigational abilities of homing pigeons. Firstly, it may be that pigeons, like ants and honey bees, are able to track their outward journey so that they can later retrace their path home. Secondly, pigeons could home based on a cue gradient system which is centered at the home loft. Such a system, however, would not require a compass mechanism. Lastly, the pigeon may home via a true map sense. This would enable the pigeon to determine its location directly through site specific cues which would describe its position in relation to the home loft. Because pigeons deprived of outward journey information are able to home well, the first hypothesis has been largely disproved (Wallraff, 1980). Furthermore, pigeons have been shown to possess an internal compass mechanism which they use for spatial navigation (Kramer, 1950). It appears, then, that the last theory is most likely the correct one in the context of pigeon homing. In fact, the theory of true navigation in pigeons has been almost unanimously supported by the experimental literature. This type of navigation is often described as using a map and compass. Kramer (1953) proposed this two-step mechanism based on the theory that in order to find its way home, the pigeon must first be able to extrapolate its map position. From this determined map position, the pigeon can then orient itself in the home direction via some sort of compass mechanism. Although there is much controversy surrounding the various environmental stimuli that may be used by pigeons, the evidence indicates that the map component used for site localization is based on olfactory, magnetic, and visual cues while the compass sense appears to be guided by the sun and magnetic information (Wallraff 1990).

The importance of visual cues in pigeon homing has received little attention compared to that given olfactory and magnetic cues. Perhaps this is a result of some early experiments in which pigeons equipped with frosted contact lenses were found to home well and were able to locate their home lofts with excellent accuracy when denied access to visual information (Keeton, 1974). Given these results and the fact that pigeons are able to home from unfamiliar areas where there are no apparent familiar landmarks, experiments naturally focused their attention elsewhere. Recent experiments, however, have shown that visual landmarks are an important aspect of the pigeons' spatial map of the familiar area (Chappell and Guilford, 1997; Burt et al., 1997). Using an indoor, food searching task Chappell and Guilford investigated the pigeon's ability to use visual landmarks to locate food. They found that pigeons were indeed able to use visual landmarks in such a task, but only if these landmarks were three and not two dimensional. Because use of two dimensional visual cues all together prevented the pigeons from acquiring the task, it seemed that the pigeons were not able to use magnetic field information in this experiment. These results suggested that pigeons can use visual cues in a spatial learning task without access to the sun. The authors further suggest that their findings lend support to Wallraff's conclusion that anosmic pigeons are able to home from familiar sites due to their ability to use visual landmarks to navigate home (Chappell and Guilford, 1997). Burt et al. (1997) also found that pigeons use visual information when released from familiar sites distant from the home loft. This experiment allowed the experimental pigeons visual access to the release site for five minutes prior to the actual release. The homing speeds of these pigeons were then compared to those of the control group which was not allowed visual access to the site before release. It should be noted that both groups of birds were allowed access to olfactory information during the previewing period. The experimental birds homed, on average, sixteen percent faster than the controls indicating that their previewing experience allowed them to home more efficiently. Visual landmarks, then, seem to be a component of the pigeon's ability to discern the homeward direction at a familiar release site. The types of landmarks and the specific features of those landmarks that are used to form this visual map, however, are not known with any degree of certainty.

William Keeton (1969) noted that pigeons must be using another system in addition to the sun compass to determine directional information. On overcast days, when the sun is not visible, pigeons are able to home quite well. In fact, he found that clock shifted pigeons deviate from normal pigeons only on sunny days. During overcast, however, the orientation of the clock shifted and control pigeons were both homeward bound and there was no significant difference between the groups. Thus, he concluded that the sun is used to obtain directional information when it is visible, but that pigeons must have a secondary compass sense. What directional information might they be obtaining on cloudy days? Keeton hypothesized that pigeons may have the ability to detect the earth's magnetic field. In 1971, he attempted to disrupt this detection by gluing a bar magnet onto the pigeons. He found that these pigeons were often unable to navigate home from unfamiliar release sites under overcast conditions but experienced little difficulty on sunny days when compared to control birds that were equipped with a brass bar.h magnet. In five out of the six releases, the mean vanishing bearing of birds with magnets was either random or not homeward oriented while control birds were oriented well in all but one release. Thus, the magnets often interfered with the normal navigation mechanism of the pigeons. Keeton concluded that there seem to be two compass systems utilized during navigation with the primary system relying on the position of the sun and the secondary one on the earth's magnetic field. From these magnet experiments came an interesting finding regarding the development of the compass sense in pigeons. Keeton found that young, inexperienced pigeons were often disoriented even in sunny conditions as a result of the magnets. He, and later others, attributed this result to the theory that the magnetic compass is innate and used to calibrate the sun compass through homing experience. These first flight birds were therefore unable to use the sun compass to obtain directional information because of their inexperience and could not navigate via magnetic field detection because of the attached magnet (Gould, 1982; Keeton, 1974).