INTRODUCTION

I'm sure you have experienced it too; as soon as you mention to a non-fancier that you race pigeons, invariable you are asked: "how do they find their way home? It is a topic that has fascinated people down the years. Pigeons are brought to places they have never been before, like Talbenny in South Wales, Penzance in the south-west corner of England, Thurso right on the northern tip of Scotland and even the north-west of France, and find their way home. Not just find their way back to Ireland, but to a particular place and their own loft. When reared in a particular loft, a young pigeon can be transported hundred of miles away and successfully find its way home. Because it requires the pigeon to pinpoint a specific location, this behaviour necessitates more than the compas orientation system of migratory birds. The pigeon must be able to determine its position relative to the location of the home loft in order to orient itself in the proper direction. In doing so, pigeons use a variety of external cues such as the sun, visual landmarks, olfactory cues, and the earth's magnetic field. Examining when and how different cues are utilized by  racing (homing) pigeons has provided neuroethologists with a better understanding of avian spatial navigation. 

HOMING  BEHAVIOUR

Because the natural environment of the racing pigeon is a domestic one, it is easier to observe and manipulate them in their normal surroundings without drastically affecting their natural behaviour and setting. As a result, pigeons have become a favourite subject for the study of avian spatial navigation.

Many ingenious methods for investigating the use and importance of various external cues have been developed. From these experiments, we really have gained no definitive explanation for the ability of racing pigeons to navigate back to their home loft, but rather we have learned that the mechanism underlying this ability 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 odours, the magnetic field and visual landmarks.

Gould (1982) described the behaviour of a homing pigeon as it is released from a site distant from its home loft. Initially, the pigeon circles around the release point several times. It then orients itself in a particular direction and flies in a relatively straight path away from the releasr 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 maintainedduring 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. This is the 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.

Gould describes three theories that have been proposed to explain the navigational abilities of the racing pigeon.

Firstly, it may be thay 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 centred on the home loft. Such a system, however, woyld not require a compas mechanism.

Thirdly, 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.

Becaused pigeons deprived of outward journey information are able to home well, the first hypothesis has been largely disproved.  Furthermore, pigeons have been shown to possess in internal compas mechanism which they use for spatial navigation. It appears, then, that the third theory is most likely the correct one. In fact, the theory of true navigation in pigeons has been almost unanimously supported by experiments. 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 figure out 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 contraversy surrounding the various environmental stimuli that may be used by pigeons, the evidence indicates that the map component used for site localisation is based on olfactory, magnetic and visual cues, while the compass sense appears to be guided by the sun and magnetic information (Wallraff 1990).

VISUAL  CUES

The importance of visual cues in pigeon navigation has received little attention compared to that given to olfactory and magnetic cues. This is probably because 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 ecxellent accuracy (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 pigeon's spatial map of the familiar area (CHappel and Guilford, 1997; Burt, 1997). Using an indoor, food searching task, Chappel 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 dimentional. Because use of two-dimentional visual cues altogether prevented the pigeons from success in the task, it seemed that they were not able to use magnetic field information. These results further suggest that pigeons can use visual cues without access to the sun.

Experiments showed 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 actual release. The homing speed 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 per cent 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 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,

MAGNETIC  CUES

William Keeton (1969) noted that pigeons must be using another system in addition to the sun compas to determine directional information.

On overcast days, when the sun is not visible, pigeons are able to home quite well. What directional information might they be obtaining on cloudy days?

Keeton hypothesised 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 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 six releases, the vanishing bearings of the birds with magnets was either random or not homeward orientated, while the control birds were orientated well in all but one release. Thus, the magnets often interfered with the normal navigation mechanism of the pigeons.

Keaton concluded that there seems to be  twp 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.Keaton found that young, inexperienced pigeons were often disoriented even in sunny conditions as a result of the magnets. They were 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.

Another famous set of experiments was conducted in which an artificial magnetic field was created around the pigeon's head using a Helmholtz coil (Walcott and Green, 1974). When this artificial field was oriented with the north pole pointing upwards, pigeons often flew 180 degrees away from home under overcast conditions, while their homing ability was unaffected under clear skies. The pigeons equipped with coils which had the magnetic south pole pointing upwards were able to home accurately on both overcast and sunny days.

Not only did these results further implicate the use of the geomagnetic field under overcast, but they also provided some insight as to what components of the field are used to infer direction. In the light of Wiltschko's finding that the European robins use the direction at which the magnetic field and gravity vectors make the smallest angle to indicate north, Walcott determined that pigeons may also define north by the steepest inclination of the magnetic field.

Gould (1982), Wiltschko (1991), and Walcott (1996) discuss the possibility that magnetic cues could be used by pigeons in the context of map sense. Since the earth's magnetic field varies in strength and dip angle over its surface, a pigeon able to detect small variation in either of these components could create a spatial map using this information.

It was discovered that pigeons released from the same site on different days vanished at slifgtly different bearings on each day. This effect was termed the "K-effect". The degree to which the average vanishing bearing changed from one day to another was correlated with the variation in the magnetic field before release.  The exaggeration of release site bias during magnetic storms (natural magnetic disturbances that result from solar activity) shows that pigeons are indeed sensitive to tiny changes in magnetic field properties.

Interestingly, groups of pigeons raised in separate loft locations experience magnetic anomalies differently. Walcott(1996) discusses his findings that pigeons raised at one loft were entirely disoriented when released site, while pigeons raised at a different loft nearby oriented well when released at the same site. He goes on to suggest that this difference may result from differences between lofts, rather than pigeons. It seems that pigeons reared in different lofts learn to use different external cues during navigation.

Much of the behavioral evidence implicates the use of the geomagnetic field in pigeon homing behaviour. The search for the mechanism of detection of the magnetic field in birds has therefore been under investigation for some time.

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