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The world record pigeon flight home was
set on 15th June 1932. The bird disappeared after being released
in Arras, France, and it was not for 24 days afterwards before
it arrived back at its original loft in Saigon, Vietnam, after
flying 10,640 kilometres (in a straight line). This is the
longest known distance and the actual world's record and was
reported in the paper "La Prensa" of Barcelona.
How do pigeons find their way? How can they
orient themselves for such long distances? What is the secret
of the pigeon's navigation? Most probably there is not "just
an answer" but a combination of several factors. Scientists
and pigeon' fanciers of the entire world are still working
on this amazing fact that makes pigeons so fascinating and
such useful birds.
There is a very interesting article about
pigeon navigation written by Verner P. Bingman (Racing Pigeon
Pictorial, September 1993) that explains the actual knowledge
on this matter. I hope you find it as interesting as I did.
Geomagnetism
Given the well-documented finding that birds can rely on the
earth's magnetic field for compass orientation (Wiltschko,
1983), it is not surprising that hypotheses citing a role
for geomagnetism in the functioning of the navigational map
of pigeons have also appeared. Interestingly, the possible
importance of geomagnetism in permitting a bird to determine
its location in space was already suggested long before it
was established as compass cue (Yeagley, 1947). The supporting
evidence indicating a role for geomagnetism for the navigational
map includes:
1 - Correlation between variations in the
initial orientation of pigeons following release with naturally
occurring fluctuations in geomagnetic field.
2 - Impaired initial orientation of racing
pigeons when released within naturally occurring magnetic
anomalies.
3 - Disrupted initial orientation following
experimentally induced magnetic disturbances.
The results of these studies link magnetic
stimuli with pigeon homing performance. The effects are plausibly
attributed to affect a map mechanism rather than compass orientation,
as the experiments are performed under sunny conditions when
the birds are known to rely on the sun for compass orientation.
However, the possibility of an interaction between magnetic
stimuli and sun compass orientation, which manifests itself
under conditions of magnetic disturbance, cannot be entirely
excluded as an explanation for the reported results. Indeed
until an experiment is performed whereby manipulating the
ambient magnetic field exposed to a bird results in predictable
shifts in orientation that are interpretable in the context
of a map, uncertainties will remain regarding its importance,
and the question of a geomagnetic role in the pigeon navigation
map will remain open.
We have discussed olfactory and geomagnetic
cues as independent sources of potential map information.
It remains conceivable, however, that they may work in some
collective fashion to permit a pigeon to locate its position
in space. For example, if the map does take the form of a
bico-ordinate gradient system, olfactory stimuli may characterise
one axis, and magnetic cues could characterise the other.
Although this is as yet a highly speculative notion, Wallraff,
Papi, Ioalé & Benvenuti, (1986) have reported that
olfactory and magnetic stimuli may interact at some level
of the homing process. Racing pigeons exposed to oscillating
magnetic fields coupled with deprivation of their sense of
smell prior to release fail to show the usual deviation away
from the homeward direction characteristic of the magnetic
field treatment alone (Benvenuti et al, 1982).
Regional and Individual Differences
Given the possibility of multiple sensory mechanisms associated
with the pigeon navigational map, it seems important to consider
to what extent regional differences may account for these
findings. The currently available evidence indicates that
wherever it has been extensively studied, olfaction plays
a critical role in the proper functioning of the pigeon navigational
map. Nonetheless olfactory treatments have been reported to
be less affective in altering navigational performance in
other locations in Germany and the Unites States.
At this time it is important to consider
how and why the importance of smells for navigation may vary
regionally. Indeed, potential regionally differences could
be based on differences in atmospheric dynamics that may render
curried odours more or less reliable as sources of navigational
information.
The situation with geomagnetism is somewhat
more difficult, insofar as it remains unclear to what extent
it may be involved in the navigational map. Nevertheless findings
indicate that pigeons from different lofts are differentially
affected at the same magnetic anomaly.
In looking at baseline individual behaviour
performance, pigeons in different regions are not equally
good at returning home following release from unfamiliar locations.
For example, birds generally perform better in Italy than
in Germany. Raising Italian birds in Germany and German birds
in Italy has indicated that baseline differences are, at least
in part, environmentally based, possibly reflecting differences
in the way environmental cues are employed in the formation
of a navigational map. The available evidence indicates that
such may be the case, supporting Keeton's (1974) belief that
environmental factors may determine the extent to close this
section by emphasising that current available evidence implicates
smell as the primary source of navigational map information,
with the demonstration of alternative sources awaiting further
empirical support.
Landmark Navigation
So far we have focused on true navigation, or that mechanism
that permits a pigeon to approximate its position in space
relative to home from places it has never been before. However,
when a racing pigeon is in an area where it has been before,
either when returning to the loft from an unknown site or
when released in a familiar area, it then has available an
additional source of potential navigational information in
the form of familiar landmarks, which may or may not be visual.
Despite its intuitive appeal, the extent
to which landmarks are used by racing pigeons in generating
goal-orientated responses has been one of the most empirically
elusive issues in avian navigational research. Nonetheless
there are strong indications that racing pigeons do use landmarks
for locating their position with respect to home when they
are in a familiar area but outside the range of direct sensory
contact with their loft. Racing pigeons approaching the familiar
area around the loft have been recorded to make appropriate
course corrections. Pigeons wearing frosted lenses that affect
their vision are successful in orienting homeward from a distant,
unfamiliar release site, but appear impaired in returning
to their loft once they get to within three to six miles.
Finally, racing pigeons which are not permitted free-flight
experience around their loft prior to their first experimental
release, orient homeward from a distant location, indicating
an ability to perform true navigation, but are impaired in
locating their loft once within its vicinity. This suggests
that flight experience is important for adequate local navigation
near the loft, presumably by permitting familiarisation with
local landmarks.
Another source of evidence indicating that
racing pigeons attend to landmarks for navigation comes from
experiments where racing birds are initially impaired at orienting
homeward from a distant, unfamiliar release site. However,
upon repeated exposure to the, now familiar, release site,
the birds become better at orienting homeward following the
same experimental procedure that originally rendered them
impaired. Familiarity with an area enables pigeons to compensate
for disruptions in their navigational map, and it seems reasonable
to suppose that this familiarity effect is based on the use
of landmarks that they had been exposed to. This familiarity
effect has been observed under diverse conditions:
1 - Olfactory-deprived pigeons which generally
fail to orient homeward from unfamiliar locations, do orient
homeward when released from locations they had been previously.
2 - When released within a magnetic anomaly,
racing pigeons are often disoriented. However, there is considerable
improvement in homeward orientation on subsequent releases
from the same anomaly.
3 - Racing pigeons that are clock-shifted,
a procedure that result in predictable change in an animal's
monitoring of the sun for sun compass orientation, typically
display a deviation in orientation with respect to home when
released from either familiar or unfamiliar locations. However,
they compensate at least partially for that deviation when
released under clock-shift conditions a second time from the
same location.
These results suggest that racing pigeons
do indeed use landmarks for navigation. However, it should
be emphasised that they are likely used under normal conditions
primarily to locate the loft once in its vicinity, that vicinity
depending on the previous experience of a pigeon (perhaps
up to fifteen miles to the loft). The possible use of landmarks
from distant, unfamiliar locations should be viewed as an
auxiliary mechanism to be employed when use of their navigational
map is disrupted or has previously been associated with incorrect
orientation.
Landmark Navigation - Theoretical Considerations
Given that racing pigeons use landmarks for navigation, it
becomes important to consider how they may be used. In the
following discussion, we consider landmarks within the context
of local navigation near the loft, but the ideas can be generalised
to landmarks use at distant familiar locations as well. Two
important questions emerge: (a) What is the nature of the
spatial or location purposes (e.g., "my location is north
of the loft") or are they also employed in guiding orientation
response (e.g., "therefore I should fly to this side
of the water tower").
With respect to the first question, two
possibilities occur. First, pigeons may rely on landmarks
in the form of a mosaic map, whereby a pigeon learns the fixed
directional relationships of landmarks or landmark complexes
with respect to the loft and/or other landmarks. For example,
a pigeon relying on such a system, acquired through experience
and coded in terms of compass directions, could learn that
the water tower lies north of the loft, or perhaps that the
water tower lies north-east of the rubbish dump/truck garage
complex, with pigeons associating a single compass directions,
or a limited set of compass directions, with respect to any
given landmark. In this model each landmark is used as an
independent source of information. Use of a mosaic landmark
map has been hypothesised to occur at familiar release sites
where pigeons are able to orient homeward by having presumably
associated a site-specific homeward orientation response with
a single set of local landmarks during previous visits.
A second possibility is that landmarks are
not used independently as signposts, but collectively in the
form of an integrated map. Such a system is characterised
by the use of at least a subset of the same group of landmarks
from effectively any location where sensory contact with them
would be possible. Such a mapping system is advantageous because
the same pool of landmarks could be used over an infinite
number of locations within an area of familiarity. The superiority
of such a cognitive mapping system compared to the mosaic
system is effective only from specific locations, for example,
the water tower, where a pigeon would have had the opportunity
to associate a limited set of directional responses with respect
to goal areas. The cognitive mapping system confers considerably
more flexibility in getting home from anywhere within a known
area.
*This series of articles has been adapted
from Spatial Navigation in Birds, Chapter 14, Neurobiology
of Comparative Cognition". (R. Kesner & D. Olton,
eds. 1990, Erlbaum, Hillside, N.J.)
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All Pictures © Gary
Hardy (1990)
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José P Ribas
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