Homing behavior in pigeons, Columba livia, is interesting because pigeons
find their way home from unfamiliar sites up to thousands of kilometers from
their roost. Pigeon races may feature releases of birds from France, for example,
which then find their way home to sites in England or the Netherlands. The extraordinary
reliability of homing pigeons makes them excellent subjects for studies of navigation.
How do pigeons find their way home when deposited in an unfamiliar location?
To do this, they must have two kinds of information. The first, called "map
sense" is their geographic location. The second, "compass sense" is the bearing
they need to fly from their new location in order to reach their home. If either
information source is disrupted, then homing fails or is delayed.
The map sense
In familiar surroundings--locations from which pigeons have previously
homed or landscapes through which they have flown--landmarks play a
predominant role in homing. Pigeons learn visual features of the landscape
and use these visual features to determine their current position (map
location) relative to their roost.
While pigeons clearly use visual landmarks, because pigeons orient
better in familiar landscapes when other sensory inputs, such as olfaction,
are eliminated, direct tests of landmark usage are diffiicult. Experiments
manipulating visual landmarks are generally not feasible. One can hardly
bulldoze mountains or cut forests as part of an experimental design, and
interference with the eyes, such as using contact lenses, may be so much of
a general disruption to the pigeon that it confouds tests of landmark
perception in orientation.
How do pigeons produce a map sense when they are released in a
completely unfamiliar location? The answer is that they use olfactory cues.
In their roost, they associate odors with wind directions. When released,
they assess the odor of their new location and extrapolate the map location
from their roost-gained knowledge of winds and odors.
Pigeons in visually unfamiliar territory whose sense of smell has been
disrupted (by cutting olfactory nerves or treatment of the nasal passages
with zinc sulfate solution) have a great deal of difficulty homing. Similarly, if
the roost is blocked from winds and provided with filtered air, homing fails.
Pigeons may home better if they have some time to olfactorily experience
their new surroundings prior to release.
One orientation mechanism that seems to have been experimentally
eliminated is path integration. There is no evidence that pigeons track their
movement when being transported in cages and then use path integration to
determine the distance and direction needed to reach home. Tests of this
possibility involve eliminating sensory inputs during transport. Because this
conclusion relies on a negative result (continued orientation ability in the
absence of a sensory input) there is a small possibility that pigeons use an
unknown (and consequently experimentally uncontrolled) sensory input for
path integration during transport.
The compass sense
The primary compass information of pigeons comes from the position
of the sun in the sky. By integrating their internal clock with the sun's position,
they compensate for the apparent movement of the sun across the sky. Pigeons
whose time sense is shifted by keeping then under artificial lights display
incorrect orientations when released. For example, if "sunrise" comes for the
pigeons 6 hours prior to actual sunrise, then their orientation is shifted counterclockwise.
If their "sunrise" is later than the actualy sunrise, then their orientation
shifts clockwise.
Like migrating birds, the pigeon's sun compass interacts with a magnetic compass.
Under some conditions, experimental modification of the magnetic field around
pigeons causes problems in homing.
Experiments with clock shifts and magnetic disruption often do not interfere
as much as expected with homing. This is because the olfactory and landscape
information used in establishing their map sense can be used to correct for
compass misinformation.
Another possible source of landscape information is the pattern of ultrasonic
reflections from mountains. Ocean waves generate infrasound which then reverbrates
through the atmosphere. However, the only evidence for use of infrasound is
the inference that sonic booms from jets may have disrupted pigeon races; the
hypothesis is that the sonic booms impaired the pigeons' hearing, causing them
to be unable to use ultrasound in orientation.
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copyright ©2001 Michael D. Breed, all rights reserved