Pigeon Homing

Homing in Pigeons

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.