Navigation
After their release, the birds may circle overhead a number of times while they get their bearings. Our white release birds navigate home using a number of techniques:
- Internal Map & Compass: Pigeons can compare the position of the sun to a mental map and their internal time-clock to determine which direction to fly to get home. If their internal time-clock is shifted by artificial lights on timers, they may travel the wrong direction. If they are released in a new location and they think it is noon, but they see the sun rising in the east, they will fly in the wrong direction. «more»
- Magnetic Sense: Homers have a tiny piece of magnetite in nerve endings near their upper beak. These neurons are very sensitive to changes in earth's magnetic field. This sense is not well understood. When Mt. St. Helens erupted, it caused racing pigeons problems navigating home, possibly because the rising magma altered the magnetic field. When a sun spot 'burps' out charged particles (protons & electrons), the particles disrupt earth's magnetic field and create auroras. These geomagnetic storms often interfere with a homing pigeon's ability to navigate home.
- Vision: Most birds have fantastic vision and a far better visual memory than humans. Buildings, roads, and terrain are important landmarks homing pigeons learn as they navigate home. Some homers have been GPS tracked following roads home, even if the road is not the most direct way. But homing pigeons can navigate home from places they have never before seen and can even navigate back to their home town when their vision is blocked by frosted contact lenses. That tells us that pigeons use more than just visual landmarks to return home.
- Magnetic Vision: Certain cells in some bird's retinas (in their eye) sense and respond to the Earth's magnetic field. Since this sense requires light to function, it appears that our birds may actually be able to see which way is north. Migratory birds seem to get one type of visual signal when they are on the correct path and a different signal when they deviate from their migratory path. «more»
- Violet Vision: Birds can see colors that humans don't recognize. We have three color receptors: red, green, and blue (RGB). Birds have a fourth color receptor sensitive to violet (or, in some species, ultraviolet) light. They also have extra sensitivity in the blue region, and color droplets on their color receptors (cones) that we don't have. These additional sensitivities may give them a whole dimension of color and contrast we lack.
- Polarized Light Vision: Birds have a second type of
color receptor not found in mammals-- double cones.
I couldn't find any definitive information about double cones,
but one hypothesis is that they may be able to
detect the polarization of sunlight.
As a result of their extra sensitivity to color, UV, and polarized light, birds may be able to detect color gradients in the sky that are invisible to us. Where we see a solid blue or overcast sky, they may see colors oriented around the sun similar to what we see during a gorgeous sunset or colors towards the poles like we see during a display of the northern lights. «more» - Smell: Some pigeons who have had their sense of smell blocked do not home as well. Salmon, too, use their sense of smell to navigate from the ocean back to the stream location where they were born.
- Hearing: Our birds can hear far lower sounds than we can. In some areas, the sound of waves crashing on shore or the wind hitting local structures like cliffs, trees, buildings, or other obstructions may create unique low-pitched sounds that travel long distances that help some pigeons find their way home. When the supersonic Concord flew over pigeons raceing across the English Channel many birds were lost. Perhaps the sonic boom disoriented them.
- Other? Some birds that migrate at night use the stars as a guide instead of the sun.
Different birds rely on different clues.
Young birds have a tendency to rely heavily on magnetic clues to navigate. Our first year birds have a tougher time navigating during solar eruptions. As they age, the homers learn to use other clues, like landmarks, roads, and smells. Birds who grow up in lofts exposed to winds from all sides rely more heavily on odors to navigate home than birds who grow up in more sheltered lofts.
Puzzle
This is one reason why understanding navigation is so difficult. Genetically similar birds raised and/or trained under different circumstances may rely on different senses and may perform differently during navigation experiments. That certainly helps keep bird navigation a fascinating puzzle.