NAVIGATION IN BIRDS
Birds use a number of methods to find their way during migration. Many use celestial navigation, a method of orienting the body to the arc of the sun, to the phases of the moon, or to the pattern of stars in a particular season, which is called menotaxis. Hummingbirds and pigeons are able to determine the position of the sun even on overcast days because they can detect the ultraviolet radiation it emits.
Experiments in planetarium on night migrant birds, such as white throated warblers and indigo buntings reveal that they orient themselves by the position of stars in the night sky.
Some birds are sensitive to coriolis force that arises by deflection of winds in the northern hemisphere by earth’s rotation.
Some diurnal birds use topographical landmarks such as mountains, river valleys, and forests to orient themselves on the migration route. Some are able to detect infrasound or low-frequency sounds that are produced by ocean waves. Many birds, particularly seabirds, identify their destinations by characteristic odours.
Many birds possess instinct or some kind of internal compass or biological clock that guide them through the route of migration. Young birds follow the migration route accurately without previous training or experience by their inherent capacity to navigate.
Some birds such as oil birds of South America and Himalayan cave swift possess echolocation and can be guided by it.
The classic experiment proving the internal-clock theory was done by German Gustav Kramer during the early 1950’s. He placed Starlings wanting to migrate in a cage from which they could see the sun. The birds would sit looking in the direction toward which they wanted to fly. Significantly, if the Starlings couldn’t see the sun, they didn’t face in any particular direction.
Also during the 1950’s, the German Franz and Eleonore Sauer did a similar experiment with birds that could and could not see the night stars and arrived at similar results. Certain species can orient themselves according to the sky’s major stars. In fact, an experiment with Mallard Ducks found that if the moon is so bright that important stars are hidden by glare, released ducks can’t orient themselves as well as on darker, moonless nights.
Some birds, such as pigeons, are sensitive to changes in the earth’s magnetic field because of the presence of magnetite in their head and neck muscles. During early 1970’s, W.T. Keeton tied small, bar magnets on the backs of pigeons. When released at locations the birds had never seen before, the pigeons with non-magnetic bars found their ways home but those with bar magnets got confused.
In a 2007 German scientists found tiny iron oxide crystals in the skin lining of the upper beak of pigeons, which might be of help to the birds to sense the earth’s magnetic field and assist them to identify their geographical position.
The researchers also discovered cryptochromes, which change their chemistry in the presence of a magnetic field, in the retinas of migratory birds’ eyes. The molecules might then affect light-sensing cells in the retina to create images due to magnetic field and help the bird to navigate during flight.
Infrasound travels much farther than ordinary sound and it comes from many different natural sources, including ocean waves, surf, winds, storms, earthquakes and other geologic events. It is believed that birds can hear infrasounds that we cannot hear and hence they possess this accessory navigational capability.
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