Navigational Aids of the Returning Snowbirds
A researcher who studies dark-eyed juncos once told me a particular junco would, after a journey of several thousand miles each spring, perch in the same Alaska valley. In the same tree. On the same branch.
How did the junco find the way back to its favorite Alaska tree branch after wintering as far away as Missouri? After conducting a simple experiment with garden warblers, a group of German researchers suggest migratory birds might use both the stars and Earth's magnetic field to navigate. Their study was detailed in a recent issue of Nature.
Biologist Peter Weindler and several colleagues at J.W. Goethe University in Frankfurt, Germany, studied garden warblers, a species of bird that migrates to northern Europe from Africa in springtime. As do many birds that migrate to Alaska, garden warblers fly great distances at night.
In autumn, garden warblers in northern Germany trek back to Africa, but they don't fly due south. Perhaps avoiding such unpleasant obstacles as the Alps, the Mediterranean Sea, and the Sahara Desert, the birds fly southwest from Germany to Spain. From the southern tip of Spain, the warblers cross the narrow gap of ocean between Spain and Africa, and continue on to southern Africa, where the birds spend the winter.
This several-hundred mile journey could be shortened significantly if the birds flew directly south, rather than making the detour through Spain. Whatever the reason for the garden warblers' dog-leg route, it provided the researchers with an opportunity to gain insight on how birds find their way during migrations.
The scientists captured several warblers when the birds were four to six days old. They raised a few of the birds using a method that exposed the birds to the stars but denied them the effects of Earth's magnetic field, the force that makes compasses work.
When it was time to migrate, the garden warblers that experimenters exposed to both the stars and Earth's magnetic field flew southeast, just as wild garden warblers do. The warblers that didn't feel the effects of Earth's magnetic field as they were developing flew due south.
The researchers concluded that the warblers used Earth's magnetic field to fine-tune their sense of migratory direction. As garden warblers and other birds develop, they may combine the visual image of star patterns with magnetic information they feel to ingrain a migration map in their brain.
Migrating birds may also use visual clues other than stars to wing their way north. In his paper, "The Visual Problems of Nocturnal Migration," G.R. Martin, of the University of Birmingham, in England, points out that birds may also orient themselves by observing the setting sun, the moon, and familiar mountain ranges, rivers, and other obvious landscape features.
The mallard duck and other birds have eyes that allow the bird total panoramic vision, Martin wrote. On cloudless nights, the mallard's eye placement allows the bird to see the stars while perhaps simultaneously relating it to the darkened landscape below. Martin, and Alaska researchers I talked to, suggested that birds probably find their way using a combination of clues: stars, Earth's magnetism, visual landmarks and possibly even their senses of smell and hearing. These senses and sensations all may help guide a snowbird on its incredible spring journey; a trip that may end with tiny feet clasping a familiar black spruce branch.
