Many species of birds that migrate are clearly born with a sense of direction that often improves with experience. However, they need accurate navigational cues for orientation.
Although we still have much to learn about animal migration, the evidence suggests large shifts in the direction of the geomagnetic field may not seriously impact bird migration, as birds usually do not navigate using just one kind of compass. In fact, birds use a variety of different directional information cues when they migrate, including the sun, stars, and polarized skylight patterns — as well as the geomagnetic field. Rays of polarized light vibrate in one plane. They are difficult for humans to see, but birds use them, especially at sunrise and sunset.
Before their migration starts, birds often use sky cues to set direction and recalibrate the geomagnetic compass. Until recently it was thought that birds do not recalibrate the compass during travel but rely on other cues for accurate navigation. However, scientists have found that birds will recalibrate it, provided they have enough access to other aids for orientation such as polarized skylight and the horizon.
This flexibility is probably an important adaptation that helps migrating birds account for the difference in the direction of the geomagnetic pole compared with the geographic pole at any given location on the earth’s surface. So it is probable that many bird species that migrate would be able to adapt to changes in the geomagnetic field and still be able to migrate successfully.
Also, a reversal of the magnetic field takes several hundred years. Although that is quick in geological terms, it is ample time for many bird species to adapt while continuing to use the geomagnetic field as a major part of their navigational toolkit.
—Nigella Hillgarth, biologist, executive director, Birch Aquarium at Scripps
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