Map-Free Travel

Most people stop and check a map when they take a long trip, or even on a short trip into a new area. You first double-check that you are where you think you are, then look at the route ahead to see what to expect in terms of route changes, distances, and obstacles.

In comparison, the map-free travel plans of birds have long been a mystery. How do they know where they are going, how far to go, or what route to take? Year after year, ornithologists have answered many questions about how birds get oriented.

They have learned birds use stars to navigate by. They have learned that birds have a built-in compass to orient by the magnetic poles of the planet, although they can’t tell magnetic north from south. (A very significant point for long-distance migrants that cross the equator.) And they know that birds can see polarized light, which enables them to orient by the position of the sun even on a fairly cloudy day.

In 2004, another piece of the migratory puzzle was solved. A team of scientists from the Illinois Natural History Survey, Princeton University, and Germany’s Oldenburg University, followed a series of Swainson’s and Gray-cheeked Thrushes that were tagged with radio transmitters and released at sunset for their nocturnal migratory flight. Some of the birds were simply “tagged” and released, while others were first exposed to artificial magnetic fields that were “off” by 80 degrees.

Control birds, as expected, flew due north when released. The birds whose magnetic “readings” had been skewed flew west for the first night, then rested during the day. The next evening, however, the control birds all corrected their course and headed north.

So what does it mean? Apparently birds are able to calibrate their internal compass using the location of the sun, and probably do so every evening before they take flight. It’s a significant factor for those long-distance migrants, who can re-orient after crossing the equator. Question #312 answered, two-and-a-half million to go!

-Janea Little, Senior Naturalist