Other Environmental Themes

Tracking Migrators

In 1899, a Danish biology teacher named Chris Morton became curious about the movements of starlings. He captured some birds in his backyard and attached bands to their legs along with instructions for contacting him if any of the creatures were later found. The practice of bird banding has expanded dramatically since then. Today, there are roughly 6,000 licensed banders in the United States and Canada alone. More than 60 million birds have been banded in North America, representing almost all of the continent's hundreds of species. The technique, which has transformed our knowledge of avian migration, has one significant drawback: Only about two percent of banded birds are caught later on.

For this and other reasons, investigators have developed a variety of sophisticated methods for studying the travel habits of migratory animals. Radar, for example, has yielded great insights on bird movements since World War II, when British radar systems recorded the passage of unidentified targets subsequently shown to be birds. Today, doppler radar technology provides relatively inexpensive estimates of the number of birds flying over large areas, their flight speed, altitude, and direction. Radar is useful for tracking bird migration since most birds travel at night when turbulence is lowest, some flying more than 20,000 feet above the ground. The technology has also become an important conservation tool for identifying critical stopover and roosting areas.

Researchers at Wake Forest University have attached tiny radio transmitters to the backs of albatrosses, learning that one bird flew nearly 25,000 miles in a three-month period. The Wake Forest team relied on Argos—a tracking system installed on two U.S. weather satellites that can determine the position of a signal beamed from anywhere on Earth to within about half a mile. Similar devices have been attached to Atlantic bluefin tuna and California's great white sharks. A 2001 study in the journal Science found that bluefin tuna were capable of rapid long-distance migrations; their elevated body temperatures, even in frigid water, enhanced the power output of their muscles. A 2002 article in Nature revealed that great white sharks migrated thousands of miles, traveling to Hawaii and beyond—toppling claims that the predators stayed near the California coast.