Do Birds Use Magnetic Field to Plan Migration Routes?

Bijal P. Trivedi
National Geographic Today
November 2, 2001

A new study suggests that birds use the Earth's magnetic field to plan dining locations along their migration route. These culinary stopovers are critical to the success of the migration because, according to a second study, the fatter the bird, the more efficiently it is able to fly.

How birds obtain precise information about their geographical location and determine exactly where to stop and eat has puzzled scientists for decades. A study published in the November 1 issue of the journal Nature says that the cue may be magnetic.

Swedish researchers led by Thord Fransson, of Stockholm University, found that when thrush nightingales were exposed to a magnetic field simulating the four locations along their migratory route from Sweden to Egypt, the magnetic field around Northern Egypt caused the birds to stock up on food.

The stopover in Northern Egypt and increase in body fat is probably in anticipation of the long flight over the Sahara Desert. Fransson and his colleagues found that birds exposed to a changing magnetic field increased their mass by 3.5 grams (an eighth of an ounce) after reaching the magnetic field of Egypt. The 'control birds' that were exposed only to the local Swedish magnetic field gained only 1.1 grams.

While most migrating birds typically remain on the lean side, accumulating only small fat deposits—20 to 30 percent of lean body mass—physical challenges like crossing the Sahara or the Gulf of Mexico require much greater fuel reserves. Some birds, like the blackpoll warbler, are known to double their body mass before such excursions.

Understanding which locations are critical feeding spots along migration routes could prove to be valuable in efforts to conserve these habitats.

Fatter Birds Fly More Efficiently

Another study published a couple of weeks ago in the same journal brought to light a surprising aspect of bird migration: the fatter the bird the more efficiently it is able to fly.

A central theory of aerodynamics states that the energy needed to fly increases dramatically as the payload increases. However, researcher Anders Kvist, of Lund University in Sweden, and his colleagues found that the energy spent to carry the extra body mass is not as high as previously thought.

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