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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.

Kvist's team trained four red knots, each of a different body mass, to fly in a wind tunnel for between six and ten hours and measured how much power was required for each bird to fly.

The researchers found that heavier birds did not expend as much energy as expected to carry the extra weight.

Apparently the heavier birds use their muscles more efficiently, says bird conservation specialist Trevor Lloyd-Evans of the Manomet Center for Conservation Sciences in Massachusetts. Flying with a greater fat, or fuel, load on long migrations is worth the energy required to carry the extra weight, added Lloyd-Evans.

A third team of researchers led by Henri Weimerskirch, of the Centre National de la Recherche Scientifique in Villers en Bois, France, measured the energy saved by flying in formation.

Weimerskirch's team trained great white pelicans to fly after a motorboat and an ultralight plane while they measured the animals' heart rates during flight and filmed the birds to determine the frequency of wing beats.

The researchers found that pelicans flying in formation had a heart rate that was between 11 and 14 percent lower than that of pelicans flying solo. They also found that pelicans flying in formation beat their wings less frequently than solo birds and were able to spend more time gliding.

Migrating birds take note: fatten up, fly in formation, and pay attention to the Earth's changing magnetic fields.


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