Magnetic field tells nightingales to binge.

Having to cross the Sahara in the middle of migration means a bird can't count on food for some 5 nights of flying. So, how does a first-timer know to take on extra fuel?

For thrush nightingales traveling from Sweden to southern Africa, the cue for a life-saving eating binge may come from changes that birds sense in the magnetic field, propose Thord Fransson of the Swedish Museum of Natural History in Stockholm and his colleagues.

They built lab equipment to simulate strengths and directions of the magnetic field along the route to Africa. When their magnetic field mimicked that in northern Egypt--the place to pack on fat for the trip across the desert--birds gained extra weight, the researchers report in the Nov. 1 NATURE.

Their work is "quite different" from earlier studies of magnetic cues, says Thomas Alerstam of Lund University in Sweden. Previous experiments showed that birds can use their magnetic sense much as a person tramping through the woods uses a magnetic compass--reading direction but not location. The new study, however, raises the possibility that the bird pinpoints a location by magnetic cues. "It's a very exciting possibility," Alerstam says.

Thrush nightingales, or Luscina luscina, don't flock to migrate, explains Fransson. Each bird travels by itself, flying up to 450 kilometers at night and resting during the day. The trouble comes near the end of this journey, when a bird has to cross some 1,500 km of Sahara. Lucky birds may find food and shelter along the way, "but this is not a strategy that birds can rely on," Fransson says. Fattening up to 20 percent above lean body weight is typical for most migrating birds, he says, but the nightingales probably need to double their weight before crossing the desert.

Fransson and his colleagues collected lean, 20-gram youngsters on the brink of their first migration and caged four at a time with abundant food inside a big magnetic coil. For comparison, the scientists kept eight birds in similar housing but with no changes in the magnetic field

By altering the coil's magnetism, the researchers mimicked changes during four hops in a 6-day trip to northern Egypt. During their simulated stay in Egypt, the birds gained 3 g of weight while the control birds gained 1 g.

"It makes excellent sense that birds should do this," comments Kenneth Lohmann of the University of North Carolina at Chapel Hill. He and his colleagues recently showed that hatchling turtles change their orientation when a magnetic field shifts from "Florida" to "Portugal."

Fransson cautions that the researchers need to do more experiments to see whether the birds respond to Egypt's magnetic qualities or to the difference between Sweden's and Egypt's.

Just how animals sense the magnetic field is "very poorly resolved," says Pavel Nemec of Charles University in Prague, Czech Republic. He found areas in the mole rat brain that become more active when the animals are using their magnetic sense while nesting. Salmon and newts can also detect magnetic fields.

"The most striking question is whether [magnetic sensing] has an ancient origin or whether it has evolved independently," says Nemec.