It takes two compasses to fly right.

Drivers who never admit they're lost or ask for directions may fancy themselves as skilled as migrating birds. These winged travelers can stay on course for thousands of miles, thanks to two internal compasses that track magnetic and celestial cues.

Sometimes, however, these compasses provide conflicting information. Magnetic and geographic north do not coincide, particularly in upper latitudes. In these regions, birds heed the celestial advice; the magnetic cues appear to be redundant-or so scientists had believed.

Actually, "the interaction between the two cue systems is far more complex than this," Peter Weindler and his colleagues at J.W. Goethe University in Frankfurt am Main, Germany, report in the Sept. 12 Nature. Certain types of migration appear to require the magnetic compass.

"Just when we thought we understood [avian navigation], a new set of experiments . . . adds a new level of complexity," asserts Princeton University's James L. Gould in an accompanying comment. The team's findings are "remarkable," he concludes.

Migratory birds are born with magnetite crystals above their nostrils that enable them to detect magnetic north. If they grow up under normal conditions, they also develop their ability to navigate using the stars. By the time they take their first trip, young birds have somehow acquired a sense of their route.

Weindler and his colleagues studied garden warblers (Sylvia borin), which breed in central and northern Europe and winter in Africa. The birds from central Europe don't migrate directly south, because they must veer around such troublesome barriers as the Alps. So on their autumn trips, they first fly southwest. At the end of September, when they reach the Iberian peninsula, they head southeast.

The researchers removed warbler chicks from their nests in the wild and raised them by hand. During the summer before their first migration, the birds stayed in cages equipped with an artificial, rotating sky containing small lights that mimicked the constellations. For one group of birds, the researchers blocked the local magnetic field.

In August, at the beginning of their normal migration period, both groups of birds were placed in cages that provided only celestial cues. The team then monitored the direction toward which the warblers oriented themselves. Birds that had been exposed to both celestial and magnetic stimuli "showed the seasonally appropriate tendency to orient to the southwest," the authors report. The others faced due south.

This suggests that garden warblers-and probably similar birds-must be exposed to both Earth's rotating sky and its magnetic field before their first migration if they are to establish their population's unique course, Weindler and his colleagues conclude.

Neither group of warblers changed its orientation later in the season, when the migrating birds ordinarily turn to the southeast. This supports previous studies showing that birds alter their course appropriately when given only magnetic cues.

Although early exposure to the stars allows a bird to migrate south even under unusual conditions, the magnetic field enables them to deviate from this basic direction, the team says. Celestial information clearly "provides only a default direction for migrants," Gould notes.