How come birds know which way to fly for the winter? asks reader Daniel Dreher.Some birds journey hundreds to many thousands of miles on their twice-yearly journeys. Familiar landmarks may play a part. And some birds, like sailors, look for patterns in the stars. But birds also sense the Earth's invisible magnetic field, arcing out from the Poles.

For many years scientists have been studying -- and hotly debating -- what's behind the uncanny ability of migrating birds to find their way to their winter and summer homes. One thing on which scientists agree: Migratory baby birds are already equipped with a kind of preset inner compass. This compass leads them to make their very first autumn flight in the direction carved out by the rest of their species. So scarlet tanagers, cerulean warblers and purple martins are born knowing which way is home.

However, if the fledgling birds are moved far away from their usual starting points, they never reach their warm winter homes in the South. Experienced adult migrators, however, remember their routes, making corrections as needed.

How do migratory birds develop such remarkable inner maps, combining information from many different sources? Some tantalizing clues have emerged from decades of studies.

The most important navigational tool of all, many scientists say, may be magnetism. The Earth is an enormous magnet, field lines looping out and connecting North and South poles. To migrating birds, the planet's magnetic field may be like a map of the world, readable even on the darkest, stormiest nights.

How do birds sense the force fields of magnetism? Recent studies point to chemical reactions. Tiny chemical changes triggered by shifting magnetic fields may occur in the birds' eyes, and involve a blue-light receptor called cryptochrome. Scientists say birds may actually see the Earth's magnetic fields, perhaps as a kind of blue glow in the Southern and Northern skies.

A 2006 study found birds may calibrate their inner magnetic compass using sunlight. At sunset and sunrise, an intense band of polarized light extends across the sky at a 90-degreee angle from the sun. (Polarized light is light whose vibrations occur in a single plane.) Since the sun sets or rises more to the south or north as the seasons pass, the polarization band shifts too. Birds look at the horizon, the study shows, and may use the light to recalibrate their built-in GPS.

Seeing magnetic force lines may be yet another enviable bird ability, only trumped by the ability to fly. But a 2010 study suggests that a sense we can relate to -- smell -- may be even more important to navigation. Researchers found that smell-impaired birds chose a southerly route when they should have flown southwest, unlike birds who could follow familiar odors.

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