Migrating songbirds use light patterns at sunrise and sunset to calibrate their internal compasses, scientists have found.

Researchers knew birds used directional cues from the sun, star patterns, Earth's magnetic field and polarized light patterns in the sky to guide their migration — but how the birds combine the information to navigate has been a mystery for 30 years.

No longer: U.S. researchers report in Friday's issue of the journal Science that the crucial navigational tool for birds stems from their use of polarized light near the horizon.

Biologist Rachel Muheim of Virginia Tech and her colleagues showed that captured Savannah sparrows from the Yukon used polarized light patterns to set their compasses when migrating south.

Polarized light is light that oscillates in one direction.

At dawn and dusk, a band of polarized light intersects with the horizon 90 degrees to the right and left of the sun.

Migrating birds average the intersections from sunrise and sunset to find the Earth's north-south axis — forming a reference that doesn't depend on latitude or time of year, the researchers said.

Previous experiments used funnel cages that blocked the birds' view of the lower 20 degrees, leading to conflicting results.