for National Geographic magazine
The bobolink, a North American relative of blackbirds, finds its wintering grounds in South America by reading the sun and stars, Earth's landscape and magnetic fields, and polarized light humans can't even see.
This songbird travels 6,000 miles (9,656 kilometers) with no guide—and he's only a few months old.
He's just one star in the sky. Millions of palm-sized songbirds streak south on fall nights, clouding weather radar displays like storms.
Geese, birds of prey, and shorebirds migrate, too, sometimes crossing entire continents.
Pale terns and albatross zigzag across oceans, covering millions of miles in a typical 50-year lifespan and completing some of the longest flights known to science.
Even though migratory birds are among the world's most accomplished air travelers, the mysteries of how they do it and where many species go remain unsolved.
Up to 80 percent of all North American forest birds make seasonal treks, but ornithologists have only recently started tracking long-distance migrants beyond their summer breeding zones.
And they are only beginning to fill in major information gaps to answer basic questions about these birds' abilities, their habits throughout the year, and the risks they may encounter.
Even though scientists still don't know where many avian migrants spend their non-breeding season—tracking full migrations is a relatively new field—they do understand these birds face unprecedented, serious threats.
And that's of urgent concern since migratory birds are experiencing precipitous declines.
"In the 1970s and 1980s, there was this epiphany that, 'Oh my God, these birds not only migrate, but they might spend six months in the tropics and another few months getting to the tropics,'" said ornithologist Russell Greenberg, director of the Smithsonian Migratory Bird Center in Washington, DC.
"Now that migratory bird populations seem to be declining or changing a lot, everyone's desperate."
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