The Dawn of Polar

Fossils hint at when birds began making their mind-blowing journey to the Arctic to breed

- BY LAUREN N. WILSON AND DANIEL T. KSEPKA

GOLDEN AUTUMN SUNLIGHT GLINTS through the sedges and shrubs of the tundra in northern Alaska.

Winter is approaching, and soon the region will be buried under snow and ice. For the past three months the chatter of the Arctic Tern colony has served as the soundtrack of the summer breeding season. But now, with daylight waning, the terns need to head south. In an instant, the usually noisy birds will fall silent, a behavior known as "dread." Moments later the entire colony will take to the skies to begin its 25,000-mile journey to Antarctica—the longest known migration of any animal on Earth.

The Arctic Tern is not the only bird that spends its breeding season in the Arctic. Billions of birds belonging to nearly 200 species—from small sparrows such as the Smith's Longspur to large waterfowl such as the Greater White-fronted Goose—make their way to the far north every spring to reproduce and then make the return flight south for the winter. It's no easy feat. Migration is costly. Even under ideal conditions, such an epic journey requires huge amounts of energy and exposes the travelers to dangerous weather. The mortality risk is high.

But undertaking these trips allows the birds to take advantage of the seasonal conditions in these environments. The endless summer sun supports lush plant growth, flourishing insect swarms, and plentiful fish populations nourished by zooplankton blooms. With 24 hours of light a day, the birds can more easily catch food such as slippery fish and tiny insects. The round-the-clock daylight also means many of the animals that prey on birds are less likely to sneak up on a nest unnoticed.