The incredible journeys of migratory animals 

It isn't unusual for animals to migrate to avoid harsh winters, but the sun-seeking Arctic tern takes this strategy to extremes. The small seabird flies between the Arctic and Antarctic to take in two summers each year and more daylight hours than any other animal. Making a round trip of 35,000 kilometers (21,748 miles), the tern breaks all records for migratory distances traveled.

If the tern takes prize for distance, the salmon surely deserves special commendation for effort. Hatching in rivers, they swim downstream to spend most of their lives at sea. But the hard work comes as they battle against the current and leap up waterfalls to make it home. If this wasn't struggle enough, hungry bears, eagles and people lie in wait for the exhausted fish as they near their goal.

By day, straw colored fruit bats hang from Africa's city trees like assemblages of broken umbrellas. But at twilight, they take gracefully to the air, traveling up to 180 kilometers before dawn and dispersing seeds and pollen as they feed. They span even greater distances by season, and in colossal numbers. Each fall, around 10 million of these "megabats" arrive in Zambia's Kasanka National Park.

Many whales hunt in polar regions but can travel 18,000 kilometers each year to enjoy warmer waters. Scientists had assumed they prefer to give birth in the tropics. But new research suggests they might be migrating for the sake of their skin. Whales need to molt, and in icy waters where their blood supply is drawn away from the skin, dead cells build up and put them at risk of infection.

It's hard to imagine a creature as tiny and fragile as a butterfly undertaking epic migratory journeys. And yet, surfing air currents, the monarch butterfly can travel up to 3,000 kilometers. In summer, they're at home in northern regions of the US, but when temperatures fall, they head south to California or Mexico to overwinter. Roosting together in large numbers helps them keep warm all year.

Leatherback turtles travel up to 10,000 kilometers, from Canada to the Caribbean and Alaska to Indonesia. No one knows how they find their way from feeding grounds rich in their favorite foods like jellyfish to the beaches where they breed. But scientists believe a spot on top of the animal's head may allow light to reach its pineal gland, triggering its journey at the right moment of the year.

The mass movement of wildebeest across the African plains is perhaps the planet's greatest migration spectacle. With no real beginning or end, their circular route takes 1.5 million wildebeest, and a good number of zebras, gazelles and other grazers, through the Serengeti-Mara — crossing crocodile-infested rivers and dodging lions and packs of painted wolves —- in search of fresh food and water.

Famous for their parental devotion, emperor penguins lay their eggs a good 100 kilometers from the Antarctic ocean where they feed. Mother and father must take turns to travel across the ice, fill up on fish, and make the long shuffle back to regurgitate it for their young. Their partner, meanwhile, goes hungry for weeks to shield the chick from cold that would otherwise kill it in minutes.