About 1,800 of the world's bird species are migratory. Some of these journeys are among the longest in the world.
The tiny Arctic tern (pictured here) undertakes nature's longest migration, spanning as many as 44,000 miles annually as it zigzags the distance between the Arctic and Antarctic. (Sooty shearwaters deserve an honorable mention for making a similar journey.) Seabirds like the albatross spend more of their lives flying than they do at rest, and bar-tailed godwits undertake the longest non-stop flight of any bird, between New Zealand and China.
Penguins also migrate, and they deserve credit for making their incredible journey through the ocean, rather than through the air.