National Geographic photographer Joel Sartore documents birds making their annual migrations north -- comets of color that splash across the landscape in Spring.
Left: Sandhill Cranes flying over the Platte River in Nebraska.
Many have just flown in from South America. How do they do it? We haven't got a clue.
Left: A Whooping Crane (Grus americana) and chick.
Biologists think warblers may have a celestial chart in their heads to navigate by - or, rather, a double map, because the stars are different in Spring than in Fall.