Every year, the survival of grizzly bears in North America depends on a spectacular natural event: the return of hundreds of millions of salmon from the Pacific Ocean to the mountain streams where they were born.

This great return provides food not only for bears, but for killer whales, wolves, bald eagles and the very forest itself. Will the salmon return in time to keep the hungry bears alive?

The story of a lion family's struggle to survive until the return of the great migration.

A look at the sardine run, which happens each winter along the South African coast.

Millions of animals time their migration across the Kalahari Desert to coincide with the the annual flooding of Botswana's Okavango Delta.

The summer plankton bloom in the north-east Pacific draws in animals in great numbers, from the billion-strong shoals of herring to migratory humpback whales.