Food and water

When the supply of food or water starts to run out, animals have to move. During long dry spells, for instance, herbivorous animals - ones that feed on plants - will migrate to greener areas.

Weather

Colder weather chases many animals to warmer climates. The minke and humpback whales migrate because their newborns do not have enough fat to keep them warm in cold winter water. So whale mothers give birth to calves in warm waters.

Insects

Because insects are so small, many cannot make the whole journey during long migrations. This means insects die on the way but new ones are born in their place. The monarch butterfly's trip comprises more than 1,600 kilometres between North and Latin America.

In Hong Kong, large groups of the common 'storm chaser' dragonfly form when a typhoon approaches. Crow butterflies - black or dark brown with white dots - also migrate.

Birds

Migrating birds are thought to use the stars, the sun and landmarks for navigation on their route.

The Arctic tern is the champion bird migrant, going all the way between the Arctic and the Antarctic.

Most local birds are migrants. More than 200 species make Hong Kong their part-time home.

Migrating together

Different species sometimes migrate together, as the zebra and wildebeest in Africa. Hyenas and lions prefer wildebeest meat to zebra so zebras have a better chance of survival if they stay with wildebeests.

Numbers

Migrating animals tend to move in large numbers. Wildebeests may number 1.5 million and zebras 200,000. Imagine lost calves trying to find their mothers in those big herds. Newborn wildebeests can run within minutes and after three days can keep up with the herd.

Swimming migration

Pacific walruses often float on ice floes during their 3,000km migration in the Bering Sea.

On TV: National Geographic Channel's Great Migrations will start running on November 7.