Attend to me, and not to them said the stork-mother: after the grand manoeuvre we fly away to a warmer country, far, far from here, over mountains and forests. To Egypt we fly, where there are three-cornered stone houses, which rise up into a point above the clouds; these are called pyramids, and are older than a stork has any notion of. In that country is a river, which overflowing its banks, turns the whole land into slime, and all one has to do is to pick up the frogs ("The Stork" Hans Christian Andersen).

a. Moving with the seasons

Birds, such as the storks in Hans Christian Andersen's tale, migrate to feast on food supplies that increase in warmer climates as they dwindle in others.

Life on Earth is characterised by change: as the earth rotates on its axis around the sun we are caught in the cycles of day and night and the changing seasons. Even in the tropics, seasonal change causes a variation in temperature and food supply. All creatures must adapt to these cycles if they are to survive. Faced with environmental variability, animals have a choice: (1) to remain at their breeding grounds in the winter, withstanding diminished food availability and lower temperatures (some animals hibernate to overcome this), or (2) to move with the seasons. Several attributes pre-dispose birds to migrate³:

• the ability to fly
• a relatively long life-span
• year-round homoiothermy