c. What is migration?

She vowed to marry the King of the Cranes,
Leaving the Nile for stranger plains;
And away they flew in a gathering crowd
Of endless birds in a lengthening cloud.

Animals spend hours every day on the move, looking for food or for a mate, defending a territory, or avoiding animals higher up in the food chain. Other phases of life-history, such as juvenile dispersal, also inherently involve movement. It is not always easy to define migration as distinct from such movements.

Because of this, many definitions of migration have been advanced. In the following pages, however, migration is generally used to refer to the regular seasonal movement from breeding areas to resting grounds and back3. Birds that undertake such movements are sometimes described as obligate or annual migrants. At the opposite end of the spectrum lie resident birds that spend most of their year in one home range - so called resident birds. Some birds adopt halfway strategies, migrating during some years but not during others - for instance in particularly cold winters: these are referred to as facultative migrants.

Annual migrations can broadly be categorised according to distance: