Migration

The migration behaviors of different species of animals can be categorized as either complete, partial, differential, or irruptive. In complete migration, all of the members of a population will travel away from their breeding habitat at the end of that season, often to a wintering site hundreds or even thousands of kilometers away. The arctic tern (Sterna paradisaea) is an example of a complete migrant. Individuals of this species travel from the arctic to the antarctic and back again during the course of a year, a round-trip migration of more than 18,600 mi (30,000 km).

In other species, some individuals will remain at the breeding ground year round, while other members of the same species migrate away. These species are called partial migrants. American robins (Turdus migratorius), considered indicators of spring in some areas, are year-round residents in others.

Differential migration occurs when all the members of a population migrate, but not necessarily at the same time or for the same distance. The differences are often based on age or sex. Herring gulls (Larus argentatus), for example, migrate a shorter and shorter distance as they grow older. Male American kestrels (Falco sparverius) spend more time at their breeding grounds than do females, and when they do migrate, they do not travel as far.

Irruptive migration occurs in species which do not migrate at all during some years, but may do so during other years, when the winter is particularly cold or food particularly scarce. For example, some populations of blue jays (Cyanocitta cristata) are believed to migrate only when their winter food of acorns is scarce.